# Grandfather paradox solved?



## Andrew Green (Jun 18, 2005)

http://news.bbc.co.uk/2/hi/science/nature/4097258.stm

 Interesting stuff...

 Quantum theory is the reason Advil stocks are where they are today :S


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## sgtmac_46 (Jun 18, 2005)

I read that article and I can't help but believe that the "research" had a lot to do with repetative bong hits. Let me surmise the argument as I understood it: 

"We don't know that the laws of the universe will allow travel backwards in time, but we do know that you can't do anything to change your present, like kill your father...at least the part of your present you know about....if you are only partially sure, then you might be able to change the present....we think....because, for example, if you know your father is alive, you certainly can't kill him...that wouldn't make sense...but if you're only partially sure he's a live, then you probably can't kill him....probably."

That was enlightening.


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## Sam (Jun 19, 2005)

wait, I want to know about advil stocks.

what?!?!

I am so confused lol.


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## FearlessFreep (Jun 19, 2005)

I think it skirts on the Shroedinger's Cat issue, which I could never quite by. I mean, at a quantum level there is a probability that a particular particle is spinning one way or another and you don't know until you look, but to extrapolate that into saying that the cat is both dead and alive? I think the cat is dead or alive and will be so regardless of whether you observe it. I think the Cat is just an analogy for quantum probabilities and shouldn't be taken too seriously in it's own right.

 To extrapolate that into "You can't go back in time to kill your father because you know your father is alive and therefore *something* would keep it from happening" strikes more of mysticism in what that "something" is (or maybe psychology that you wouldn't do something you know is a contridiction).

 I think one sci-fi writer to the tact that the past was involitile, but this meant the that it was *really* involitile.  Evervy blade of glass was rock hard and razor sharp to the time traveller because for the traveller to walk on grass and bend the blades of grass was changing the past and the past could not be changed


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## Bob Hubbard (Jun 19, 2005)

It is the paradox of time travel.
You can't go back in time and kill your father because then you wouldn't be alive to go back in time to kill him.

By the same token, you can't go back in time and kill Hitler as a boy, because then you wouldn't have wanted to go back in time to kill him since he was never a problem so therefore he wouldn't be killed by you causing you to want to kill him.

Y'all with me on this?


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## Andrew Green (Jun 19, 2005)

yup 

 Of course I do believe that this impies that once we gain the ability to see the future, the future is no longer uncertain and we won't be able to change it.


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## bignick (Jun 19, 2005)

Unless you see more than one probable future?


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## arnisador (Jun 19, 2005)

All speculation and interpretation, I say. Maybe there's more science buried somewhere in it, but I'm not convinced.

 As for me, I _liked_ my grandfathers, so it's all moot anyway!


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## sgtmac_46 (Jun 19, 2005)

FearlessFreep said:
			
		

> I think it skirts on the Shroedinger's Cat issue, which I could never quite by. I mean, at a quantum level there is a probability that a particular particle is spinning one way or another and you don't know until you look, but to extrapolate that into saying that the cat is both dead and alive? I think the cat is dead or alive and will be so regardless of whether you observe it. I think the Cat is just an analogy for quantum probabilities and shouldn't be taken too seriously in it's own right.
> 
> To extrapolate that into "You can't go back in time to kill your father because you know your father is alive and therefore *something* would keep it from happening" strikes more of mysticism in what that "something" is (or maybe psychology that you wouldn't do something you know is a contridiction).
> 
> I think one sci-fi writer to the tact that the past was involitile, but this meant the that it was *really* involitile. Evervy blade of glass was rock hard and razor sharp to the time traveller because for the traveller to walk on grass and bend the blades of grass was changing the past and the past could not be changed


 I think you're on the right track there. I actually think they simply based it on Shroedinger's cat. I'm not so sure you can come to a conclusion about what IS or IS NOT possible to do in theoretical time travel (a monumental problem in it's own right) simply be discussing Particle/Wave duality.


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## arnisador (Jun 19, 2005)

Shroedinger's cat is from quantum physics...this is a relativity issue, isn't it?


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## sgtmac_46 (Jun 19, 2005)

Kaith Rustaz said:
			
		

> It is the paradox of time travel.
> You can't go back in time and kill your father because then you wouldn't be alive to go back in time to kill him.
> 
> By the same token, you can't go back in time and kill Hitler as a boy, because then you wouldn't have wanted to go back in time to kill him since he was never a problem so therefore he wouldn't be killed by you causing you to want to kill him.
> ...


Actually, I heard a far better example of this paradox by a physicist than the killing of an ancestor.

He stated that if you take two ends of a worm hole, and pull one of them at or near the speed of light for a distance, one end will open moments earlier than the other end. Worm holes are a widely accepted theory. Moreover, moving at or near the speed of light is understood to cause temporal differences based on Einstein's theory of relativity. So this presents LESS problems than simply traveling backwards in time, and is more likely possible.

In effect, anything that goes in one end, will come out the other end before it went in.

He proposed this experiment. If we line up the worm holes near each other, what happens if we toss a marble through one end of the wormhole, what happens if the marble comes out the other end and strikes itself, thereby preventing itself from entering the wormhole? Moreover, what happens when an astronaut flies through our imaginary worm hole and comes out before he went in, but this time pre-trip astronaut sees post trip astronaut, something post-trip astronaut didn't see before entering, and decides not to enter the wormhole, thereby making it paradoxically impossible for him to be there?

I always felt this was a more straight forward version of the problem that side steps a lot of irrelavent conjecture and gets to the core of the problem. No killing, no long term historical questions, just one HUGE PARADOX.

For example, are we now stuck with TWO marbles?  If we are stuck with two marbles, where did it come from?  There would appear now to be more mass in the universe that existed before, one marbles worth.

If there is only one marble, where did the other marble go?  If you are post-trip marble, do you simply disappear, die?  Irregardless, there were two marbles present at the same place and time where only one previously existed.


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## MA-Caver (Jun 19, 2005)

yeah... I've read versions a plenty of this concept. One of them was very complex involving a story where a guy was (all at once mind you) his father, grandfather, brother, cousin, uncle, aunt, grand mother and sister. Seems that when he traveled through time he was also changed alternately from a woman to a man... thus through the course of time became either or the other grandparent/sibling.  How's that for a mind bender? Wish I can rememeber who wrote that.. I keep thinking Heinlien or Foster. 

Another one of my favorite time travel (short-stories) involves a big game hunter that goes back 65 million years to hunt a T-rex... he accidently stepped on a catapillar and didn't say anything about it... when he came forward through time the world was destroyed and he couldn't go back because the device relied on modern technology to run it.  

Lots of the Grandfather paradoxes can be found... but is it really solved? I dunno. The theory still has some discrepencies to it.
But it is fun to bend one's thought waves all twisty and such once in a while.


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## Deuce (Jun 20, 2005)

I've always thought the infinite time lines idea of solving the paradox problem to be interesting. 

Within the universe there are infinite dimensions or time lines in which every possible "time" or situation exists. By travelling back through time, you are actually entering a different dimension that is very similar to your own, but at an earlier perceived time. In this time line, if you kill your grand father, you will not have been born, and the future will be changed in this particular time line only. So you can alter the paths of other dimensions, which would have been very similar to your present if interferance was non-existant. 

With this theory, you can't travel back in time in the traditional sense (in one particular dimension or time line), but can visit other dimensions that are very similar to our past. So, if you return to the exact dimension that you originally came from, the amount of time that you were gone will have passed in that dimension. Or, you could go to another dimension very similar to your own where you had just left. To the people of this dimension, it would appear that you returned the instant you left, even though, the two "yous" (the one that left, and then one that returned) are actually from different dimensions, but nobody would notice the difference because the two dimensions would be nearly exactly the same. 

Does anyone know if current physics supports or discouarages this theory? I have very limited knowledge on the principles of black holes, space-time, etc.


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## Andrew Green (Jun 20, 2005)

Deuce said:
			
		

> Does anyone know if current physics supports or discouarages this theory? I have very limited knowledge on the principles of black holes, space-time, etc.


 Yes, it's usually refered to as the many worlds theory in Quantum Theory. http://en.wikipedia.org/wiki/Many_worlds


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## still learning (Jun 20, 2005)

Hello, Why change things?  Do you think if we change some bad guys in the past that life might be easier?  Maybe we may have a more worst person than Hitler or Stalin.  Who knows?  Maybe the bad guys erase the good guys?  Who knows?  Arn't you glad we can't change the past but the future? .....Aloha


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## Tgace (Jun 20, 2005)

I once put instant coffee in my microwave oven...I almost went back in time.

If they invented powdered water, what would you add?


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## Sam (Jun 20, 2005)

Tgace said:
			
		

> If they invented powdered water, what would you add?


oxygen, obviously.

( I don't know why that is obvious but what the hey it was the first thing that popped to mind)


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## Andrew Green (Jun 20, 2005)

still learning said:
			
		

> Hello, Why change things? Do you think if we change some bad guys in the past that life might be easier? Maybe we may have a more worst person than Hitler or Stalin. Who knows? Maybe the bad guys erase the good guys? Who knows? Arn't you glad we can't change the past but the future? .....Aloha


 Well its not always a wanting to change things...

 If time travel does become a reality, and historians go back a couple thousand years to watch the Crucifiction of Jesus... The fall of Rome...The building of the pyramids... Dinosaurs...  or any other historical event, what danger to the present  (if any) does that pose.

 I don't think any scientist would go back and try to change a bad event in our past, the bad things help define who we are, and getting out of them made us stronger.  Any changes could be either very bad, very good, or do nothing at all.

 50 years ago there was a question of "If we walk on the moon, what will happen?" Fortunately scientists figured out enough to keep the people doing it alive before they stepped out of the ship 

 Let's hope they figure out enough about time before sending someone back...


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## Flatlander (Jun 20, 2005)

Travelling backwards in time is categorically impossible.  I say this because, were we to develop this capability in the future, would we not have already travelled back in time and screwed things up already?


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## Andrew Green (Jun 20, 2005)

Can you prove that we haven't?

 Not to say that anyone can prove that someone has come back.  But hopefully by the time that sort of technology is around those using it will know enough not to go back and start messing with things.  Or, if that multiple worlds theory is true, it doesn't matter.  As all you will do is spawn off new dimensions anyways.  And they would have been spawned no matter what you did, as long as there was the possibility of you doing them.


