I get your point, but you're only proving a different context of FTL than I was trying to prove. Could you prove something can move FTL in the direction of its magnitute? (does that even make sense?)
I can't prove that, nor am I trying to. Hell, I think it would be hard even to explain how photons do what they do at the speed of light. For the most part, we simply observe that photons travel (by definition) at the speed of light and go from there, not worrying about how bizarre the universe would appear from a photon's point of view.
You can contemplate hypothetical FTL phenomena without worrying about the mechanism by which FTL would be achieved. There's some value to this, because a lot of people don't realize that FTL is still very troublesome even without worrying about how you get up to or past the speed of light.
In sci-fi, the typical solution to going faster than light is something like "hyperspace", or some sort of warping or folding of space. Speed of light too slow for you? No problem... jump out of this slow spacetime neighborhood we're stuck in, get into a faster one, travel there for a while, then hop back into normal space when you're ready.
If you want to be true, however, to what we know about "normal" space once you've turned off your fictitious hyperspace drive, hyperspace has only solved half of your problem. Writers tend to focus on the problem of how to move several light-years to visit the next star, and do it in a day or two, without realizing that, no matter how you accomplish this tricky motion, simply being there so fast is in-and-of-itself problematic.
For ease of discussion, let's just talk about abstract "messages" being sent rather than physical travel.
If you can send a message faster than light, no matter how you get the job done, there will be frames of reference from which it could be observed (by a technical meaning of the word "observed" which is quite a bit more abstract than "seeing") that you had not merely sent your message faster than light, but that your message had gone backward in time, that it had arrived even before it was sent.
You might think this kind of time travel is already more than enough trouble for FTL. If there's any sort of time travel at all, don't we already have shoot-your-own-grandpa paradoxes that make no sense? Can't the recipient of your message send you another message telling you not to send your first message, that arrives before you've sent that first message? Then you might never send the first message, in which case your partner wouldn't send the "don't send" message, in which case you will send the first message...
The trouble isn't with the first message, it's with possible response messages. One FTL message alone can't get us in trouble as long as one thing is true: As far as you, the sender, are concerned, your message doesn?t go ?too far? backward in time. What?s constitutes ?too far?? Further backward in time than the light-travel time between you and your recipient. (In terms of a spacetime diagram, reception must occurs anywhere but within the past light cone of the sending event -- if not, even one FTL message can get you in trouble.) As long as this backwards-in-time limit isn?t broken, then your recipient can't contradict your FTL message in any irresolvably paradoxical way using light speed or slower means for a response.
But what about an FTL response to an FTL message? Without special rules governing how the FTL messaging system works, the situation gets dicey. You might get a response to a message before you've even sent that message, and all temporal paradox hell can break loose.
However -- and this is what I find fascinating myself -- with the right set of rules, you can have your cake and eat it too. You can have FTL, but avoid the irresolvable paradoxes. Rather than go over how that might work again, I'll simply recommend PhysGuyJWH's web page on the subject, and perhaps also going over what's already been written in this thread and its parent thread.
I dont have the time to read that site, could you sum it up in a few lines?
On the other hand, while not at all critising your interest in this kind of FTL stuff, what is the point? What does it prove?
I have quite a good interest in this area, but when the discussion turns to these kind of paradoxes, i kind of tell myself, "well yeah, so what?" Am i missing something? Does it matter? If you had the answer would it make the slightest difference?
Isn't it just a clever curiosity? Doesn't it just trivialize into the chicken and egg problem? If (keeping it simplistic)the chicken came first, wouldn't you go "yeah, now I know, and um, oh...well...yes" and never give another thought to it the rest of your life.
Would you wonder why the chicken came first? Would the answer strangely become the question to the exact same problem leading you in an endless loop for infinity?
Well, I think I took care of defining that well enough yesterday.
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but there is no point. It isn't necessary.
This is something I wanted to come back to today...
"Necessary" is a tricky word, so let's just say it's easy to see how FTL would be desirable, and desirable in a meaningful way.
Sure, even without FTL, you can get anywhere you want in this universe, as quickly as you want in your own frame of reference -- so long as you can find a sufficient source of energy to power your trip, and so long as you can survive some pretty brutal acceleration, radiation, etc.
But just try to see the other side of this galaxy, and then come back home to tell your friends and family about it. They'll have been long dead, for nearly 200,000 years.
What you want out of FTL is not only for your own perceived travel time to be short, but for your travel time to be brief in the frame of reference of the galaxy at large as well. What this would mean for the traveler's frame of reference is unclear, but the typical sci-fi answer comes down to things like taking a shorter route, like a wormhole, between the two points, or somehow moving a special bubble of warped space through space, or "hyperspace", etc. -- basically, some fanciful notion that protects you from needing to violate the speed of light within the immediate area space around you.
Easier to think about (at least for me) is FTL communication. If someday we have a colony on Mars, it would sure be nice to conduct a real-time phone conversation with a friend on Mars, without 40 minute gaps between saying "Hello, is Dave there?", and hearing "Who's calling?". Although we don't normally think of these things as separate issues, the one-way travel time of each message is less important to us than the round-trip time between sending a message and getting a response to that message. We want that round-trip time to be as close to instantaneous as possible.
The laws of physics probably make such convenient conversation over large distances impossible, but you can bet that our scientifically illiterate descendants (as now, likely the vast majority of the populace) would constantly be complaining "When are they going to do something to fix these #@*^&!! delays?" -- at least if Earth/Mars conversations ever become a commonplace thing.
