Is the theory of relativity applicable only at the speeds of light?
467316
That version of Einsteins theory of general relativity is only for particles with 0 momentum.  for ones that do wave momentum but no mas it is\nE= pc (p=momentum, c= speed of light)\n\nbut yes, it is only applicable for at the speed of light hence it only has c, the speed of light, and not v, another speed.
447950
1.  According to the Special Theory of Relativity you can't travel as fast as the speed of light.\n\n2.  The General Theory of Relativity has to do with gravity, not what happens when you approach the speed of light.\n\n3.  If you could travel at the speed of light, the light from the sun could not catch up with you, no more than a pulse of light can catch up with another pulse of light.  So, you couldn't see the sun again.
1084318
Actually, the way modern physicists think of it, the question doesn’t quite make sense.  The problem is the “traveling at the speed of light” part, because one of the conclusions of Einstein’s Special Theory of Relativity is that nothing can travel that fast.\n\nInterestingly, though, its was consideration of a similar question that started Einstein down the path toward relativity… Light is best described by Maxwell’s Equations, which predict that any experiement will measure same speed for light in a vacuum.  When Einstein thought about traveling at the speed of light, he realized that there must be a problem.  In fact, Maxwell’s Equations don’t allow for a “standing wave” solution, which is what a light wave would look like if you were traveling at the speed of light. \n\nSo Einsten took things back to basics, which is what he was good at.  Let’s start with what he actually said… His Special Theory of Relativity started with two postulates (or assumptions): 1) that the laws of physics are observed to be the same in any non-accelerated “reference frame” and 2) that the speed of light (c) is observed to be the same for all observers.\n\nThe first assumption basically says that as long as you’re traveling at a constant speed, all your experiments will give the same results.  That seems reasonable.  But the thing about the speed of light always being the same sounds a little odd: the Michelson-Morley experiment (and others) had detected no change in the speed of light when measured from a moving frame of reference, however, and as I already mentioned, Maxwell’s Equations yield a specific value for the velocity of light.\n\nStarting from there, Einstein worked out how to add velocities together, and it comes out to be not simple addition, but a fraction that never gets larger than c. Basically, what happens is that the measurement of time and distance changes, so a physical object (e.g., a spaceship) can never achieve lightspeed.\n\nSo”  That’s a long-winded way of saying that your headlights themselves can’t travel at the speed of light.  If we want to think about them traveling very, very close to the speed of light, however, we realize that the light they emit still travels at lightspeed (as measured by the fast-moving headlights themselves), so they stil work the same way headlights would work on a car traveling much slower.\n\n\nRyan Wyatt\nRose Center for Earth & Space\nNew York, New York
236873
According to Einsteinian theory, yes, you would see the light.  Einstein postulates that the speed of light is relative to the viewer.  If you were travelling at the speed of light and turned on a flashlight, the light traveling out of the flash light would be traveling away from you at the speed of light relative to you.
148002
It's technically impossible for something with mass (like a spacecraft) to go the speed of light in our universe under the laws of physics.  As something approaches the speed of light, it gains mass.  In theory, if something were to actually go the speed of light, it would have infinite mass, which is impossible.  \n\nOf course, no one can say for sure.  Science fiction on television and in books have gotten around these rules by using wormholes, subspace, etc., none of which are known to actually exist.
306574
The important principle in this theory is:\n\nThe speed of light is not affected by the speed of the source emitting this light.\n \nThat is: “Once light is emitted from a source it moves in vacuum with a constant speed.\n\nSpeed of a matter and the speed of light are measured by measuring length and time.   But light is energy; whereas the source is a matter.\n\nIf both observers; one at rest and another moving; says that light’s speed is same for both, there are only two possibilities; 1. Both must be at rest or 2. Time and length must be different for the two observers. \n\nBut one observer sees that the other is moving with a speed relative to him. Therefore, time and length must be different for the two observers.\nThe other consequence is that mass is also affected by its speed. \n\nThe one at rest will observe that time and the length are shortened for a moving object such that the speed of light is the same for both frames of reference. \n\nSince the moving observer observes that the other one is moving and he is at rest he will say in the other man’s reference frame time and length are shortened such that the speed of light is the same for both frames of reference. \n\nThis is the theory of relativity.
