Will Time Travel ever be possible? and what if it is?

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Alberti_Devoveo said:
No!!! Not the Alcubierre warp drive (AD)! What a cruddy piece of ‘science.’ It is not at all real life, it is only a mathematical hypothetical. To ‘make’ the AD, one needs to have exotic matter (which does not exist) and force it into a specific geometry that would make the drive. The link you give states rather clearly that it is “speculative” and has the insuperable problems of

Discounting our own second-by-second progression forwards in time, there is no way to time travel. It just cannot be done.
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Of course, the AD is unfortunately still a half-formed theory. We don’t know enough about the boundary between QM and GR to make any conclusive statements about the AD, but the AD is still our best bet.
 
Not in the phyical world… BUT when we are in Heaven I think we will be able to witness anything in history that we wish… I want to see what real dinosaurs look like.
 
BobCatholic said:
It is impossible.

Suppose we were to go back in time and go back far enough, enter the Garden of Eden and tie up Adam and Eve so they wouldn’t fall.

I’m sure God would not want that, as that would stop all suffering in the world.
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I’m not sure that God originally wanted suffering; he allows it as punishment for original sin; but it, and death, were originally not in the playbill.

ICXC NIKA
 
TheRealJuliane said:
Considering no one has ever come “back” from the future, I’d say it’s not possible, or it would have already happened.

😉
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You are assuming that “futurites” would not bother to disguise themselves effectively. I think they would, as they would be loath to interfere with “the past”.

Assuming, of course that it was physically possible.

ICXC NIKA
 
empther said:
Somebody, possibly Isaac Asimov, wrote a short science fiction story about that.
A scientist invented a machine to pick up “M-rays” that showed scenes from the past. You could see Ceasar conquering the Gauls etc.
The government kept the device secret. But of course, some idiot got the blueprints and published them to the world.
Then people realized why the government had been keeping the design secret. When does the past end? A nanosecond ago. Now that anybody could get his hands on one of these machines, they could look into what you were doing one second ago, including the most intimate moments. :eek:
Welcome to the goldfish bowl! 😊
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It was indeed Asimov.

And you couldn’t in fact see more than 200 years back, however, one person’s dsire to do so let the cat out of the bag.
 
itullian said:
Warp drive? Would we be able to go both forward and backward? Future doesn’t exist yet?
if we go back could we change things? if yes, who’s changes would stick or would things change constantly.

what do you see would be the problems if we could time travel?

could we go back to Eden? ahead to the 2nd coming?:eek:

mind boggling stuff.:confused:

do you think it will ever be possible?
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Not w/o a Delorean going 88 mph & 1.21 gigawatts of energy.
 
aquohn said:
Of course, the AD is unfortunately still a half-formed theory. We don’t know enough about the boundary between QM and GR to make any conclusive statements about the AD, but the AD is still our best bet.
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AD can only be a theory, it has no ability to become a reality. The stress energy tensor necessary to make the drive itself requires a negative energy density, something that violates the energy condition.

A paper by D H Coule (pdf here) is worth looking at as to the impossibility of the Alcubierre warp drive. There really isn’t any tensor mathematics in the paper, as one might expect in a paper discussing GR, it’s purely analytic arguments. The main part to look at is the 2nd paragraph of Section 3 of the linked PDF.
 
GEddie said:
I’m not sure that God originally wanted suffering; he allows it as punishment for original sin; but it, and death, were originally not in the playbill.
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I don’t think it is just to hold a descendant (ANY descendant) responsible for the sins of the ancestor.
 
I read if you go faster than the speed of light you can go into the future but you cant go back.

And if it was possible, then there should be people from the future visiting us right now.
 
PumpkinSeed said:
I read if you go faster than the speed of light you can go into the future but you cant go back.

And if it was possible, then there should be people from the future visiting us right now.
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I still say, if human beings from “the future” were visiting, they would be too well disguised to be recognized as such.

ICXC NIKA
 
@Alberti_Devoveo Heh, you’re initial are AD. Just saying.

But seriously though, our dear friend Wikipedia comes to the rescue:
While the intent of the energy conditions is to provide simple criteria which rule out many unphysical situations while admitting any physically reasonable situation, in fact, at least when one introduces an effective field modeling some quantum mechanical effects, some possible matter tensors which are known to be physically reasonable and even realistic because they have been experimentally verified to actually fail various energy conditions. In particular, in the Casimir effect, in the region between two conducting plates held parallel at a very small separation d, there is a negative energy density

between the plates. (Be mindful, though, that the Casimir effect is topological, in that the sign of the vacuum energy depends on both the geometry and topology of the configuration. Being negative for parallel plates, the vacuum energy is positive for a conducting sphere.) Fortunately, various quantum inequalities suggest that a suitable averaged energy condition may be satisfied in such cases. In particular, the averaged null energy condition is satisfied in the Casimir effect. Indeed, for energy-momentum tensors arising from effective field theories on Minkowski spacetime, the averaged null energy condition holds for everyday quantum fields. Extending these results is an open problem.