Whether FTL communication is actually possible or not, there are at least ways to imagine it happening in a meaningful way that might not tread too badly on our current understanding of physics. Should the universe actually be so kind as to be hiding some usable, causal (and hopefully inexpensive) form of FTL that we haven't discovered yet, it would be an enormous boon to a potential human future as a space-faring species.
Come to think of it, we could make good use of FTL even today. One example: to reduce those annoying pauses in conversation between <insert favorite news anchor here> and "our correspondent in Baghdad". A good portion of these delays is due to the light-travel time of radio signals bouncing up and down several times along the 22,500 mile distance between Earth and geosynchronous communications satellites.
I stand corrected, thanks for the insight. I might have phrased my last post differently in hindsight, but i'll leave it as is.
Doesn't it just make sense for a man on mars not to transmit a signal so much FTL that the reply is received before the question? Supposing the answer is received before the question, is there any point debating wether the answer changed the question in the first place, because if the two (or however many relays) were not related 100%, the question would never have produced the answer
On the subject of long distance Tv relays, surely c is not the reason for the delays. 22000 miles is like 1/8 a second for the signal, hardly something anyone would notice.
I have quite a good interest in this area, but when the discussion turns to these kind of paradoxes, i kind of tell myself, "well yeah, so what?" Am i missing something? Does it matter? If you had the answer would it make the slightest difference?
Hopefully the response I was writing, while you were apparently writing this question, answers the question you're asking here.
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I dont have the time to read that site, could you sum it up in a few lines?
It's not too difficult to sum up in a few lines, but any short summation is likely to leave open a bunch of "Yeah, but isn't that impossible?" or "Doesn't that violate...?" SO don't say I didn't warn you -- and please don't be lazy about doing a little link-clicking to make sure the questions that might pop up in your mind haven't been answered already.
The (somewhat) short summation:
If FTL effects are somehow carried by a medium of some sort, which could be a localized phenomenon of limited spacetime extent (a good idea I hadn't thought of until visiting PhysGuyJWH's site), or (and a little more upsetting to the philosophical frame-invariance of relativity) a Universe-permeating "aether"-like medium, such that this medium can be said to have a frame of reference, then, as long as FTL messages do not go backwards in time within the special medium's frame, then all observers, in any frame, will be able to utilize FTL in a way that preserves causality. It is also necessary to stipulate that the FTL medium must be exclusive over the spacetime region in which it functions, not overlapping with any other FTL medium, or causality-breaking loops could be formed.
Under this type of FTL scheme, sometimes you might not be able to travel or send a message much faster than light, but you also might be able to send an object or message so fast that it goes backward in time (but only backward in time to a place so far away that its light-time distance is greater than the time regression of the effect). The results balance out in such a way, however, that round-trip travel or communication can be as close to instananeous as you like.
I kind of understand that, but my understanding is very limited, sorry if I have derailed your thread a bit with my ignorance. I tried to learn this a while ago, but realised my education is very lacking, so this year I have returned to university to get the basics before I try to tackle this stuff.
IDoesn't it just make sense for a man on mars not to transmit a signal so much FTL that the reply is received before the question? Supposing the answer is received before the question, is there any point debating wether the answer changed the question in the first place, because if the two (or however many relays) were not related 100%, the question would never have produced the answer
This is someplace where I'll just have to ask you to re-read the thread or follow the other links -- suffice to say there are indeed rules and limits you can establish for FTL that neatly and cleanly avoid the kind of temporal paradoxes that you're worried about here.
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On the subject of long distance Tv relays, surely c is not the reason for the delays. 22000 miles is like 1/8 a second for the signal, hardly something anyone would notice.
But it's not just one trip, one way to one satellite, that the signals have to make. If you're talking to someone on the other side of the world via satellite, the signal first has to go up 22,500 miles, jump between at least two satellites (traveling as much as a distance of 2 * 22,500 * sqrt(2) or more), and then another 22,500 miles to get back down to the ground. Double that for round-trip time. That gets you up around 220,000 miles round-trip -- more than a full light second.
Add to this the fact that there are often multiple earth/satellite bounces of a signal, and it adds up. There are of course latencies in signal relays and other signal processing equipment as well, but light-travel time actually adds up to a perceivable part of the delay you see in satellite conversations.
Originally posted by audiopollution (in two different message)
I'm still waiting to see the answers to the 'how much time has passed' questions. I know that perception of time/actual time is affected by theoretical light speed trvael. Just not how much...
...Can't someone just answer the questions posed on the previous page about how much time passes? I'm dyin' here.
This is a bit of an open-ended question, with no one answer, but I'll try to answer what I think you're getting at...
First of all, although time is relative for different observers going at different speeds, for each observer, there's only one kind of time. Unlike in bad sci-fi, there isn't a separate category for perceptual time and biological time and clock time, etc. If your sci-fi hero enters a "time anomaly" or whatever, it makes very little sense for him to come out it with gray hair and looking forty years older, unless he actually experienced the feeling of forty years going by, unless he managed to have enough supplies on hand to eat and drink for forty years without starving, and unless he managed to keep his mind occupied for forty years without going crazy and killing himself.
Sci-fi breaks the rules a little or a lot for plot purposes, of course, but it's way, way on the "a lot" side of that bargain to pretend that time for your hair pigment follows utterly different physical laws than time on your watch or time for your traveling stomach.
Setting aside speculative FTL stuff for now, plain old relativity tells us that everyone in an inertial frame of reference (free-floating, not being acted upon by external forces) will observe all clocks that are both free-floating as well, and at relative rest to the observer, as ticking away at the same speed as his own clock. All other clocks will be observed as running slow -- although it takes a substantial difference in speed or acceleration for the clock rate difference to be obvious, hence the fact that relativistic effects are not apparent in our normal realm of experience.
