FTL and time travel

[jb_farley]

damn. i understand the physics behind no FTL travel. what i don't get is why going faster than c necessitates backwards time travel. everything i read on the subject just restates that 'if you go faster than the speed of light, you will travel backwards in time'. or that going faster than c would start to screw with causation, but without explaining why.

here's my understansing of the situation, and my peeve. (disclaimer- no, i know one can't actually travel faster than c) i'm in my vessel departing from the colony of New New New Jersey, which is one light year away from Earth. If i went at the speed of light, i would get there in one year (*ignoring time dilation and reference frames). from my understanding, if i left going faster than c i would get to earth before i left(and thereby messing with cause and effect)? this makes no sense to me. why must i arrive on earth before i left? wouldn't i just get to earth in less than a year? and if that's true, would that still give cause and effect the finger?

*this is the only fly in the ointment. is this the catch, that my reference frame would start going in the opposite direction? if so, how would that impact my arrival time? many thanks,

jb

[Nanook of the North Shore]

Funny you bring this up. I was just reading an atricle in todays Newsday(NY paper) about an experiment where some scientists got light to go faster than light speed. From the description what happened was the beam left the chamber before it entered. Kinda boggles the mind.

[tcburnett]

I am pretty good at explaining things like Yogi Berra. I try to make sense at least part of the time but this won't be one of them.

I'm a causality chauvinist so my contention is that nothing can happen until it happens. Which means you can't arrive if you haven't left yet. The best I will allow is that you can arrive at your destination at EXACTLY the same time you left your departure point. In my (personal) convoluted theory, that is the ONLY time you can arrive if you travel through a wormhole, but you can make the journey in zero time, just not in reverse time. Oh...I am also a timeline chauvinst. I don't think Al was wrong, I just don't think he had the whole story.

But the REAL problem with your question is the location. Who the hell would ever live in NEW NEW NEW JERSEY?!?!?!? I mean, time travel is one thing, but New New New Jersey is right off the wall. :)

[tcburnett]

Originally posted by Nanook of the North Shore
Funny you bring this up. I was just reading an atricle in todays Newsday(NY paper) about an experiment where some scientists got light to go faster than light speed. From the description what happened was the beam left the chamber before it entered. Kinda boggles the mind.

Er....maybe you need to read that again:

http://www.foxnews.com/science/060500/times_speed.sml

[DrMatrix]

You are right that if an object travels faster than light from your frame of reference, it does not travel back in time in your reference frame. However, in some reference frames, the object travels back in time. This is why FTL is equivalent to traveling back in time.

Consider this: If two events A and B occur at the same time according to you, in some frames A occurs first, in others B occurs first. So a supposed signal from A to B travels infinitely fast, faster than light, or travels back in time depending on your view point.

[The Ryan]

According to relativity, the amount of time through which an object travel is equal to the square root of the distance traveled squared plus the time experienced squared i.e.
(time distance)^2=(space distance)^2+(time experienced)^2

So suppose you travel at twice the speed of light. for one light year. The equation above becomes:

(half a year)^2=(light year)^2+(time experienced)^2

solving for time experienced:

time experienced=sprt(1/4 - 1)=(sqrt 3/2)i


So actually, you would experience not negative time but imaginary time. The only way in which you are going "backwards in time" is that no matter which place you choose to consider you starting place, and which your ending place, there will be some reference frame in which you arrived at your destination before you left your starting point (and there will also be a reference frame in which you left before you arrived). Relativity shifts around time and space distances, but does not change their signs as long as the velocities involved are less than c.

[Chronos]

Put another way, "the same time" only has absolute meaning if it's also the same place. If the location is different, then you've got to specify a frame of reference, as well, and there are no particular reference frames preferred by the laws of physics.
I wrote a lengthy post on this back in May, in the thread The Past through Tomorrow (http://boards.straightdope.com/sdmb/showthread.php?postid=446307#post446307). It's a bit in-depth, but I hope I made it clear.

[Bear_Nenno]

I've gotta stop opening these threads...

Hopefully one day someone will answer these questions before they are asked!!

[JonF]

If i went at the speed of light, i would get there in one year (*ignoring time dilation and reference frames)

But you can't] ignore time dilation and reference frames, or you get a totally wrong answer.

If you traveled at the speed of light, you would see yourself (that is, in your personal reference frame) arriving in zero time.

