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,
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.
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.
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.
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.
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. It’s a bit in-depth, but I hope I made it clear.
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.
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.
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.)
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?
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)
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.
[QUOTE]
*Originally posted by 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 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.
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.
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.
**
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?