If two inertial observers are moving relative to one another, each will deem the other's clock to be slower than his own. This runs counter to common sense: you'd think if A deems B's clock to be slow, B would have to deem A's clock to be fast. It's hard to explain -- especially just with words and without pictures and diagrams -- but there really is no contradiction here, and both claims about which clock is slow are equally valid and equally true.
As Earth-bound observers, we aren't actually in an inertial frame of reference. The electromagnetic forces at work that keep us from falling through chairs and floors and solid ground apply an acceleration to us. You might think of someone who's free-falling toward the ground as someone who's accelerating, but in a relativistic sense, the free-falling observer is in an inertial reference frame, while those of us who aren't falling are in an accelerated frame. The time-dilation effect due to resisting Earth's gravity isn't much, but it's enough to make Earth-bound clocks slower than falling or orbiting clocks. This is a circumstance in which one observer actually deems someone else's clock as going faster than his own clock. At least this scenario fits a little better with common sense -- astronauts in orbit consider our clocks down on Earth to be running a little slow, we deem their clocks to be running a little fast.
By the way, if someone starts talking about relativity like this -- "Train A is traveling at the speed of light and..." -- stop them right there. Train A can travel near the speed of light, not at the speed of light. Unless were getting into very speculative FTL games, no physical object can travel as fast as light.
So "how much time has passed", as you've asked, for, say, someone traveling very quickly to a nearby star? There is no one answer for that, because you can make that time as arbitrarily short as you like -- the closer you push to the speed of light, the shorter the time becomes. If you have enough energy and can withstand the acceleration, you can cut your own travel time to a distant place to as close to zero as you like.
For the sake of argument, consider the Earth and some fictional Planet G (it doesn't always have to be X, now does it?) 10 light years away to be relatively at rest with one another. Of course, these planets are spinning and revolving around their stars, and those stars are probably moving toward or away from each other -- but as far as relativistic effects are concerned, these effects are small and we can ignore them for now. The same for gravitational effects on clocks for people on either planet -- not big enough to worry about.
You can travel from the Earth to the other planet as fast as you like. As far as anyone on Earth is concerned, however, you aren't going to get there any faster than 10 years.
As you start your trip, people on Earth will see your clock as running slow, and you'll think the same thing about Earth clocks. But here's where acceleration makes all the difference, and distance amplifies the effect: half way to the other star, you switch on your rockets in order to slow down -- you want to land on Planet G, not hurtle right past it, or into it, at some hideous velocity. This acceleration causes you to observe Earth clock's as speeding up, and eventually running faster than your own. By time you reach Planet G -- let's say after one year of your own time (average speed, 0.99c) -- your accounting of the rate Earth clocks have been running vs. your own clock all works out so that it makes sense that, for people on Earth, it has taken you a little over 10 years to get to Planet G, even though you only experienced one year going by.
shetline, I kind of understand that during the big bang, there were like 21 dimesions and most of them contracted into a single point.
Im leaning towards the idea (ok its my own, so its prolly shite), that these dimensions are very small because they contain no matter. However, i think it is possible that our own universe is the same size, but we perceive it bigger, because it does contain matter travelling less than c, however as I said earlier, I reckon that matter is just a byproduct of travelling less than c, and from a photons reference, the universe we live in is just a singular point connected to the other singular point dimensions. SO, if signals were to propogate on more dimensions than we can humanly perceive, we could in effect claim to solve the spooky action distance by concluding, not that signals travel backwards in time, or instantly, but through the smaller dimensions, giving the illusion that it has travelled much faster than light in our own universe.
Does this make any sense? Also, what do you make of my idea that the universe will expand into a singularity?
shetline, I kind of understand that during the big bang, there were like 21 dimesions and most of them contracted into a single point...
...Does this make any sense? Also, what do you make of my idea that the universe will expand into a singularity?
Out of my realm to really comment.
I've studied the basics of Newtonian physics. I feel fairly confident discussing issues of Special Relativity as I have some solid practical experience there. (It only takes simple high school algebra, by the way, to handle the basics of SR.) Conceptually, I feel reasonably confident talking about some parts of General Relativity, although I have no mathematical experience at all with things like tensor calculus. At least you can get at some aspects of GR just by playing with SR and trying to handle accelerated frames of reference, which I have done.
Quantum mechanics, second law of thermodynamics... I'd generously rate myself at the higher end of what you can get out of studying popularized accounts of these subjects. A good intuitive grasp, I think, but I wouldn't recognize a related equation if one bit me.
I'm starting to slog through a book the covers the Standard Model right now... what you might call a "popular account", but a bit more mathematical than most such things. (I wish I could remember the author or title right now. My father gave me the book, so, not having picked the book out myself, that info isn't sticking with me.)
As for anything involving multiple "curled up" dimensions and the like... about as far as I can get with any of that stuff is the kind of intution that comes from reading books like "Flat Land" and "Planiverse", teaching yourself to imagine higher dimensions and non-Euclidian geometries by analogy with lower dimensions and curved surfaces.
So, could you play FTL games using these extra dimension? I haven't a clue. But this much I can say: be it extra dimensions or pixie dust that gets you FTL, the FTL effects you get either need to be confined to mutually exclusive FTL regions of spacetime, or, if FTL phenomena act through a consistent, space-permeating medium with no boundaries within normal space, then you'll be mucking with the frame invariance of relativity. This last kind of FTL would create a preferred frame of reference, something that's at least philosophically opposed to relativity, even if you can make it work within the known experimentally-verified aspects of relativity.
