Time Travel is Science Fiction

[Billy T]

Billy T, please don't use mathematical terms you do not understand.

Two sets are said to be "disjoint" if they have no common elements. The set of "all things prohibited" is disjoint from the set of "all things not prohibited", quite obviously. I cannot see that the set of "all things NOT prohibited" is in any way distinct from the set of "all things permitted". Therefore the latter pair cannot be disjoint

I understand this simple set theory and the union of sets perfectly. The problem is the meaning of English words. I understand the words "not prohibited " to equal "is permitted" or conversely "not permitted" to be the same as "is prohibited" so these are two identical ways to describe this pair of disjoint sets. Only differenc is which gets the "not." I. e. Not X is is disjoint from X and X is disjoint from not X; and their union is everything, at least in their domain.

I would also say that the set of "all things permitted" is not disjoint from the set of "all things possible"...

I certainly agree. In fact if their union is the null set, there is nothing permitted that is also possible! - A strange state of affairs: You are only permitted what is impossible!

 

[paddoboy]

The subject is whether GR predicts it's theoretically possible. Not whether it's technologically attainable.

100% correct. All the silly red herrings and irrelevant pedantic nonsense infesting this thread makes no difference to the validity of that sentence.

 

[Billy T]

... The GPS satellites clock tick rate is > the earth based clock tick rate. ...

I had forgotten which was faster. If you are correct, then the slowing of the earth based clock by stronger gravity dominates the satellite speed's slowing.

I have a post some pages back with footnote suggesting that as gravity falls off inverse square it might be possible to define a new type of "geosynchronous" orbit altitude than has nothing to do with the Earth's spin rate (duration of the day) such that the orbiting clock and the earth based clock stay in synchronization.

It is my bed time so I don't want to go find that old post (may not be in this thread) or use the Kepler law (period)^2 proportional to (semi major axis)^3 again to see if it is indeed posbbile to define this new type of "geosynchronous" orbit altitude. It gets a little messy as the slowing with relative speed is under a square root and related to (v/c)^2 inside.

... The subject is whether GR predicts it's theoretically possible. Not whether it's technologically attainable. ...

No one (at least not me) is saying anything about tehnology limitations (we speak of a very advanced civilations). What was Kip Thone's POV and I quote from the link given in post 1249 was:
"We simply don't have the understanding of the laws of physics (3:26) to be able to speculate about that quantitatively. (3:33) We have to get a much deeper understanding of quantum gravity to get the answer whether that is possible.(3:41 – the end of the video – Kim Thorne's final answer to the question posed by the interviewer and the title of the video: “Is time travel possible”)" The time insert are when in the video he says that (from the "closed captions" the video has.)

 

[paddoboy]

http://arxiv.org/abs/1405.1283v1

ABSTRACT:
We calculate the Casimir energy-momentum tensor induced in a scalar field by a macroscopic ultrastatic spherically-symmetric long-throated traversable wormhole, and examine whether this exotic matter is sufficient to stabilise the wormhole itself. The Casimir energy-momentum tensor is obtained (within the R×S2 throat) by a mode sum approach, using a sharp energy cut-off and the Abel-Plana formula; Lorentz invariance is then restored by use of a Pauli-Villars regulator. The massless conformally-coupled case is found to have a logarithmic divergence (which we renormalise) and a conformal anomaly, the thermodynamic relevance of which is discussed. Provided the throat radius is above some fixed length, the renormalised Casimir energy-density is seen to be negative by all timelike observers, and almost all null rays; furthermore, it has sufficient magnitude to stabilise a long-throated wormhole far larger than the Planck scale, at least in principle. Unfortunately, the renormalised Casimir energy-density is zero for null rays directed exactly parallel to the throat, and this shortfall prevents us from stabilising the ultrastatic spherically-symmetric wormhole considered here. Nonetheless, the negative Casimir energy does allow the wormhole to collapse extremely slowly, its lifetime growing without bound as the throat-length is increased. We find that the throat closes slowly enough that its central region can be safely traversed by a pulse of light.
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also.....

http://edition.cnn.com/2013/05/13/opinion/opinion-time-travel-paul-davies/
where he says in part......