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## Shizen Shigoku (Jun 21, 2005)

*AG: "Interesting stuff..."*


That article's ideas weren't all that new; at least they've been around, as far as I know for ten or more years. It's certainly been around longer in science-fiction, as well as armchair speculation among the past century's scientists.

And it only covers quantum theory as it applies to the probabilistic state of the universe.

*arnis: "**Shroedinger's cat is from quantum physics...this is a relativity issue, isn't it?"*

Both theories, while not completely compatible with eachother (especially in regards to forces such as gravity - sheesh!), can be used simultaneously to look at problems involving thought-experiments to gain different perspectives.

*sgt: "**I read that article and I can't help but believe that the "research" had a lot to do with repetative bong hits."*

Better get a four-footer for this one!

- As I understood the argument:

You probably can't go back in time and affect a change in the present because the uncertainty of the past no longer exists, but the uncertainty of the present makes it highly unlikely that anything you do now (such as travel backward in time) will affect what is happening now, as the probability (uncertainty) collapses (diminishes) on the time travel event . . . 

I think someone once said that there are only two people in the world that understand quantum mechanics. That was a long time ago and there may be more than two now, but still I - on the subject of QM and time-travel - am not one of those people.

So, here is . . .

Relativity Theory as I understand it: 

Energy is proportional to mass by a relationship that only involves velocity (speed). For total mass to energy convergence that velocity is light-speed, therefore any material body (mass) acheiving light-speed becomes light by definition. 

This also implies that matter and energy are in-fact the same (M=E), and the only difference in effect on space-time is how fast M=E is moving.

Effects of near light speed (NLS), light speed (LS, or c) and (hypothetical) faster-than-light speed (FTL) according to relativity are that NLS bodies* undergo little significant departure from Newtonian physics until very close approaches on the limit of c. 

(* - for this discussion, a body is defined as a mass, force field, or quantity of energy as E=M, proportional to speed.)

Approaching c (light speed), bodies experience simultaneous contraction and dilation of space-time, i.e. space contracts (distances become shorter), and time dilates (passage of time slows down). At c, all bodily mass will have been converted to energy, and will in effect have become light. 

Theoretical FTL speed effects on E=M bodies seemingly would follow the pattern of space-time contraction & dilation. At the limit c, the following characteristics of a body have been fixed with a sort of 'ultimate inertia' - distance becomes zero, time has stopped, and mass is infinite. Of course, a particular body - a quantity of light (quanta of photons) - is traveling at c over non-zero distances, in measurable time, and with no mass besides it's potential in equivalent E->M conversion. 

So what happened to all the mass that acheived 'ultimate inertia'? Following the relativistic pattern into FTL, distance goes less-than-zero (hyperspace dimensions, etc.), time runs in reverse, and the infinite, ultimately-inert, mass-energy body continues to increase speed in the now real alternate space, but since it is traveling backwards in time is slowing down in imaginary space or vice-versa (for math geeks: review "i" and complex numbers, and skim through some Hawking). 

What happens to the body (if we can call it that anymore in regards to its mass-energy relationship) is uncertain, but I believe theoretically (under the assumed limits) that the corresponding changes in space-time behavior would be dependent on velocity.

So, relativistic-theoretically, time travel to the past is possible - if you're willing to give up all your mass, become light, and put up with an alternate dimension of reality in which to travel.


Travel to the future on the other hand is plainly possible as we are experiencing that ride right now. 

In accordance with the mathematical extrapolations of relativity theory, increasing bodily speed slows the rate of travel into the future. To travel into the future faster than we are presently experiencing it would then require slowing down. 

If actual time travel forward is possible it must involve some other theoretical split into alternate space-time, as to decrease speed less than zero in our real universe, would acheive another sort of 'ultimate inertia' - distances are infinite, time passes instantaneously, and body (mass&energy) effectively dissapears. So you couldn't get anywhere, but you'd get there really fast!


*Freep: "**To extrapolate that into "You can't go back in time to kill your father because you know your father is alive and therefore *something* would keep it from happening" strikes more of mysticism in what that "something" is (or maybe psychology that you wouldn't do something you know is a contridiction)."*

I think you somehow extrapolated the paradoxical idea of going back in time to kill one's own ancestor with the QM postulation referred to as Shroedinger's (sp.?) Cat. 

I do agree, however that the paradox itself contains mystical and psychological propositions: A transendent suicidal, patricidal experience - combined with the necessary giving up one's body to "become light" according the the above relativistic ramifications - I'll leave that philosophical discussion alone for now.


"The Cat" is an analogy for the indeterminate nature of quantum-level phenomena. This is on the scale of Heisenberg's Uncertainty Principle (HUP) - very very small - electrons and photons and such.

The deal with that is that in the Uncertain quantum-level region of space-time, the state (position and velocity) of bodies is an uncollapsed wave-function of probability. Measuring that body's state would necessarily involve collapsing the wave as to observe the state of an electron, or a photon, e.g. 

A quantum of energy (at least) is required to interact with the body-to-be-measured; this interferes with the uncertain state and makes it certain, but only insofar as the one measuring caused the state and fixed the outcome in the process of observing the state of the body, so the original (premeasured) state of the body is uncertain in the range of 100% unknown down to the limit imposed by HUP, depending on the precision and invasiveness of the measurement.
The Cat is an example of 100% unknown, which implies all possibilities exist simultaneously. Though I'm sure if you put a living cat in the 'box of uncertainty' - and don't blast it with a huge quanta of energy - it will almost certainly come out alive. At the non-QM level, an infinite number of possibilities occuring in instantaneous time doesn't ammount to much noticeable change.



*Sam: "wait, I want to know about advil stocks."*

Study QM, encourage others to do so, and then start investing.



*AG: "Of course I do believe that this impies that once we gain the ability to see the future, the future is no longer uncertain and we won't be able to change it."*

Reminds me of the movie, _Paycheck;_ brings up the philosophical subject of determinism as well, which hard science is not in disagreement with.


So we can't change the past; we can't change the future. What does that leave, hard determinism aside?

. . . ooh very Zen . . .



BTW, All of the above are my own speculations based on limited recall of my University Physics courses taken many years ago.


p.s. If you're interested, I can tell you how to acheive light speed in under a year at a comfortable pace. No charge!


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## heretic888 (Jun 21, 2005)

For what its worth...

I personally don't believe "time travel" is possible in that much of what we call time is a perception (or, if'n you wanna be Buddhist about it, an illusion). How "time" is experienced changes as the individual's awareness changes. A pre-pubescent child does not experience "time" in the same way a functional adult does. Piaget's developmental structuralism looked into a lot of this kinda stuff (the constructing of personal ontologies thing, not the quantum mechanics thing) and is worth looking into.

Enough of that. Laterz.  :asian:


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## sgtmac_46 (Jun 21, 2005)

I personally find the multiple universe theory appealing.  The idea of an infinite number of universes, one for every possibility, compelling.  It does seem to fit in with the quantum physics probability theories in that, even when reality has chosen one path, the other still exists.  What an interesting possibility.


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## Rich Parsons (Jun 22, 2005)

arnisador said:
			
		

> All speculation and interpretation, I say. Maybe there's more science buried somewhere in it, but I'm not convinced.
> 
> As for me, I liked my grandfathers, so it's all moot anyway!



And



			
				arnisador said:
			
		

> Shroedinger's cat is from quantum physics...this is a relativity issue, isn't it?




Arnisador,

I am with you, I have some problems with this. First, I use models in what I do for a living and each model is only as good as the rules, you impose. 

Let us look at the Sun and Light.

Light is represented as a wave length that we can see.

Light is represented as a beam of energy that is sent from its source.

Light is a particle and has mass and can be force to bend around large massive objects.

All three are models. All three descibe a system, but all three cannot be used at the same time with out creating some inconsistancies. 

So the article discusses using Quantum Mechanics to Model Time Travel. And from this model they have found that per the rules of Quantum Mechanics you cannot go back and change the present. 

Anyone see the logical problem here, besides me?

Let us continue with their line of thought or arguement. If you know your father or grand father is alive you cannot go back and do something to kill him. That would be directly do something. What about indirectly? According the model it is not allowed, becuase it would change the current known state of when your father of grand father was alive.

So let us look at someone who has Amnesia or adopted, and cannot remember or know their parents or grand parents. This same model would allow for the death of them because it is an unknown in the current state.

Personally, if you can obtain the speed of light and actually go backwards in time, I believe anything is possible. If you changed the current state aka the future of the time you went back too, then everyone else would know no difference. But this is theory and I did not right a big computer program using big words to make my point, so it is not news worthy.

Here is a question I have:

Assumption:
1) We have a vehicle that can travel the speed of light.
2) We have two particapants in this experiment.

The first particpant is in the vehicle and the second is outside of the vehicle.

The vehicle is traveling at the speed of light and the person inside turns on an exterior light source. Would the light source be visable to the person in the vehicle or outside the vehicle?

If you used the three models from above about light, you can get different answers. 

Second question:

Assumptions
1) one normal car
2) one normal baseball
3) two people in the car (* One driving and the other in the back seat, both wearing seat belts of course  *)
4) A person standing outside the vehcile

If the vehicle speeds up to 60 MPH, and you throw the ball from the back seat to the front, how fast is the ball traveling? Assume you person in teh back seat can throw the ball 60 MPH. What happens if you stop the vehicle while the ball is moving? What happens if you throw the ball at the person outside of the vehicle?


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## Shizen Shigoku (Jun 23, 2005)

*Rich Parsons: "*
*Quote:*
*Originally Posted by arnisador*

_*Shroedinger's cat is from quantum physics...this is a relativity issue, isn't it?*_

*Arnisador,*
*I am with you, I have some problems with this. First, I use models in what I do for a living and each model is only as good as the rules, ...*

*All three are models. All three descibe a system, but all three cannot be used at the same time with out creating some inconsistancies. "*

I still have problems with it (the QM angle). I think it is a relativity issue, but since QM models well what is at the foundation of everything, it plays a part.

About the different models of light; like I said here: "Both theories, while not completely compatible with eachother . . ., can be used simultaneously to look at problems . . ."

