Is Time Travel (going backwards) Possible?

[Morrus]

Both of you are correct, and both incorrect.

Relativity says a couple things on the subject of time travel:

1) If you can travel faster than light (you can't, but if you could), then you can travel in time. There's no "time machine" other than the FTL ship, and it can travel to times prior to its own creation.

2) There are certain curvatures of spacetime (usually around "compact objects", things like black holes and neutron stars) that could allow you to fly what freyar mentioned - a "closed timelike curve".

Aha, but this makes me completely correct and not at all incorrect. You've kindly elaborated upon my point in more detail. Indeed, it's exactly what I said - Relativity does not forbid time travel prior to the creation of the device; the fact that it may allow for some solutions which are so restricted does not negate the fact that it allows for solutions (FTL, for example) which are not.

I admit, though, I was being mildly stubborn in not bringing up potential solutions such as FTL because I was being obstinate about the burden of proof.

 

[Bullgrit]

However, there's a catch - the wormholes of general relativity are not stable. Flying a spaceship through one is likely enough of a disturbance to cause the thing to snap shut, destroying you and your ship in the process. One can theoretically stabilize a wormhole with, get this, "negative energy".

What does "destroying" a spaceship mean in this kind of context?

Bullgrit

 

[Umbran]

the fact that it may allow for some solutions which are so restricted does not negate the fact that it allows for solutions (FTL, for example) which are not.

Except, of course, that both special and general relativity explicitly disallow FTL travel. Bit of an issue there to you being right on that count.

I mentioned it because it is the usual thing brought up in time travel discussions: when you put a speed greater than c into the equations of relativity, time travel falls out. But let us be clear - this is not relativity "allowing" for the solution. It is you putting in something that doesn't fit in the model.

I admit, though, I was being mildly stubborn in not bringing up potential solutions such as FTL because I was being obstinate about the burden of proof.

You were, however, entirely correct on the burden of proof. The burden of proof sits on the one who makes the assertion, not on those who question the assertion.

 

[Umbran]

What does "destroying" a spaceship mean in this kind of context?

Well, honestly, this is something that's outside solid models and mathematically-grounded theory.

The typical expectation is that the hole actually is perturbed enough to snap shut before you really enter it. There is gravitational energy tied up in the wormhole's existence. It will be released as the hole snaps shut, with a burst of energy in the form of photons and elementary particles from both ends of the wormhole*. For a wormhole large enough to pass a ship through, that should be enough energy to simply vaporize your ship.

If you do manage to be deep inside the wormhole, far from either end, the common expectation is that the mass of your ship just joins with the energy tied up in the wormhole, so you get squirted out both ends of it as elementary particles and light.


*For those who know about Hawking radiation, the effect is similar to the final moments of a black hole's life.

 

[Umbran]

Going back to address some of the other questions that cropped up....

But how about being able to observe the past (i.e. without being able to change anything)?

Is there any argument against that being possible?

There is at least one, and it is subtle...

It is thermodynamically impossible to transmit information without also transmitting energy. Therefore, to get information out of the past, you also take energy out of the past. That energy was in the past before you observed, and now it isn't - the past is thus changed (perhaps a miniscule amount, perhaps not in a way you intend to control, but change nonetheless) by your observation. Ergo, there is no such thing as "observe, but not change".

I have this hypothesis, that I'd love to talk to a qualified physicist/cosmologist about someday, regarding a possible explanation of the increasing rate of the expansion of the universe.

The current wisdom is that this is explained by the presence of sufficient "dark energy" (which, I know, sounds like, "It's magic!!!1!"). This happens to be pretty consistent with General Relativity, and doesn't have profound implications for time travel.

The problem with that is that every single event (on a quantum level, no less) creates an entirely new universe - duplicating everything that exists. Where does all the energy come from?

There are a couple of ways to think of that answer, but here's a simple one: Let us consider the case where there are not Many Worlds. Where does the energy for the one single Universe come from?

The energy for the Many Worlds ensemble of universes comes from the exact same place. Of course, "place" is not the right word, and here we get into one of the places where cosmology and quantum mechanics give people headaches...

I suppose it's possible that there could be a finite (and fixed) number of parallel timelines, constantly splitting from and converging with one another, in some sort of ball of wibbly-wobbly timey-wimey stuff. But that does seem rather unlikely.

It does seem unlikely, but probably not for the reason you think it does. Recall that the ideas of "where" and "when" (as in, "Where does the energy come from when the universe splits?") are *internal* to the universe. The idea of conservation of energy (even the phenomenon of "energy") is likewise internal to the universe, and only holds within our spacetime. Our clocks and rulers (by which we define energy) do not function, apply, or exist outside our spacetime.

Basically asking where the energy comes from is rather like asking where time comes from, or where space comes from. In a sense, the energy does not come from anywhere - it is a function of the mere existence of the universe.

 

[Thotas]

Going back to address some of the other questions that cropped up....

The current wisdom is that this is explained by the presence of sufficient "dark energy" (which, I know, sounds like, "It's magic!!!1!"). This happens to be pretty consistent with General Relativity, and doesn't have profound implications for time travel.

Not so much "it's magic" but my understanding is that the reason it's called "dark" and put in quotations like that is that nobody really knows for sure what it is. That's what my layman's/crackpot hypothesis addresses, although even if there is something to it I'm sure it would have implications I totally wouldn't understand.

 

[Umbran]

Not so much "it's magic" but my understanding is that the reason it's called "dark" and put in quotations like that is that nobody really knows for sure what it is.

Close. It is called dark for the same reason "dark matter" is called dark - it doesn't actively shine in a way we can see it.

As for what it is - it is energy that is in a form that causes no impact currently measurable on short distances, but that in aggregate has a repulsive effect over very large distances.

 

[Nagol]

Close. It is called dark for the same reason "dark matter" is called dark - it doesn't actively shine in a way we can see it.

As for what it is - it is energy that is in a form that causes no impact currently measurable on short distances, but that in aggregate has a repulsive effect over very large distances.

And short means "within the heliosphere". There was an anomaly detected on the Pioneer craft's speed, but it was shown to be function of heat radiation. http://en.wikipedia.org/wiki/Pioneer_anomaly

 

[Umbran]

And short means "within the heliosphere".

Actually, I think short is still "within the galaxy". The effects of Hubble expansion are not generally measured on things within the Milky Way, but only when you start looking out at the rest of the Universe.

 

[Morrus]

Close. It is called dark for the same reason "dark matter" is called dark - it doesn't actively shine in a way we can see it.

I was given to understand that's the name which was catchy enough to catch no with the newspapers and the like and that there were dozens of names floating round until the media effectively made the call.