So travel into the future is not only possible, we have done it, although so far in only paltry amounts. How about going back in time? That is far more problematic and remains an active area of research. Einstein found that not only speed affects time, gravity does too. Time runs a little bit faster on the roof, where gravity is imperceptibly weaker, than in the basement, for example.
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It's these "paltry" amounts that some here appear to agenda burdened to accept.
Time goes slower for us when we walk down the street, but the effects are infinitesimal and paltry and obviously mean nothing in the greater scheme of things.
We don't use GR mathematics and methodology to calculate Earthly effects in general, because at these non relativistic effects, the accuracy is not needed, and again, makes no difference in the greater scheme of things.
As I have said, and as most sensible people will agree, achieving time travel is not going to be easy, if it can be done at all.
"But by the same token, no one with any sense can say logically that IT WILL NOT BE DONE OR CANNOT BE DONE"

 

[brucep]

The subject is whether GR predicts it's theoretically possible. Not whether it's technologically attainable. GR predicts the wormhole thought experiment IS theoretically possible. I don't think you understand what constitutes a local proper frame. You're living in one right now. Your wristwatch time is invariant [frame independent]. The local proper time is an invariant. For the relativistic rocket example and Professor Thornes wormhole the tick ratio between two different local proper frames are being compared. The tick rate [dTau] for the rockets local proper frame over its path to Andromeda compared to the tick rate of the local proper frame on eart. For the wormhole it's the tick rate for the local proper frame at the mouth of the wormhole compared to the tick rate of the local proper frame where the wormhole terminates. One you'd be traveling into the future earth local proper frame and the other the past. The GPS satellites clock tick rate is > the earth based clock tick rate. In essence that is a micro analysis that correlates with the wormhole result. While the folks on the ISS are time traveling into the earths future. So just what do you mean when you say something like 'jump to another time'? Think about what I just told you and explain why that isn't jumping to another time. Neither one of you guys know what your arguing about. The examples I listed are Iime traveling into the future or past of a different local proper frame from the one you're living in. The OTHER example your referring to is time traveling in your own local proper frame. The one that is abused in the movies and science fiction stories written by folks who are as clueless as you on this theoretical physics. Time traveling into a different local proper frame has been empirically confirmed. The one where the principle of causality is violated hasn't been confirmed in reality or thought experiment.

Maybe I have that wrong. The thought experiment.
http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf

 

[Trippy]

It's a physics discussion. People like Krash661 and PhysBang are the trolls, not people like Billy and me.

Physbang is doing a better job of addressing you in a reasoned and reasonable manner than you are to physbang.

 

[LaurieAG]

Unfortunately, the renormalised Casimir energy-density is zero for null rays directed exactly parallel to the throat, and this shortfall prevents us from stabilising the ultrastatic spherically-symmetric wormhole considered here. Nonetheless, the negative Casimir energy does allow the wormhole to collapse extremely slowly, its lifetime growing without bound as the throat-length is increased. We find that the throat closes slowly enough that its central region can be safely traversed by a pulse of light.

So the problem with this model is that nothing with mass can travel through it let alone time travel through it?

As I have said, and as most sensible people will agree, achieving time travel is not going to be easy, if it can be done at all.
"But by the same token, no one with any sense can say logically that IT WILL NOT BE DONE OR CANNOT BE DONE"

Once your probability gets into the same remote range as flying pigs you should probably grab one of them (if they happen to come along first) and use it to fly through the wormhole and solve your problems.

 

[arfa brane]

One problem with travelling backwards in time is that you (first) have to leave your own future lightcone; you have to accelerate to > c to do this.

I saw it on the TV.

 

[PhysBang]

What might they be - give an example.

I already gave 4 in one post and many more in other places.

I have several times noted (now in more technical terms) the two disjoint sets are "not forbidden" and "is permitted"

There is a problem in reasoning right there: you response seems to have absolutely nothing to do with anything I'm discussing.

Not Paddoboy's "not forbidden" and "is possible."

If you are imagining that GR merely "doesn't forbid" time travel, then you are quite mistaken. I believe that there are other reasons to forbid time travel, but GR alone is not the reason, since it allows for closed, time-like causal loops.

Also I mainly quote what others, like newton and more modern physicists have actually said, not tell in my own words what I understood them to be saying.

Sure, Farsight claims this too. However, you got Newton horribly wrong, in your own words, and stuck to that mistake. Like Farsight does with Einstein and others. You don't seem to fall into this pattern as often as Farsight, but it's not great to do it once.

I hope to soon return by edit with an example or two.

Whatever that means. You aren't the great communicator you imagine yourself to be. Often you post things that do not make sense.

 

[Landau Roof]

One problem with travelling backwards in time is that you (first) have to leave your own future lightcone; you have to accelerate to > c to do this. I saw it on the TV.