Without the above kinds of restrictions, FTL, regardless of the means by which it might be accomplished, runs into serious problems with causality. Real time travel, not just some weird non-local timestamps on a sequence of events, becomes possible.
While I think that trying to preserve causality and rule out time travel leads to the most interesting way of imagining FTL, I've heard other kinds of speculation: causality not being preserved, because troublesome time-travel simple jumps you from one slice of a quantum multiverse to another, or, in another formulation, mysterious quantum forces interfering with any attempt to alter the time line if you go back in time and try to change things.
The problem with that logic is it doesn't take into account the equations that go with the theory of relativity. You can't add velocities that large without taking into account relativty. Velocities that large must be added using relativistic addition. When you use this proven formula, no two velocities will add to over c. Even with the two rocket ships, the observer will only see a velocity close to the speed of light.
The problem with that logic is it doesn't take into account the equations that go with the theory of relativity. You can't add velocities that large without taking into account relativty. Velocities that large must be added using relativistic addition. When you use this proven formula, no two velocities will add to over c. Even with the two rocket ships, the observer will only see a velocity close to the speed of light.
You can't simply add velocities in order to calculate velocities in other frames of reference. It is perfectly valid, however, to say that in your own frame of reference that two objects are moving relative to one another faster than the speed of light.
Consider frame of reference X, where you are standing along side a straight east-west track. Train A is traveling east at 0.6c, Train B is traveling west at 0.6c. In your frame, frame X, it's quite okay to say Train A is moving away from Train B at 1.2c, in your frame of reference.
However, if the question is "In Train A's frame of reference, how fast is Train B moving?", that is when you invoke relativistic addition, and find that, relative to Train A, Train B is moving west at about 0.882c.
It has less validity than saying god created the world in six days. At least that is testable.
You might be right that some of what's going on in the world of string theory may well turn out to be nothing more than mental masturbation with high-level math, but I think you're being quite unfair dismissing the whole field of string theory out of hand.
A lot of physicists aren't happy with the current state of physics. The Standard Model lacks much in the way of elegance and simplicity. There are a lot of "just so" values for physical constants with no good explanations for why the lucky values needed to make the universe work come out just the way they have to. Relativity and Quantum Mechanics don't mesh together well in all ways, especially in the realms of very high energy and very short distances. There's definitely room to hope for improvement in our understanding of physics, as well as to hope for potential new discoveries.
So, what is a physicist to do? Accept the status quo and say "good enough"? Give up and not explore any further, simply because it looks like we can't yet test some theories at the energies we'd need to test them at?
If the best science can do for now is look for mathematical consistency and mathematical beauty in areas like string theory, why not? As long as physicists working on such things keep some perspective on the limitations they're functioning under, what's the problem?
Perhaps there will turn out to be low-energy consequences of string theory that don't need solar-system sized particle accelerators to validate. Perhaps cosmological consequences can be discovered, and then tested by looking at astronomical data -- like the way some theories of the big bang have been tested by looking for patterns in the cosmic background radiation.
But if no one does the work, if no one takes the chance of exploring a few blind alleys and dead ends, how will we ever make any progress?
When we were arguing about FTL and thermodynamics, you brought up work that Stephen Hawking had done with thermo and relativity. I'm not sure if this is the work you're thinking about, but one such thing Hawking worked on was relating the entropy of a black hole to its diameter.
The idea was this: entropy is always supposed to increase over time, so what happens if I heat up a cup of tea -- converting high level energy into less ordered heat energy -- and then toss the cup of tea into a black hole. Can I "hide" the entropy inside the black hole, thus lowering the entropy of the universe?
The answer Hawking came up with was that the radius of the black hole increases slightly when you dump your cup of tea into it, and that the radius corresponds to the total entropy of the black hole. You haven't managed to hide entropy by somehow throwing it away beyond this universe, all you've done is change the expression of that entropy within this universe.
If this is the kind of work you admire, guess what? It's completely untestable noodling around with equations. We've just barely confirmed the existence of black holes as real physical objects. We're no where near being able to travel to one, and even further from being able to measure the radius of one with such demanding precision as would be needed to test this kind of theory.
what would ship A measure if was moving at exactly c and ship b was exactly c, at 0, 90 and 180 degrees to each other
It's a question that you can't really answer, because frames of reference become meaningless at exactly at c. If the universe is zero thickness in your direction of travel, and all other clocks have slowed to a stop, what meaning does speed have in a time-stopped, less-than-paper-thin universe? By having infinite mass relative to the rest of the universe, you'd probably be cut off inside some mathematically degenerate infinite black hole anyway. Perhaps you'd have already converted the entire remainder of the universe into energy in order to power your mad light-speed dash, so Ship B would be toast anyway
Maybe QM, if it can be used here at all (and I'm just wildly guessing about all of this), would tell you the the Universe can only be said to be 0 thickness +/- a Planck length, and that clocks are only stopped +/- a Planck time -- but I don't know if you can extract anything meaningful out of any of that.
I was interested in seeing what the formula produced. Could you tell me what the answers are at 0.99 c, and if poss, what the answers are at 100%c, assuming no div by 0, or other strangities.
I completely agree with all of that, however, string theory isn't based upon a working metric like black holes are. Hawking readily admits that until black holes are proven phenomena all of what he does in his mental flexing is a what if situation that is based upon known working physics. String theory doesn't even have that. It has existed for the better part of two decades more or less and there are no predictions of physical phenomena...
