Will true, Sci-Fi level interstellar travel require time travel?

[G'th]

As is often pointed out when discussing interstellar travel, the issue of time dilation is a huge factor against its practicality. Namely because by the time you traveled somewhere and came back, the Earth you left might not even be around.

But, what if instead of trying to directly combat that problem (By somehow preventing the dilation), why not address it via time travel? You would use a method of travel thats fast, and obviously induces heavy time dilation, but then you use a time travel method to bring yourself back to where you left. 

It would certainly still induce the same weirdness, as to non participants you'd basically would disappear and then reappear, whereas to a participant several years could have passed, but it would be otherwise a functional system of interstellar travel if you could also figure out a working method of backwards time travel. 

But anyway thats my thought for the day. Should be noted that this is just a thought I had and is not educated in any sense of the word lol.

[sevenperforce]

I've used this as a plot device in fiction before.

The problem is that if you travel to, say, Alpha Centauri at a high enough fraction of c to reach it in one week from your perspective, then time-travel back five years less one week, you could then turn around and intercept yourself before you arrived. Which is a problem. For most people anyway. 

In my fictional 'verse I had the FTL drive travel back in time as it went forward in space, so that the time dilation was counteracted in real time. Which introduces its own set of challenges....

[Bill Phil]

Time dilation is free stasis (after going real fast), and is already technically time travel, but only to the future. 

Anyhow, the time dilation issue is only huge across huge distances. Going to Tau Ceti and back at relativistic speeds would only result in a few decades of displacement.

[SomeGuy123]

TLDR, in some models of FTL travel, yes.  Other models, the https://en.wikipedia.org/wiki/Chronology_protection_conjecture prevents any time travel.

For example, if the only working model of FTL travel is constructing wormholes.  Wormholes link 2 points in both space and time.  So you can build a wormhole pair at earth, load one of the pair onto a very very massive starship, and accelerate it to near the speed of light.  Since time AND space are linked, technically you only have to wait on earth the amount of "ship time" elapsed.  So if you get it going fast enough, the wormhole could arrive at a star 100 lightyears away in just a few years and you could send explorers back and forth no problem.

BUT, if you then load that wormhole back on the ship and send it back, you can intrude on your own past.  The CPC in this cases is supposed to be that the very instant the wormhole pair has created a bridge in time even 1 planck second long, virtual particles of light can now make a round trip and interfere with themselves.  This causes infinite constructive interference and essentially both wormhole mouths would detonate spectacularly, exhausting all of their stored mass-energy all at once.  (because the energy to create the infinitely bright photon of light came from the wormhole mass itself)

Note that even if wormholes are possible, they would not be human traversable nor would you be able to send significant amounts of matter one way because of this mass-energy balance problem.  The way you would explore using this kind of wormhole is to convert your mind to computer data files, then send them through on a very very thin beam of light through the wormhole.

[PB666]

2 hours ago, SomeGuy123 said:

TLDR, in some models of FTL travel, yes.  Other models, the https://en.wikipedia.org/wiki/Chronology_protection_conjecture prevents any time travel.

For example, if the only working model of FTL travel is constructing wormholes.  Wormholes link 2 points in both space and time.  So you can build a wormhole pair at earth, load one of the pair onto a very very massive starship, and accelerate it to near the speed of light.  Since time AND space are linked, technically you only have to wait on earth the amount of "ship time" elapsed.  So if you get it going fast enough, the wormhole could arrive at a star 100 lightyears away in just a few years and you could send explorers back and forth no problem.

BUT, if you then load that wormhole back on the ship and send it back, you can intrude on your own past.  The CPC in this cases is supposed to be that the very instant the wormhole pair has created a bridge in time even 1 planck second long, virtual particles of light can now make a round trip and interfere with themselves.  This causes infinite constructive interference and essentially both wormhole mouths would detonate spectacularly, exhausting all of their stored mass-energy all at once.  (because the energy to create the infinitely bright photon of light came from the wormhole mass itself)

Note that even if wormholes are possible, they would not be human traversable nor would you be able to send significant amounts of matter one way because of this mass-energy balance problem.  The way you would explore using this kind of wormhole is to convert your mind to computer data files, then send them through on a very very thin beam of light through the wormhole.

Or even more simply because wormholes can potentially violate, do the highest numer of communication events is one, in other words they are quantum entanglements at the upper limit of size. 

[CptRichardson]

I have a suspicion that all the talk of causality violation/time travel involved with FTL travel is basically people getting too drunk on their own cleverness. It relies on light still being magically the fastest reference frame in the universe while still being exceeded. In that case, it's no longer the fastest reference frame in the universe, and whatever you're using would be/is the 'true' reference frame. 

