That's correct as I understand it.  As Gio said in the other thread, the theory is -- approximately -- that your speed through space and your speed through time always add up to the speed of light.  That's what you mean, I think, when you say "you can't travel faster than the speed of light because you cannot move any slower."  But that alone doesn't mean that you can't move faster than light through space; it just means that to do so, you have to move backwards through time to cancel out the extra speed.

Which is not on its face impossible, but does create odd paradoxes.  For one thing, what do I see if I'm watching your spaceship?  Well, if you take off at 12:00 and arrive at 11:00 (an hour before you started), I see your ship "take off" from its destination at 11:00 and fly backwards to its origin point, which it reaches at 12:00, when you get in and start the engine (or whatever time-traveling light-defying spaceships have).  That's not the paradox, though.  The paradox is that if I take out a phaser and shoot your ship down at 11:30, you don't have a ship to get into at 12:00 -- right?  But then how did I shoot your ship down if it never took off in the first place?

This is a version of the well-known "grandfather paradox" -- can I go back in time and kill my own grandfather before he meets my grandmother?  If so, and I thus prevent my father's birth and thus my own existence, then who killed my grandfather?  This is said to prove that faster-than-light travel is impossible.

Asano calls bull on this one.  She points out that even if you're traveling faster than light relative to me, you're still at rest relative to your ship.  That is, you may be going into my past, but you're always going into your own future.  That means you can't change anything you've already experienced, or that's necessary to anything you've experienced.  You can't kill your grandfather before he meets your grandmother because, at the time you set out, he already had.  Asano is hand-wavy on how exactly the universe keeps you from killing him, however.

Futurama once had a great take on this.  The crew accidentally goes back in time to 1947 Roswell because Fry puts metal in the microwave.  (Long story; don't try this at home.)  There he meets his grandfather as a young man.  Someone explains the grandfather paradox to him: Fry must, at all costs, not let Grampa die, or Fry himself will blink out of existence.  This ends with Fry isolating his grandfather in a little cottage deep in the New Mexico desert where nothing bad can ever befall him -- except, as it turns out, a nuclear test explosion that vaporizes the house and Fry's grandpa too.  Fry is sure he'll soon disappear.  Desolate, he takes out the cute waitress who he saw eyeing his grandfather earlier in the episode.  Well, you see where it's going from here; Fry becomes his own grandfather later that evening.

I don't know enough about string theory to educate anyone on it, sadly!  I think the "one universe for each possible choice" theory is one that's been endorsed by a few scientists, though.  It's one way of understanding the idea of Schrodinger's Cat.

For those not familiar with Schrodinger's Cat, it's a (rather sadistic) thought experiment that goes something like this.  You seal a cat inside an opaque metal box attached to a machine.  The machine flips a hidden coin.  If the coin comes up heads, it silently pumps a lethal gas into the box that kills the cat instantly.  If it comes up tails, the cat lives.  Say you do this experiment; you've hit the button and the machine has flipped the coin.  According to the model that quantum mechanics uses to study electrons, before you open the box, the cat is simultaneously alive and dead in a 50/50 probability distribution.  It's not until you actually open the box and check on the cat that "the waveform collapses" and the cat is 100% alive, or 100% dead.

Now, Schrodinger wasn't seriously saying that the cat was 50% alive until the box was opened; he was using a metaphor, acknowledging that what is true of quantum mechanics isn't necessarily true of macro-physics.  But sci-fi writers, and perhaps some scientists?, have taken the idea to its logical conclusion: when that coin is flipped, two new universes spin off, one in which the cat dies and one in which it lives.  The cat isn't 50% alive in either universe, but it is 50% alive on average.

What is the argument behind nothing traveling faster than the speed of light?

And (it's kind of weird physics) if anyone can explain dimensions 8+ to me, I would greatly appreciate it. I've seen a very good video on the topic, but that's the extent of my knowledge and it got confusing after a point. A line. A fold.