[bibliophage]

Originally posted by tcburnett
Originally posted by Nanook of the North Shore
Funny you bring this up. I was just reading an atricle in todays Newsday(NY paper) about an experiment where some scientists got light to go faster than light speed. From the description what happened was the beam left the chamber before it entered. Kinda boggles the mind.

Er....maybe you need to read that again:

http://www.foxnews.com/science/060500/times_speed.sml The authors of the study themselves in Nature (http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v406/n6793/abs/406277a0_fs.html) say We measure a group-velocity index of ng = -310(±5); in practice, this means that a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it. The key phrase is "appears to leave the cell before entering". They go on to say The observed superluminal light pulse propagation is not at odds with causality, being a direct consequence of classical interference between its different frequency components in an anomalous dispersion region.

[ZenBeam]

If the universe has a preferred frame of reference (say, an expanding frame in which the galaxies are more-or-less stationary throughout the universe), you could have FTL travel with respect to that frame of reference without causality violations.

There's no evidence at all for a preferred frame in any measurements, and it's extremely unlikely, but it can't be ruled out completely. So you can go read your SF without worrying about it.

[Chronos]

Nice try, ZenBeam, but an expanding coordinate system doesn't work as a frame of reference, for purposes of relativity. You are correct, however, that if there were a preferred frame of reference in the Universe, then FTL would not neccesarily imply time travel-- That's the only possible chink in Special Relativity's armor. Of course, we can't prove absolutely that there's no preferred frame, but every experiment done so far on the topic is consistent with that assumption.

As for science fiction, I usually just suspend my disbelief-- Even I don't think in such relatavistic terms that causality-preserving FTL is immediately obvious as a problem.

[Phobos]

Originally posted by Chronos
Nice try, ZenBeam, but an expanding coordinate system doesn't work as a frame of reference, for purposes of relativity. You are correct, however, that if there were a preferred frame of reference in the Universe, then FTL would not neccesarily imply time travel-- That's the only possible chink in Special Relativity's armor. Of course, we can't prove absolutely that there's no preferred frame, but every experiment done so far on the topic is consistent with that assumption.


Another possible chink is if gravitomagnetism is not confirmed (e.g., by Gravity Probe B)...I suspect Einstein will prevail though.

Why can't Cosmic Background Radiation be used as a standard universal reference frame? (I don't think it can, but I don't know the reason as to why not.)

[ZenBeam]

Nice try, ZenBeam, but an expanding coordinate system doesn't work as a frame of reference, for purposes of relativity.

I would think that in an expanding universe, a comoving, expanding coordinate system would be the most likely candidate for a preferred frame of reference. Why wouldn't the appropriate Robertson-Walker metric work as the "universal" preferred frame of reference?

[Kyberneticist]

Originally posted by Phobos

Another possible chink is if gravitomagnetism is not confirmed (e.g., by Gravity Probe B)...I suspect Einstein will prevail though.


Hang on one second... gravitomagnetism? I thought Gravity Probe B was testing General Relativity? Have we finally come up with a theory that unites Electro-Magnetic-Weak with Gravity? That's quite an accomplishment!

[Phobos]

Originally posted by Kyberneticist
Hang on one second... gravitomagnetism? I thought Gravity Probe B was testing General Relativity? Have we finally come up with a theory that unites Electro-Magnetic-Weak with Gravity? That's quite an accomplishment!

Unfortunately, I don't know the details of the experiment. But I think some of the links from the Gravity Probe B website talk about it.
http://einstein.stanford.edu/index.html
(At least the New Scientist link mentions gravitomagnetism...I saw it elsewhere too, but I forget which link)

[jayron 32]

*ignoring time dilation and reference frames

This makes no sense. It's like asking "What is the chance of my running through that brick wall over there unscathing, ignoring for a moment that it's brick and I'm going to run into it." You can't just ignore "reference frames." Where are you making measurements? I've said it before and I'll say it again:

YOU CANNOT GO FASTER THAN THE SPEED OF LIGHT. It is NOT a symptom of not having enough thrust to accomplish the speed. It is not allowable by the physical existance of the Universe. It makes no sense to say "Ok, if we ignore this fundemental property of the Universe, can i go backwards in time in this manner." No, you can't. It's not a property you can ignore, because speed and size and time and mass and all that DOES NOT WORK in that manner. Damn it, why is this so hard to understand.

