# Time Travel Yeah I know! maybe some parallel universe talk too!

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Andrew Mason
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Quantum1332 said:
Ok...I know there have been tons of threads on this, but I would like to hear some opinions on this. So, after reading and reading about this I have learned that there are tons of theories and reasoning as to why it is possible, but what I really want to know is what all of your opinions are about this.
Are you asking whether Special Relativity and its prediction of time dilation is true? Or are you asking whether it will be technologically possible for a person to experience significant time dilation by travelling at relativistic speeds?

AM

My favorite among all the time travel devices we could construct is the Tipler Cylinder:
http://en.wikipedia.org/wiki/Tipler_Cylinder

Mostly because, even though it would be incredibly difficult to build, it provides a way to travel backwards in time without violating ANY laws of physics (At least to my understanding).

pervect
Staff Emeritus
Guillochon said:
My favorite among all the time travel devices we could construct is the Tipler Cylinder:
http://en.wikipedia.org/wiki/Tipler_Cylinder

Mostly because, even though it would be incredibly difficult to build, it provides a way to travel backwards in time without violating ANY laws of physics (At least to my understanding).
Tipler's paper was groundbreaking at the time, but it inspired further research to show that it won't work, at least with a finite cylinder. (Note that the math in Tipler's paper was for an infinite cylinder, as that case was easier to analyze).

Specifically, as Steve Carlip says:

Hawking's chronology protection'' result
(Phys. Rev. D46 (1992) 603) ... shows that creation of
closed timelike curves from a compact region of spacetime
requires that the weak energy condition be violated.
So you need either an infinite cylinder, or some sort of exotic matter (which violates the weak energy condition).

It appears that it may be possible to violate the weak energy condition, however.

pervect
Staff Emeritus
Quantum1332 said:
I think that any answer to this would have to be highly speculative.

I am asking, if you think that it is theoretically possible to travel through time, and if so, how you think this goes along with parallel universe theory?

pervect
Staff Emeritus
Certain classical solutions to Einstein's equations suggested that closed timelike curves (commonly called time-travel) could exist, i.e. would not be inconsistent with Einstein's equations.

At this point, quantum mechanics hasn't entered the picture at all.

Sometime after this, I think, partially due to the science fiction movie contact, wormholes were hypothesized.

It was soon realized that wormholes, if they existed, could be turned into time-machines. This is a bit more user friendly than the CTC's - with a wormhole turned time-machine, a person could just step through the wormhole and travel into his past, or future.

Here, interestingly enough, quantum mechanics seems to act to make time travel more difficult, perhaps even impossible. "Quantum vacuum fluctuations" would tend to act to close any wormholes that were on the verge of becoming time machines.

The situation is unclear enough that is hard to make any definitive statements. I don't think time travel can be totally ruled out at this point, nor can it be said with a certanity that it's possible.

As far as "parallel universes" and quantum mechanics go, I would try the QM forums. Quantum mechanics doesn't necessarily imply "parallel universes", at least in the usual sense. There is quite a bit of debate about that topic, too.

pervect said:
Certain classical solutions to Einstein's equations suggested that closed timelike curves (commonly called time-travel) could exist, i.e. would not be inconsistent with Einstein's equations.
If we view the spacetime continuum as a static object through which we pass then theoretically past, present and future all exist at the same time. If the past and future exist in concert with the present then there is no reason that we should not expect to be able to move from one part of time to another.

pervect
Staff Emeritus
On the other hand, if we vew time from the point of cause / effect, time travel gets to be very confusing.

Some primitive versions of the grandfather paradox involving billiard balls havea been discusssed in the literature and shown to not create logical paradoxes.

see for instance http://www.pbs.org/wgbh/nova/transcripts/2612time.html

KIP THORNE: Billiard balls provided us a way to study paradoxes with time travel without getting into the nasty business of free will of human beings.

NARRATOR: Billiard balls behave in predictable ways according to the laws of physics. Cause always precedes effect. But what happens if time travel is possible?

KIP THORNE: If I have a time machine the story is quite different.

