FTL in my book

[tomandlu]

Hi,

Apart from obviously being nonsense, does the following sort-of make sense (and, for bonus points, has anyone ever used this idea in fiction before):

“Let us suppose you travel at a little below the speed of light, and that for every second of ship time, a hundred seconds would pass externally.”

“Okay,” said Louie, concentrating hard and trying to ignore the constant discomfort of the ship’s acceleration.

“So, you complete a return journey that takes you a month, but you find that a hundred months have passed for everyone who stayed behind,” continued the machine. “In effect, the problem that cronium has to solve is not how long the journey takes for you, but how long it takes for a everyone else.”

“And that’s where the time-travel comes in?”

“Exactly – in the example I gave you, a cronium-shift during flight would gradually move you ninety-nine months into the past. The result would be that the journey would now have taken a month from all perspectives – stationary and moving.”

“I think I get it now,” said Louie. “But isn’t it dangerous? I mean, couldn’t you end up changing history or something?”

“Not at all,” said Cassita. “The shift available never allows you to arrive prior to your departure. Causality is always preserved”

“So I don’t get to kill my own grandfather?”

“Why would you want to do that?” said Cassita. “Was he unpleasant?”

Louie laughed. “No, he was really nice – well, one of them was. It’s just what people always say when time travel comes up.”

“Ah, I see. A paradox.”

... and if it does make sense, anyone want to tell CERN?

[Robert'); DROP TABLE *;]

It does make sense in a handwave capacity, but if you were clever, you could violate conservation of energy with it. (Since the "speed" of time is related to redshifitng of photons.)

[Macbi]

“So, you complete a return journey that takes you a month, but you find that a hundred months have passed for everyone who stayed behind,” continued the machine. “In effect, the problem that cronium has to solve is not how long the journey takes for you, but how long it takes for a everyone else.”

I don't understand. Your doing something to the people on the ship, and this makes the folks at home age more slowly?

[scarecrovv]

“So, you complete a return journey that takes you a month, but you find that a hundred months have passed for everyone who stayed behind,” continued the machine. “In effect, the problem that cronium has to solve is not how long the journey takes for you, but how long it takes for a everyone else.”

I don't understand. Your doing something to the people on the ship, and this makes the folks at home age more slowly?

As I understand it, the idea is that you go on your journey and come back, so you age 1 month and everybody else ages 100. Then you jump back in time by 99 months using handwavium so your history and everybody else's sync up. Except that the jump back in time is distributed across the whole journey.

[Izawwlgood]

The obvious question is what happens when/if you make a return journey along the same path. Or what happens if someone in the new timeline affects the ship in transit/arrival prior to it's jump back in time.

[tomandlu]

I don't understand. Your doing something to the people on the ship, and this makes the folks at home age more slowly?

As I understand it, the idea is that you go on your journey and come back, so you age 1 month and everybody else ages 100. Then you jump back in time by 99 months using handwavium so your history and everybody else's sync up. Except that the jump back in time is distributed across the whole journey.

Spot on...

[tomandlu]

The obvious question is what happens when/if you make a return journey along the same path. Or what happens if someone in the new timeline affects the ship in transit/arrival prior to it's jump back in time.

I'm not sure if either of those present a problem. You might 'see' yourself (since your image is essentially slower than you are), but you are never in the same time, same place as yourself from an earlier or later part of the journey. Bear in mind that you are not really going back in time in the ordinary sense - for every second of time that passes, the most you can go 'back' is <1... (if X is the amount of time the journey takes you, and Y is the time for the stationary observer, then the maximum shift available over the entire journey is Y-X).

I think it would look odd - you could pass the image of yourself on the way back, but not on the way out (just as you can pass a bit of rubbish you threw overboard on the way back into port, but not the reverse).

... of course my wavehandium avoids this rather confusing bit by making anything shifted invisible...