Or, to a blind person, a sniper bullet from a mile away seemingly violates causality. You're hit by the bullet before the sound reaches you. BUT, that's not true, because there is still light, even if the blind person can't interact with it properly. They can't see light. They might be able to understand that there is a heat/energy hitting them, but lack enough information to realize what's going on.

Hell, FTL energy might well be a good explanation for 'dark energy'/'dark matter'. We can see the effects, see what's going on, recognize that there is something, but lack the reference frame to explain it.  

So yeah, almost certainly not. Lots of very weird effects, but not time travel.

[lugge]

We're talking about SciFi, not real physics, right?

If you need a plot device for getting some characters to distant stars, just go with some FTL drive. Call it warp drive, wormhole-drive, whatever you want. It is mainly accepted in SciFi media.

A FTl drive with no time dilation is easier to accept as your solution, which involves time dilation AND time traveling. When you introduce time-traveling into your universe, you are at the risk of introducing some plotholes.

Imagine some hero dying: "Hey why don't they just go back in time and prevent the hero from dying?? Come on, they have a time machine!!!"

Edited May 3, 2016 by lugge

[shynung]

6 hours ago, lugge said:

Imagine some hero dying: "Hey why don't they just go back in time and prevent the hero from dying?? Come on, they have a time machine!!!"

Grandfather's paradox.

Say some important guy died. The hero obtains a time machine and uses it to save the guy. After all happened, important guy lives. Then, the hero would have no reason to go back in time to save them. The hero never used the time machine, since he sees no need to. Because of that, the important guy dies. Repeat ad infinitum.

[HebaruSan]

Let's make a rule that we only launch one interstellar ship at a time (they'll be hugely expensive anyway). Then we can induce artificial (gravitational?) time dilation on Earth to keep them in sync. A sci fi author could probably come up with something involving dark matter and gravity waves. Sixty years passes in a few weeks on the ship? No problem, it's only been a few weeks on Earth by the time they get back.

[kerbiloid]

No problem for many-worlds interpretation of quantum mechanics, but this is obviously beyond the range of applicability of the modern physical theories.

[qeveren]

On 2016-05-03 at 1:48 AM, CptRichardson said:

I have a suspicion that all the talk of causality violation/time travel involved with FTL travel is basically people getting too drunk on their own cleverness. It relies on light still being magically the fastest reference frame in the universe while still being exceeded. In that case, it's no longer the fastest reference frame in the universe, and whatever you're using would be/is the 'true' reference frame.

Time travel as a result of FTL is used as a fairly straightforward argument for why we're pretty sure (local) FTL isn't a thing in real life.  It basically works out to be "Relativity, Causality, FTL; you only get to pick two."

Also, light isn't the fastest reference frame; it isn't a valid reference frame at all.

[problemecium]

I've gone into depth about the theories and hypotheses involved before and I'm not in the mood to do so now, but as a matter of opinion, I doubt it.

I doubt first that warping through space is going to actually result in anyone traveling backwards in time relative to anything meaningful. Perhaps a reference frame could be constructed in which someone's ship goes back in time, but I don't foresee much likelihood that such a reference frame will actually apply to any of the stars or planets anyone's visiting.

I also doubt that if it does result in massive time discrepancies, that it will be an unsurmountable problem for communications or result in egregious causality violations. One thing I've learned about the universe is that it's a lot less fragile than Hollywood science or people in general make it out to be. Just like you can't destroy the universe by looking up "dictionary" in a dictionary or dividing by zero, I don't see much real probability that trying to warp places is going to spawn dramatic time loops and universe-breaking paradoxes.
And on the topic of communication in particular, people have already demonstrated that they can handle information being sent and received out of order. Quite often people send chat messages that end up being listed before the messages to which they were responding, and thus they have to decipher the "real" order of communications. Before we had that, similar events frequently occurred in the mail system - letters would be out of date by the time they arrived, or people would arrive at destinations before their letters arrived to announce them. While I understand that differences exist between these examples and real relativistic problems, I have a good feeling that people will figure out a way to get past it all.

P.S.:

Grandfather's paradox.

Say some important guy died. The hero obtains a time machine and uses it to save the guy. After all happened, important guy lives. Then, the hero would have no reason to go back in time to save them. The hero never used the time machine, since he sees no need to. Because of that, the important guy dies. Repeat ad infinitum.