OnionPrince, bothersomely, I've never found or created a thought experiment that convincingly demonstrates why stuff gets heavier as it gets faster, or why FTL travel is time travel.  The effect Daytripper describes in the curveball example is caused by a change in momentum in response to velocity, not mass, so as far as I know it's unrelated to relativity's predictions about mass.  As for the time travel side of things, you can easily create awesome thought experiments that demonstrate bizarre side effects of going faster than light, some of which appear to be time travel; unfortunately, though, I can't think of a way to demonstrate time travel without assuming the part where time gets slower as you get faster.  As far as I can tell, these are just predictions made by Einstein's equations.  Mind you, these predictions have held up to a lot of empirical scrutiny.  For instance, we've accelerated particles to over 99% of the speed of light, but we just can't get them to a hundred.  As far as thought experiments go, though... it's slim pickings.  This bothers me too.  If anyone does know a way to explain why relativistic effects must exist using thought experiments, I hope they'll fill us in!

About the best I can do is something else Daytripper said.  Light is massless.  There's no way that something with mass, which has to spend energy to accelerate, will ever be able to compete with the speed that light can attain.  That means that our intuition, which is that we can go as fast as we want by building better and better rocket engines, must lead us to faulty conclusions as we get faster and faster.

Last edited by satyreyes (07-05-2008 10:51:15 PM)

satyreyes wrote:

If anyone does know a way to explain why relativistic effects must exist using thought experiments, I hope they'll fill us in!

I don't! And this thread is totally beyond me, and likely I'm not even getting what you're asking for...

The way I think of it is: It takes eight minutes for light from the sun to get from its origin to Earth. Which is to say Earth is eight light minutes from the sun. SO! If we fly from earth to the sun in four minutes; then turn around and come back from the sun in four minutes; for an eight minute round trip: We'll get back to Earth at the same time as the rays of light that we saw when we were taking off. Meaning we get back at the same moment we left, so time travel on a small scale. Because... because we can gauge time by the distance light has traveled.

I keep forgetting to post here, but now I remembered! Sorry, here comes some theory.

First and foremost to an understanding of what the so-called speed of light MEANS is that speed is not a property that a single object is capable of possessing. In other words, a baseball CANNOT travel at 60 mph. That statement, a baseball is traveling at 60 mph is in fact logical gibberish. Hogwash. Nonsense. What makes sense is to say that the baseball is traveling AWAY from my hand at 60 mph, or TOWARD the batter at 60 mph. In other words, to describe motion at all, TWO objects are necessary. This point seems a bit trivial on its face, but it helps to explain a lot of the wonky stuff about light and its "speed"...I was about to show this with a complicated example using geometry and calculus, but maybe I can get by without that. Let's look at a javelin toss.

A man runs at oh...15 mph. Then he hurls a javelin forward at...60 mph. The javelin travels forward at 75 mph, right? This example only makes the tiniest bit of sense because everything is actually moving in relation to the field he's running on, or the earth, or any number of other nearby stationary objects. OK, the nearby thing isn't that important, and stationary is a lie, but...whatever. As long as the man is running, the javelin is still moving WITH RESPECT TO HIM at 60. So you see how speed can be different depending on what you compare it too. When I'm in the car, I'm stationary with respect to the car, but I'm moving at 70 with respect to the road. This is why falling from a moving vehicle will almost certainly kill you.

OK, now light. You would think that if a spaceship were moving at say .5c that is half the speed of light, and turned on its headlights, that light would leave the ship and travel away from the ship at c. In this you would be correct, according to Einstein. You would also think that since the ship is moving away from earth at .5c and the light is moving away from the ship in a straight line at c, that the light would be moving away from earth at 1.5c. But here you would be wrong, according to Einstein. The light is also traveling at c away from the planet. In fact, light always travels at c with respect to any object, anywhere, ever. That's the underlying foundation of relativity, from which all the other crazy Lorenz shit follows. Working from this premise, it is necessary that an objects mass RELATIVE (boy that's a hard one to wrap your brain around) to another object CHANGES with respect to their RELATIVE velocity. In other words, when I'm on the ship, I DO NOT get heavier. I never notice, because I'm on the ship, which is moving the same speed I am. If they somehow measured my mass from earth, I would be heavier (not look heavier, be heavier), but then for me, EARTH would be heavier. It goes both ways, because velocity isn't something my ship has, it's something it shares with its reference point. Same goes for size and also time. From earth, time is going slower on the ship, but for me, time is going slower on earth.