Observation of dark energy, or the cosmological constant, demonstrates that even the averaged strong energy condition must be false in cosmological solutions.
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It’s not impossible. You’ve weakened my hopes somewhat, but it’s not impossible.

Secondly, I didn’t look at the PDF because it’s pay-to-view. Could you summarise the arguments or something?
I read if you go faster than the speed of light you can go into the future but you cant go back.
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Wrong. If you go faster than zero, you travel into the future. If you travel at the speed of light, time would seem to stop for you. If you travel at superluminal speeds, then you would travel back in time.

As to the problem about ‘no time travellers’, I have two solutions.

Perhaps no-one comes to this time period. After all, it’s already so well-documented. There wouldn’t be anything to learn; no information to take back. No changes could be made, of course, thanks to the Novikov self-consistency principle. So there would be no point coming here to observe our everyday life. Since our best bet for time travel is the Alcubierre Drive, which probably isn’t going to be too cheap, time travel will probably be used to observe critical events or retireve vital information, rather than trivial purposes liek visting one’s ancestors on a whim.

Another thing is due to the ontological paradox. This one seems to indicate that time travel is either limited or impossible, in the sense that you cannot put anything ‘in’ (but you can still take things ‘out’). An example:
A man builds a time machine. He goes into the future and steals a valuable gadget. He then returns and reveals the gadget to the world, claiming it as his own. Eventually, a copy of the device ends up being the item the man originally steals. In other words, the device is a copy of itself and it is not possible to state where the original came from.
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So therefore, people travelling to the past probably cannot interact with the physical objects there. A more general case can be given for objects moved between points in time.
Let’s say I travel from point B on this timeline to earlier point A. The universe has a mass M.

I take an object O from B and leave it at A, then return to B.

Now, placing the future object O into A increases the mass of the universe at A by M(O).

But since I am in precisely the same universe at B, when I left, the mass of universe was M + M(O).

So that means travelling back caused the mass of the universe to become M +2(M(O)).

You can repeat the process infinitely, and there goes mass-energy conservation. The universe now has infinite mass-energy.
 
Those neutrinos that are faster than light are clearly the devil, that’s my thoughts on it. 😃

In all seriousness though, I honestly think time travel is neat to speculate about, but I don’t think it will ever be possible.
 
Sorry for the delay, I have been traveling the last couple days and have had no internet access.
aquohn said:
@Alberti_Devoveo Heh, you’re initial are AD. Just saying.

But seriously though, our dear friend Wikipedia comes to the rescue:

It’s not impossible. You’ve weakened my hopes somewhat, but it’s not impossible.
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The wiki article is discussing the Casimir effect, which is a nano-scale quantum effect. Once you get into the millimeter-scale range, it no longer plays a role.
Secondly, I didn’t look at the PDF because it’s pay-to-view. Could you summarise the arguments or something?
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Sorry! 😊😊 Just one of those things about having access to journals online through the University that sometimes occurs. The paper is called “No warp drive” by D H Coule, but the relevant item is here:
Consider a spacetime that is to be altered by a distribution of matter T[sub]µν[/sub] . The specific details of this metric will not be necessary for our argument. In the warp drive spacetime the disturbance is to move at speeds v greater than the speed of light c. But this also means that the matter has to be distributed at this hyperfast speed before the geometry G[sub]µν[/sub] is likewise affected. One cannot do this without first being able to distribute matter at hyperfast speeds: exactly what one is trying to accomplish in the first place. The warp drive is impossible, see figure 1. Why was this not realized initially in [1]? There Alcubierre considered the motion of the spaceship within the warp drive and found that it travelled on timelike curves which looked to outside observers to be hyperfast. But this is premised on the metric existing in the first place which in turn is caused by the (impossible) distribution of matter: the ‘need one to make one’ paradox was overlooked.
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QED: The Alcubierre warp drive is an impossibility.
 
PumpkinSeed said:
I read if you go faster than the speed of light you can go into the future but you cant go back.

And if it was possible, then there should be people from the future visiting us right now.
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Scientists have determined that galaxies (determined though analysis of red shift) at the far reached of the universe are actually traveling much faster than the speed of light, Which means relative to them , WE are also traveling faster than the speed of light.
 
wcknight said:
Scientists have determined that galaxies (determined though analysis of red shift) at the far reached of the universe are actually traveling much faster than the speed of light, Which means relative to them , WE are also traveling faster than the speed of light.
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No we are not. Fraser Cain has a nice article that expresses it nicely:
Fraser Cain:
The galaxies themselves aren’t actually moving very quickly through space, it’s the space itself which is expanding away
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The better way to think of this is that we and the receding galaxy are both moving away from each other due to space expanding; the distance per year that we and the galaxy are separating is larger than a lightyear, but neither us nor the galaxy is actually moving faster than c.
 
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