931618
Simultaneity in Special Relativity is based on 2 assumptions:\n\n- the laws of physics apply equally to all inertial reference frames\n- the speed of light in empty space is constant and independent of the relative motion of its source\n\nThis implies that there is no absolute simultaneity, meaning that two events that happens at the same time in one inertial frame may not happen at the same time in another inertial frame.\n\nSo Einstein is NOT defining Simultaneity based on the invariance of the speed of light in all inertial frames, but instead the failure of absolute Simultaneity is a consequence of the invariance of the speed of light in all inertial frames.\n\nSo why does the invariance of the speed of light in all inertial frames imply the lack of Simultaneity? The argument for this is exactly the same as why there can be no absolute reference frame if we assume that the speed of light is constant in all reference frames.\n\nThe only way for the speed of light to be constant in all reference frames is for distance and time to be relative and be a function of speed of the reference frame, i.e. speed = distance/time implies that distance must contract and time must dilate if the reference frame is moving with a certain speed.\n\nSo if distance and time is different in one reference frame than another, then there is no way that two observers, each in separate reference frames, can agreed on when some event has happened. Thus simultaneity is violated.
452909
Believe it or not, we already have it and use it everyday. I want to answer this in detail, but I cant disclose the true nature of how light travels until I publish.\nI will say that light that is propagated within a closed system can and does move at superluminal speeds relative to a location outside the closed system.\n\nI know-Everybody has a theory, but I can also explain why light bends at every instance without once referring to the fabric of space/time.\n\nWhat if Einstein was WRONG...errr...ok... (just so I dont offend anyone) not completely correct? What if light actually didnt travel by propelling itself into the electromagnetic field created by....itself? What if there was no such thing as a pure vacuum with zero atomic content? (by the way, there is no such vacuum.)\nResearch it, its true, there are always atoms left over to fill the space left by the vacated ones.\nDon't you find that at least a little interesting?  No such thing as a pure vacuum?
185055
The equations imply that it is impossible for any object with >0 rest mass to accelerate from below the speed of light up to exactly the speed of light. This is because, as has been mentioned, that would take an infinite amount of energy.\n\nThe equations don't prohibit an object traveling faster than the speed of light from the outset though. Photons have zero rest mass, and the instant they are created they are traveling at exactly c. If it were possible for a particle to exist that was traveling faster than c from the instant it was created, this would not be a problem for relativity. I'm not sure what it would imply about it's mass though. The name for such a particle is a tachyon (although there is no evidence these actually exist as far as I'm aware).
233087
this is where relativity comes into play. It depends from what viewpoint. speed is always relative to a "fixed" point (which might not be so "fixed" after all). get my drift?
1062755
That question goes in line with my latest thought on einstein theory of relativity.  If something approaches the speed of light (or at the speed of light) time slows for it but not the observer, so how do we measure it if the light itself is time compressed?
592101
The negative of the Speed of Light.