And yet, if I get on a ship and travel away from earth at very close to the speed of light (and the actual necessary mechanics of how this wouldn't kill me are trickier than most people realize) and then I come back at the same speed, I'm younger than my cousin Kyle when I get back. Why does this make sense if it goes both ways? The answer is that ACCELERATION doesn't go both ways. So the cases are in fact different. Acceleration moves beyond special relativity into general relativity where I have been unable to follow it.

More on Heisenberg, Schrodinger's cat and such later.

hmm...not exactly. Infinity is a very abstract concept, and it means very different things in different contexts. The one that's closest to what you describe is the extended real line, where it's capped by infinity at either end. But then, it's sort of circular, and there's only one infinity. er...anyway, the speed of light isn't infinite. It's well-defined. But as you pointed out earlier, the ENERGY required to get there is infinite. Tricky.

That reminds me of your other point about rest mass vs relativistic mass, as you pointed out zero times infinity is a little strange. In fact, zero times infinity can be anything at all, but...that has to do with limits. Nothing ever actually GETS there, you know? I can't seem to concentrate long enough to get this written out, but oh well. I'll try again some other time.

Well, photons are like everything else; they are affected by gravitational forces, and in fact one of Einstein's most important predictions (which was later borne out by experiment) had to do with starlight curving around the sun.  So no, photons don't travel in straight lines.

That doesn't mean they go into orbit, though.  Earth's escape velocity -- the minimum speed you need to escape our gravity into space -- is 11.2 km/s.  Light travels at 300,000 km/s.  The laser beam you fire will easily escape Earth's gravity and go soaring into space.  Earth's gravity well will bend it inward first, though, if only a little; someone on the receiving end of your laser beam deep in space wouldn't be able to see you, because the beam would appear to originate from a point on Earth a bit in front of you.  I think!

That the speed of light always moves at the same speed relative to any object just staggers the mind. I'd really love to know when and how we'd be able to actually test this. If you take that property of light and combine it with the whole idea of using the mass of light to move an object, wouldn't you be able to increase the speed of two objects by bouncing the speed of light off each other, or will that just dead end because of the speed relative to one another being constant?

I think what I'm trying to get at here is requiring a more specific explanation of solar sails. If the idea is to catch the mass of light particles, so to speak, than wouldn't the ship not accelerate but just blast the hell off or get torn to shreds? It can't catch and slow down the light, after all..?

I'm rusty as hell on this, sheesh.

Ragnarok wrote:

satyreyes wrote:

Right, Ragnarok; that's what I meant by "awesome thought experiments that demonstrate bizarre side effects of going faster than light, some of which appear to be time travel."  But if you think about it harder, you're not actually getting back at the same time you took off; you're getting back eight minutes later.  _Light_ is causing some things to happen that look like time travel but aren't.

In particular -- say you leave Earth at 8:00 and reach the Sun at 8:04.  Light takes eight minutes to get from the Sun to the Earth, so observers on Earth don't see you at the Sun till 8:12.  But you return from the Sun at 8:08, so you're there standing with the scientists as they observe you arriving at the Sun four minutes later.  That's not time travel, though, because you're not _actually_ at the Sun at 8:12, it just looks that way from Earth.

I realized this after I'd gone to bed. But I thought about it more, too. To make a round trip from Earth to the Sun and back again in eight minutes would mean going twice the speed of light. If I'm in my spaceship that's me gauging the time as I travel, whereas an observer on Earth wouldn't witness my flight taking the same amount of time. If the observer and I synchronized watches pre-flight at 8:00, when I returned my watch would say 8:08 and his/hers would say... no idea. This is where it runs into actual calculations I have no clue about. And there's all the stuff you and Stormcrow are talking about which I can't follow; but it seems there would be some sort of time discrepancy involved, beyond just the visual.