In the case of light, it is a situation where a body (which is both wave and particle because everything is both energy and matter) is traveling at max speed, c. All three models you mentioned can be used simultaneously. The inconsistancy is removed by M=E (relativity).

*"So the article discusses using Quantum Mechanics to Model Time Travel. And from this model they have found that per the rules of Quantum Mechanics you cannot go back and change the present. *
*Anyone see the logical problem here, besides me?"*

The logical problem I see, is first - again I think it's a relativity issue - time travel (in a practical sense) involves things of macroscopic scale such as people and vehicles; secondly, QM as a model is most useful at the scale of fundamental particles. Quantum theory does show that the ultimate behavior of bodies cannot be determined because matter/energy is always vibrating/fluxuating as a probability function. 

This uncertainty is still contrained by the limited number of events (behaviors) that are probable. E.g. it's impossible to know exactly what kind of spin and energy level an electon is going to have at any instant, but at a macroscopic level, we are pretty confident that it's not going to fly off, dance a jig, fry us some eggs and then suddenly gain a positive charge and be repelled forever away from all protons.
Assuming temporal manipulation of that kind is possible, I think relativity and the "many worlds" hypotheses provide the best description of how it would be possible. QM just helps show that there is an even higher chance of something screwing up.

*"Let us continue with their line of thought or arguement. If you know your father or grand father is alive you cannot go back and do something to kill him. That would be directly do something. What about indirectly? ..."*

Good point. 

Again, this is assuming that time travel is possible, and we want to know whether or not things that have already happened can be changed. As a proponent of the many worlds theory, I don't think going back to kill your own father (directly or indirectly) will paradoxically erase your existance. You as the time traveler will still carry on after the event, but now this current time line will be different from the original.

The article doesn't seem to show how QM theory can be used to make time travel possible, and comparing the uncertainty of fundamental particles with the uncertainty of knowing things in the present is really stretching the analogy in order to make changing the past possible. It's more of a sci-fi hook than anything else.

From article: 

"these constraints exist because of the weird laws of quantum mechanics even though, traditionally, they don't account for a backwards movement in time. "

"So either time travel is not possible, or something is actually acting to prevent any backward movement from changing the present. For most of us, the former option might seem most likely, but Einstein's general theory of relativity leads some physicists to suspect the latter. "

I suspect the latter too. The constraints I see are the same I mentioned above:

"At c, all bodily mass will have been converted to energy, and will in effect have become light. "

So there goes your time machine and anyone in it, they would just be added to the background radiation that we are recording coming in from the far-flung edges of the universe - which has its origin billions of years in the past.

IOW, possible but not practical for anything.

*"Personally, if you can obtain the speed of light and actually go backwards in time, I believe anything is possible. "*

Yeah, that's a big if. Obtaining the speed of light is certainly possible, as light itself does it all the time. If traveling FTL is a requirement for retrograde time travel, there would have to be a way around that whole 'imploding-into-another-dimension-as-radiant-energy' thing to be able to do anything useful. 

*"Here is a question I have:*

*Assumption:*

*1) We have a vehicle that can travel the speed of light.*

*2) We have two particapants in this experiment.*

*The first particpant is in the vehicle and the second is outside of the vehicle.*

*The vehicle is traveling at the speed of light and the person inside turns on an exterior light source. Would the light source be visable to the person in the vehicle or outside the vehicle?*

*If you used the three models from above about light, you can get different answers. "*

I don't think any of those models conflict with any others in that scenario. First problem is the first assumption: we have to assume that our vehicle will still be functional as a useful machine after experiencing relativistic effects - size approaching zero, mass converting to energy, etc.

If everything is fine after that, then according to relativity, both participants would experience the same thing: A light source going by (wave, particle, or beam - depending on how it is measured).

Both participants are moving relative to eachother and relative to other external bodies. Each experiences themselves as being stationary (constant velocity = no forces = no feeling of change). The vehicle traveling at light speed is traveling at the same speed as the exterior light source - assuming it is attached to the vehicle - so both combine into a single light source, and no one notices any change. The vehicle and its passenger experience themselves as stationary, and see the participant outside the vehicle as a beam of light.

*"Second question:*

*Assumptions*

*1) one normal car*

*2) one normal baseball*

*3) two people in the car *

*4) A person standing outside the vehcile*

*If the vehicle speeds up to 60 MPH, and you throw the ball from the back seat to the front, how fast is the ball traveling? Assume you person in teh back seat can throw the ball 60 MPH. What happens if you stop the vehicle while the ball is moving? What happens if you throw the ball at the person outside of the vehicle?"*

The speeds of the vehicle, the ball, and anyone outside the vehicle are all relative to eachother and to any outside motion they are measured in comparison too. Compared to light speed, they are all small fractions and are additive without significant error. I.e. 60mph ball relative to vehicle + 60mph vehicle relative to person outside = 120mph ball relative to person outside

light speed ball relative to light speed vehicle is not additive to any outside point of reference, as both ball and vehicle have become light and travel together at c.


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## heretic888 (Jun 23, 2005)

Shizen Shigoku said:
			
		

> [...] but since QM models well what is at the foundation of everything



Everything _material_, anyway...


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## Shizen Shigoku (Jun 23, 2005)

Well, sure. There has to be limits. Material things are easier to measure, and scientists like measuring things.


Everything else (non-material) can only have meaning in our lives if it interacts with matter/energy, so in order to understand the non-material, we still need to measure the material to infer its effects.

Even for something like "I think therefore I am." - The thinking, and the idea of existence matters not (are immaterial), but both depend on and have effects on the material world, otherwise there would be no evidence for either.


p.s. After all that I had written, "... QM models well what is at the foundation of everything" was the only thing you could nit-pick?

Bringing up a case against materialism would change the parameters of the whole discussion, and would require introducing more theories - I'm having a hard enough time remembering and discussing what I speculate just about Relativity from a long ago Physics class, while paying a little lip-service to QM (which still creeps me out).


I'll break out my old philosophy books and brush up if you really want to go down that way.


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## Andrew Green (Jun 23, 2005)

Can I toss in a little anti-realism, just to spice things up?  Or is there not enough cyber...umm... thought provoking inhalents to go that way yet?

 "There is no spoon"


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## heretic888 (Jun 23, 2005)

Shizen Shigoku said:
			
		

> I'll break out my old philosophy books and brush up if you really want to go down that way.



Yikes. Take it easy there, tiger.  :supcool: 

The only problem I had was the notion that physics (QM or not) can somehow account for "everything". It most assuredly cannot, not anymore than, say, semiotics can account for, say, genetic coding.

Really, though, this deals with the relationship between objective phenomena and subjective phenomena, a subject I had discussed here in the past. The truth is that something "mattering" or having "importance" is entirely subjective phenomena --- qualia --- and is separate (yet not independent of) material existence. Matter only matters if there's mind to make it matter. 

Personally, I think subjectivity and objectivity are co-substantial, but that's just me.

As to the nitpicky thing, I don't know enough about physics to even begin...

Laterz.


----------



## Rich Parsons (Jun 24, 2005)

Shizen Shigoku,

I respectfully disagree wiht some of what you posted.




			
				Shizen Shigoku said:
			
		

> In the case of light, it is a situation where a body (which is both wave and particle because everything is both energy and matter) is traveling at max speed, c. All three models you mentioned can be used simultaneously. The inconsistancy is removed by M=E (relativity).[/font]



I disagree in that you can use the Wave Length but you cannot use the beam at the same time. The beam requires that the particle travels in a straight light (* unless afected by some other mass of input force *), while the Wave length has no mass and is a wave. I know this is a very simple explanation, but it is what I remember from many a discusions with my Physics instructors in college. As to the relativity discussion of where M=E when C is one (1).




			
				Shizen Shigoku said:
			
		

> *"So the article discusses using Quantum Mechanics to Model Time Travel. And from this model they have found that per the rules of Quantum Mechanics you cannot go back and change the present. *
> *Anyone see the logical problem here, besides me?"*
> 
> The logical problem I see, is first - again I think it's a relativity issue - time travel (in a practical sense) involves things of macroscopic scale such as people and vehicles; secondly, QM as a model is most useful at the scale of fundamental particles. Quantum theory does show that the ultimate behavior of bodies cannot be determined because matter/energy is always vibrating/fluxuating as a probability function.



I agree with your statement, but, maybe it is the application engineer or application physicist in me, I would add the following comments. The model is using QWuantam Mechanics rules to prove that Quantum Mechanics is at work and will prohibit you from affecting time in the past. I am surprised the code compiled with such circular dependancies, but with the probabilities ranging from 0 to 1, this would allow for that compile to occur. As the article stated, as the probability of an event being known goes to one(1), the chance of changing it goes to away. I would have said approach zero myself. 

So let us design a simple system.

In our simple model all cars will be Blue. This is the laws of making cars.

We build cars, and look they are Blue. Hmmm What if I decide to build a new car. As the car gets closer to being built the probability of it being Blue goes to one as this is in the original "Rules" we used to define the system. So, lets say we go back in time and build a new car, and I take Red Paint with me, but the model will not allow me to then show that there is a red car. Why? Because the certainty of the event of a car being Blue is 100% or 1. 

This is the problem I see with this arguement as they presented it. Now, I am sure if the probaility of an event is not one, then we can go back and change things, (* assumming Time Travel is possible *)




			
				Shizen Shigoku said:
			
		

> Again, this is assuming that time travel is possible, and we want to know whether or not things that have already happened can be changed. As a proponent of the many worlds theory, I don't think going back to kill your own father (directly or indirectly) will paradoxically erase your existance. You as the time traveler will still carry on after the event, but now this current time line will be different from the original.



I never said it would erase yourself. All I said was that you could change they events and thereby history or the flow of time. 




			
				Shizen Shigoku said:
			
		

> The article doesn't seem to show how QM theory can be used to make time travel possible, and comparing the uncertainty of fundamental particles with the uncertainty of knowing things in the present is really stretching the analogy in order to make changing the past possible. It's more of a sci-fi hook than anything else.



I never said the article said that. I said assuming Time Travel is possible. 



			
				Shizen Shigoku said:
			
		

> From article:
> 
> "these constraints exist because of the weird laws of quantum mechanics even though, traditionally, they don't account for a backwards movement in time. "
> 
> ...



