The Game of 9's

[TMac3000]

The recent thread about relativistic travel inspired me to finally speak out on an idea I've been kicking around for a while.

Okay, so time slows down when we go really fast. Fair enough. But what if there were a way to "cheat" the speed of light without actually "breaking" it? I'm not talking about warp drive here, but rather about time travel. Here is my idea.

Suppose a ship bound for Alpha Centauri accelerates to a high enough percentage of light speed that the time for the journey as experienced by the travelers is, oh, say 1 year.

But our ship has a device that allows it to slip backward through the timeline as it decelerates, so that it emerges in objective time (time as experienced by outside observers) exactly where we wish to be, which is one year from the date of departure. We have effectively acheived "sooner than light" travel without actually exceeding the speed of light.

I came up with this "time drive" for a story I was working on a while back. I think astronauts traveling this way would develop a kind of slang for describing their journey in which their speed is given in terms of how close they got to the speed of light. For example "three nines" means 99.9% of c, "four nines" means 99.99%, etc.

 

[Linguofreak]

The recent thread about relativistic travel inspired me to finally speak out on an idea I've been kicking around for a while.

Okay, so time slows down when we go really fast. Fair enough. But what if there were a way to "cheat" the speed of light without actually "breaking" it? I'm not talking about warp drive here, but rather about time travel. Here is my idea.

Suppose a ship bound for Alpha Centauri accelerates to a high enough percentage of light speed that the time for the journey as experienced by the travelers is, oh, say 1 year.

But our ship has a device that allows it to slip backward through the timeline as it decelerates, so that it emerges in objective time (time as experienced by outside observers) exactly where we wish to be, which is one year from the date of departure. We have effectively acheived "sooner than light" travel without actually exceeding the speed of light.

I came up with this "time drive" for a story I was working on a while back. I think astronauts traveling this way would develop a kind of slang for describing their journey in which their speed is given in terms of how close they got to the speed of light. For example "three nines" means 99.9% of c, "four nines" means 99.99%, etc.

Well, if time travel is OK, then warp drives are OK. The problem with any form of FTL travel that allows a return option (diving into a black hole is a form of FTL with no return option: you can and will exceed the speed of light going in, but you can't get out, and you aren't really going anywhere) is that it allows your whole round trip journey to travel backwards in time without any individual part of it going backwards in time. Since time travel has not been observed to occur, it's generally assumed that two-way FTL is impossible. (This is a bit of a simplification, but it gives the general idea)

I believe I have seen a system similar to yours used in a novel, though.

 

[jedidia]

But our ship has a device that allows it to slip backward through the timeline as it decelerates

Your missceonception lies right there. Time dilation does not happen during acceleration or deceleration (actually it does exactly NOT happen at these times, because the ship is no own frame of reference while energy is being aplied).
Finally, acceleration and deceleration are one and the same. Of course one is defined as adding energy and one as substracting energy, but this definition is relative of the observer, since energy is only a sensible concept in interaction between two objects. So what might be acceleration relative to one body might be deceleration relative to another. Hence, there is no difference.

The time dilation itself happens while you are traveling, i.e. when your ship is it's own frame of reference. You cannot revert it by braking. And it depends on your relative speed to whatever body, so you cannot move "backwards". The lowest relative speed possible is 0, by further deccelerating you are actually acceleraing again (hence the two are completely the same, as explained above). Even if time dilation would work the other way around, i.e. time would pass faster the faster you move (which is NOT a mathematical possibility!), you couldn't use this to travel back in time, it would merely mean that on a planet barely any time would have passed between your start and your return.

Hence your question could be compared to the following:
"If pines were apples, could we make applejuice of them?" The answer is "yes, IF". And that's a pretty big if.

 

[Linguofreak]

Your missceonception lies right there. Time dilation does not happen during acceleration or deceleration

He's talking about a time machine on the ship that takes it back in time to compensate for the time dilation. Why he chooses to do this in the deceleration phase, rather than at any arbitrary point before, during, or after the trip, I don't know.

In any case, this is an example of "FTL by time travel," which I have seen suggested a few times before, although relativity doesn't really draw much distinction between the two in any case, unless you make certain restrictions to how the FTL can be used.

(actually it does exactly NOT happen at these times, because the ship is no own frame of reference while energy is being aplied).

Time dilation in relativity applies to any two objects that are moving relative to one another, whether or not they are accelerating, for as long as they are moving relative to one another.

 

[the.punk]

The better question would be: Is timetravel possible?
No.
There are too many paradoxes on the timetravel thing.
I don't think timetravels are possible. Maybe someone else has a different opinion.
But I also think it is a good idea though.

---------- Post added at 09:01 AM ---------- Previous post was at 08:51 AM ----------

Maybe we could say timetravel is possible (just for now, because I don't think so).
What would see the outside observer when the ship beginns timetravel during decelerate?
Or how would it look for the outside observer when the ship beginns timetravel during decelerate?