Or would it be the other way around? It'd be 8:08 on Earth, but the watch on the spaceship would read differently?

Well, if relativity was false -- if time didn't slow down and then reverse itself as you approached and passed the speed of light relative to Earth -- then your watch and my watch would both say 8:08 when you got back.  No reason why they shouldn't, after all; it's no different from your taking a train from San Francisco to Boston and back, where you wouldn't expect your watch to say anything different from ShatteredMirror's watch when you returned.  (Mind you, if there were a way for me to read your watch while you were in flight, weird things would happen -- but that again is just because light would be traveling slower than you do, not because time would necessarily warp itself.)

But relativity isn't false, at least as far as we can presently tell.  If you asked Einstein about your superluminal journey to the sun and back, he'd chuckle and explain it's not possible.  Basically, there's a number called the Lorentz factor that tells you, among other things, how many times faster time is moving on Earth than it appears to be for you.  You calculate that factor this way:

G = 1 / sqrt(1 - B^2)

Where B is the fraction of the speed of light you're going and G is the Lorentz factor.  The problem with going twice the speed of light is that then B is 2 -- which, if you plug it in, turns the demominator of the fraction into the square root of negative three.  But there's no such thing as the square root of a negative number, so this speed simply doesn't exist.  According to Einstein, anyway, but he died over fifty years ago, so maybe his theories are due for an upgrade.

Last edited by satyreyes (07-06-2008 04:36:23 PM)

The good news, OnionPrince, is that scientists at large are less dogmatic than Internet people about that particular piece of relativity, because they are sci-fi fans too.  Scientists want to be able to travel faster than light.  If it can be done, they'll find a way.

satyreyes wrote:

The good news, OnionPrince, is that scientists at large are less dogmatic than Internet people about that particular piece of relativity, because they are sci-fi fans too.  Scientists want to be able to travel faster than light.  If it can be done, they'll find a way.

You could swing me off into a whole rant about how worried I am that science fiction isn't being as technologically creative as it used to be, which means it's inspiring the new scientist crowd less. It says something that even a young scientist still is a greater ST fan than anything else, nine times out of ten.

String theory was brushed on here but kinda passed over. It has the most inspiring (mathematical) argument for faster-than-light travel because of the folds in space that string theory predicts. Actually it evolved to the point where this version of string theory is called M-Theory, for membrane, or branes, which are fundamental to the explanation of string theory as it currently exists. The idea being that this dimension, the 3rd, exists folded in dizzying ways in membranes that build up several dimensions. These dimensions in most versions of string theory have predicted multiple universes, but the key idea is that because this dimension's warped in these folds that it cannot perceive at all, the possibility stands that we could someday sense these folds and learn to jump from one spot on this dimension to a totally different one based on a short hop between higher dimensions that might be folded closer together.

Let's see....think of a bolo lanyard:



The third dimension is the string. To get from one end of the string to the other in this dimension we'd have to travel the full length of the string. But you can see this string that to us would be a straight line from point A to point B is actually curved in the space of another higher dimension. If we learn to jump across this higher dimension where the two points in our dimension meet, it's a shorter distance. How this would be done is of course not even guessed at yet, but it may not require faster than light travel. -OR- it may be that in other higher dimensions, the speed of light is different, or completely irrelevant, so that we may travel faster in those places.

BTW this is all the higher, larger scale stuff in string/M-theory. The basic idea is that at the subatomic level, everything is comprised of vibrating 'strings', like a guitar, and the nature of the atom/whatever else is decided based on the frequency of the vibration. When it first burst onto the scene people took the 'string' part to mean we're all connected and each of us has a string connecting us to everyone else, and this hokey explanation meant that absolutely no one took it seriously for a very long time.