The Grandfather Paradox isn't real, by the way. It results from an easy and all-too-common misunderstanding of the basic concept of time. Always remember: the past happened first, the present happens next, and the future will happen afterward. Period. So:
- Some important guy exists.
- MYSTERIOUS THING
- Some important guy dies.
- With the important guy dead, a time machine is made to save him, but since the important guy is dead, we can surmise that he did not survive (since, uh, he didn't).
There is no sequence of events following a loop of "he un-dies, then the time machine is un-invented, so he re-dies." That would require time to pass, and therefore would occur in the future. We could of course invent a machine to travel into a parallel universe in which he lives, but that wouldn't do anyone much good in this universe.
What this problem really is is a philosphical one - if we have free will, why can't we time travel and save him, or murder our grandparents? But the laws of physics aren't concerned with our free will and work just the same with or without it.

Edited May 5, 2016 by parameciumkid
Did you know the new post editor sucks?

[qeveren]

2 hours ago, parameciumkid said:

And on the topic of communication in particular, people have already demonstrated that they can handle information being sent and received out of order. Quite often people send chat messages that end up being listed before the messages to which they were responding, and thus they have to decipher the "real" order of communications.

...

The Grandfather Paradox isn't real, by the way. It results from an easy and all-too-common misunderstanding of the basic concept of time. Always remember: the past happened first, the present happens next, and the future will happen afterward. Period.

FTL signaling is a problem not because messages arrive out of order, but rather that one can use it to trivially send signals into one's own past light cone... which, when the message is "don't send this message", is the Grandfather Paradox in simplified form.

As for time, observers in different frames of reference won't necessarily agree on the wheres and whens of the same events.  The order of causally connected events is preserved for all reference frames... right up until FTL gets introduced.

[Bill Phil]

4 hours ago, parameciumkid said:

I've gone into depth about the theories and hypotheses involved before and I'm not in the mood to do so now, but as a matter of opinion, I doubt it.

I doubt first that warping through space is going to actually result in anyone traveling backwards in time relative to anything meaningful. Perhaps a reference frame could be constructed in which someone's ship goes back in time, but I don't foresee much likelihood that such a reference frame will actually apply to any of the stars or planets anyone's visiting.

I also doubt that if it does result in massive time discrepancies, that it will be an unsurmountable problem for communications or result in egregious causality violations. One thing I've learned about the universe is that it's a lot less fragile than Hollywood science or people in general make it out to be. Just like you can't destroy the universe by looking up "dictionary" in a dictionary or dividing by zero, I don't see much real probability that trying to warp places is going to spawn dramatic time loops and universe-breaking paradoxes.
And on the topic of communication in particular, people have already demonstrated that they can handle information being sent and received out of order. Quite often people send chat messages that end up being listed before the messages to which they were responding, and thus they have to decipher the "real" order of communications. Before we had that, similar events frequently occurred in the mail system - letters would be out of date by the time they arrived, or people would arrive at destinations before their letters arrived to announce them. While I understand that differences exist between these examples and real relativistic problems, I have a good feeling that people will figure out a way to get past it all.

P.S.:

The Grandfather Paradox isn't real, by the way. It results from an easy and all-too-common misunderstanding of the basic concept of time. Always remember: the past happened first, the present happens next, and the future will happen afterward. Period. So:
- Some important guy exists.
- MYSTERIOUS THING
- Some important guy dies.
- With the important guy dead, a time machine is made to save him, but since the important guy is dead, we can surmise that he did not survive (since, uh, he didn't).
There is no sequence of events following a loop of "he un-dies, then the time machine is un-invented, so he re-dies." That would require time to pass, and therefore would occur in the future. We could of course invent a machine to travel into a parallel universe in which he lives, but that wouldn't do anyone much good in this universe.
What this problem really is is a philosphical one - if we have free will, why can't we time travel and save him, or murder our grandparents? But the laws of physics aren't concerned with our free will and work just the same with or without it.

The grandfather paradox is referring to events exactly the way you specify.

But that only works if you don't assume the many worlds theory, or if you don't assume the multiverse hypothesis, is correct.

[crazyewok]

I doubt we will ever break past light speed.

 

Any interstellar travel will be done sub light I think.

 

It’s possible in the future humans could be living hundreds  even thousands of years so journeys of 50-100 years would hardly be insurmountable. Its more likely humans will have made massive strides genetic engineering and gene therapy before we reach interstellar travel.

[Sereneti]

Maybe...
 

Not for the people who are traveling, Je faster something is, the slower the time...

For the rest of the universe...

Because, if you are 2 jears older, but the universe is 200 years older, i think that isnt sooo much fun...

[Bill Phil]

9 hours ago, Sereneti said:

Maybe...
 

Not for the people who are traveling, Je faster something is, the slower the time...

For the rest of the universe...

Because, if you are 2 jears older, but the universe is 200 years older, i think that isnt sooo much fun...

But you just traveled a bunch of light years. That's pretty neat.