[AvatarIII]

the only problem i can see is if the ship is constantly travelling 9/10 of an instant back in time every instant, it's going to be continually occupying the same space, so you are going to have to do something that says it only shunts back in time once it has cleared the space that the ship occupied at the time you are going to be going back to,

so say the ship is going at 3x10⁸ms^-1 and the ship is, 300m long (3x10²) it will have to travel 10^-6s back in time every 2x10^-6 seconds.... i think.

[Izawwlgood]

Yeah I'm not sure I understood this properly: are you suggesting that while traveling at whatever speed you're going at, you are also constantly shifting backwards in time by exactly the same amount you lose due to time dilation?

Or, are you suggesting you make a trip, and upon arriving, instantly travel backwards in time by the equivalent length lost to relativity?

[tomandlu]

This one...

Yeah I'm not sure I understood this properly: are you suggesting that while traveling at whatever speed you're going at, you are also constantly shifting backwards in time by exactly the same amount you lose due to time dilation?

Not this one...

Or, are you suggesting you make a trip, and upon arriving, instantly travel backwards in time by the equivalent length lost to relativity?

Soon, their velocity reached half light-speed – over ninety thousand miles every second.

“Would you like to activate the shift?” asked Cassita, indicating a control.

“Sure,” replied Louie.

Cassita moved aside and Louie turned the simple switch. Outside, the universe returned to its normal shape and colour, the stars ahead and behind suddenly shifting away from them, the blue and red turning back to white. It was both impressive and slightly anticlimactic.

“That’s it then?”

“That’s it.”

“We’re travelling back in time?”

“Essentially, yes.”

Their speed was staggering and it was still increasing. The numbers were so large that Louie didn’t even like to try and think about them anymore. They made him feel light-headed and dizzy.

... at which point, for every second of ship time, they would be travelling back about 0.14 seconds externally

[Soralin]

The tricky part is avoiding simply going back into your own past. I mean, how would you limit that? If you tried limiting it to your speed, the question becomes: your speed, relative to what? I mean, in the story section there, it says they're moving at half light-speed, but half-light speed relative to what? (I suppose you could have some universal reference frame that your speed is compared against, although that's breaking relativity even further, which says that no such thing exists, and velocity (and thus rate of time passing) is all relative.)

For example, say that you're moving at .5c relative to Earth, and another object is also moving away from Earth at .5c in the same direction. Relative to that object, you're just sitting motionless and going back in time. And conversely, if you're sitting motionless relative to Earth, that means that you're traveling at a speed of .5c away from the aforementioned object, and since you're traveling at .5c (relative to that object), you should be able to activate the shift, and go back in time, relative to it.

[tomandlu]

The tricky part is avoiding simply going back into your own past. I mean, how would you limit that? If you tried limiting it to your speed, the question becomes: your speed, relative to what? I mean, in the story section there, it says they're moving at half light-speed, but half-light speed relative to what? (I suppose you could have some universal reference frame that your speed is compared against, although that's breaking relativity even further, which says that no such thing exists, and velocity (and thus rate of time passing) is all relative.)

For example, say that you're moving at .5c relative to Earth, and another object is also moving away from Earth at .5c in the same direction. Relative to that object, you're just sitting motionless and going back in time. And conversely, if you're sitting motionless relative to Earth, that means that you're traveling at a speed of .5c away from the aforementioned object, and since you're traveling at .5c (relative to that object), you should be able to activate the shift, and go back in time, relative to it.

Hmm... tricky...

Let's see...

You're travelling along the same path, same speed (relative to earth), as the other object (give 'em a wave). Then you turn on your shift, and at this point you start travelling back in time, relative to earth, 0.14 seconds for every second of earth-time. However, relative to the other ship, no shift should be available...

Yeah, I think you need a universal reference frame (gravitational centre of the universe?). Thank god for handwavium...

[tomandlu]

T(I suppose you could have some universal reference frame that your speed is compared against, although that's breaking relativity even further, which says that no such thing exists, and velocity (and thus rate of time passing) is all relative.)