It is math and should be treated as such. Placing any significant weight on it as the solution ot the universe and all of life (42) is simply stupid....
Comments
Originally posted by MarcUK
I get your point, but you're only proving a different context of FTL than I was trying to prove. Could you prove something can move FTL in the direction of its magnitute? (does that even make sense?)
I can't prove that, nor am I trying to. Hell, I think it would be hard even to explain how photons do what they do at the speed of light. For the most part, we simply observe that photons travel (by definition) at the speed of light and go from there, not worrying about how bizarre the universe would appear from a photon's point of view.
You can contemplate hypothetical FTL phenomena without worrying about the mechanism by which FTL would be achieved. There's some value to this, because a lot of people don't realize that FTL is still very troublesome even without worrying about how you get up to or past the speed of light.
In sci-fi, the typical solution to going faster than light is something like "hyperspace", or some sort of warping or folding of space. Speed of light too slow for you? No problem... jump out of this slow spacetime neighborhood we're stuck in, get into a faster one, travel there for a while, then hop back into normal space when you're ready.
If you want to be true, however, to what we know about "normal" space once you've turned off your fictitious hyperspace drive, hyperspace has only solved half of your problem. Writers tend to focus on the problem of how to move several light-years to visit the next star, and do it in a day or two, without realizing that, no matter how you accomplish this tricky motion, simply being there so fast is in-and-of-itself problematic.
For ease of discussion, let's just talk about abstract "messages" being sent rather than physical travel.
If you can send a message faster than light, no matter how you get the job done, there will be frames of reference from which it could be observed (by a technical meaning of the word "observed" which is quite a bit more abstract than "seeing") that you had not merely sent your message faster than light, but that your message had gone backward in time, that it had arrived even before it was sent.
You might think this kind of time travel is already more than enough trouble for FTL. If there's any sort of time travel at all, don't we already have shoot-your-own-grandpa paradoxes that make no sense? Can't the recipient of your message send you another message telling you not to send your first message, that arrives before you've sent that first message? Then you might never send the first message, in which case your partner wouldn't send the "don't send" message, in which case you will send the first message...
The trouble isn't with the first message, it's with possible response messages. One FTL message alone can't get us in trouble as long as one thing is true: As far as you, the sender, are concerned, your message doesn?t go ?too far? backward in time. What?s constitutes ?too far?? Further backward in time than the light-travel time between you and your recipient. (In terms of a spacetime diagram, reception must occurs anywhere but within the past light cone of the sending event -- if not, even one FTL message can get you in trouble.) As long as this backwards-in-time limit isn?t broken, then your recipient can't contradict your FTL message in any irresolvably paradoxical way using light speed or slower means for a response.
But what about an FTL response to an FTL message? Without special rules governing how the FTL messaging system works, the situation gets dicey. You might get a response to a message before you've even sent that message, and all temporal paradox hell can break loose.
However -- and this is what I find fascinating myself -- with the right set of rules, you can have your cake and eat it too. You can have FTL, but avoid the irresolvable paradoxes. Rather than go over how that might work again, I'll simply recommend PhysGuyJWH's web page on the subject, and perhaps also going over what's already been written in this thread and its parent thread.
On the other hand, while not at all critising your interest in this kind of FTL stuff, what is the point? What does it prove?
I have quite a good interest in this area, but when the discussion turns to these kind of paradoxes, i kind of tell myself, "well yeah, so what?" Am i missing something? Does it matter? If you had the answer would it make the slightest difference?
Isn't it just a clever curiosity? Doesn't it just trivialize into the chicken and egg problem? If (keeping it simplistic)the chicken came first, wouldn't you go "yeah, now I know, and um, oh...well...yes" and never give another thought to it the rest of your life.
Would you wonder why the chicken came first? Would the answer strangely become the question to the exact same problem leading you in an endless loop for infinity?
Originally posted by MarcUK
Not only is there no definition of FTL...
Well, I think I took care of defining that well enough yesterday.
but there is no point. It isn't necessary.
This is something I wanted to come back to today...
"Necessary" is a tricky word, so let's just say it's easy to see how FTL would be desirable, and desirable in a meaningful way.
Sure, even without FTL, you can get anywhere you want in this universe, as quickly as you want in your own frame of reference -- so long as you can find a sufficient source of energy to power your trip, and so long as you can survive some pretty brutal acceleration, radiation, etc.
But just try to see the other side of this galaxy, and then come back home to tell your friends and family about it. They'll have been long dead, for nearly 200,000 years.
What you want out of FTL is not only for your own perceived travel time to be short, but for your travel time to be brief in the frame of reference of the galaxy at large as well. What this would mean for the traveler's frame of reference is unclear, but the typical sci-fi answer comes down to things like taking a shorter route, like a wormhole, between the two points, or somehow moving a special bubble of warped space through space, or "hyperspace", etc. -- basically, some fanciful notion that protects you from needing to violate the speed of light within the immediate area space around you.
Easier to think about (at least for me) is FTL communication. If someday we have a colony on Mars, it would sure be nice to conduct a real-time phone conversation with a friend on Mars, without 40 minute gaps between saying "Hello, is Dave there?", and hearing "Who's calling?". Although we don't normally think of these things as separate issues, the one-way travel time of each message is less important to us than the round-trip time between sending a message and getting a response to that message. We want that round-trip time to be as close to instantaneous as possible.
The laws of physics probably make such convenient conversation over large distances impossible, but you can bet that our scientifically illiterate descendants (as now, likely the vast majority of the populace) would constantly be complaining "When are they going to do something to fix these #@*^&!! delays?" -- at least if Earth/Mars conversations ever become a commonplace thing.