529605
I'll try to explain this in simple terms, but it's rather mind-twisting.  \nThese are the basic principles of Relativity:\n\n1. The speed of light is constant for all frames of reference.\n  Light goes toward you at the same rate no matter how fast you're moving away from the source.  As an example, think about someone throwing a ball at you.  If they throw the ball at 25 miles an hour and you run away from them at 10 miles an hour, then from your perspective, the ball will be approaching you at only 15 mph, because you are running away from it.  But with light, no matter how fast you run away from the approaching light, you will always "see" it coming after you at 186,000 meters per second.  But the light isn't speeding up.\n\n2. Time can speed up and slow down.\n  If you were to get in a rocket and zoom off into space going really fast (nearly the speed of light), and if you were to stay up there for a couple of years (still going really fast), you would come back to Earth to find that a couple of decades had passed.  In other words, even though you would have lived only two years, the people on Earth would have lived 20 years already.  This is because the faster you are moving physically, the slower time moves.  This is not noticeable in everyday situations, because the time change is less than a billionth of a second.  It's only a noticeable difference at really high speeds or with really sensitive atomic clocks, but it has been proven to be true.\n\n3.  No matter (physical thing made out of atoms or sub-atomic particles) can move at the speed of light, or any faster.\n  The faster something is moving, the more mass it has.  So if you were trying to accelerate something to the speed of light, the faster it got, the heavier it would get, and the harder it would be to make it go any faster.  Eventually, it would be too heavy to accelerate.  An object moving at the speed of light would theoretically have infinite mass.  Since light waves aren't matter (they're energy), they can move at the speed of light.  Same goes for gravity waves - they also move at the speed of light.\n\n4.  Space and time are meshed together in something called spacetime, which is sort of the "framework" for the universe - like the paper on which you draw a picture.  Matter (mass) bends spacetime.\n  Picture a trampoline.  If you were to place a ping pong ball on the trampoline, it would make the area below it sag a little bit, but it would hardly be noticeable, because the ping pong ball is so light.  But if you were to put a bowling ball there instead, the indentation would be much larger.  In the same way, larger masses cause more curvature in spacetime.  This concept has two consequences:\n\nA: The closer you are to a large mass, the slower time gets.  So if you were to get really close to a black hole, then time would slow down for you.\n\nB: Remember that going really fast gives you more mass?  Well, this why #2 works.  If you're in a rocket going nearly the speed of light, then your mass is going to be bigger.  That in turn means that spacetime will curve more around you, so then time will pass slower.\n\nThere's a lot morer to it than that, but those are the basics.
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You can't reach the speed of light in any conventional way, but you can get as close as you want, provided you can keep applying acceleration.  \n\nThe best way to do a thought experiment of time dilation for special relativity (special relativity being the case in which you consider high velocities, general relativity just applies this same model to all spacetime trajectories with the observation that gravity is indistinguishable form a physical acceleration) is to consider the case of a "light clock".  \n\nFor a light clock, you have a laser pulse generator, a mirror at some distance, and a pulse counter next to the laser.  Everytime a pulse is recieved at the counter, another pulse is sent from the laser, and so you have a clock that "ticks" at a rate determined by twice the distance from the laser to the mirror.  So, what happens when you move this clock?  If the clock is arranged such that the pulse bounces back and forth perpendicular to the direction of travel, it's easier to get the basic idea.  \n\nThe speed of light is always constant, but as the clock moves at higher and higher speeds, from the perspective of a stationary observer the total distance traveled by the light increases, since now it's going diagonal.   But, to a person traveling along with the clock, it doesn't appear to travel any greater distance.  \n\nIt's something like a person standing in a train car or bus tossing a ball up and down.  While the ball is up in the air, it travels horizontally by the distance the bus has moved while it's been up in the air, so the total distance traveled by the ball is much longer if the bus is moving rapidly instead of stationary.  For normal speeds, the Gallilean transforms are sufficiently accurate, so a stationary observer and the person tossing the ball can agree how fast it's moving in each axis, and how far it moves, just by adding the horizontal velocity of the bus to the vertical velocity the ball is tossed with, and come up with the same wider parabola for higher speeds and narrow parabola for low speeds, or stationary cases.  \n\nBut, light has an interesting property...  it must remain at the same velocity for all observers no matter what their speed relative to the light.  So, in the case of a ball, we can simply say that it's moving faster than it looks to the person on the bus with it, because there's the added background horizontal velocity.  