Are there other phenomena besides light whose speed is invariant?
815552
It's all about quantum mechanics. Molecules can have a temporarially set speed at which they rotate, expand and contract, or move, but they can not go just a little bit slower or a little bit faster, their speeds are constant unless there is not enough energy to stay at a speed, or enough to "jump" speeds. I believe Philip Ball can describe it best:\n\nWhen a molecule absorbs light, it's energy increases- it gets "hotter," by which I mean that the molecule may shake, jiggle, or spin more vigorously. It is then said to have been "excited" by absorption. The reason why molecules absorb photons of light at some frequencies but not at others is that their motions are determined by the rules of  quantum mechanics, which place restrictions on which motions are allowed. We saw in chapter one (My note: I'm typing this straight from the book, but I can't show all of chapter one) that the electrons in atoms and molecules are not free to adopt any old orbit around the nuclei, with correspondingly arbitrary energies; instead, they are confined to a ladder of specific energy levels, with the energies in between the rungs being forbidden. The electronic energy levels are said to be quantized. In the same way, the energies of the motions that the molecules execute in space may also be quantized. The two-atom oxegyn molecule O2 for example, both rotates (tumbles end over end) and vibrates (the bond between the two atoms stretching and contracting) in free space. The energies associated with these motions may take only certain values, forming a ladder of energy levels like those for the electron energies. As the energies of rotation and vibration depend respectively on the speed of rotation (the number of revolutions per second, say) and the frequency of vibration (the number of in-and-out oscillations per second), the latter two quantities are quantized too. The o2 molecule, terefore, can vibrate only at specific frequencies and spin only at specific speeds. \n\nPhew, that was alot of typing! Yes, I did get that straight out of the book, I didn't copy-paste. He then goes on to explain how the larger an object is, the colser the energy levels are, so there is no real "forbidden zone", and therefore the object can accelerate gradually.\n\nEdit : Sorry about the typos, I'll pick them out as I find them.
811313
I believe it is expanding.  The speed og light, c, is the cosmic speed limit for objects or radiation within spacetime.  Spacetime itself can (hypothetically) expand faster than c, though what the limit to this might be I don't know.  Special relativity is not violated since there is no object being accelerated.
883279
light has constant speed... a big zero acceleration....\nlight does not have mass so no inertia... but amazingly has momentum...........
811109
You misunderstand Einstein's Theory of Special Relativity. It doesn't say that you can't have something move faster than the speed of light. Only "information" can NOT move faster than the speed of light. By information, I mean any "signal", like electromagnetic waves (which includes light, radio waves, etc. etc.). Any material object within the Universe are also restricted by Special Relativity, since material objects also carry "information".\n\nFor the Universe or the fabric of spacetime to be expanding faster than the speed of light is NOT a violation of Special Relativity, and the Universe will not be moving backwards in time.\n\n--------------------------------------------------------\n\nEdit - Ok, here let me clarify why I said that Special Relativity (SR) does NOT apply to the expansion of the Universe:\n\nVelocity of something depends on which definition of distance we use. So, if we assume that the distance of an object at time t is the distance from our position at time t to the object's position at time t measured by a set of observers moving with the expansion of the Universe, and all making their observations when they see the Universe as having age t, then the velocity (change in D per change in t) can definitely be larger than the speed of light. This is not a contradiction of special relativity because this "distance" is not the same as the spatial distance used in SR, and the age of the Universe is not the same as the time used in SR.\n\nIn the special case of the "empty Universe", where one can show the model in both special relativistic and cosmological coordinates, the velocity defined by change in cosmological distance per unit cosmic time is given by v = c ln(1+z), where z is the redshift, which clearly goes to infinity as the redshift goes to infinity, and is larger than c for z > 1.718.\n\nFor the "critical density Universe", this velocity is given by v = 2c[1-(1/sqrt(1+z))], which is larger than c for z > 3. \n\nFor the "concordance model" based on CMB data and the acceleration of the expansion measured using supernovae, a flat Universe with Omega = 0.27 (this would be our Universe), the velocity is greater than c for z > 1.407\n\nAll this just means that for any expanding universe, there will be a value of z (i.e., redshift or cosmological distance) that will give the appearance of galaxies moving at velocities greater than c.
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?
341703
Yes, and no- The theory of relativity states that light moves at the same speed regardless of the viewer-it is "special relativity".  If you are riding a beam of light, and switch on a flashlight, that light will speed away from you at the speed of light.  However, to a person standing still behind you, both you and the beam of light you just sent out, are traveling at the same speed.  So, light is always in motion, but it is not relative to the source.  Light, no matter how fast you are moving or in what direction, always travels relative to you (and everything else) at the same speed.  All that will happen is a change in the frequency of the light.
811141
The Speed of Light is measured to any observer, preferably "at rest" or not moving on an intersteller ship.  As said above, there is no such thing as absolutely stationary anywhere.  Everything in this Universe moves relative to something else.  We move relative to the local star systems, they in turn move relative to the galactic center.  The galactic center moves realative to the nearby Galaxies, such as Andromeda Galaxy (which in less than 6 Billion years the Milky Way will collide with to form one larger Galaxy).  So, everything moves.  Even if it looks like it doesn't.
