Grandfather paradox solved?

[Shizen Shigoku]

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!

 

[heretic888]

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:

 

[sgtmac_46]

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.

 

[Rich Parsons]

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

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?

 

[Shizen Shigoku]

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.

 

[heretic888]

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

Everything material, anyway...

 

[Shizen Shigoku]

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.

 

[Andrew Green]

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"

 

[heretic888]

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]

Shizen Shigoku,

I respectfully disagree wiht some of what you posted.

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).

"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 *)

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.

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.

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.

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.

"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. "

My Arguement Above your reply below

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.

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.

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.

"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

This is what I expected.

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]

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]

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]

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.

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]

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]

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.

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]

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]

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 = mc2. 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]

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.

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.

"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.

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.

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]

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):

Thanks I browsed some, will read in detail

"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 = mc2. 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.

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]

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.