NARRATOR: Imagine the pockets of this billiard table are the mouths of a wormhole time machine.

KIP THORNE: In this case I have only one billiard ball and I send that billiard ball into this mouth of the wormhole and it will then come out of that mouth before it entered this mouth, hit itself and prevent itself from going into the first mouth. Voila, a paradox. It's the billiard ball version of going back in time and changing history.

IGOR NOVIKOV: Of course, this problem was discussed a lot in literature, in movies, in science fictions, but I am talking not about fantasies but real science.

KIP THORNE: Having developed simple versions of the paradox, we then set about trying to figure out how to solve these. How will the laws of physics behave if time machines are permitted? In searching for a resolution of the paradox we were led by a principle introduced by Igor Novikov, which said that nature will only allow those behaviors that are absolutely self-consistent.

NARRATOR: In order to be self-consistent, a ball emerging in the past must knock itself into the time machine so it can come out again along a trajectory that will knock itself back into the pocket. Novikov wrote mathematical equations for the possible outcomes of a ball hitting itself, and found that a self-consistent solution always exists. This implied that nature would not allow a paradox to arise.

IGOR NOVIKOV: This is the main principle. All these events must be in self-consistency with each other. It's so simple, so obvious, but more of that we gave the strict mathematical proof that this principle is the consequence of the basic ideas of the physics.

NARRATOR: By the same token, Novikov's findings implied that the laws of physics would prevent a time traveler from rewriting history.
So this leads to one possilbe resolution of the issues with time travel and causality. Interestingly enough, there can be more than one possible solution for classical billiard balls when a time machine is introduced - the intial conditions do not necessarily pre-determine the outcome anymore.

A good semi-popular source for this information is Kip Thorne's excellent book "Black Holes and Time Warps: Einstein's outrageous legacy".

I believe some work has been done with replacing the classical billiard balls with quantum particles.

Google finds a aource of somewhat unknown provenance, though it is an .edu site and it appears to make sense:

http://www.isr.umd.edu/Labs/CSSL/simonlab/pubs/physics_world_ctcs.pdf [Broken]

The classical behavior of particles and fields is sufficiently rich and confusing that one naturally turns to quantum mechanics to try and resolve the question of “what happens” when something goes through a time machine (since the classical world of our everyday experience is actually the limiting case of the underlying quantum mechanical world). It turns out that quantum mechanics does give a more definite answer than classical mechanics. Unfortunately, however, one must first pick a
definite formulation of quantum mechanics to get a definite answer, and there are several distinct formulations of quantum mechanics which are equivalent without closed timelike curves but are inequivalent in their presence.

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George Jones
Staff Emeritus
Gold Member
pervect said:
Google finds a aource of somewhat unknown provenance, though it is an .edu site and it appears to make sense:

http://www.isr.umd.edu/Labs/CSSL/simonlab/pubs/physics_world_ctcs.pdf [Broken]
From looking at the Publications link at

http://www.isr.umd.edu/Labs/CSSL/simonlab/

it seems that this paper was published in Physics World, which is roughly the British equivalent of Physics Today, and that this guy was a very serious researcher in relativity before experiencing a major change in research direction somewhere along his timelike worldline.

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Quantum1332 said:
Time travel is easy. I do it all the time. But not backwards time travel. Can't be done. Nope. And anyhow, there isn't really any time. All this stuff like time and past and future is just a bunch of concepts to do with measuring motion. And you can't move a negative distance...

Oh, Lordy me! Negative velocity:

http://www.msnbc.msn.com/id/3077368/

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Here's something about Time that I found on the internet. I share the sentiment that time is not something real, it's like heat in that it's a "derived effect".

http://www.beotel.yu/~mmalovic/newtime.htm [Broken]

"So we build an instrument to do the simple task of demonstrating a standard uniform motion, and we cause this instrument to record its accruing motion in some manner, which we calibrate to mimic the Earth's rotation. Since this instrument accomplishes no other task, but to move uniformly, we call it a clock and say it is measuring time. We then declare this TIME, that we claim to be measuring, to be a universal property of nature and a separate dimension. We are violating Occam's razor, because we are inventing a TIME concept that is unnecessary and confusing. Our motion-clock is not measuring Time; rather it is measuring Motion, its own Motion."