If you add the vertical and horizontal velocity vectors, the sum is longer than just the vertical velocity.  \n\nBut, in truth, how can you know if you're moving at a constant velocity?  There are no pseudoforces of acceleration if you're moving at a constant velocity, but if light behaved oddly by traveling faster in one direction that the other, you would be able to tell if there was a fixed frame of reference outside of your motion, and this would allow you to determine a fixed location relative to the universe as a whole, and even determine where the center of the universe is.  But it doesn't have a center, or all points are equally the center, depending on how you want to look at it, so there can't be any way to tell if you're moving at a constant velocity, and so light must travel at the same speed in any direction when viewed by any set of observers, no matter what their relative velocities are.    Not surprisingly, we find this also comes out from Maxwell's equations describing the propagation of light, the velocity winds up being proportional to one over the square root of the product of the electrostatic permittivity and the magnetic permeability of empty space, a number which is absolute and gives no preference for directions or relative motions.  This isn't surprising since, like I said, it's a necessary condition for a centerless universe.    \n\nSo, the speed of light must remain absolute, but something has to give.  Since the light clock is ticking slower becaus
442563
There's a number of ways to answer this.  Einstein's theory of Relativity says that all things are moving at the speed of light through spacetime, with stationary objects moving wholely in the time dimension.  As you start moving, some of your velocity is directed away from time and in a physical dimension.  When you approach the speed of light, you are traveling very little in the time dimension and mostly in the physical dimesions.  If you actually reached the speed of light you would experience no time whatsoever; your entire trip across the universe would be one instantaneous flash.  However, the faster you travel, the more difficult it is to travel even faster.  When you are traveling relativistically, Newton's second law of F=ma no longer applies.  You continue adding force, but rather than creating acceleration, you'll begin gaining mass.
651526
Einstein’s Special Theory of Relativity started with two postulates: 1) that the laws of physics are observed to be the same in any non-accelerated reference frame and 2) that the speed of light (c) is observed to be the same for all observers.\n\nThe first assumption isn’t too worrisome, but the second seems trickier.  It’s certaonly not intuitively obvious, but it makes sense from both a theoretical and an observational perspective.  Observational because the Michelson-Morley experiment (and others) detected no change in the velocity of light when measured from a moving frame of reference; theoretical because Maxwell’s Equations yield a specific value for the velocity of light.\n\nWhen Einstein considered Maxwell’s Equations, he realized that they don’t allow for a “standing wave” solution, which is what a light wave would look like if you were travelling at the speed of light.  So he already sensed there would be a problem with travelling at very high speeds.  Indeed, when you re-work the addition of velocities, it comes out to be not simple addition, but a fraction that never gets larger than c.  \n\nThus, in the photon-photon collision you describe, each photon would “see” the other one travelling toward it with a velocity c.  (Although measuring velocities from a photon’s perspective is a little non-intuitive; you have to take the limit of the aforementioned addition formula, which causes some trepidation.)\n\nOh, and the ten-dimensional idea is just one among many…  No way yet to prove (or disprove) that one.\n\n\nRyan Wyatt\nRose Center for Earth & Space\nNew York, New York
830585
Y'know, I've been trying to get a handle on that for a long time, but so far I understand it to mean that post-Einstein, once you  include time as a fourth axis to the 3 spatial ones, the universe makes sense in terms of our physical laws working everywhere and everytime.  That is a neccessity of a good physical law - it must always work no matter where in the universe you test it or how fast you're travelling.  In other words, translations and rotations of objects or fields or whatever must have no effect on the validity of the law.  If you do not include time as an axis perpendicular to the 3 spatial ones (mathematically, of couse), what we predict won't agree with what we see, especially in such cases as high relative velocities (as in approacing the speed of light), and strong gravitational fields.  Amoung the oddest result of this view is that, in truth, all time actually exists simultaneously, and there is some not-quite-understood mechanism that only lets us see one instant at a time.\n\nAnyway, to answer your question, "How does spacetime work?", the answer is, much better than space or time alone and separate.  They are, in fact, as inseparable as the pair of poles of a magnet.\n\nThis all might be not quite accurate, especially since I'm so far just an "armchair physicist" with no formal education in the area, (tho I'm looking forward to taking physics, including the higher levels, if I can ever get thru these friggin' calculus classes).  Anybody else have something to contribute or set me straight on??