I do not think that E=mc^2 mans that all mass will be converted into energy. It means you can mesure the energy and their is a correlation between the two, not necessarily a transformation. 




			
				Shizen Shigoku said:
			
		

> *"Here is a question I have:*
> 
> *Assumption:*
> 
> ...



My Arguement Above your reply below



			
				Shizen Shigoku said:
			
		

> I don't think any of those models conflict with any others in that scenario. First problem is the first assumption: we have to assume that our vehicle will still be functional as a useful machine after experiencing relativistic effects - size approaching zero, mass converting to energy, etc.



Did I not say that Assumption one is having a vehicle to travel the speed of light? I guess I should have said a functional vehicle at any speed or condition.



			
				Shizen Shigoku said:
			
		

> If everything is fine after that, then according to relativity, both participants would experience the same thing: A light source going by (wave, particle, or beam - depending on how it is measured).



I am confused by this maybe I wrote something wrong. I said one particpant in the vehicle and the other outside the vehicle, from simple frame of reference how can it be the same between the two? Even Einstein stated if you traveled a year at the speed of light and returned to the earth one would find that the participant who remained behind would have aged significantly, while the one who travelled only aged a year. 



			
				Shizen Shigoku said:
			
		

> Both participants are moving relative to eachother and relative to other external bodies. Each experiences themselves as being stationary (constant velocity = no forces = no feeling of change). The vehicle traveling at light speed is traveling at the same speed as the exterior light source - assuming it is attached to the vehicle - so both combine into a single light source, and no one notices any change. The vehicle and its passenger experience themselves as stationary, and see the participant outside the vehicle as a beam of light.



Once again this makes no sense. I must not have written the question properly. See above. 

If the person in the vehicle is in a different frame of reference from the person outside, then they would experience different reactions.

What I expected was the following:

Person outside the vehicle would see:
1) A blur of light as the vehicle is travelling the speed of light.
2) Nothing because the wave lengths of the light source was in direct opposite magnitude to that of the vehicle and they would cancel each other out.

Person inside the vehicle would see:
1) Wave Length - a light source that would visible, as the wave length would be actually twice the speed of light, but in reference to person inside the vehicle it would be only the speed of light. 

2) The Beam - a light source that would seem very weak, as the wave length would be actually twice the speed of light, but in reference to person inside the vehicle it would be only the speed of light. Although it would loose it's energy pretty fast and therefore would not be a Head Lamp affect.

3) The Particle with Mass - As the particle left it would loose the energy and also would collide with vehicle. This would cause a very weak light source, or no light source depending upon if you believed the light source had enough power to move the mass far enough from the source.



			
				Shizen Shigoku said:
			
		

> *"Second question:*
> 
> *Assumptions*
> 
> ...



This is what I expected.



			
				Shizen Shigoku said:
			
		

> light speed ball relative to light speed vehicle is not additive to any outside point of reference, as both ball and vehicle have become light and travel together at c.



This is where we disagree. Do you recommend some reading to educate myself on the issue of all mass becoming energy at the speed of light?


----------



## Shizen Shigoku (Jun 24, 2005)

*Rich Parsons: "I respectfully disagree wiht some of what you posted.*

*I disagree in that you can use the Wave Length but you cannot use the beam at the same time. The beam requires that the particle travels in a straight light ..."*

Whether you model or measure a collection of photons as either particles or waves, they exist as both simultaneously. Even a massive particle like a baseball can be described in terms of its energy and wave-length. Maybe its my visualization of it, but I don't see why a beam of light particles (a collection of photons all moving in the same general direction) cannot be simultaneously understood as a beam-shaped collection of wave-forms.

It has been a while since I studied any of this stuff - I've been working on more practical matters lately. I'll have to review that whole wave-partical duality thing.

I'll have to brush up on my QM too, because I just don't understand it at a deep enough level yet to speculate further on its possible effects on time travel.

*"I never said it would erase yourself. All I said was that you could change they events and thereby history or the flow of time. "*

I never said you said that either - it's a possible consequence of the Grandfather Paradox if alternate realities do not exist.

*"Originally Posted by Shizen Shigoku*

*The article doesn't seem to show how QM theory can be used to make time travel possible, and comparing the uncertainty of fundamental particles with the uncertainty of knowing things in the present is really stretching the analogy in order to make changing the past possible. It's more of a sci-fi hook than anything else.*

*I never said the article said that. I said assuming Time Travel is possible. "*

I also never said that you said that. I was only relaying my opinion of the article. 

*"I do not think that E=mc^2 mans that all mass will be converted into energy. It means you can mesure the energy and their is a correlation between the two, not necessarily a transformation. "*

True, the equation doesn't state that explicitly, but the consequences of relativity show that the only things in the universe that travel at c are massless particles. If something had mass to begin with, it would necessarily have to lose all of it in order to reach c.

_*"light speed ball relative to light speed vehicle is not additive to any outside point of reference, as both ball and vehicle have become light and travel together at c.*

_*This is where we disagree. Do you recommend some reading to educate myself on the issue of all mass becoming energy at the speed of light?"*

I'll look, but it's just my personal feeling on the matter for now. As I've only known light to travel at light speed, I assume anything at that speed to be light (or otherwise massless - that's a free pun; your welcome). 

IOW, for an object of non-zero mass to even approach c would require greater and greater amounts of energy to accelerate it as relativistic mass effects become more significant. As stated before, an object at c would acquire 'ultimate inertia' - time stops, distance becomes zero, and mass is infinite. 

This condition of course is not possible because it would take an infinite amount of energy to move an infinite mass. However, if the object lost mass as it accelerated - at a rate that left it massless by the time it reached c - then that is not a problem. 

Photons (light) are one of few massless particles that could pull that off. What I mean by that is as a body approaches luminal speed, it would have to lose all of its mass. As mass is converted to energy, photons are released. A good example is an atomic bomb - all of its mass (theoretically at 100% efficiency) is accelerated to light speed by the high-energy chain reaction and is converted to radiation.

There's an old theory called Autodynamics that tries to explain velocity in terms of lost mass such that when light speed is reached, all mass has been exhausted (body is pure radiant energy) and is therefore unable to increase speed. That theory hasn't gotten much respect lately, but the math involved ain't too bad.

Try: http://en.wikipedia.org/wiki/Category:Special_relativity

*"Did I not say that Assumption one is having a vehicle to travel the speed of light? I guess I should have said a functional vehicle at any speed or condition."*

Yes, I just wanted to reemphasize the relativistic effects, because that is what I'm describing in the outcome of the experiment. Making the assumption that the vehicle does not experience RE is like asking how much water will a towel soak up if I threw in it a pool - assuming it doesn't get wet . . .

*"Originally Posted by Shizen Shigoku*

*If everything is fine after that, then according to relativity, both participants would experience the same thing: A light source going by (wave, particle, or beam - depending on how it is measured).*

*I am confused by this maybe I wrote something wrong. I said one particpant in the vehicle and the other outside the vehicle, from simple frame of reference how can it be the same between the two? "*

By "simple frame of reference" do you mean a third FoR separate from the first two? 1st frame is person inside vehicle - they will experience themselves at rest (constant velocity = inertial reference frame) and everything else is whizzing by at light speed. 2nd frame is person outside vehicle - they also feel no motion as long as they are not accelerating, they would see the vehicle whizzing by at light speed. That's what relativity means - motion is relative. 

The further RE would have each participant see the other's time as slowing down. The only way to determine which one ages more slowly would be to compare them both to a third frame of reference, e.g. one that is motionless relative to the first.

It makes my head hurt thinking about it. Try this: http://en.wikipedia.org/wiki/Speed_of_light

*"Once again this makes no sense. I must not have written the question properly. See above. *

*If the person in the vehicle is in a different frame of reference from the person outside, then they would experience different reactions."*

As long as both frames of reference are inertial, they are both relativisticly equivalent.

*"What I expected was the following:*

*Person outside the vehicle would see:*

*1) A blur of light as the vehicle is travelling the speed of light."*

That would be my guess too

*"2) Nothing because the wave lengths of the light source was in direct opposite magnitude to that of the vehicle and they would cancel each other out."*

Not sure I understand this - sounds like destructive interference?

*"Person inside the vehicle would see:*

*1) Wave Length - a light source that would visible, as the wave length would be actually twice the speed of light, but in reference to person inside the vehicle it would be only the speed of light. "*

I get ya' now. The person in the vehicle still experiences their headlights working normally (perhaps blue shifted though), but the 2nd FoR would not see light emitted because the vehicle and anything it emits would appear to have zero length.


Stuff sure gets confusing!

BTW, if you would like to try going at light speed, I figured out that if you accelerate at the same rate as earthly gravity (an amount of force we are all more-or-less comfortably used to), then you can acheive c in little under a year.


----------



## Kane (Jun 24, 2005)

Andrew Green said:
			
		

> http://news.bbc.co.uk/2/hi/science/nature/4097258.stm
> 
> Interesting stuff...
> 
> Quantum theory is the reason Advil stocks are where they are today :S


 Can't believe I missed this this thread.

 I might be able to shed some light on the info provided, since I am studying in the feild.

 First off the info provedided does not exactly fit in science, like much of time travel info. The reason being is that they are going back to the whole idea that you cannot chance the past because something would happen when you go back and you will decide not to do it. This would defy lscience in that sense. In order for your mind to stop yourself from killing your father in the past, divine intervention must occur. This does not fall in physics but metaphysics.

 If I went back in time in intent on killing my dad, I would do it (if time travel to the past is possible). My mind is not going to with magic change and decide not to do it, regardless on whether I know whether he is alive or not.

 As for the possibility of time travel it depends. Travelling to the future is very much possible. In fact, it is rather easy according to Eistein. In General Reletivity, all you need to do is travel close to the speed of light and you will travel to the future.

 However, traveling to the past has currently labeled mathematicly impossible (unless you use strings). In order to travel back in time you would need to travel above the speed of light, but right now that is impossible. If you were to plug the speed of light into the energy-mass equation you would come up with an undefined equation: 1/0. Anything divided by zero is defined, like its slope so it is impossible.

 Sorry about the spelling errors, my keyboard is acting up.