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Here is a time machine i built earlier...

Its a little slow, but it is a comfotable ride.

pervect
Staff Emeritus
Farsight said:
Here's something about Time that I found on the internet. I share the sentiment that time is not something real, it's like heat in that it's a "derived effect".

http://www.beotel.yu/~mmalovic/newtime.htm [Broken]

"So we build an instrument to do the simple task of demonstrating a standard uniform motion, and we cause this instrument to record its accruing motion in some manner, which we calibrate to mimic the Earth's rotation. Since this instrument accomplishes no other task, but to move uniformly, we call it a clock and say it is measuring time. We then declare this TIME, that we claim to be measuring, to be a universal property of nature and a separate dimension. We are violating Occam's razor, because we are inventing a TIME concept that is unnecessary and confusing. Our motion-clock is not measuring Time; rather it is measuring Motion, its own Motion."
This appears to me to be a crank website. The quote isn't the crankiest thing I've seen on time, it's not even the crankiest thing on the website....

Given the direction that this thread is moving, I was thinking that it could perhaps find a better home in the PF lounge. While relativity does have a few things to say about time travel that are in the peer-reviewed literature, the responses and the OP's question do not seem to be focused around the technical papers.

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ZapperZ
Staff Emeritus
Thanks to pervect for bringing it up. I will caution everyone involved to please remember our guidelines. Please do not make any references to some dubious website to support whatever it is you are trying to present. This is not acceptable as a valid citation. If you cannot cite something that is based on accepted physics, or peer-reviewed publications, then don't!

If anyone wishes to deal with some untested or unproven ideas, you are welcome to submit it to the IR forum. I will leave this thread here for the moment. However, if it starts going south again, it will be the end of it. So all of you decide the fate of this thread.

Zz.

Pervect: I would be grateful if you could point out what's cranky about the website I linked to. Apologies in advance.

You may want to have a peek at my thought experiment thread https://www.physicsforums.com/showthread.php?t=124037"

I believe its one of the first theoretically plausible means of 'universe travel'. Though I'm still ironing out issues (which is why I joined here).

The idea is basically, if you surround a traveller with event horizon, without the traveller ever passing through it, the only option left, is that the traveller emerges in a different universe. Your more than welcome over on the hollow spherical blackhole thread.

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Perhaps slightly aligned towards philosophy, but relevant nevertheless...

Farsight said:
Pervect: I would be grateful if you could point out what's cranky about the website I linked to. Apologies in advance.
Well, I didn't quite read the whole text, but the parts I did read, it didn't seem to cross the line IMHO. Although the article was somewhat badly written, he didn't seem to be talking about any alternative model to relativity at all (QUOTE: "my notions about time may have little or no impact on today's science").

Rather he seemed to be merely talking about the very important philosophical notion that there is no metaphysical sense in time, which I think is a relevant notion to OP. This is not some testable claim, it is a simple fact that we only observe motion, and we derive time by comparing the motion of one physical thing to the motion of another physical thing.

That we use the concept of time in our mathematical and semantical models of reality has no metaphysical meaning to reality itself. For reality to exist, it doesn't require any metaphysical "fabric/dimension" of time to exist.

Perhaps people were put off by the claim "time dilation is as much a fantasy as time itself." I didn't interpret this as a claim to invalidate relativity or to claim time dilation has not been observed, but rather as an assertion that the observation of time dilation is not an observation of the time dimension, but an observation of physical motion.

Some may say this is not science but a philosophy, but the matter of the fact is that this is very, very important notion to the philosophy of science and objective thinking overall. It is important to understand that reality is not "like" the spacetime graph we use to understand it.