242501
Relative motion between objects was not new to Newton and his contemporary scientists.\n\nEvery experimental result proved and confirmed again and again Newton’s laws of motion even in the frames of reference moving relative to each other.\n\nLatter with the improvement in measuring technology and particles moving nearly equal to the velocity of light, their behavior contradicted Newton’s laws.\n\nEinstein with a firm belief that any physical law (including Newton's law) must be the same in any reference frame moving relative to each other postulated that speed of light is independent of the source of light.\n\nThis explained every experimental result well and obeyed Newton’s laws.\n\nThe only difference is that  the basic quantities mass , length and time which during Newton’s time was thought of absolute in the sense that  they will not change with the velocity of the frame in which they are , is now considered to vary with the speed of the reference frame, and the velocity of light is independent of the motion of the source.
77099
it would be called superluminal.\n\nyou can read more about it here - \nhttp://en.wikipedia.org/wiki/Faster_than_light
619887
How can I answer that?  If I didn't know then how could I tell you what I didn't know?\n\nHowever, the ten thing that I do know that I think are interesting are:\n\nThe speed of light is constant no matter the respective velocities of the source and the observer.\n\nThe rate of time passage is not constant.\n\nHot water will freeze faster than cool water.\n\ne=mc^2\n\nWhen you stand still, you are moving in 8 different directions.\n\nL’ = L sqrt(1-v^2/c^2)\n\nM’ = M/sqrt(1-v^2/c^2)\n\nAbsolute Zero (not my physics grade, but close)\n\nClarke's Orbit\n\nAvagadro'w number\n\nPlanket's Constant\n\nThe Big Bang\n\nBlack Holes\n\nString Theory\n\nGravity Bends Light (not really, but the effect is the same)\n\nNeutrinos\n\nGirls (Ok, that's biology not physics, but I am listing more than 10 anyway)\n\nThe expanding universe\n\nrefraction of light\n\nSpectrograms\n\nAnd, I could go on and on.  When you look deep enough to understand things, it is all interesting.
347791
Your example is actually a good one for how we know no one can travel faster than the speed of light.  The people on the boat would measure the same speed for the beam of light as those standing on the shore.\n\nEinstein's theory of special relativity states that, regardless of your motion, light's speed remains constant.  Space and time warp in order to make light's speed constant from all perspectives.  As matter gets closer and closer to the speed of light, space stretches and time contracts, requiring infinite energy to reach the speed of light.  Only things with no mass (i. e. energy only) can travel at the speed of light, and nothing we know of can go faster.\n\nHowever, Einstein's theories do suggest that we could bend space and time to allow us to get around the speed of light.  We can't travel a trillion miles per second, but, with enough energy, we could shift some spot a trillion miles away to right next to us.
799784
In theory, yes.  Because the light particles that were traveling slower than you would have been slower.  Very good question.  However, speeding past the speed of light would mean the fundamentals of E = Mc^2, would cease to make sense.
24020
Neglecting the lengthy derivation of why light speed travel is impossible, the first postulate of relativity is that light has the same velocity regardless of how fast you're moving.  So, at ANY speed you can achieve in your car, when you turn the lights on you'll see the waves moving away from you very very quickly.
1170536
I will attempt an answer that is somewhat factual.  \n\nThe theory does not stipulate that nothing can travel faster than the speed of light.  It stipulates that matter can not approach the speed of light.  In fact a hypothetical partical called a tachyon has been named that has a speed faster than the speed of light.  That partical has an imaginary mass. \n\nThe entire work comes from the equations developed by Einsten,\n\nE= MC^2/ sq rt (1- c^2/v^2)
711259
I have always been a fan of this discussion. I propose that ANYTHING can travel faster than the speed of light (SOL). I also believe that was the driving force behind Einstein's assumptions. When we start talking about time slowing down as we approach SOL, we are talking about time RELATIVE to something else...If two people are together on a SOL spacecraft, then the time they experience is the same as if they were standing still. \n\nBefore I ramble on too much, What stops light from going any faster? If we had a theoretical spacecraft that could travel at 1/2 SOL, and we shot a laser off the front of it, how fast would the light be travelling? To the observers on the SOL-craft, the light would be travelling at absolute SOL, but to someone standing next to the craft when the laser was activated, would the light NOT be travelling at 1.5 SOL?
270366
In a sense, time may be relative to size.  If you look at a time dilation equation, t' = t*(1/(1-(v^2)/(c^2))^(1/2)) you can see that time is related to velocity.  The larger (i.e. more massive) an object it, the more space it displaces and the greater the gavitational (velocity) effect.\n\n---EDIT---\n\nThe more massive an object the greater it's gravitational force.  From experimental fact, the speed of gravity is much faster than light. (Yes, I know, this contradicts Special Relativity, but that's another discussion.)\n\nAdditionally, the mass-square of the electron neutrino, measured from tritium beta decay experiments, has been found to be negative. (Yes, this too contradicts Special Relativity.)\n\nSo, from one extreme to the other, time is only relevent for a object that is moving, not for a stationary object.\n\nThat being said, there is nothing that tells us how time flows for a "moving" observer, only for a "fixed" observer.  Therefore, time must actually a constant.
271484
yeah, you get a bit off on a deep tangent towards the end there... but I don't think Einstein asserted anything more than the idea that time, space, and matter are related and not independant.  two systems of matter travelling in wildly different velocities relative to one another also have different processions of time and different masses, from the perspective of the other.  It only applies to things moving close to the speed of light- I think it's the Schroedinger Equation that covers the inter-relation.  But there's no real analogy to it in the observable world, unless you work at a particle accelerator, so I leave it to the doctorates of physics to debate.