Whether FTL communication is actually possible or not, there are at least ways to imagine it happening in a meaningful way that might not tread too badly on our current understanding of physics. Should the universe actually be so kind as to be hiding some usable, causal (and hopefully inexpensive) form of FTL that we haven't discovered yet, it would be an enormous boon to a potential human future as a space-faring species.
Come to think of it, we could make good use of FTL even today. One example: to reduce those annoying pauses in conversation between <insert favorite news anchor here> and "our correspondent in Baghdad". A good portion of these delays is due to the light-travel time of radio signals bouncing up and down several times along the 22,500 mile distance between Earth and geosynchronous communications satellites.
Doesn't it just make sense for a man on mars not to transmit a signal so much FTL that the reply is received before the question? Supposing the answer is received before the question, is there any point debating wether the answer changed the question in the first place, because if the two (or however many relays) were not related 100%, the question would never have produced the answer
On the subject of long distance Tv relays, surely c is not the reason for the delays. 22000 miles is like 1/8 a second for the signal, hardly something anyone would notice.
Originally posted by MarcUK
I have quite a good interest in this area, but when the discussion turns to these kind of paradoxes, i kind of tell myself, "well yeah, so what?" Am i missing something? Does it matter? If you had the answer would it make the slightest difference?
Hopefully the response I was writing, while you were apparently writing this question, answers the question you're asking here.
I dont have the time to read that site, could you sum it up in a few lines?
It's not too difficult to sum up in a few lines, but any short summation is likely to leave open a bunch of "Yeah, but isn't that impossible?" or "Doesn't that violate...?" SO don't say I didn't warn you -- and please don't be lazy about doing a little link-clicking to make sure the questions that might pop up in your mind haven't been answered already.
The (somewhat) short summation:
If FTL effects are somehow carried by a medium of some sort, which could be a localized phenomenon of limited spacetime extent (a good idea I hadn't thought of until visiting PhysGuyJWH's site), or (and a little more upsetting to the philosophical frame-invariance of relativity) a Universe-permeating "aether"-like medium, such that this medium can be said to have a frame of reference, then, as long as FTL messages do not go backwards in time within the special medium's frame, then all observers, in any frame, will be able to utilize FTL in a way that preserves causality. It is also necessary to stipulate that the FTL medium must be exclusive over the spacetime region in which it functions, not overlapping with any other FTL medium, or causality-breaking loops could be formed.
Under this type of FTL scheme, sometimes you might not be able to travel or send a message much faster than light, but you also might be able to send an object or message so fast that it goes backward in time (but only backward in time to a place so far away that its light-time distance is greater than the time regression of the effect). The results balance out in such a way, however, that round-trip travel or communication can be as close to instananeous as you like.
Originally posted by MarcUK
IDoesn't it just make sense for a man on mars not to transmit a signal so much FTL that the reply is received before the question? Supposing the answer is received before the question, is there any point debating wether the answer changed the question in the first place, because if the two (or however many relays) were not related 100%, the question would never have produced the answer
This is someplace where I'll just have to ask you to re-read the thread or follow the other links -- suffice to say there are indeed rules and limits you can establish for FTL that neatly and cleanly avoid the kind of temporal paradoxes that you're worried about here.
On the subject of long distance Tv relays, surely c is not the reason for the delays. 22000 miles is like 1/8 a second for the signal, hardly something anyone would notice.
But it's not just one trip, one way to one satellite, that the signals have to make. If you're talking to someone on the other side of the world via satellite, the signal first has to go up 22,500 miles, jump between at least two satellites (traveling as much as a distance of 2 * 22,500 * sqrt(2) or more), and then another 22,500 miles to get back down to the ground. Double that for round-trip time. That gets you up around 220,000 miles round-trip -- more than a full light second.
Add to this the fact that there are often multiple earth/satellite bounces of a signal, and it adds up. There are of course latencies in signal relays and other signal processing equipment as well, but light-travel time actually adds up to a perceivable part of the delay you see in satellite conversations.
Originally posted by audiopollution (in two different message)
I'm still waiting to see the answers to the 'how much time has passed' questions. I know that perception of time/actual time is affected by theoretical light speed trvael. Just not how much...
...Can't someone just answer the questions posed on the previous page about how much time passes?
This is a bit of an open-ended question, with no one answer, but I'll try to answer what I think you're getting at...
First of all, although time is relative for different observers going at different speeds, for each observer, there's only one kind of time. Unlike in bad sci-fi, there isn't a separate category for perceptual time and biological time and clock time, etc. If your sci-fi hero enters a "time anomaly" or whatever, it makes very little sense for him to come out it with gray hair and looking forty years older, unless he actually experienced the feeling of forty years going by, unless he managed to have enough supplies on hand to eat and drink for forty years without starving, and unless he managed to keep his mind occupied for forty years without going crazy and killing himself.
Sci-fi breaks the rules a little or a lot for plot purposes, of course, but it's way, way on the "a lot" side of that bargain to pretend that time for your hair pigment follows utterly different physical laws than time on your watch or time for your traveling stomach.
Setting aside speculative FTL stuff for now, plain old relativity tells us that everyone in an inertial frame of reference (free-floating, not being acted upon by external forces) will observe all clocks that are both free-floating as well, and at relative rest to the observer, as ticking away at the same speed as his own clock. All other clocks will be observed as running slow -- although it takes a substantial difference in speed or acceleration for the clock rate difference to be obvious, hence the fact that relativistic effects are not apparent in our normal realm of experience.
