Is Time Travel (going backwards) Possible?

[jonesy]

In the many worlds interpretation, assuming time travel to the past is possible, you have already gone back and killed your grandfather. You have also, at the same time, not have gone back at all, and have gone back and not killed him. Many worlds accounts for all possibilities. But nothing would change for your grandfather in your original starting point, because that starting point exists in a timeline where you didn't go back to kill him. The timeline where you did go back to kill him isn't your original starting point, and the 'changes' there were technically already there and lead through a different crossroads.

It's like a choice between being Doctor Manhattan or Paul Atreides. Doc can't change anything because he experiences every point of change at the same time and he is already having been making every one of his choices at the same time in every single point. Paul on the other hand sees every possible and impossible choice at the same time, even the ones he has already passed (like recognizing the other potential Kwisatz Haderach who were not to be).

 

[Umbran]

This resident physicist will be back at his keyboard on Sunday or Monday, and will be able to address some of th questions then.

 

[Corathon]

You've got your burden of proof backwards. You made the assertion - the burden upon you is to prove it, not in me to disprove it. I cannot prove a negative. I cannot prove what relativity *doesn't* assert. Failure to provide a real fireball spell does not prove that Sesame Street lyrics are a fireball spell; similarly, failure to provide a theoretical example of a time machine does not prove that your claim is true.

The logical flaw in your cited evidence is that it does not directly address the question. The question is what General Relativity stipulates about time travel; your cite does not address the overall question of time travel, merely one suggestion for a time machine. It at no point claims that this would be only method of time travel; thus it can not validate your assertion.

I've made a statement - let's call it a hypothesis, "statement" is too definite - of the form "x does not exist". Citing 1 example of an x would surely prove me wrong. Not logically impossible at all.

 

[Morrus]

I've made a statement - let's call it a hypothesis, "statement" is too definite - of the form "x does not exist". Citing 1 example of an x would surely prove me wrong. Not logically impossible at all.

I can't think of how to present the logical flaw here more clearly: I recognise it, but my literary style is not up to the task. But there's a fundamental logical flaw in those two sentences, located in the logical fallacy of demanding proof of the negative (impossible). All I can say is - demanding that I disprove you is not proof of your assertion. Don't say "prove me wrong"; prove you're right.

There's an alien from Neptune behind my couch. Prove me wrong. Alpha Centauri is made of chocolate. Prove me wrong. The ghost of Henry VII visited me last night. Prove me wrong. I am Superman. Prove me wrong. Dark Energy is created by the psychic fields of aliens working at Walmart. Prove me wrong.

I can't prove you wrong. But I don't have to. A claim has to be proved right.

 

[KarinsDad]

See (coincidentally), Morris et al, 1988 (ref below) in which the authors speculate that a wormhole could be turned into a time machine. Such ideas may never be physically realizable, but they do exits in theory. General relativity is most definitely not my field, and maybe ideas have changed since I last paid much attention to it - but ideas for time travel have been discussed in physics journals.


Morris, Michael S., Kip S. Thorne, and Ulvi Yurtsever, Wormholes, Time Machines, and the Weak Energy Condition, Physical Review Letters, 61, 1446, 1988.

Kip and the boys were sitting around the pool smoking some pretty strong stuff when they thought this up.

"Hey, I know. We can take a wormhole, yeah, yeah, I know, we have no evidence that they exist, hear me out, and we find both ends of this microscopic object in different parts of the galaxy. Well, we just do, ok? Then, we make it really really big on the inside and both ends. Of course that will take tons of energy. Yes, I don't know how to do that. But listen, listen. Then, we rotate one end near the speed of light in order to slow time down on that end. No, we'll rotate it so that the wormhold doesn't break or disintegrate or something. Just pretend for a minute that we are billions of years in the future and have all of this cool tech, ok? Then we send a spaceship into the end that is moving at normal time and it goes through and comes out the other end where time was slowed up. So, we start this thing up at 1 PM on both sides, send the ship through at 2 PM on the normal time side and on the other side, the ship comes out at 1:01 PM. And Hey! I know where we get the gazillion dollars to do this. We'll get a grant from the federal government."

Just because mankind can think of an idea does not make that idea doable, even in theory.

There is a difference between the word "theory" in the fantasy sense (like Star Trek), and "theory" in the scientific sense (like reality). This idea wasn't anywhere near the scientific sense of the word. It just made for amusing TV science shows.

 

[freyar]

I've skimmed this thread with some interest and a bit of bewilderment for a while but, like Umbran, haven't had time to comment yet. I can give a few comments right now (as a practicing physicist working in gravitation).