Strange as it may seem, not everything on TV, the movies and comic books is true. In fact, the main reason this thread is so long and full of controversy is that time travel was depicted on TV and hence many people take it as gospel truth. Would old H.G. Wells late-night TV and Michael J. Fox movies lie!? Not about what general relativity does and does not forbid surely! That would be sacrosanct!

 

[PhysBang]

Strange as it may seem, not everything on TV, the movies and comic books is true. In fact, the main reason this thread is so long and full of controversy is that time travel was depicted on TV and hence many people take it as gospel truth. Would old H.G. Wells late-night TV and Michael J. Fox movies lie!? Not about what general relativity does and does not forbid surely! That would be sacrosanct!

No, you don't get it.

The reason this thread is so long is because of people who don't want to address the hard parts of the science and just dismiss it on prejudice. Or, in the case of Farsight, who want to use the rejection of time travel as evidence that their own pseudo-theories are correct.

Time travel is an element of science fiction. It is not likely that we can actually do anything like that fiction. Yet many physicists have outlined plausible scenarios where something similar to time travel is possible and there is no definitive argument against these scenarios.

 

[OnlyMe]

If you are imagining that GR merely "doesn't forbid" time travel, then you are quite mistaken. I believe that there are other reasons to forbid time travel, but GR alone is not the reason, since it allows for closed, time-like causal loops.

I think you mentioned something along this line before, during that short discussion of the Godel solutions, but I missed what the nut of the issue really was. These discussions seem very often to get caught in conceptual quagmires, meaning argumentative loops where the opposing positions are not just based on different interpretations of the same information, they are arguing from different theoretical or practical perspectives. I did not really catch the full significance of your introduction of the Godel solutions and then admission that they did not represent reality until I read the quote above. IOWs until I could step back and see the argument as a third party.

If you restrict your world view to any limited theoretical or even practical POV, what is and is not acceptable speculation is different. I think I now get your point!

GR does allow for the theoretical possibility of time travel, but it is also not an complete model of reality, so that theoretical caveat is important. And the issue remains the same no matter what theoretical basis we choose to base an argument on.

I generally try to run theoretical assumptions through a reality check, in discussions like this. Probably one reason I return to the, what is theory and what is known issue as often as I do.

I don't believe that all things which are theoretically possible, will ever be practical reality. Time-travel is one of those! But as I believe you have been trying to point out the reason(s) the theoretical possibility, will probably never become reality, cannot be found in the theoretical basis, of GR.., if you are, confining your argument to GR, nothing prohibits the possibility, of time-travel. However, once the idea is subjected to practical, philosphical and even issues that arise within the context of QM, those theoretical possibilities begin to become restricted in ways that are not associated with the initial theoretical basis.

We all, or most of us anyway, tend to see things in these discussions from a myopic point of view, originating with our own predispositions. Sometimes based on the subject of the discussion itself and sometimes based on who is making the argument.

 

[krash661]

IMO , part of the problem is anthropocentric[1] thinking.
[1] from point of view of humankind,seeing things in human terms, especially judging things according to human perceptions, values, and experiences

 

[OnlyMe]

IMO , part of the problem is anthropocentric[1] thinking.
[1] from point of view of humankind,seeing things in human terms, especially judging things according to human perceptions, values, and experiences

Even when we disagree, as long as we assume all posters are human, how could we avoid anthropocentric thinking? (As defined)

 

[krash661]

Even when we disagree, as long as we assume all posters are human, how could we avoid anthropocentric thinking? (As defined)

this i can not help with, this is something one has to develop themselves.
but i can suggest, understanding anything involved, the whole picture[thorough as possible], and then look at it, experience it [ in your mind if possible] from the point of view of what ever is being discussed.in a sense, you have to be able to connect with the situation as if you are the situation. once this connection is established, then it is possible to see things nonanthropocentric. other than that, i cannot explain this in another way.
it's little things like this [yes, it sounds ridiculous] that helps with understanding. it's why einstein mentioned, imagine riding on a photon, or something similar to that.

 

[brucep]

I had forgotten which was faster. If you are correct, then the slowing of the earth based clock by stronger gravity dominates the satellite speed's slowing.

I have a post some pages back with footnote suggesting that as gravity falls off inverse square it might be possible to define a new type of "geosynchronous" orbit altitude than has nothing to do with the Earth's spin rate (duration of the day) such that the orbiting clock and the earth based clock stay in synchronization.

It is my bed time so I don't want to go find that old post (may not be in this thread) or use the Kepler law (period)^2 proportional to (semi major axis)^3 again to see if it is indeed posbbile to define this new type of "geosynchronous" orbit altitude. It gets a little messy as the slowing with relative speed is under a square root and related to (v/c)^2 inside.