In fact, the popular claim that time dimension exists in reality as it does in 4D block of spacetime (where nothing moves), is immediately problematic in a philosophical sense, because metaphysically, the only thing we know to exist for certain, is our conscious experience, and we know that in our experience there exists motion, and such experience has to have a physical cause one way or another. Such physical cause is not offered in the block time interpretation of relativity at all. In short, it must be the objective physical reality that causes our subjective experience of reality, and if our subjective experience is really caused by the motion in our brain, there must in fact exist motion in a metaphysical sense.

Or to suppose that time dimension metaphysically exists, and there is a metaphysical flow/motion to this dimension, is also philosophically problematic because this motion would be self-referential. Motion requires time, and for time dimension to flow, there would need to be another time dimension to allow the motion to the flow. Any such infinite regress is always caused by a naive model of reality where semantical ideas are thought to really exist as fundamentals (spirit/soul, language, math, thoughts...).

This should in fact prove that time is not a dimension in a metaphysical sense, and that makes no claim whatsoever about the validity of the mathematical framework of relativity.

I also had a related question of my own;

I understand that there has been teleportation experiments with quantum particles. How/why is it that no information can be passed through this way faster than C? (I suppose this to be so, because otherwise it would be immediately quite trivial to send messages to the past or to the future, if relativity is true and the past and the future do in fact exist in some metaphysical sense)

-Anssi

Farsight said:
Here's something about Time that I found on the internet. I share the sentiment that time is not something real, it's like heat in that it's a "derived effect".

http://www.beotel.yu/~mmalovic/newtime.htm [Broken]

"So we build an instrument to do the simple task of demonstrating a standard uniform motion, and we cause this instrument to record its accruing motion in some manner, which we calibrate to mimic the Earth's rotation. Since this instrument accomplishes no other task, but to move uniformly, we call it a clock and say it is measuring time. We then declare this TIME, that we claim to be measuring, to be a universal property of nature and a separate dimension. We are violating Occam's razor, because we are inventing a TIME concept that is unnecessary and confusing. Our motion-clock is not measuring Time; rather it is measuring Motion, its own Motion."
I read all of this article and I must say the guy who wrote this definitley has a realistic view of things. Especially time. Time doesn't exist. We humans made it based on the rotation of the earth. What time really is, is just MOTION and the measurement of it.

There is simply no physically possible way to travel "back in time." Science fiction my friends!!!

Although ONE thing I don't understand is, for example. I am in a space ship traveling 99 percent c, and come back to earth, why has my twin aged 7 years faster than I have?? Again, years is a measurement of motion according to the article, but I am comparing the biological processes of the human body. Why is it that motion changes biological clocks so to speak? This is all very hard to visualize.

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Event_Horizon said:
I read all of this article and I must say the guy who wrote this definitley has a realistic view of things. Especially time. Time doesn't exist. We humans made it based on the rotation of the earth. What time really is, is just MOTION and the measurement of it.
Yeah, it tends to clarify things a bit to consider that it is physical motion that has a metaphysical/fundamental existence, instead of time. Unfortunately it is very popular interpetation of relativity that time exists as a dimension and metaphysically past and future exists at all times.

Only, conscious experience cannot be explained this way. This really is where strong understanding of philosophy of the mind is needed to evaluate the ontology of physics.

Although ONE thing I don't understand is, for example. I am in a space ship traveling 99 percent c, and come back to earth, why has my twin aged 7 years faster than I have?? Again, years is a measurement of motion according to the article, but I am comparing the biological processes of the human body. Why is it that motion changes biological clocks so to speak? This is all very hard to visualize.
It may be more important to explain the observed phenomenons, not so much the predictions of existing models. The prediction is based on relativity of simultaneity (between inertial frames), and is pretty straightforward to visualize if one assumes the existence of time as a metaphysical dimension as implied by Minkowski spacetime.

As for the actual observations of time dilation between inertial frames, we have, let's see... Long lifetimes of cosmic myons at least. And probably a handful of others? In any case, a careful investigation of RAW data of these experiments is required, instead of just going with the interpretations and assumptions that are done based on the model w/ time dimension.