... thinking about it, and a bit of a cheat, but there is a difference between rest and movement, otherwise the twin-paradox wouldn't work. e.g. two ships moving away from each other and 1/3 of C each does not produce the same result as one ship moving away from a stationary thing at 2/3 C. In the former, both twins age equally, but in the latter, the travelling twin will be younger than his stay-at-home sibling. This does seem to imply that there is an absolute 'rest'... or have I misunderstood?

[Robert'); DROP TABLE *;]

The twins paradox is resolved by considering that one of the twins must accelerate. It doesn't imply an absolute reference frame.

[tomandlu]

The twins paradox is resolved by considering that one of the twins must accelerate. It doesn't imply an absolute reference frame.

Ah, of course...

... okay, I think the solution is to say that the shift is relative to one's self at the time of turning on the shift... or something

[PM 2Ring]

Yeah, I think you need a universal reference frame (gravitational centre of the universe?).

Sorry, there isn't one. But you could use the comoving frame as a defacto rest frame.

While general relativity allows one to formulate the laws of physics using arbitrary coordinates, some coordinate choices are more natural (e.g. they are easier to work with). Comoving coordinates are an example of such a natural coordinate choice. They assign constant spatial coordinate values to observers who perceive the universe as isotropic. Such observers are called "comoving" observers because they move along with the Hubble flow.

A comoving observer is the only observer that will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

Most large lumps of matter, such as galaxies, are nearly comoving, i.e., their peculiar velocities (due to gravitational attraction) are low.

I think it might be a good idea to have the time-shifting mechanism integral to the ship's drive, rather than having it as something that gets switched off & on (although that would mean getting rid of the scene quoted above). That way, your ship's world-line will always be a light-like geodesic, as far as the rest of the universe is concerned.

Thank god for handwavium...

Definitely!

[tomandlu]

I think it might be a good idea to have the time-shifting mechanism integral to the ship's drive, rather than having it as something that gets switched off & on (although that would mean getting rid of the scene quoted above). That way, your ship's world-line will always be a light-like geodesic, as far as the rest of the universe is concerned.

Yeah, I've been coming round to this opinion too. Not too much of a switch to handle...

[Kaelith]

But we do have a reference point, the atoms themselves. Our problem in understanding has been that we see this in the perspective of time, while the subject is of the particles that are moving. See, the closer you travel to the speed of light the slower your atoms age in relation to other atoms that are stationary, this we understand and has been proven with atoms clocks in real-life (atomic clocks being measured off energy pulses from atoms, which is universal and unchanging)..this being said, lets solve some problems.
No, you wouldn't ever see yourself (or grandfathers you may accidentally kill) because you never go back in time. Correctly, you are moving SLOWER through time. As stated n the original text, you cant ever go further back in time than your point of origin. There only seems to be a relative disparity in past/present/future when you have arrived back to where you were. Furthering this, as you accelerate, you accelerate the time dilation effect, and as you decelerate the opposite happens. There is no sudden shift in time.
So, with all this, we can say with certainty that you wouldn't occupy the same space as yourself. This would imply a staggering effect in space, where you would be in two places at once, which happen to be overlapping. By logic, we are time traveling, affected by space traveling within normal bounds of physics, not 'teleporting' which would be a travel through space while time remains constant, so no overlap would happen (just the same as, say, if you were to move your hand (theoretically) fast enough there is a 'blur' of where your hand was after you moved it. simply, that's not how space works). The act of traveling at the faster speed is the "switch" you would need to turn on 'time travel' mode.
Lastly, anyone moving at the speed you are at is also traveling time at the same speed...because the aging is only relativistic, someone going the same speed as you would be aging the same rate as you are. Doesn't matter in what direction, with you against you, doesn't matter because it is the same speed, and the only way to know if you would be aging faster or slower is to compare yourself to someone who is moving at a different rate. If you would to travel with a companion at near-light speed, both you and your companion would age at the same rate. then his side of the cabin detaches and you two are traveling side by side at NLS, same age rate (the distance in between you doesn't affect aging) then he turns his craft and heads in another direction, since the distance doesn't matter and you both are traveling at the same speed, you both are aging the same in reference to the point of origin(and to each other, same speed so you're practically standing still).