 

[Ghostrider]

There are too many paradoxes on the timetravel thing.

Only if you travel to YOUR OWN past. What happens if you go back one second and you don't find your world of one second ago, but a wholly different reality?

 

[the.punk]

Maybe we could say timetravel is possible (just for now, because I don't think so).
What would see the outside observer when the ship beginns timetravel during decelerate?
Or how would it look for the outside observer when the ship beginns timetravel during decelerate?

 

[jedidia]

Time dilation in relativity applies to any two objects that are moving relative to one another, whether or not they are accelerating, for as long as they are moving relative to one another.

Ah yes, you'd be right about that. I was confused because I never actually found a formula that allowed calculating time dilation while acceleration, only for static speeds.

 

[TMac3000]

Maybe we could say timetravel is possible (just for now, because I don't think so).
What would see the outside observer when the ship beginns timetravel during decelerate?
Or how would it look for the outside observer when the ship beginns timetravel during decelerate?

In a future when a lot of these ships are being used, ships with radar might see brief radar "ghosts" that blink in and then disappear quickly. They would probably say "Ah, a time-drive ship in deceleration."

As far as what the other posters are saying, it is unfortunate that time, space, and the speed of light are all intimately connected in a way that seems to conspire to prevent us from ever reaching another star system in a reasonable amount of time. It was just a thought experiment, and a fanciful one at that.

 

[Andy44]

But our ship has a device that allows it to slip backward through the timeline as it decelerates,

So you've substituted time travel handwavium for warp drive handwavium. Either way, not plausible.

 

[ar81]

I had a crazy idea to cheat speed of light.

If B is the point of reference, and A moves almost at speed of light, put a C oberver moving at the same speed of A (using B as reference) but in opposite direction. That way you travel faster than light from observer C and not from B.

When A and C get back to B they would be both older than B.

Why is it crazy? Because I never got to understand how aging may rely on relative points. If A travels too fast using B as reference, B would age slower, but from B it may look like A ages slower. I never understood that dilemma.

 

[Ghostrider]

I hope you're all aware that "cheating" the speed of light usually leads to:

- crew going insane and feasting on their own innards
- unseen entities going BOOM! BOOM! down the corridors of your ship driving people crazy
- phantom apparitions of crippled kids driving their mothers to suicide
- your lead scientist hallucinating, ripping his own eyes out, blowing up the rescue craft and generally acting unwholesomely
- something friggin' evil coming out of your gravity drive

But you don't have to take my word for it.

 

[jedidia]

If B is the point of reference, and A moves almost at speed of light, put a C oberver moving at the same speed of A (using B as reference) but in opposite direction. That way you travel faster than light from observer C and not from B.

I'm not QUITE sure I understand what you're saying. If I understand you right, you say that if A and C move in opposite directions at allmost the speed of light, then they would be travelling faster than the speed of light relative to each other.

This is a missasumption. Observer A would NOT apear to travel FTL to observer C, and vice versa. It is a looong time ago since I did the math, but in the end it actually made sense, allthough I cannot explain anymore how that conclusion is reached.

It would however be correct to say that for the "static" observer B the relative speed between A and C would apear to be faster than light, a classical case of apparent FTL travel, of which we have lots (mostly galaxies that can be observed to move away from each other at FTL speeds. However, when you do the math, you learn that they actually do not).

Why is it crazy? Because I never got to understand how aging may rely on relative points. If A travels too fast using B as reference, B would age slower, but from B it may look like A ages slower. I never understood that dilemma.

If you'd like to understand and have enough time on your hand, here's the web page that tought it all to me (allthough I have forgotten most of it again by now):

http://galileoandeinstein.physics.virginia.edu/lectures/lecturelist.html

for relativity, start at lecture 20 and work your way up. It is VERY comprehensive, and even someone like me that never had any higher math than trigonometry in school could follow it without problem (as long as you do the math examples for yourself. It's allmost impossible to really comprehend what's going on if you don't). The nice thing about SPECIAL relativity (as opposed to general relativity) is that the math involved is quite simple and straight forward.

 

[ijuin]

I'm not QUITE sure I understand what you're saying. If I understand you right, you say that if A and C move in opposite directions at allmost the speed of light, then they would be travelling faster than the speed of light relative to each other.

This is a missasumption. Observer A would NOT apear to travel FTL to observer C, and vice versa. It is a looong time ago since I did the math, but in the end it actually made sense, allthough I cannot explain anymore how that conclusion is reached.

It would however be correct to say that for the "static" observer B the relative speed between A and C would apear to be faster than light, a classical case of apparent FTL travel, of which we have lots (mostly galaxies that can be observed to move away from each other at FTL speeds. However, when you do the math, you learn that they actually do not).

To put it briefly, at high sublight speeds, velocity does not add in a linear manner, but instead in an asymptotic manner.