Does light accelerate; does it start from a standsill and eventually reach, well, the speed of light?
90556
Always exactly the same speed, c = 299 792 458 meters/second, no acceleration or deceleration or anything.\n\nThe *apparent* speed of light through glass is less than c, but that is because the photons (the light) bounce off all the atoms and that slows down the speed through the glass. The speed of light from one atom to the next is exactly c.\n\nAlso, sometimes you will read about experiments where the popular press version says some bunch of light has been made to faster than c, but this is just the phase speed and so isn't quite the same thing. You can't send any information faster than c with this method.\n\nThere is an apparent something that goes faster than light in what is called the EPR-effect, a wierd property of quantum mechanics often misleadingly called "quantum teleportation," but again, no information travels faster than c.\n\nThere are hypothetical particles called tachyons which *always* go faster than light - they can't go slower. But no evidence for these have been found and most physicists think they can't exist.\n\nAs for warp drives and other faster-than-light science fiction type things, that, in my opinion isn't possible. Certainly everything we know about physics and the universe now says it is impossible, and it is unlikely that any future theory will show we are so wrong now that we missed this one. Also if FTL is possible then you can have a sort of time travel, and then cause and effect get ruines and, well, its a mess.\n\nBe glad that c is absolutely the same to all observers in every situation - the universe wouldn't work without it.
411862
According to Einstein's theories, if you took a particle with a non-zero mass mi on the earth, and accelerated it to a speed v respect to the earth, then the mass would increase according to something like m = mi/(1-v^2/c^2), where c is the speed of light.  There may be a square root in there.  The point is that the mass becomes infinite as v approaches c, because the denominator goes to zero.  As the mass increases it takes more and more energy to accelerate it, so that the amount of energy required getting to the speed of light becomes infinite.  Clearly, there is not an infinite amount of energy available for this experiment.\n\nBut, if Einstein was wrong then it might be possible to do.
979220
Albert Einstein answered this question a long time ago.  He theorized (and every experiment since then has borne him out) that the speed of light is constant from ANY inertial (non-accelerating) frame of reference.  This includes the speed of light relative to your headlights.  \n\nLight speed is constant.  What changes is time and length.  Weird but true.
144613
light rays do not move faster or slower than the speed of light.  the speed of light is the speed of light, period.\n\nthat said, light often travels *slower* than c, which is a constant defined as the speed of light IN A VACUUM, approximately 3x10^8 m/s.  here is a way you can actually observe some honest-to-god light traveling slower than c:\n\nstep 1.  find a window.\nstep 2.  look at it.\n\ncongratulations, you have observed light traveling slower than c.  in ANY medium (air, glass, water - you name it) the speed of light is slower than the speed of light in a vacuum.  \n\nin fact, the so-called index of refraction (n, which equals 1.4 for glass) is defined as the ratio of c to the speed of light in that material.  generally, denser materials have a higher index of refraction, and people have been able to make light go REALLY slow by making some very dense materials.  all of this is perfectly consistent with general relativity.\n\nas for FASTER than c:  under certain conditions, it is possible to make photons with a "phase velocity" that is greater than c.  however, when people speak of the speed of light, they usually refer to the "group velocity".  the distinction between phase velocity and group velocity is kind of technical; but phase velocity refers to the speed at which the individual phase fronts propagate through space.  \n\nit is possible to make these phase fronts go faster than c; but because the energy or information or whatever carried by the light travels at the group velocity, the actual speed (as far as stuff like general relativity is concerned) is still less than c.\n\nanyway, all of these results are entirely consistent with general relativity.  in fact, i don't know of any published results of a scientific experiment which are not consistent with GR.  \n\nthe fact that einstein's theory, largely concocted out of the blue from basic principles and logical reasoning, has stood the test of time for so many years is a testament to the genius of the man's work.\n\nhere are some links to learn about phase velocity, group velocity and the speed of light:\nhttp://en.wikipedia.org/wiki/Group_velocity\nhttp://physicsweb.org/articles/news/4/7/8/1
11066
well first, read my other answer to your question about the speed of sound.  You would probably expect the answer to be the same, and the light to reach the other car, but this is a special circumstance because nothing can travel faster than the speed of light, not even light itself.  when a beam of light is shone ahead of a rapidly moving space vehicle, and is expected to be moving at the speed of light plus the speed of the vehicle, it is still only moving at the speed of light, which is puzzling.  Physicists have defined the speed of light to be the ultimate cutoff point for speed.\n\nAnd also, you wouldn't be able to travel at the speed of light anyway because time would probably stop since it does get slower and slower the faster you go.  Velocity warps time, the faster you go, the slower time moves for you relative to a stationary object.  I've read this, and it is definately true, we had to derive the formula in physics from, and calculate the time, lenghth(which decreases with speed), and mass (which increases with speed) distortion as well.  The effects of this are very negligable unless you are moving at speeds very very close to the speed of light, at least over 90% of the speed of light, and we haven't gotten anywhere close yet.  The denominator of this factor that time, length, and mass are distorted by actually is 1 minus the square root of the speed of light(c) over the speed in which the object is traveling (v), so the reason you can't go faster than the speed of light is because if v was greater than c, then that square root would be greater than one, meaning the denominator would be negative, meaning the factor would be negative, and the light would actually be travelling backwards in time...\n\nIf you find any of this interesting, I highly recommend "The Elegant Universe" by Brian Greene.
