Is Time Travel (going backwards) Possible?

tomBitonti · Nov 7, 2012

Umbran said:
That there *must* be a consistent behavior is an assumption. It may well be that no consistent behavior is strictly required. Of course, if it isn't required, one can quickly come up with scenarios where perpetual motion machines and other previously impossible situations become possible.

It is imaginable that we live in a universe where time travel is not strictly prohibited, but that it can be used in such ways that actually destroy the cosmos.

Yeah, the *must* is strong, but, the alternative seems to be a universe where science doesn't work. Quantum mechanics, although random for individual events, fits distributions, and (as I gather) is a consistent theory. When attempting to understand time travel, should we start by assuming that it completely breaks other principles?

Thx!

TomB

KarinsDad · Nov 7, 2012

Umbran said:
It is imaginable that we live in a universe where time travel is not strictly prohibited, but that it can be used in such ways that actually destroy the cosmos.

One would be hard pressed to imagine the amount of energy required to destroy the cosmos, assuming that it cannot just unravel at the seams. It seems more likely that the energy required to go back in time (i.e. to force everything everywhere to reverse itself) would be infinitely great (or at least so great that it is more than all of the energy in the universe) and hence, not achievable.

Umbran · Nov 7, 2012

Site maintenance prevented me from posting this yesterday:

freyar said:
Absolutely! Although, to be honest, people use WIMP to mean anything with a weak-strength interaction, not just the weak nuclear force any more. And these interactions with the Standard Model are reasonably well-motivated. But that part has always been a bit of a guess; maybe there's a whole dark sector with weak-strength interactions and even weaker interaction with the Standard Model.

Oh, of course. The primary characteristic for the stuff is "doesn't interact much with the rest of the Universe, except by gravity" - outside of that, we can imagine all sorts of exotic behavior. You just try the basics first, is all.

Or DM might interact relatively strongly with the Standard Model but not with the detectors we've built for some reason (yes, I can give some good ones if you want to know).

I'd be interested in hearing, yes.

I have to go do some work now, so just briefly: compared to the number of graduate programs in the world, there are very very few graduate course lectures available on-line, and only a percentage of those would relate to dark matter or anything similar.

And, if you found one, you'd likely find that if you didn't already have the equivalent of an undergraduate degree in math or physics, you'd likely not be able to follow the presentation - grad school lectures in the area are very math-heavy.

KarinsDad said:
From the videos I've seen online (and granted, that's a very tiny subset of class and lecture material), I haven't seen much push back by students questioning what they are taught.

And, you take this to mean that nobody questions?

The process of hard sciences is dominated by peer review. That's PEER review, not students reviewing teachers. The students generally don't know the details well enough to question them on the spot in lecture - remember that each notable model or result will probably take the student some to many hours of work through the math before they have a real handle on it.

And that's assuming that interrupting a speaker in the middle of the presentation (a presentation that's being recorded for public consumption, even) was the socially acceptable way to go about it.

It's not a matter of merely learning the ideas and theories, especially in theoretical areas, it's a matter of learning where these ideas come from and also on how to evaluate these ideas (i.e. learn how to think).

The bulk of a graduate student's career is building up just that. One topic builds on the next - you don't see that looking at isolated lectures.

If the evidence for mankind's influence on it is even somewhat substantial, how come so many scientists from unrelated branches are declaring a side? There either should only be one side that most of them line up on, or these scientists should say that we don't yet have enough data.

You picked an example that's hard to discuss without touching politics.

But, let's consider it this way: how many individuals are actually responsible for the Edition Wars? Modern communications make it possible for a small number of very vocal individuals to make it seem as if there's a fundamental controversy, when none really exists.

Umbran · Nov 7, 2012

tomBitonti said:
When attempting to understand time travel, should we start by assuming that it completely breaks other principles?

We should assume it *might*, yes. Do remember that time travel implies events coming before their causes - there's not much more fundamental-principle-breaking than that.