No one (at least not me) is saying anything about tehnology limitations (we speak of a very advanced civilations). What was Kip Thone's POV and I quote from the link given in post 1249 was:
"We simply don't have the understanding of the laws of physics (3:26) to be able to speculate about that quantitatively. (3:33) We have to get a much deeper understanding of quantum gravity to get the answer whether that is possible.(3:41 – the end of the video – Kim Thorne's final answer to the question posed by the interviewer and the title of the video: “Is time travel possible”)" The time insert are when in the video he says that (from the "closed captions" the video has.)

The reason the satellite clock tick rate is faster than the earth clock tick rate is the gravitational time dilation component is > the SR [relative velocity] component. For the ISS the SR component 'rules'.
You can check it for yourself by working through this project modeling the GPS near earth spacetime with Schwarzschild coordinates. Physics 'fun'. Relativity is really fun.
Choose the 'Student Project for the GPS.
http://www.eftaylor.com/download.html#general_relativity
The following is the derivation for a circular orbit in the near earth spacetime modeled with Schwarzchild coordinates. This renders the same result for the satellite path that the GPS project formula does. The earth based clock path isn't an orbit.


The derivation

Put the derivative of the effective potential term (from the equation of motion) into quadratic form (to find critical values)

r*^2 - L*^2r + 3L*^2 = 0

Where

r* = r/M, and L* = L/mM

Then divide through by L*^2 and manipulate to get

r*^2/L*^2 = r* - 3 [saving this for a later
substitution]

Setting dr = 0 in the Schwarzchild metric and
substituting dphi = (L*/r*^2)dTau the metric becomes

dTau^2 = (1 - 2/r*)dt^2 - (L*^2/r*^2)dTau^2

To find the ratio dTau^2/dt^2 divide through by the
bookkeeper time dt^2 and simplify to

(dTau/dt)^2 = (1 - 2/r*) / (1 + L*^2/r*^2)

Now substitute 1/(r*-3) for L*^2/r*^2 and simplify to

dTau/dt = (1 - 3M/r)^1/2

This equation accounts for both the GR and SR time dilation component.

 

[phyti]

paddoboy#1225

I don't see my stance as arrogant or stubborn...I'm not giving any 100% faitre complei certainty that time travel, spacetime manipulation, matter transportation etc etc are certain.
I'm saying they are possible by an advanced civilisation.

I don't see you as arrogant, maybe just opinionated.
People may see me as such, but if they can prove me wrong based on some evidence, I'll change.

And in all reality, if we don't achieve all those possibilities, along with inter-stellar travel, then we are doomed as a species.
In summing, we were not born to stagnate on this fart arse little blue orb""".

This is your gloom and doom conclusion, based on what, other than your life experiences.
D0n't you think the problems of the world can be solved?

The point of the fly scenario is perspective. If they had lived longer, they could have reconciled the differences in their interpretation of their world.

The marvelous human mind, is not going to understand the complex universe after two centuries of concentrated scientific study. Knowledge is a continuous process of refinement. There are still more questions than answers.

To me it's foolishness to predict a long term outcome based on short term limited information. As a real world example, suppose someone living a few thousand years ago, after becoming aware of ice and snow accumulation in the artic region, predicted the earth surface would freeze over, destroying all life. It didn't happen.

Long term predictions assume a trend will continue, without any intervening phenomena, and are based more on ignorance than fact.

The entropy issue, ignores the fact that there are constructive processes, genetic code, gravity, etc. making new life forms, and stars and galaxies, i.e. adding more order. So much for a dying universe.

Global warming, even those who saw it coming, didn't expect it to accelerate as it is doing.

 

[brucep]

One problem with travelling backwards in time is that you (first) have to leave your own future lightcone; you have to accelerate to > c to do this.

I saw it on the TV.

You might find this discussion from Kip Thorne, on this very subject, interesting. Lots of cool geometry with the conclusion that GR predicts these paths, CTC, are possible. With the caveat that a realistic analysis would require quantum gravity. Professor Hawking thinks there is some possible quantum mechanism that forces the causal path [chronology] to hold.

Right up your alley
http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf

 

[paddoboy]

Once your probability gets into the same remote range as flying pigs you should probably grab one of them (if they happen to come along first) and use it to fly through the wormhole and solve your problems.

To compare GR and its predictive equations to "flying pigs" is pretty silly and naive in the extreme.

 

[paddoboy]

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