----------



## Rich Parsons (Jun 24, 2005)

Shizen Shigoku said:
			
		

> *Rich Parsons: "I respectfully disagree wiht some of what you posted.*
> 
> *I disagree in that you can use the Wave Length but you cannot use the beam at the same time. The beam requires that the particle travels in a straight light ..."*
> 
> ...




I see your points and I have heard them before, but the particle theory requires mass, and you state we can model mass in wave lengths, but under your own arguements if mass must be converted to energy at c then you cannot have the particle theory for it has mass at c. Yet, we know there is mass no matter how small it might be, and this is why all three cannot work at the same as well as why I do not believe that E=Mc^2 means all mass is turned into Energy at the speed of c. There can be mass and a correlating energy associated with that mass, even at the speed of c. You nuclear reaction comment still has alpha and beta and gamma particles that all have mass. 

I have not checked your links yet, I will do so, and think upon this issue,

Thanks


----------



## Marginal (Jun 24, 2005)

Kane said:
			
		

> First off the info provedided does not exactly fit in science, like much of time travel info. The reason being is that they are going back to the whole idea that you cannot chance the past because something would happen when you go back and you will decide not to do it. This would defy lscience in that sense. In order for your mind to stop yourself from killing your father in the past, divine intervention must occur. This does not fall in physics but metaphysics.
> 
> If I went back in time in intent on killing my dad, I would do it (if time travel to the past is possible). My mind is not going to with magic change and decide not to do it, regardless on whether I know whether he is alive or not.


In the case you're discussing, what people were saying previously was that if you were to go back in time and kill your dad, he'd be dead, and then you'd then have no reason to go back in time and kill your dad, because he was already dead. Your mind isn't changed in that case. 

The problem is, if he's dead, you don't have to kill him, but if you don't kill him, then he isn't dead.


----------



## sgtmac_46 (Jun 26, 2005)

Kane said:
			
		

> Can't believe I missed this this thread.
> 
> I might be able to shed some light on the info provided, since I am studying in the feild.
> 
> ...


 
I think the point is going back and killing your father before you are born, thereby not only NOT having a reason to go back and kill him, but also no way to do it....since you never existed.  Of course the point made by the Quantum physics crowd was that the knowledge that your father is alive makes it impossible to kill him based on some bizarre interpretation of the Schroedinger's cat principle.


----------



## Shizen Shigoku (Jun 28, 2005)

*Rich: "..., but the particle theory requires mass, and you state we can model mass in wave lengths, but under your own arguements if mass must be converted to energy at c then you cannot have the particle theory for it has mass at c. ..."*

Remember these are models. Photons (massless particles) can impact other bodies and impart energy as if hit by a particle of equivalent mass.

At near light speed, bodies retain their original rest mass, but also gain an equivalent (relativistic) mass. By using E=mc2, all moving bodies can be expressed in terms of energy and momentum.

I don't believe that a body loses its rest mass according to autodynamics, but for a body with non-zero mass to acheive light speed, it has to overcome the obstacle of needing infinite energy.


*"You nuclear reaction comment still has alpha and beta and gamma particles that all have mass. "*

But are those particles moving at c, or near-c?

Alpha particles are Helium nuleii, beta particles are electrons, and gamma radiation is a massless EM wave.

The gamma rays and photons emmitted can travel at c (in a vacuum), electons can travel pretty close to that, and alpha particles a bit slower.

The particle model for light is just that - it models the energy and momentum of photons (quanta of EM radiation) as an equivalent mass, and has some analogies to mechanics (motion, force, reflection, etc.).


----------



## Shizen Shigoku (Jun 28, 2005)

See also:


http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html
&
http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html (addresses confusion of terminology):

"The case of photons and other particles that move at the speed of light is special.  From the formula relating relativistic mass to invariant mass, it follows that the invariant mass of a photon must be zero, but its relativistic mass need not be.  The phrase "The rest mass of a photon is zero" might sound nonsensical because the photon can never be at rest; but this is just a side effect of the terminology, since by making this statement, we can bring photons into the same mathematical formalism as the everyday particles that do have rest mass."


"Radioactive decay results in a loss of mass, which is converted to energy (the _disintegration energy_) according to the formula _E_ = _m__c_2. This energy is commonly released as photons (gamma radiation)." (http://en.wikipedia.org/wiki/Radioactivity)


My general feeling still is that masses do not necessarily lose mass in order to travel at light speed (nuclear radiation is an example where they do), but the only particles I know of that do travel at c are photons - which are massless. So either it is impossible for a massive body to travel at light speed, or said body must convert all rest mass to energy in order to do so.

That's pretty much what I meant.


----------



## Rich Parsons (Jun 28, 2005)

Shizen Shigoku said:
			
		

> *Rich: "..., but the particle theory requires mass, and you state we can model mass in wave lengths, but under your own arguements if mass must be converted to energy at c then you cannot have the particle theory for it has mass at c. ..."*
> 
> Remember these are models. Photons (massless particles) can impact other bodies and impart energy as if hit by a particle of equivalent mass.
> 
> ...



Yes they are models. That was my point not a single model can represent this case, and that the model has limitations built into the design.

BTW: Photons: http://en.wikipedia.org/wiki/Photon

The first instance and or reference is light. Hence back to the three models I presented, some show mass and others show no mass in there descriptions. 



			
				Shizen Shigoku said:
			
		

> *"You nuclear reaction comment still has alpha and beta and gamma particles that all have mass. "*
> 
> But are those particles moving at c, or near-c?
> 
> Alpha particles are Helium nuleii, beta particles are electrons, and gamma radiation is a massless EM wave.



Yes I know what Alpha and Beta and Gamma, particles are, hence my reference of them and how people refer to them, but they still have mass. So once again not all models define the system. Sometimes you can only define part of the system and there by you create a model for that part, and then you describe that, and you create another model to describe other parts of the system. Then you put them together trying to understand the whole, but they are still separate models, as you pointed out, trying to fit a situation. 



			
				Shizen Shigoku said:
			
		

> The gamma rays and photons emmitted can travel at c (in a vacuum), electons can travel pretty close to that, and alpha particles a bit slower.



So the Gamma rays from the sun are traveling less than the speed of light or less than 'c'.  The Sun being a Fisson Fusion reactor, in as close to a vacuum as you can get. 



			
				Shizen Shigoku said:
			
		

> The particle model for light is just that - it models the energy and momentum of photons (quanta of EM radiation) as an equivalent mass, and has some analogies to mechanics (motion, force, reflection, etc.).



Once again this was my point that it modeled certain physical aspects we can measure or "see" and then we use other models to describe other datapoints. Thereby trying to describe the system, but sometimes the basis of those models may not agree, but we tend to ignore those because we know from application or experience that neither model is 100% correct. 

Hence back to my original point, the model of using Quantum Mechanics to time travel and show that Quantum Mechanics will not allow you to go back and change the past because in qunatum mechanics as the probability goes to 1 that we "know" something, means it was true. So, by stating that we know our Grand father was alive in 1982, we know we cannot travel back in time and change it so he will be dead in 1982. Why. because the model shows that it is impossible from the fact that we have a probabilit of 1 that we know out Grand Father was alive in 1982.

Now once again this assumes Quantum Mechanics in a Linear Time Line, and not alternate time possibel lines.


----------



## Rich Parsons (Jun 28, 2005)

Shizen Shigoku said:
			
		

> See also:
> 
> 
> http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html
> ...



Thanks I browsed some, will read in detail



			
				Shizen Shigoku said:
			
		

> "The case of photons and other particles that move at the speed of light is special.  From the formula relating relativistic mass to invariant mass, it follows that the invariant mass of a photon must be zero, but its relativistic mass need not be.  The phrase "The rest mass of a photon is zero" might sound nonsensical because the photon can never be at rest; but this is just a side effect of the terminology, since by making this statement, we can bring photons into the same mathematical formalism as the everyday particles that do have rest mass."
> 
> 
> "Radioactive decay results in a loss of mass, which is converted to energy (the _disintegration energy_) according to the formula _E_ = _m__c_2. This energy is commonly released as photons (gamma radiation)." (http://en.wikipedia.org/wiki/Radioactivity)






			
				Shizen Shigoku said:
			
		

> My general feeling still is that masses do not necessarily lose mass in order to travel at light speed (nuclear radiation is an example where they do), but the only particles I know of that do travel at c are photons - which are massless. So either it is impossible for a massive body to travel at light speed, or said body must convert all rest mass to energy in order to do so.
> 
> That's pretty much what I meant.



The link I provided above, states that Photons, "The photon is one of the elementary particles, along with the electron. Together with the particles that make up nuclei, their interactions account for a great many of the features of matter, such as the existence and stability of atoms, molecules, and solids. These interactions are studied in quantum electrodynamics (QED), which is the oldest part of the Standard Model of particle physics."

When you take away all the elements except the Photons, there is still mass not accounted for.  I am still not of the idea that Photons are massless.


----------



## Shizen Shigoku (Jun 28, 2005)

I don't know what you mean by *"some show mass and others show no mass."*

The two models are 'wave' and 'particle.' A 'beam' of energy can be either. In fact it is both. All things that exist (matter = energy) are both waves (with wavelengths and frequencies) and particles (discrete quanta with momentum) *simultaneously*. That is why either model can be used. It depends on what aspect you want to look at.

*"..., but the particle theory requires mass, ..."*

Why?

*"Yes I know what Alpha and Beta and Gamma, particles are, hence my reference of them and how people refer to them, but they still have mass."*

Gamma 'particles' do not have mass. They are photons.

Originally Posted by *Shizen Shigoku* 

_The gamma rays and photons emmitted can travel at c (in a vacuum), electons can travel pretty close to that, and alpha particles a bit slower._

[should have read: "...gamma rays and *other* photons . . ."

*"So the Gamma rays from the sun are traveling less than the speed of light or less than 'c'. The Sun being a Fisson Fusion reactor, in as close to a vacuum as you can get. "*

Not sure what this follows from. Gamma rays from the sun do travel at c - same as all other forms of EM radiation.