KarinsDad said:
One would be hard pressed to imagine the amount of energy required to destroy the cosmos, assuming that it cannot just unravel at the seams.

Well, I am not convinced that the assumption that the universe cannot "unravel at the seams" is at all outlandish, once we start considering time travel at all.

Plus, with a time machine, you can have perpetual motion, which means you have access to infinite energy. And, with that same time machine, you can have that energy all in one time. It might, subjectively, take a number of loops through time to get the ball rolling, but once that ball is rolling, from the point of view of the rest of the universe, the energy all just pops out of nowhere.

Again, with the "not much more fundamental-principle-breaking than that".

tomBitonti · Nov 7, 2012

Umbran said:
We should assume it *might*, yes. Do remember that time travel implies events coming before their causes - there's not much more fundamental-principle-breaking than that.

Well, I am not convinced that the assumption that the universe cannot "unravel at the seams" is at all outlandish, once we start considering time travel at all.

Plus, with a time machine, you can have perpetual motion, which means you have access to infinite energy. And, with that same time machine, you can have that energy all in one time. It might, subjectively, take a number of loops through time to get the ball rolling, but once that ball is rolling, from the point of view of the rest of the universe, the energy all just pops out of nowhere.

Again, with the "not much more fundamental-principle-breaking than that".

Perhaps, but all that seems very unlikely. Even the advances of relativity and quantum mechanics play nice at the macro level. I'm more inclined to imagine folks tripping themselves over misunderstandings of very basic principles and common notions which don't carry very well into either relativity or quantum mechanics, and probably don't serve very well, either, when looking more closely at causality.

From a pure physical point of view, what happens when field equations are extended through a closed time-like curve? If the field is subject to a device which projects the field if the field is absent and is quiet when the field is present, and that is placed on a point on the curve, that seems to cause problems. Would the field reject placement of the device?

Thx!

TomB

Umbran · Nov 7, 2012

tomBitonti said:
If the field is subject to a device which projects the field if the field is absent and is quiet when the field is present, and that is placed on a point on the curve, that seems to cause problems. Would the field reject placement of the device?

This is logically equivalent to the Grandfather Paradox - we don't know the answer. The equations of physics don't handle recursive conditional statements.

KarinsDad · Nov 7, 2012

Umbran said:
We should assume it *might*, yes. Do remember that time travel implies events coming before their causes - there's not much more fundamental-principle-breaking than that.

We should assume that it *might not*.

Why?

Because the universe has not yet been destroyed.

Even time travel where the observer can go back and observe, but not interact, would imply that at least light leaves the past in order to be observed in the future (which also on some level infers that s/he is interacting anyway). This has some minor possibility of not breaking actual laws (who knows if energy cannot be created or destroyed in a single instant of time, or over all time).

Umbran · Nov 7, 2012

KarinsDad said:
Even time travel where the observer can go back and observe, but not interact

As I noted upthread, this is not possible. Thermodynamically, transfer of information requires transfer of energy - taking information from the past means taking energy from the past - so if you observe and come back, you did interact.

Unless, of course, the laws of thermodynamics don't hold - and then we're back with the perpetual motion machines.

Janx · Nov 7, 2012

Umbran said:
As I noted upthread, this is not possible. Thermodynamically, transfer of information requires transfer of energy - taking information from the past means taking energy from the past - so if you observe and come back, you did interact.

Unless, of course, the laws of thermodynamics don't hold - and then we're back with the perpetual motion machines.

Conceptually, when we observe stars, we are looking at their state from millions of years ago (1 year in the past, per light year).

technically, the star was broadcasting the info, and we just happen to catch it in the future.

Though by Umbran's more technical assertion, we did not actualy go back in time and capture that info (thereby removing that energy).

I suppose what Umbran means my Information = Energy, and what most of us mean might also be different.

If I could go back and watch Licoln's assasination, the rays of light in the room are bouncing around and hitting me and my eyes, where previously there was an empty gap. I can assert that my brain is merely copying the information of what happened, but technically, I was blocking somebody's view of the play, and "taking" energy from the event.