disclaimer time: im not a physicist, and have only looked up facts as needed/wanted, but the relativity theory and time travel have fascinated me for a long time and this is a subject that i have thought through many many many times before. theory crafting, logic, and a bit of evidence is all i have and if there's a flaw in my thought line please point it out. I hope this helps in your writing ;D
(ps, wrote late at night/early in morning so my apologies for grammar and spelling mistakes)

[Tass]

disclaimer time: im not a physicist, and have only looked up facts as needed/wanted, but the relativity theory and time travel have fascinated me for a long time and this is a subject that i have thought through many many many times before. theory crafting, logic, and a bit of evidence is all i have and if there's a flaw in my thought line please point it out.

There is. It is called theory of relativity for a reason. There is no absolute speed, only relative speed. If you are going at some speed one way and someone else is going "the same speed" the other way, then you are very much not aging at the same speed (both of you will see the other age slower).

Try to read the sticky thread about relativity in the main science forum.

[BlackHatSupport]

Interesting idea. And we'll just avoid the whole "occupying the same space" bit with some handwavium.

[Quizatzhaderac]

Hi,

Apart from obviously being nonsense, does the following sort-of make sense (and, for bonus points, has anyone ever used this idea in fiction before):

In Robin Cook's Abduction has time machines used similarly. However all of the action occurs on or inside the Earth so the book doesn't even go into as much detail as the sample from your book.

... and if it does make sense, anyone want to tell CERN?

It makes sense, but you're not actually solving the problem of FTL just restating it. I won't tell CERN because I don't think anyone's closer to time travel than FTL. However from a literary sense restating the problem like this seems like a good idea. Rephrasing the problem has been an important step in solving many problems. Also showing things from new perspectives is a big benefit of literature.

Regarding the reference frame: you can't actually have a absolute reference frame, but you can pick one and stick with it. Outside of a galactic core stars don't move at relativistic speeds next to their neighbors. Just have everything happen within a few hundred light years, don't have ships launch from each other, and don't talk about what would happen if you did.

Regarding the same space at the same time: Instead of continuously shifting back in time, you could do many tiny time shifts. Say the ship is 300 m long with a .01 dilation. To the observer on Earth the ship completely changes position every 3m/ c = 10 ns. So every 10 ns it shifts back in time 9.9 ns. To an outside observer the ship appears as 100 ships of 1/100 normal length sandwiched together. Looking out the front you'd see the back of the ship from 9.9 ns in the future, however they're be no paradox as it'd take 1000 ns to get a message to the back of the ship.

If you wanted to be trippy the ship could dock with itself, the actual experience being more like going in circles than moving through time.

[speising]

IMHO, saying you have a working time machine is on par, if not even equal to stating you have FTL.
so explaining an FTL drive with time travel is a tautology.

[tomandlu]

IMHO, saying you have a working time machine is on par, if not even equal to stating you have FTL.
so explaining an FTL drive with time travel is a tautology.

In terms of hard science, you're almost certainly correct - however, in this case, we don't have an FTL drive, we have a sub-FTL drive with time-travel used to offset the twin-paradox... oh, and Quizatzhaderac, I wasn't serious about CERN - it's just that the original post coincided with CERN having those odd results where particles seemed to have travelled FTL... (no longer the case, I'm sad to say...)

[bouer]

One thing I am concerned with is:

Soon, their velocity reached half light-speed – over ninety thousand miles every second.

I certainly would not want to accelerate to 1/2 c in a short time, accelerating at 1G would take over 4000 hours to reach that speed.