*"So once again not all models define the system. Sometimes you can only define part of the system and there by you create a model for that part, and then you describe that, and you create another model to describe other parts of the system. Then you put them together trying to understand the whole, but they are still separate models, as you pointed out, trying to fit a situation. "*

&

*"Once again this was my point that it modeled certain physical aspects we can measure or "see" and then we use other models to describe other datapoints. Thereby trying to describe the system, but sometimes the basis of those models may not agree, but we tend to ignore those because we know from application or experience that neither model is 100% correct. "*

I understand well the usefulness and limitations of modeling. In this case, I believe that wave-particle duality can be understood as a single non-conflicting model.


*"Hence back to my original point, the model of using Quantum Mechanics to time travel and show that Quantum Mechanics will not allow you to go back and change the past because in qunatum mechanics as the probability goes to 1 that we "know" something, means it was true. "*


This is a case where the model doesn't even closely resemble the phenomenon to be predicted. I don't think using QM as a model to predict the possibility of changing a past event is sound.

I see the analogy between quantum probability, and the statistical probability of knowing someone is alive, but I really don't think one can be used to model the other.

*"When you take away all the elements except the Photons, there is still mass not accounted for. I am still not of the idea that Photons are massless."*

I think you are talking about 'mass defect.' When an atom loses energy (electron moves to a lower orbit) it emits photons that carry away that energy. Since mass and energy are the same, the atom loses mass. The photons themselves do not have mass other than their equivalent in 'relativistic mass' (M=E/C^2).

Photons definitely are massless.

Again, from: http://math.ucr.edu/home/baez/physi...ty/SR/mass.html ,

"The case of photons and other particles that move at the speed of light is special. From the formula relating relativistic mass to invariant mass, it follows that the invariant mass of a photon must be zero, ..."

I was really hoping I wouldn't have to do math for this thread, but maybe this will help:

Variables and constants defined:

M, mass (kilograms) 
V, velocity (meters / second)
F, frequency (1 / seconds)
L, wavelength (meters)
C, speed of light (meters / second) ~ 2.997*10^8 
H, Planck's constant (Joules * seconds) ~ 6.626*10^-34
E, total energy (Joules) = sqrt((M*C^2)^2 + (P*C)^2) = H*F
P, momentum (kilograms * meters / second) = M*V/sqrt(1 - V^2/C^2)
U, wave energy (Joules) = P*C 

Photons have energy & momentum, but no mass:

1. EM radiation (photons) has energy = U, which is equal to total energy, E when M=0.

So, for M=0,

E = U --> H*F = P*C

--> P = H*F/C (momentum = energy / velocity)

2. photons have both energy and velocity - therefore, radiation carries momentum, P.

3. P is proportional to V/R. 

[ where R = sqrt(1-V^2/C^2) ]

As V approches C, R approaches 0, and V/R approaches infinity for any non-zero mass. 

Infinite momentum yields infinite energy which I am assuming does not exist. For photons to have non-zero & non-infinite momentum, they must have zero mass.

While I'm at it . . .

On particle-wave duality:

EM waves travel at C in a vacuum. C = L*F

The velocity of any wave = wavelength * frequency

E = P*V 

E = P*L*F

P*V = P*L*F

L = P*V / P*F

L = V/F

V = E/P

L = E / P*F

E/F = H

L = H/P (The de Broglie hypothesis)

The wavelength of any body (with or without mass) = Planck's constant divided by the body's momentum.

Therefore, any massive particle is therefore also a wave; all waves are also particles; not all particles have mass.


Anyhoo, . . . The conclusion as it relates to this thread is that time travel is more likely not possible because of the impossibility of any massive body achieving light speed, let alone exceeding it. FTL motion is the only thing I can think of that would lead to that sort of temporal retrogression.

I do believe that time travel to the past is physically possible, but one would be in a parallel universe and will have become radiant energy, so it isn't very practical.


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## elder999 (Jun 30, 2005)

Shizen Shigoku said:
			
		

> I
> Gamma 'particles' do not have mass. They are photons.
> 
> 
> ...


Mass of photon = [h^2/(8*(2r)2c^2)]1/2 = *2 x 10-69 kg *. (10d) 

Photons are definitely _relatively_ massless-they do, indeed, have mass which can be measured in a variety of ways-and is every day. 

I'll address the rest of this at length somewhat later, but I wasn't going to let this go......'cause it's wrong.


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## arnisador (Jun 30, 2005)

Well, they have _effective_ mass, but photons are massless strictly speaking. Try plugging a velocity of _c_ into the formula for mass as a function of velocity and see what happens to things with mass that travel at lightspeed...


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## Xequat (Jul 1, 2005)

Wow, this got awfully technical.  I'm in a training class right now (boring), so I only have time to go back to the philosophical side of the paradox.  Here's an idea...there can't be a paradox.  Ever.  No matter what.  Because there are cause-and-effect realationships for things.  Let's take the grandfather killing thing.  If I go back to 1940 and kill my grandfather, then my dad wouldn't exist.  Thing is, he existed for me to exist.  I've heard that time is a dimension, so a person's placement on the timeline is a dimension of sorts.  Right now, I know that I exist; therefore, I can go back and kill grandpa.  But if I were to go back into the future from that point, then I would not be able to meet myself in 2004.  But if I went back and truly did not influence anything, then I should be able to go along that timeline and meet my other self in 2004.  I guess I believe that we all travel along our own individual timeline, just like we all have our own length, width, and height.


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## elder999 (Jul 3, 2005)

arnisador said:
			
		

> Well, they have _effective_ mass, but photons are massless strictly speaking. Try plugging a velocity of _c_ into the formula for mass as a function of velocity and see what happens to things with mass that travel at lightspeed...


This is one of the problems that comes up when laymen try to discuss quantum physics.

The texts all read, and everyone seems to forget (excpet for physicists like myself) that the photon has _zero *rest* mass_.

Ever seen a photon at rest?


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## arnisador (Jul 3, 2005)

You seem to be the one guilty of having a layperson's opinion here.

   Look at the formula for mass...it's got rest mass as a multiplicative _factor_. No rest mass, no mass. That's not what 'effective' mass means. (Afetr all, we're now mixing relativity and quantum, which is a dicey business.) It's not the mass 'gained' by motion (a cheesy description, but forgive me), it's the mass the photon acts as though it has for the purposes of, e.g., gravitational bending.

 A photon has zero rest mass and hence has zero mass. If you're a physicist and you disagree, you're out of the mainstream. It can, howeveer, have (finite) momentum, which goes to the point about it being in motion.

  The image is from here:


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## elder999 (Jul 8, 2005)

> *A photon has zero rest mass and hence has zero mass. If you're a physicist and you disagree, you're out of the mainstream. It can, however, have (finite) momentum, which goes to the point about it being in motion.*


 


Well, no. Not only does it depend on your definition of mainstream, i.e., Does Special Relativity hold true, does one embrace the Copenhagen interpretation or the many-universes interpretation or the transactional interpreationl, etc., etc., etc., but a quick look 

here will show that there are a variety of papers-literally hundreds-detailing experiments and calculations directed at, or at least touching on determining the _upper limit_ of photon rest mass. Heres an abstract for just one of them:





> *Because classical Maxwellian electromagnetism has been one of the cornerstones of physics during the past century, experimental tests of its foundations are always of considerable interest. Within that context, one of the most important efforts of this type has historically been the search for a rest mass of the photon. The effects of a nonzero photon rest mass can be incorporated into electromagnetism straightforwardly through the Proca equations, which are the simplest relativistic generalization of Maxwell's equations. Using them, it is possible to consider some far-reaching implications of a massive photon, such as variation of the speed of light, deviations in the behaviour of static electromagnetic fields, longitudinal electromagnetic radiation and even questions of gravitational deflection. All of these have been studied carefully using a number of different approaches over the past several decades. This review attempts to assess the status of our current knowledge and understanding of the photon rest mass, with particular emphasis on a discussion of the various experimental methods that have been used to set upper limits on it. All such tests can be most easily categorized in terms of terrestrial and extra-terrestrial approaches, and the review classifies them as such. Up to now, there has been no conclusive evidence of a finite mass for the photon, with the results instead yielding ever more stringent upper bounds on the size of it, thus confirming the related aspects of Maxwellian electromagnetism with concomitant precision. Of course, failure to find a finite photon mass in any one experiment or class of experiments is not proof that it is identically zero and, even as the experimental limits move more closely towards the fundamental bounds of measurement uncertainty, new conceptual approaches to the task continue to appear. The intrinsic importance of the question and the lure of what might be revealed by attaining the next decimal place are as strong a draw on this question as they are in any other aspect of precise tests of physical laws.[/B]*


 

*That does a fair job of at least explaining the debate, and the reasons why the rest mass of the photon is not necessarily a hard and fast figure of zero.*



*The mass of the photon depends on the theory you apply. For example, in quantum field theory it is* defined to be the massless particle which is due to gauge symmetry of the field equations, and its _relatively_ seamless merger with electromagnetism , while in string theory it is a string with a certain normal mode vibration which makes the string having the properties of a photon. The only reason the photon is asserted to be massless is that it makes the wave/particle interactions work-in fact, for string theory, it can and does have one of several defined rest mass*es* I wont even get into supersymmetry or other assorted models, though I will distinguish the photons behavior in the three (some might say two) prevailing interpretations of the quantum universe.Suffice it to say that to assert that the figure is zero, and that photons must therefore be massless is to accept a sort of mathematical convenience-a _conceit_, if you will-that allows the majority of the math to work. 


Of course, I can also find a bunch of papers asserting that the gamma ray is not photons, or ones like the original bong-hitty article posted for this thread.Its also a fairly common mistake to think of the photon as a particle-rather than the quantification of lights particle-like attributes.


Arguments about effective mass vs rest mass notwithstanding, the figure for the upper limit of rest mass is constantly moving away from zero-the problem being that it makes various parts of Special Relativity-which is inherently flawed, though _sometimes_ correct-not work. This is somewhat important given the orginal intent of this thread, an exploration of time travel paradoxes.

What should really bend your noggin, given the data, is whether the photon takes up space..


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## arnisador (Jul 9, 2005)

The link didn't work for me. However, we seem to be in agreement. Mainstream physics, of the sort taught in colleges and universities, teaches that the photon has zero rest mass. Experiemental studies don't contradict it but also aren't as confirming as might be hoped, and unaccepted theories like string theory say any number of wild things. Sure, that could turn out to be right...but if you currently accept string theory but reject QCD then you're not in the mainstream.