Us layman see that as a pretty passive event. Especially if I stood someplace out of the way. Scientists who pay attention to observation protocols would probably nitpick it based on the concept that the act of observing tends to interfere, if only slightly in the event being observed. Plus, I recall some phenomenon in quantum physics where you tend to find what you were looking for, but that trended toward hippy quantum physics.

tomBitonti · Nov 7, 2012

Some interesting links

http://arxiv.org/pdf/gr-qc/0703100.pdf

28-May-2007

Stability of Closed Timelike Curves in the G¨odel Universe

Val´eria M. Rosa
Departamento de Matem´atica, Universidade Federal de Vi¸cosa, 36570-000 Vi¸cosa, M.G., Brazil

Patricio S. Letelier†
Departamento de Matem´atica Aplicada-IMECC, Universidade Estadual de Campinas, 13081-970 Campinas, S.P., Brazil

PACS numbers : 04.20.Gz, 04.20.Dw, 040.20 Jb

Stability of Closed Timelike Curves in the G¨odel Universe

From the top of page 2:

"All our experience seems to indicate that the physical laws do not
allow the appearance of CTCs. This is that, essentially, says the
Chronology Protection Conjecture (CPC) proposed by Hawking in 1992
[16]."

http://physics.stackexchange.com/qu...ke-curves-to-be-impossible-in-quantum-gravity

There are a lot of other links which I haven't scanned. That second one has this:

These are some notes to complement previous answers.

Your concerns are sound: a rigorous definite way to rule out CTCs has
not been found. What we have is arguments (and quite nice looking
ones) to illustrate that every known universe with CTCs looks
unphysical.

Second, there are two nicely-written pedagogical letters written by
Kip Thorne addressing your question [1],[2]. They mainly focus on
physical aspects of the known CTC solutions, and three popular
mechanisms that could prevent CTCs: violation of the averaged null
energy conditions (the first argument cited in the post), classical
instabilities of chronology horizons, and quantum field instabilities
(following the notation of [2], section 4). Although he does not seem
to believe in CTCs personally, at the end of [2] he states that this
is still an open question:

It may turn out that on macroscopic lenghscales chronology is not
always protected, and even if chronology is protected
macroscopically, quantum gravity may well give finite amplitudes
for microscopic spacetime histories with CTCs [29].

[29] Friedman J 1992 in Proceedings of the 4th Canadian Conf. on
General Relativity and Relativistic Astrophysics eds G Kunstatter
et al (Singapore: Word Scientific) pp. 183-199.

Finally, regarding the argument against CTCs that uses logical
paradoxes, which has already appeared in the post: it is not clear to
many people whether CTCs inevitably lead to causal paradoxes. Several
studies have pointed out that causal-paradoxes of time travel could
disappear once one takes quantum mechanical effects; or maybe their
meaning could simply change [3],[4],[5],[6]. For instance, in the
framework used in the first reference the grandfather's paradox does
not violate causality. In connection with this, although it is known
that some of these models of CTCs [7],[8] lead to counter-intuitive
collapes of computational complexity classes, this is not exactly the
same as a causal paradox.

The link [1] to a Kip Thorne PDF seems to be the jackpot:

http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf
"Closed Timelike Curves" Kip S. Thorne

One of the questions reviewed: Do the laws of physics prevent CTCs
from ever forming in classical spacetime?

From page 2:

"However, the combination of general relativity's laws and the laws of
quantum fields in curved spacetime may well provide a chronology
protection mechanism, though we might be sure of this until we
understand the laws of quantum gravity much more deeply than today."

Lots of complicated details to the PDF, which I am not qualified to
present. There is this end note:

"In summary, these studies are giving us glimpses of how CTSs
influence physics; but whether those glimpses are teaching us
something deep and important or just playing fun mental games, is far
from clear."

Note: The paper is from Feb, 1993.