One is that Many Worlds Theory in physics is not really about time travel. It's about measurements in quantum theory as time goes forward in the usual sense. It's also a bit of a misnomer: there is still just one world but with an increasingly complicated description (wavefunction). Nonetheless, you could speculate that some quantum theory of gravity would relate that to time travel, but I'm not aware of any serious (ie, mathematically formulated) theory that does that.

As for what general relativity says: yes, there are spacetimes in general relativity, including wormholes, that have what are called "closed timelike curves," trajectories you can travel into your future that would allow you to come back and see yourself leaving. So that's time travel in the usual sense. On the other hand, these spacetimes tend to be unstable (meaning, you move any matter around, and they will collapse into a black hole or similar object). I don't know of any theorem about that, but it's generally believed that closed timelike curves always get hidden behind black hole horizons or in some other way can never be used to time travel in actuality.

 

[Janx]

One thing to ponder is the concept that Time does not exist as a discernable place to travel to.

Let's ignore the whole physics thing for a second (like traveling really fast slows time down).

At its core, the concept of time is acknowledging the sequence of events. I sat down beneath a tree. Then I ate a sandwich under the tree that I sat down under. After the sandwich was gone, I pondered deep thoughts under neath the tree that I was still sitting under. Then an apple from the tree fell down on my head as I sat under the tree with a full belly from the sandwich that I ate.

While we remember these events as if they were places we could revisit, they are simply sequences of events AT physical places. time inherently moves forward because we observe the world and see things happen in sequence.

It is not probable that an intelligent life form exists that observes all reality and time before events happen. This Omniscient being is unlikely to know an apple is going to fall on my head in 4.5 billion years and thus get the idea for Newton's laws of physics by knowing something WILL happen, but hasn't happened yet. By "knowing" all things out of order, it literally can't move from one idea to the next because it quite literally must have all ideas at once by nature of being defined as "not experiencing time" or seeing all events at once.

Anything that observes or interacts with the world sequentially is bound to time, not as a place, but simply as the mechanic that events happen in sequence.

Now physicists can step in and mention that Time is wierder than I ascribe. Given that we can launch a ship at near light speed and when it gets back, the clock on it will read that less time passed inside the ship, than we all experienced.

Though time itself didn't re-order anything. It simply slowed down for those inside the ship. And that's not the same as time existing as a place that one can travel backwards to.

 

[the Jester]

Nobody disputes that physicists have been using the theory to discuss theoretical time travel for decades; it's your added "can only go as far back as when the time machine was built" part that we're disputing.

I've seen this asserted in a number of physics-for-the-layman books, speculative science programs, etc. It seems to be a fairly widely-accepted view amongst scientists who do scientific speculation about time travel.

 

[Morrus]

I've seen this asserted in a number of physics-for-the-layman books, speculative science programs, etc. It seems to be a fairly widely-accepted view amongst scientists who do scientific speculation about time travel.

Which ones?

We're not talking speculation here; we discussing the claim that General Relativity specifically asserts that time travel works in this manner.

 

[Umbran]

Theoretically, general relativity allows one to create a time machine. (that doesn't mean that it will ever be possible in practice). Such a time machine allows one to travel back in time, but no further back than the creation of the machine.

I don't know what version of General Relativity you've been reading, but that's not what it says about time travel at all.

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". You're never moving faster than light, but you end the route before you started it. This kind of time travel does not allow you to move back before the curvature created by the compact object existed. So, if you are flying a weird course around a black hole to travel in time, you cannot go to times before the star collapsed to create the black hole. There are possibly some compact objects that can be used that have existed since the beginning of the universe, or the first inflationary period of the universe, such that how far back you could go using them is not an issue for human terms. You would, of course, have to reach the objects in the first place, and none of them are anything like "nearby".

When we get to wormholes, things get funky.

A wormhole is a shortcut through spacetime, and it is allowed by General relativity. It is a shortcut through spacetime, so you can go to the place, and time, of either end of the wormhole.

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".

Now, general relativity doesn't tell you how to make negative energy. Quantum mechanics does. But most of you probably know that quantum mechanics and general relativity as we know them don't get along very well. They don't speak to each other. So, can you use the combination to create a wormhole time machine that can go back to some arbitrary period in time? Or would such a thing only be able to use its creation as the earliest end of the wormhole?

Nobody can say. The math to describe how to do this doesn't exist yet.

General relativity does not speak to the "many worlds" (or any other quantum mechanical) time travel. In fact, "Many Worlds" time travel, in which you don't actually travel in time, but you travel to some other branch of the many-worlds tree, is as far as I understand it, a complete fabrication of science fiction. It is a simple fictional crossing between the "many Worlds Hypothesis" and the idea of time travel, without any theoretical math supporting it, as far as I am aware.