I too am suspicious of the group-theoretic mathematical reasoning that leads to the Standard Model--it's just too pat--but it _is_ the mainstream.

Your post lists numerous contradictory theories (relativity, quantum theory, string theory). Different things happen in different subversions of these theories. But there's a clear mainstream. If you take Modern Physics as an undergrad. in college, you'll study (special) relativity and basic quantum theory. You'll be taught that photons are massless in the former since otherwise they'd have infinite mass when traveling at light speed, and that they're massless in the latter for other mathematical reasons.

Anything else may be interesting, but isn't accepted physics.


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## Rich Parsons (Jul 9, 2005)

arnisador said:
			
		

> The link didn't work for me. However, we seem to be in agreement. Mainstream physics, of the sort taught in colleges and universities, teaches that the photon has zero rest mass. Experiemental studies don't contradict it but also aren't as confirming as might be hoped, and unaccepted theories like string theory say any number of wild things. Sure, that could turn out to be right...but if you currently accept string theory but reject QCD then you're not in the mainstream.
> 
> I too am suspicious of the group-theoretic mathematical reasoning that leads to the Standard Model--it's just too pat--but it _is_ the mainstream.
> 
> ...



Jeff,

I agree that many of the theories disagree, which was my original comment, no matter how simple to these much more indepth explanations that followed. 

The question I have, is only having an Undergrad Degree, and some (few) masters courses, but nothing in Physics directly, where did I come up with a non-mainstream understanding or approach? Is it because I was studying in the late 80's before it became Mainstream? or is it something that I just picked up from some instructor(s) that may have offered non-mainstream discussions?

So, what I am really asking is when did it become mainstream, that would help me out in trying to understand, what I have learned, and read.

Thanks


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## elder999 (Jul 9, 2005)

arnisador said:
			
		

> You'll be taught that photons are massless in the former since otherwise they'd have infinite mass when traveling at light speed, and that they're massless in the latter for other mathematical reasons.
> 
> Anything else may be interesting, but isn't accepted physics.


You'll be taught that photons are massless because the math makes it agree with Maxzwellian electomagnetics-at work I regularly accelerate protons to about 84% of c, and we do see relativistic effectsAt othe accelerators, like CERN, and SNS when and if it ever goes on line, protons are accelerated to light speed, and we see true relativistic effects, where the particle continues to 'accelerate" and doesn't pick up speed, just increases in mass, and thus, energy . The only problem with the whole "infinite mass" thing, aside from just being *wrong* is that the _math_ doesn't work. 

Or are you going to tell me that the "rest mass" of a proton is zero as well?

Anyway, I want to get into the whole "time travel" thing in a bit.....


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## arnisador (Jul 9, 2005)

elder999 said:
			
		

> You'll be taught that photons are massless because the math makes it agree


 Yup, it's a model.

 Newtonian mechanics was wrong too, but it was the standard, mainstream model for centuries. Being the current theory doesn't make one right--science marches forward, constantly improving its models. Maybe the next big model will have a photon that has a nonzero rest mass--who knows? It was a big surprise when DeBroglie said a photon was both a wave and a particle, after all.

 You get a proton to _c_ exactly? That's cool.


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## arnisador (Jul 9, 2005)

Rich Parsons said:
			
		

> So, what I am really asking is when did it become mainstream, that would help me out in trying to understand, what I have learned, and read.


 The massless photon? I think it goes back to at least the 30s or so. I don't know what earlier theories were. The Standard Model started firming up in the 50s but in its current formulation is from the early 70s, I think.


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## elder999 (Jul 9, 2005)

arnisador said:
			
		

> Yup, it's a model.
> 
> Newtonian mechanics was wrong too, but it was the standard, mainstream model for centuries. Being the current theory doesn't make one right--science marches forward, constantly improving its models. Maybe the next big model will have a photon that has a nonzero rest mass--who knows? It was a big surprise when DeBroglie said a photon was both a wave and a particle, after all.
> 
> You get a proton to _c_ exactly? That's cool.


Have a look here. 



> The proton synchotron (PS) is the switchyard of CERN. All the particles used in experiments at CERN go through the PS, are accelerated to the speed of light, and fed to other machines in the complex. The PS is the oldest of CERNs accelerators, and has been running continuously since 1959.


Of course <_harrumph w/handwaving_> it's probably more like 99.9999999% of c, but who's counting?:lol:


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## elder999 (Jul 9, 2005)

Anyway, back to time travel. Anyone familiar with the two-slit experiment? you should be,it illustrates a key principle of quantum mechanics: Light has a dual nature. Sometimes light behaves like a compact particle, a photon; sometimes it seems to behave like a wave spread out in space, just like the ripples in a pond. In the experiment, light a stream of photons shines through two parallel slits and hits a strip of photographic film behind the slits. The experiment can be run two ways: with photon detectors right beside each slit that allow physicists to observe the photons as they pass, or with detectors removed, which allows the photons to travel unobserved. When physicists use the photon detectors, the result is unsurprising: Every photon is observed to pass through one slit or the other. The photons, in other words, act like particles. 

But when the photon detectors are removed, something weird occurs. One would expect to see two distinct clusters of dots on the film, corresponding to where individual photons hit after randomly passing through one slit or the other. Instead, a pattern of alternating light and dark stripes appears. Such a pattern could be produced only if the photons are behaving like waves, with each individual photon spreading out and surging against both slits at once, like a breaker hitting a jetty. Alternating bright stripes in the pattern on the film show where crests from those waves overlap; dark stripes indicate that a crest and a trough have canceled each other. 

The outcome of the experiment depends on what the physicists try to measure: If they set up detectors beside the slits, the photons act like ordinary particles, always traversing one route or the other, not both at the same time. In that case the striped pattern doesn't appear on the film. But if the physicists remove the detectors, each photon seems to travel both routes simultaneously like a tiny wave, producing the striped pattern. 

It also, oddly, demonstrates that photons-and electrons, when they're used for the experiment-can be in two places at the same time, and that particular effect, while of much interest to me in a portion of my work, is particularly germane to this conversation it doesn't exactly reconcile itself with the Copenhagen or the many-universes interpretation of quantum theory, although those are, as arnisador pointed out, also just models-one of my favorite phrases by the way, given my engineer's aesthetic. Why do you think the whole "massles photon" thing gets me so riled up? 

Anyway, Sir Roger Penrose helped come up with a third model called the transactional model. I won't bother explaining it (another aesthetic: if I can't make it so simple that my mom the shrink will understand, I don't bother messing up the explanation) but I will say that it reconciles the "many universes" and Copenhagen" interpretations, *and* explains why photons and electrons can be two places at once, and more complex bundles of quanta-like matter, which includes *us* can't. This is important to the whole "grandfather paradox" because it pretty much shows that you not only can't go back in time and kill your grandfather, you pretty much can't go back in time, because in order to do so (to my engineer's aesthetic applied to the reconciliation of "many universes") you'd have to access an alternate universe with a "backwards running time stream, (as theorized by more thatn one person) to travel back-at which time your "matter" would be in two places at "the same time". Just can't be,m at least not simply. At best, you could "travel" back in time in some sort of stasis at a fixed point, at your desk, for example, and then travel forward again.


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## Rich Parsons (Jul 9, 2005)

arnisador said:
			
		

> The massless photon? I think it goes back to at least the 30s or so. I don't know what earlier theories were. The Standard Model started firming up in the 50s but in its current formulation is from the early 70s, I think.




Well I remember both theories and models, and know that many still use the Newtonian Model(s) for they work in many ways. I just wonder how I got it stuck in my head that the mainstream model was not it.

Oh Well.


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## arnisador (Jul 9, 2005)

Certainly, engineering couldn't be done without Newtonian mechanics. The objection to it is mostly philosophical.

 If you were exposed to multiple theories, I'd say you got a _better_ education than most! As *elder999* makes clear, there _are_ other theories out there, and while they aren't accepted, they aren't crackpot theories either. String theory gets a lot of respect--but no one but its proselytizers would currently say "Yes, this is the best known model of how the universe works at a fundamental level." If you were forced to design something right now that involved elementary particles (I have no idea what this would be--a quantum computer, maybe?), you'd base your calculations on the Standard Model and all that goes with it. Anything else wouldn't stand up in court if it failed--you'd have built a failed device using unproven theories. It's much more legally defensible, of course, to build a failed device using proven theories!


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## arnisador (Jul 10, 2005)

Meanwhile...

http://news.yahoo.com/s/space/20050708/sc_space/teleportationexpresslanespacetravel



> Think Star Trek: You are here. You want to go there. It's just a matter of teleportation.
> 
> Thanks to lab experiments, there is growth in the number of "beam me up" believers, but there is an equal amount of disbelief, too.
> 
> Over the last few years, however, researchers have successfully teleported beams of light across a laboratory bench. Also, the quantum state of a trapped calcium ion to another calcium ion has been teleported in a controlled way.


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## elder999 (Jul 10, 2005)

arnisador said:
			
		

> Certainly, engineering couldn't be done without Newtonian mechanics. The objection to it is mostly philosophical.
> 
> If you were forced to design something right now that involved elementary particles (I have no idea what this would be--a quantum computer, maybe?), you'd base your calculations on the Standard Model and all that goes with it. Anything else wouldn't stand up in court if it failed--you'd have built a failed device using unproven theories. It's much more legally defensible, of course, to build a failed device using proven theories!


I can't provide a link, but:

(This is from a Lab Newsletter, and is for public dissemination.)



> Los Alamos National Lab physicist Wojciech Zurek of the Theoretical (T) Division and
> his team of students, recently proved a mathematical theorem supporting quantum Darwinism
>  a quantum form of natural selection. Quantum Darwinism sheds new light on
> the workings of environment-induced superselection or einselection  a process proposed
> ...


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## Rich Parsons (Jul 10, 2005)

arnisador said:
			
		

> Meanwhile...
> 
> http://news.yahoo.com/s/space/20050708/sc_space/teleportationexpresslanespacetravel




I liked the article, and the quantum Computer research by HP.


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## jonbey (Nov 17, 2007)

I main thing to remember is that if people could go back in time, then someone would have spotted it by now. It is all very well having a theory, but ultimately it is not possible, so any theories are really just the workings of bored/drunk astrophysics professors with little else to think about.