If two inertial observers are moving relative to one another, each will deem the other's clock to be slower than his own. This runs counter to common sense: you'd think if A deems B's clock to be slow, B would have to deem A's clock to be fast. It's hard to explain -- especially just with words and without pictures and diagrams -- but there really is no contradiction here, and both claims about which clock is slow are equally valid and equally true.
As Earth-bound observers, we aren't actually in an inertial frame of reference. The electromagnetic forces at work that keep us from falling through chairs and floors and solid ground apply an acceleration to us. You might think of someone who's free-falling toward the ground as someone who's accelerating, but in a relativistic sense, the free-falling observer is in an inertial reference frame, while those of us who aren't falling are in an accelerated frame. The time-dilation effect due to resisting Earth's gravity isn't much, but it's enough to make Earth-bound clocks slower than falling or orbiting clocks. This is a circumstance in which one observer actually deems someone else's clock as going faster than his own clock. At least this scenario fits a little better with common sense -- astronauts in orbit consider our clocks down on Earth to be running a little slow, we deem their clocks to be running a little fast.
By the way, if someone starts talking about relativity like this -- "Train A is traveling at the speed of light and..." -- stop them right there. Train A can travel near the speed of light, not at the speed of light. Unless were getting into very speculative FTL games, no physical object can travel as fast as light.
So "how much time has passed", as you've asked, for, say, someone traveling very quickly to a nearby star? There is no one answer for that, because you can make that time as arbitrarily short as you like -- the closer you push to the speed of light, the shorter the time becomes. If you have enough energy and can withstand the acceleration, you can cut your own travel time to a distant place to as close to zero as you like.
For the sake of argument, consider the Earth and some fictional Planet G (it doesn't always have to be X, now does it?) 10 light years away to be relatively at rest with one another. Of course, these planets are spinning and revolving around their stars, and those stars are probably moving toward or away from each other -- but as far as relativistic effects are concerned, these effects are small and we can ignore them for now. The same for gravitational effects on clocks for people on either planet -- not big enough to worry about.
You can travel from the Earth to the other planet as fast as you like. As far as anyone on Earth is concerned, however, you aren't going to get there any faster than 10 years.
As you start your trip, people on Earth will see your clock as running slow, and you'll think the same thing about Earth clocks. But here's where acceleration makes all the difference, and distance amplifies the effect: half way to the other star, you switch on your rockets in order to slow down -- you want to land on Planet G, not hurtle right past it, or into it, at some hideous velocity. This acceleration causes you to observe Earth clock's as speeding up, and eventually running faster than your own. By time you reach Planet G -- let's say after one year of your own time (average speed, 0.99c) -- your accounting of the rate Earth clocks have been running vs. your own clock all works out so that it makes sense that, for people on Earth, it has taken you a little over 10 years to get to Planet G, even though you only experienced one year going by.
Im leaning towards the idea (ok its my own, so its prolly shite), that these dimensions are very small because they contain no matter. However, i think it is possible that our own universe is the same size, but we perceive it bigger, because it does contain matter travelling less than c, however as I said earlier, I reckon that matter is just a byproduct of travelling less than c, and from a photons reference, the universe we live in is just a singular point connected to the other singular point dimensions. SO, if signals were to propogate on more dimensions than we can humanly perceive, we could in effect claim to solve the spooky action distance by concluding, not that signals travel backwards in time, or instantly, but through the smaller dimensions, giving the illusion that it has travelled much faster than light in our own universe.
Does this make any sense? Also, what do you make of my idea that the universe will expand into a singularity?
Originally posted by MarcUK
shetline, I kind of understand that during the big bang, there were like 21 dimesions and most of them contracted into a single point...
...Does this make any sense? Also, what do you make of my idea that the universe will expand into a singularity?
Out of my realm to really comment.
I've studied the basics of Newtonian physics. I feel fairly confident discussing issues of Special Relativity as I have some solid practical experience there. (It only takes simple high school algebra, by the way, to handle the basics of SR.) Conceptually, I feel reasonably confident talking about some parts of General Relativity, although I have no mathematical experience at all with things like tensor calculus. At least you can get at some aspects of GR just by playing with SR and trying to handle accelerated frames of reference, which I have done.
Quantum mechanics, second law of thermodynamics... I'd generously rate myself at the higher end of what you can get out of studying popularized accounts of these subjects. A good intuitive grasp, I think, but I wouldn't recognize a related equation if one bit me.
I'm starting to slog through a book the covers the Standard Model right now... what you might call a "popular account", but a bit more mathematical than most such things. (I wish I could remember the author or title right now. My father gave me the book, so, not having picked the book out myself, that info isn't sticking with me.)
As for anything involving multiple "curled up" dimensions and the like... about as far as I can get with any of that stuff is the kind of intution that comes from reading books like "Flat Land" and "Planiverse", teaching yourself to imagine higher dimensions and non-Euclidian geometries by analogy with lower dimensions and curved surfaces.
So, could you play FTL games using these extra dimension? I haven't a clue. But this much I can say: be it extra dimensions or pixie dust that gets you FTL, the FTL effects you get either need to be confined to mutually exclusive FTL regions of spacetime, or, if FTL phenomena act through a consistent, space-permeating medium with no boundaries within normal space, then you'll be mucking with the frame invariance of relativity. This last kind of FTL would create a preferred frame of reference, something that's at least philosophically opposed to relativity, even if you can make it work within the known experimentally-verified aspects of relativity.
Without the above kinds of restrictions, FTL, regardless of the means by which it might be accomplished, runs into serious problems with causality. Real time travel, not just some weird non-local timestamps on a sequence of events, becomes possible.