[Rusalka]

[QUOTE]Originally posted by jb_farley
[B]damn. i understand the physics behind no FTL travel. what i don't get is why going faster than c necessitates backwards time travel. everything i read on the subject just restates that 'if you go faster than the speed of light, you will travel backwards in time'. or that going faster than c would start to screw with causation, but without explaining why.
-----------------------
Here's an illustration: Pretend you have a very powerful telescope through which you are looking at a planet 1 light year away. You see a guy on a park bench eating a sandwich. Pretend you can snap your fingers and be sitting on that park bench the "instant" (from your reference point) you see the image. Well, that would mean you traveled faster than light, since it took the light from that event a year to get to your telescope. For all practical purposes, the light from the event is the same thing as the event.

You might say: "well what if I had a pole that reached all the way to the image I was watching, and I decided to poke the guy in the shoulder with it." The force of pushing the pole would create a wave in the material of the pole that would travel to the planet to poke the guy on the shoulder, but the thing is, this wave can't travel faster than light, so by the time the "poke" arrived he would be long gone. No electromagnetic or other type of phenomenon can travel faster than the speed of light either.

If you have mass you cannot move faster than the speed of light. This is because as you approach the speed of light, your mass approaches infinity and how can you propel an infinite mass? There's a simple equation that illustrates this but I can't remember it.

[Chronos]

The reason that you can't use an expanding coordinate system as your reference frame is that as seen from any point in it, all of the other points are moving. Any given point, taken in isolation, appears to be at rest, but that's not enough.
The rest frame of the CMB works pretty well for most purposes, but it's limited to the portion of the background that's within our lookback distance. There's no reason to assume that the same rest frame would work for a point ten billion light years from us, for instance.
As to gravitomagnetism: The term refers not to a link with electromagnetism, but to a phenomenon related to gravity in the same way that magnetism is related to electricity. It's got a lot to do with what's known as frame dragging. Incidentally, if it's not detected, that would represent a chink in the armor of General Relativity, not Special-- GR is a lot less ironclad, and there's a number of ways in which it could potentially be modified or expanded. I don't think that there's currently any proposed extensions or modfications which would accomodate a lack of gravitomagnetism, but it's still worth checking, IMO. You can bet that if we don't detect it, there'll be theories to explain that lack quickly enough.

[jb_farley]

Originally posted by jayron 32
*ignoring time dilation and reference frames

This makes no sense. It's like asking "What is the chance of my running through that brick wall over there unscathing, ignoring for a moment that it's brick and I'm going to run into it."


actually, (as i'm sure you know) there is a chance. i know this cuz i asked my high school physics teacher (who, in street parlance, was da BOMB) and got a straight answer. sure, the normal force isn't likely to give out, but it's worth a try.


YOU CANNOT GO FASTER THAN THE SPEED OF LIGHT. It is NOT a symptom of not having enough thrust to accomplish the speed. It is not allowable by the physical existance of the Universe. It makes no sense to say "Ok, if we ignore this fundemental property of the Universe, can i go backwards in time in this manner." No, you can't. It's not a property you can ignore, because speed and size and time and mass and all that DOES NOT WORK in that manner. Damn it, why is this so hard to understand.

my question had nothing to do with trying to go faster than light, or asking 'why can't i get my car to get up to c?' it was asking for clarification of one of the reasons it's impossible to do so.

if you really want to be anal, you should go back in your time machine (whoops, that's impossible in the real world, so it's out for fiction) and slap around Al Einstein for his thought experiments involving flying alongside a photon.

relax dude. if you are not allowed to nreak rules in a thought experiment, when can you?

[Averye0]

SF deals with FTL on a regular basis, and while fictional, many authors actually are Physicists.

Oddly enough, some of the best descriptions that I've read of how FTL travel would be accomplished involve the assumption that an object with a mass>0 cannot go faster than c.

In "Primary Inversion" Catherine Asaro describes a phenomenon much like the recent (and well-discussed) experiment. It involves adding an imaginary element to one's velocity, and more or less going around c without actually hitting it. Bizarre, I know, but who knows? It fits in with idea of "imaginary time" at FTL speeds.

The other author that has presented (IMHO) an acceptable method of FTL travel is David Weber. He kinda takes the easy way out though. In his imagination, since one cannot go faster than light, simply manipulate oneself into another dimension, where c is apparantly faster. Thus one can travel at what appears to be FTL speeds, without ever actually going FTL.

The preceding is all copyright it's respective authors and is only presented here to furthur intellectual discussion, and is not to be taken as anything remotely resembling truth, fact, or reality.