Wouldn't it be great though to have your own "ground hog day". Just imagine, you could learn a whole new system without aging or spending more than a day doing it! That would be cool.

This bit is just getting silly though: 

"If we don't know your father is alive right now - if there is only a 90% chance that he is alive right now, then there is a chance that you can go back and kill him. But if you know he is alive, there is no chance you can kill him."


This is all nonsense. Basically, these scientists just talk a load of gibberish and then decorate their words with a few fancy scientific terms and mention that they have a PhD and we are all expected to say "wow, that's amazing". 



I tried to do astrophysics and failed. I am not bitter though.


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## kaizasosei (Nov 17, 2007)

maybe one day, someone will invent a time machine that will go back into time for us and even be able to bring back objects like rocks or blank pieces of paper. 
  well, a time capsule sortof does this by sending' things off into the future. rather simply preparing the things, like pictures, videos, scientific data, etc to be discovered later.

of course i do believe that i could not find any timecapsule if i or someone had not prepared it. 

but once we probe the nature of those 'gateways' into the past or present, then it is almost clear that the material aspect of our being is more like a catalyst  than the main subject of the reaction known as being.

could not communication be seen as a type of motion.  thing is that communication is not always easy to measure. rather, communication by itself as we define it, is not something that is measureable in any widely accepted way. other than the present maybe, or possibly by stretching and transcending the common notions of communication.  
i think the whole problem lies in the 3rd principle.  there is the observed as we are observing it' and there is the observer as we are also the observer.  i know this sounds the same but i don't think it always is. 

so if i were to propose that like stated earlier in the buddhist way that 
there is no past or present and that it is illusion, or to say as above that: if we cannot at all travel to the past or the future in any way whatsover, then  what is left?  
i would even go further and ask then, what is the purpose of living?

as we are constantly remembering the past and some propose that a part of our perceptual apparatus is continuously scanning the future.  
 to me it seems that before one can have a strategy- a quantam strategy.  this is a huge factor.  like in any battle, one would need to be very clear as to what one is.  i mean the self, the friends and foes.  a sense of identity without which life would become unbearably cosmic and probably empty or futile.  so the illusion of self,  is greater still than the illusion of time or space. yet it may be called illusion or symbolic, but it can also be called real-this could be seen as a matter of preference.

if i were to say:  learn from the past, love or accept the present, pray for the future.  
 this is doesn't sound very scientific.  however, if we were to take it apart and ask...who or what are we learning from? what are we in the first place?, and who or what could we possibly be praying to or hoping for?- it slowly becomes clear what an active role one plays in determining ones part in everything if not the everything itself.

i can see it happening that one passes time quicker by going slower and slower by going quicker.  however, these rules not being enough, one would have to figure out strategy i mentioned and then furthermore i believe it can't be that easy.  there must be other factors (other worlds) as relatively speaking it would depend on the speed of the mind rather than the speed of the body. and the speed of the mind can with simple logic calculate many things that are probable or improbable.  and that's just simple logic with material things mostly.  the theory of relativity also stating according to my understanding that matter is a condensed form of energy. 
this seems to be reflected in my understanding that we must constantly be jumping back and forth between that which real and that which is symbolic.  
  this becomes more evident when we study language and communication. 
i think this most natural aspect of being within a timeframe is something so simple it's almost blasphemous.  

time flies when your having fun.


j


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## BrandonLucas (Nov 6, 2008)

I know this is a dead thread, but it's interesting to look at...

I don't think the theory is correct, though.

The report uses an example of going back in time to kill your father, and the simple fact that you know your father is alive now will prevent you from killing him.

The problem is what happens if someone who doesn't know your father goes back in time and kills him?

I think that, going by the theory that is stated, our reality is more of an understood perception.  We know and understand what is in front of us now, and we know what has happened in the past.

If someone or something were to alter the past, then our present would be altered without our knowledge of the change.  It would be accepted as reality, or our perception of reality, as to what has happened in the past and what is currently happening.

For instance, if someone travels back in time and kills my father before I'm born, then I cease to exist in the present time.  Everyone who ever knew me or knew of my accomplishments would forget that I ever existed, and anything that I accomplished or built or impacted would be erased.  

Basically, it's the butterfly effect.  If you travel back in time, and step on a butterfly, the ripples of the effect of you killing that butterfly can drastically change the present.  That butterfly could have flown onto the windshield of a car driven by the next Hitler, made such a mess that he cannot see and hits a tree, instantly killing him and any chance of world domination.

However, without that butterfly to obstruct his vision, he could finish his trip to whatever destination he is headed to unharmed, and continue on with his evil plan of death and destruction.

It can all get a bit deep.  But that is just my hole in that theory.


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## Marginal (Nov 6, 2008)

As I understand the classic paradox:

There's no paradox if someone else goes back in time and kills your dad before you were born. You going back in time and doing so would mean that someone who never existed killed him, which is the paradox since if you never existed, there's now nobody going back in time to kill the father. So he wouldn't die. Which means that he'd live to father you...


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## MA-Caver (Nov 6, 2008)

Bob Hubbard said:


> It is the paradox of time travel.
> You can't go back in time and kill your father because then you wouldn't be alive to go back in time to kill him.
> 
> By the same token, you can't go back in time and kill Hitler as a boy, because then you wouldn't have wanted to go back in time to kill him since he was never a problem so therefore he wouldn't be killed by you causing you to want to kill him.
> ...


Few more bong hits and I'll be catching up with ya on that one Bob....  (NOT!) 

Sigh... I think I'll stick with Doc Brown's explanation to Marty.


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## CanuckMA (Nov 6, 2008)

Andrew Green said:


> Well its not always a wanting to change things...
> 
> If time travel does become a reality, and historians go back a couple thousand years to watch the Crucifiction of Jesus... The fall of Rome...The building of the pyramids... Dinosaurs... or any other historical event, what danger to the present (if any) does that pose.
> 
> ...


 

Best example of what the 'past' is. Read "The Dead Past" by Asimov.


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## kaizasosei (Nov 6, 2008)

Most of this is fairly obvious.  It has to do with the laws of cause and effect within linear time.  Also, it also has to do with our subjective obvervation and conciousness both individual and collective.  
The underlying subject is attempting to be objective and explain the laws of the universe, happenings and time.  

Thing is though, the next step is to ask ourselves to what degree can we hope to grasp even a fraction of the univeral elements that constitute existence with our limited minds and limited language.  Our conciousness is greatly restricted by our language as much as it is created,quickened, and perpetuated by language.  We think of nouns, we are centered on verbs of doing and we cannot think outside the box of past, present and future- but what if all that is an illusion and the true nature of existence is quite independent of many of these earthly essentials that we experience as subjectively real. Mythology for example in many archetypal ways is just as real as history is and what we call history or the past is more like a type of mind trick or manipulation of conciousness.  So the question would be; what is reality in the first place and what's it to us?

I was going to mention a little more on alternative ideas proposed by religions such as that of a personal God that watches everything and is omnipotent, but i due to the fact that we most likely cannot even agree on the definitions of universe, the solarsystem, the self or the soul-manmade or not, i can see it all getting mighty complicated and taositic to the point of being unproductive.

Ofcourse, the past was and is real, the present is real and the future will become real.  So the common denominator is reality.   The only chance we have to change anything actively is the present.  The future also will become the present at a future time and the past was once the future too.  Without the glue of the present, the past and the future would not hold together and there would not be any linear time.  And when people say things like history repeats itself, it would be wise to apply this saying to the subject at hand and ponder if the future itself is related to the past in some harmonious way.

The present is something that we call present due to the experience of linear time, however, everything points to the fact that it may well be less linear and more cyclic in nature. As in a circle is whole and the largeness thereof depends on the circumference, a line can be shortened or lengthened depending on the substance and force.   

Within our realm polarity of conciousness vs reality is constantly alternating at some level.  For example, an individual is a dual thing that cannot be separated. Man and world, individual and a species, sun and sunsystem. Each part is only a half of it's collective opposite. Each sun is only a half, each person is only a half and the collective is it's other half. 

What we perceive as time, is a function of our planets size and speed as it orbits the sun.  What we are perceiving is not completely an illusion, as everything on our planet follows this and is created and destroyed within the time that we experience.  However, the aspect of creation and destruction are in direct opposition to the phenomenon of existance.  Basically science would say that energy is neither created nor destroyed. 
I see time as having layers like onion, or a seed that sprouts under certain conditions, and yet on the deeper side i see time as something irrelevant to the nature of existence.




j


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## Shizen Shigoku (Nov 6, 2008)

That's why I like the Multiple Worlds / Parallel Universes theories - cleans up that mess quite neatly.


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## Cryozombie (Nov 7, 2008)

The real question is if I put jiffy pop in the microwave while observing a star going nova, will the red and blue radiation make me go back to roswell and become my own grandfather?


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## Shizen Shigoku (Nov 7, 2008)

Cryozombie said:


> The real question is if I put jiffy pop in the microwave while observing a star going nova, will the red and blue radiation make me go back to roswell and become my own grandfather?



Yes. This phenomenon has already been observed.


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## hpulley (Nov 8, 2008)

Shizen Shigoku said:


> That's why I like the Multiple Worlds / Parallel Universes theories - cleans up that mess quite neatly.



I'm surprised it took (unless I missed this in an earlier post) three years for someone to state this.

With the multiple worlds theory, at every point where more than one thing can happen, you get multiple universes splitting off.  So to think about it, when playing craps you roll two dice.  Generally there are numbers from 2-12 which can come up so there are 11 possibilities but there are actually 36 combinations and this doesn't include all the physical ways in which they can occur.  BUT, what if you went back in time to before the dice were rolled so you would 'know' the outcome and could bet on it?  Well, this doesn't work because you don't know which universe you are returning to and in fact the universes where you went back in time are different from the universes where you didn't go back in time.

Sooo... the reason why you can't change the present is simply because it is in a different universe.  If you go back in time and then return to the present you are in a different universe than one in which you did not go back in time.

So, whenever you flip a coin or roll the dice or anything like that, rather than finding out if it is heads or tails or snake-eyes or boxcars you are really just finding out what universe you are in!!!!!


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