954004
No, not at all.\n\nProcesses in perfectly stationary nuclei turn mass into energy. For instance, the energy an emitted alpha particle has in alpha decay from a staionary nucleus is reflected by a tiny fall in mass of the nucleus.\n\nConversely, as you accelerate a particle towards the speed of light, more and more of the energy you give to the particle gets converted to mass. In fact, you can never get the particle to the speed of light - its mass will go to infinity first. This is quite measurable in a particle accelerator.
706572
The answers already given are generally correct for light travelling in a vacuum. However, the speed of light slows down when it travels through any material substance (the exact amount depends on the material).
75582
Tehnically, yes. Scientists succeeded in sending light through a light accelerator. Light beams are already going the speed of light so any acceleration would cause them to jump past the speed of light. The resuly was that the light actually came out the other end before it went in. This was actually measured. I heard this on NPR radio a couple of years ago.
292196
Particles travelling at the speed of light cannot be decelerated; any such particle is massless. Since mass is the constant of proportionality between force and acceleration, no force can cause any acceleration of such a particle. \nEinstein's fundamental equation states that the energy E of a particle is the magnitude of its 4-momentum E^2 = (mc^2)^2 - (pc)^2 = (mc^2)^2/[1-(v/c)^2] where p is the magnitude of its spatial momentum vector, m is the particle's inertial mass and c is the speed of light in vacuum. Thus (v/c)^2 = 1 -  (mc^2)^2/[(mc^2)^2 - (pc)^2]. Letting v/c=1, we get mc^2 = 0, which implies m=0.\nNote that this does not contradict general relativity; light particles lose energy in gravitational fields instead of decelerating. Also, light "slowing down" in a medium is due to photons being absorbed and reemitted; the speed of a photon is always c.
329962
Here is the correct answer....the speed of light is constant IN A VACUUM.  Light is most certainly slowed down by matter, and can be shifted by gravity.
321687
OK, to answer this question, we first have to make a few important assumptions, otherwise the question is meaningless.\nFirst, we have to assume that you can accelerate your car to the speed of light....this is not actually possible since it would have an infinfity mass and thius take an infinite amount of energy to achiece.  Secondly, wehave to assume that at the speed of light, your head lights will opperate noramlly (i.e. you turn them on and light shines out) due to [for reasons beyond the scope of this answer] the way head lights actually work.\n\nI would also like to preface my answer with say that traveling AT the speed of light is much different than traveling just UNDER the speed of light.\n\nOne VERY important thing to remember is that in ALL frames of reference, the speed of light is constant.\nNo matter who is watching the light travel and measuring its speed, it will always come out to be the exact some value, 299,792,458 m/s exactly.\n\nWhen you, inside your car traveling at the speed of light (with respect to some "stationary" observer), swtich on your head lights and look out the wind shield, you WILL see light traveling away from you at the speed of light.\n\nIf this "stationary" observer (by which I mean a someone who you are traveling past at the speed of light) watches all this happen, both you and the light are still traveling at the same speed in his reference frame....the speed of light.\n\nSo the answer depends on who's frame of reference your talking about.  From your point of view, yes, you would see the light traveling away from you at the speed of light as if nothing was out of the ordinary.\nFrom the "Stationary" person's reference frame, you and the light would be keeping up with eachother, neither moving infront or falling behind the other one...tied.
2702
Light has nothing to do with space. It is a property of the electromagnetic field when this field is undergoing changes. from its point of origin, this field radiates out into space in all directions. It is invisible, but can be felt by charged particles. When this field is changed, for instance by suddenly altering the position of its emitter, a kink develops in the field which propagates out into space at the speed of light. Under the right circumstances of acceleration, this kink becomes electromagnetic radiation - light. So space simply acts as a passive container for the electromagnetic field, which in turn is the 'medium' in which light exists.
518583
Let’s start with what Einstein actually said… His 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 certainly 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. 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\nHere’s where the infinite thing comes in.  Because the laws of physics must remain the same (remember the first postulate), then the changes in length and time at high velocities also start to affect the measurements of momentum and of energy—and since E=mc^2, mass and energy are equivalent, so mass itself has to change.  The rate of change has a term in the denominator that’s the square root of (1-(v/c)^2), so if the velocity (v) approaches the speed of light (c), then the denominator approaches zero; but if the denominator of a fraction approaches zero, then the total fraction approaches infinity.  Thus, you start to talk about problematic things such as infinite mass and energy and size.\n\nBut remember that nothing can actually travel at the velocity of light, so although this fraction should get very big, it shouldn’t actually become infinite.\n\n\nRyan Wyatt\nRose Center for Earth & Space\nNew York, New York
44895
Interesting quatum phenomenon question.  The lights would come on, but they might appear strange to an observer due to the Doppler effect.  You, inside the car, would continue to see a headlight 'beam' because light's speed is a universal constant and not dependent on the speed of the source.  However, traveling beyond the speed of light in conventional space is not possible, to our knowledge.  For one, it would require infinate energy.  It would also cause macro-scale collapses in casaulity (effects happening before the cause).  To date, these bizzare effects have only been seen on quantum scales.
718668
Neglecting the fact that it would be imposible to accelate a train (non-zero rest mass) to the speed of light,\nif you did, an observer in the train would still see light moving away from them at the speed of light.\nThe speed of light is constant in all frames of reference.
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
724874
Answer is yes.  By "seeing", what we really mean is our eyes receiving light either reflected or emitted from an object.  It doesn't matter how fast the object is moving, or if it's approaching or receeding - the speed of light is constant, therefore you will see it both coming and going.  That's one of the beautys of relativity - the speed of light is not relative to the object it's coming from, but instead is fixed and will measure the same speed for all observers in the universe, moving or not.