Flame as necessary

Averye0

[ZenBeam]

The reason that you can't use an expanding coordinate system as your reference frame is that as seen from any point in it, all of the other points are moving. Any given point, taken in isolation, appears to be at rest, but that's not enough. The rest frame of the CMB works pretty well for most purposes, but it's limited to the portion of the background that's within our lookback distance. There's no reason to assume that the same rest frame would work for a point ten billion light years from us, for instance.

I don't see the basis for your objection. All other points are moving, but the maximum velocity is WRT the preferred frame at the point the particle/person is at. It doesn't matter that the velocity is different WRT the comoving coordinate system at some point a distance away because the particle isn't there. If you can define a universal time coordinate, and the particle never travels backwards in time WRT that coordinate at the particle's location, there can't be a causality problem. If the universe is homogeneous on a large enough scale, as it appears to be, such a coordinate is definable.

[Chronos]

I'll agree that David Weber is pretty good with his physics, compared to a lot of authors, but he's no Heinlein-- he has things like FTL propagation of gravitational effects, a common misconception. The key in his system, is that although there is no preferred reference frame in n-space or in any single band of hyperspace, there is a preferred frame in any system of two or more bands, in order for the speed of light to remain invariant in each band-- Hence, he's indirectly taking advantage of that same chink that I mentioned earlier. Incidentally, the notion of a hyperspace where c is faster is by no means unique to Weber-- It's probably the most commonly used FTL method in science fiction, and is also used in Star Wars, most of Asimov's works, and a few of Heinlein's. I've never read anything by Catherine Asaro, but the idea sounds interesting; I'll have to look into that book. I'm pretty sure that her method would still be subject to the time-travel effect, though.

ZenBeam: You can't use a universal time coordinate because of things like the Twin Paradox... Were there a Universal Time, the twins' watches would have to agree after the travelling one returned. You might try using time since Big Bang as your coordinate, but when a high-energy cosmic ray slams into the Earth's atmosphere, for instance, that time is a lot lower for the cosmic ray than it is for the atmosphere.

[ZenBeam]

You can't use a universal time coordinate because of things like the Twin Paradox... Were there a Universal Time, the twins' watches would have to agree after the travelling one returned. You might try using time since Big Bang as your coordinate, but when a high-energy cosmic ray slams into the Earth's atmosphere, for instance, that time is a lot lower for the cosmic ray than it is for the atmosphere.

The twin paradox isn't relevant. The universl time coordinate is the time coordinate for an observer at rest in the comoving frame. At least one of the twins leaves this frame, so he can't expect to use the universal time. Likewise, the high-energy cosmic ray is not stationary in the comoving frame. From it's frame of reference, the universe is anisotropic. The observable mass in the universe is moving predomnantly in one direction. The CMB is blue shifted in one direction, red shifted in the opposite direction.

Here's a version of the twin paradox which may be relevant (and hopefully interesting enough to make up for its length): Consider a two dimensonal universe with one time dimension, and with a closed space dimension. You can represent this as a cylinder, with time along the axis, and space around the cylinder.

Two twins are in a frame of reference where their timeline runs along the cylinder, parallel to the axis. One gets into a spaceship, and heads off in one direction at a high (constant) velocity. Eventually, he circumnavigates the universe, and returns to the stay-at-home twin. The travelling twin is younger due to relativity, even though both were in an inertial frame during the entire trip*. You can't use General Relativity to resolve the issue, because all accelerations occurred when the twins were together. The resolution is that the stay-at-home twin is in a preferred frame of reference. He can define a spatial axis such that it forms a closed loop at a constant time T in his frame. For any other frame of reference, the spatial axis is a helix, spiraling around the cylnder. For this situation, there must be a preferred frame of reference. I don't see any way to avoid it. Yet any experiments the twins performed without circumnavigating the universe would just show normal Special Relativity, with no preferred frame of reference.

Further, in a spatially closed 3+1, spatially homogeneous dimensional universe, I don't see how you can avoid the same issue. If there can (or must) be a preferred frame of reference in a closed universe, it's hard to imagine why it isn't at least possible for a flat or open universe.

(*) You could symmetrize this by having both twins start out with some velocity around the cylnder, and have the two accelerate equally away from each other, such that one of the twins ends up in the preferred frame, and likewise have them decellerate equally at the end.

[Chronos]

You have a point there with your cylindrical universe, ZenBeam... As it happens, I have access here to one of the foremost experts on non-simply connected cosmologies... I'll have to ask him about it. Give me a little time to look into it.