While I think that trying to preserve causality and rule out time travel leads to the most interesting way of imagining FTL, I've heard other kinds of speculation: causality not being preserved, because troublesome time-travel simple jumps you from one slice of a quantum multiverse to another, or, in another formulation, mysterious quantum forces interfering with any attempt to alter the time line if you go back in time and try to change things.
Originally posted by MarcUK
shetline, I kind of understand that during the big bang, there were like 21 dimesions and most of them contracted into a single point.
String theory is shite.
Simply totally unprovable shite.
It has less validity than saying god created the world in six days. At least that is testable.
Originally posted by FashnNuggt
The problem with that logic is it doesn't take into account the equations that go with the theory of relativity. You can't add velocities that large without taking into account relativty. Velocities that large must be added using relativistic addition. When you use this proven formula, no two velocities will add to over c. Even with the two rocket ships, the observer will only see a velocity close to the speed of light.
You can't simply add velocities in order to calculate velocities in other frames of reference. It is perfectly valid, however, to say that in your own frame of reference that two objects are moving relative to one another faster than the speed of light.
Consider frame of reference X, where you are standing along side a straight east-west track. Train A is traveling east at 0.6c, Train B is traveling west at 0.6c. In your frame, frame X, it's quite okay to say Train A is moving away from Train B at 1.2c, in your frame of reference.
However, if the question is "In Train A's frame of reference, how fast is Train B moving?", that is when you invoke relativistic addition, and find that, relative to Train A, Train B is moving west at about 0.882c.
Originally posted by billybobsky
String theory is shite.
Simply totally unprovable shite.
It has less validity than saying god created the world in six days. At least that is testable.
You might be right that some of what's going on in the world of string theory may well turn out to be nothing more than mental masturbation with high-level math, but I think you're being quite unfair dismissing the whole field of string theory out of hand.
A lot of physicists aren't happy with the current state of physics. The Standard Model lacks much in the way of elegance and simplicity. There are a lot of "just so" values for physical constants with no good explanations for why the lucky values needed to make the universe work come out just the way they have to. Relativity and Quantum Mechanics don't mesh together well in all ways, especially in the realms of very high energy and very short distances. There's definitely room to hope for improvement in our understanding of physics, as well as to hope for potential new discoveries.
So, what is a physicist to do? Accept the status quo and say "good enough"? Give up and not explore any further, simply because it looks like we can't yet test some theories at the energies we'd need to test them at?
If the best science can do for now is look for mathematical consistency and mathematical beauty in areas like string theory, why not? As long as physicists working on such things keep some perspective on the limitations they're functioning under, what's the problem?
Perhaps there will turn out to be low-energy consequences of string theory that don't need solar-system sized particle accelerators to validate. Perhaps cosmological consequences can be discovered, and then tested by looking at astronomical data -- like the way some theories of the big bang have been tested by looking for patterns in the cosmic background radiation.
But if no one does the work, if no one takes the chance of exploring a few blind alleys and dead ends, how will we ever make any progress?
When we were arguing about FTL and thermodynamics, you brought up work that Stephen Hawking had done with thermo and relativity. I'm not sure if this is the work you're thinking about, but one such thing Hawking worked on was relating the entropy of a black hole to its diameter.
The idea was this: entropy is always supposed to increase over time, so what happens if I heat up a cup of tea -- converting high level energy into less ordered heat energy -- and then toss the cup of tea into a black hole. Can I "hide" the entropy inside the black hole, thus lowering the entropy of the universe?
The answer Hawking came up with was that the radius of the black hole increases slightly when you dump your cup of tea into it, and that the radius corresponds to the total entropy of the black hole. You haven't managed to hide entropy by somehow throwing it away beyond this universe, all you've done is change the expression of that entropy within this universe.
If this is the kind of work you admire, guess what? It's completely untestable noodling around with equations. We've just barely confirmed the existence of black holes as real physical objects. We're no where near being able to travel to one, and even further from being able to measure the radius of one with such demanding precision as would be needed to test this kind of theory.
what would ship A measure if was moving at exactly c and ship b was exactly c, at 0, 90 and 180 degrees to each other
Originally posted by MarcUK
shetline
what would ship A measure if was moving at exactly c and ship b was exactly c, at 0, 90 and 180 degrees to each other
It's a question that you can't really answer, because frames of reference become meaningless at exactly at c. If the universe is zero thickness in your direction of travel, and all other clocks have slowed to a stop, what meaning does speed have in a time-stopped, less-than-paper-thin universe? By having infinite mass relative to the rest of the universe, you'd probably be cut off inside some mathematically degenerate infinite black hole anyway. Perhaps you'd have already converted the entire remainder of the universe into energy in order to power your mad light-speed dash, so Ship B would be toast anyway
Maybe QM, if it can be used here at all (and I'm just wildly guessing about all of this), would tell you the the Universe can only be said to be 0 thickness +/- a Planck length, and that clocks are only stopped +/- a Planck time -- but I don't know if you can extract anything meaningful out of any of that.
Originally posted by shetline
Shetline and the current state of physics...
I completely agree with all of that, however, string theory isn't based upon a working metric like black holes are. Hawking readily admits that until black holes are proven phenomena all of what he does in his mental flexing is a what if situation that is based upon known working physics. String theory doesn't even have that. It has existed for the better part of two decades more or less and there are no predictions of physical phenomena...
It is math and should be treated as such. Placing any significant weight on it as the solution ot the universe and all of life (42) is simply stupid....