Take the image below for example:  For the shadow to travel from point A to B would be extremly fast on the surface on the moon but probably not breaking the speed of light. But if the moon were 20 times larger then it would appear that the shadow would be travelling 20 times faster appearing to break the speed of light on the surface of the moon.  So in essence the bigger an object is the quicker that shadow would appear to move on the surface of that object.  So what does this mean in terms of breaking the speed of light?  Also for interesting purposes, what is the speed of dark (if it even has a speed?)
40815
Minutephysics has a popular  called "How to break the speed of light". In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of light on the moon will travel 20 times the speed of light. Now don't get me wrong, I do like their videos, just this one seemed a bit fishy to me. At first I thought it all practically made sense, then I realised something...  In my mind, I would think that light particles (photons) travel from the laser to the moon and bounces off the moon and back to your eye (it doesn't just stay there, in place, so you can't move it around). Now, what he is stating is that if you flick your wrist these photons that have travelled to the moon will move along with your wrist. Wouldn't these photons be bouncing off of other objects or still travelling to the moon by the time you flick your wrist? i.e. dissipating, therefore new photons will be travelling to the moon (from the laser directly).  For example: let's say you point the laser at the moon, and once it reaches the moon, you wait a couple of seconds and then flick your wrist. The laser that you have flicked will emit photons in every direction that your wrist was in, correct? i.e. The photons would shoot out in a straight line (unless disrupted) continuously, with your wrist taking no affect on the speed of the photons.  So back to the question, is this video wrong?
8822
First of all, I know that light does indeed travel slower in a medium like air or water, but that's because the photons are bouncing off of the medium's particles and in different directions so the light beam as a whole is slowed down (or at least that's the classical explanation I've read somewhere).  But my question is about light traveling freely in the vacuum. What if an astronaut during a spacewalk switches on a flashlight towards the opposite direction he is moving. Will that light travel slower than normal?  If not, is there another way to make light move slower? For example, can you make a beam of light move so slow that you can visually perceive it being affected by gravity?
2809
Is it possible for information (like 1 and 0s) to be transmitted faster than light?   For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on his/her end.   The person on the opposite end should receive the pushes and pulls instantaneously as no particle is making the full journey.  Would this actually work?
55561
Is it true that the light from some galaxies will never reach us?   The explanation for that is that the Universe expanding faster than the speed of light. But, if the speed of light is constant in all reference frames, then in the reference frame of a galaxy, light must move at the speed of light and thus it must reach us (?).   Could you also explain  how is it possible that the Universe is expanding faster than the speed of light if an object can never be observed to move faster than light? (without Doppler effects).
9068
I know that if we solve the Maxwell equation, we will end up with the phase velocity of light being related to the permeability and the permittivity of the material. But this is not what I'm interested in - I want to go deeper than that. We know that the real speed of light is actually not changing, the decrease in speed is just apparent. Material is mostly empty, the light will still travel with $c$ in the spacing. The rare atoms will disturb the light in some way. So I am interested in how the atoms affect the light. Photon absorption-emission theory Some textbooks that I read explain it in a way kind of like this: In a material the photons are absorbed by atom and then re-emitted a short time later, then they travel a short distance to the next atom and get absorbed&amp;emitted again and so on. How quickly the atoms in a material can absorb and re-emit the photon and how dense the atoms decides the apparent speed of light in that material. So the light appears slower because it has a smaller “drift speed”. Interference theory But recently I realize an alternative explanation: Atoms respond to the light by radiating electromagnetic wave. This “new light” interferes with the “old light” in some way that results in delayed light (advanced in phase), this can easily be shown by using simple phasor diagram. Consequently effectively the light covers a smaller phase each second, which gives the impression of a lower phase velocity. However the group velocity is changing in a complicated way. I think that the first explanation does not explain the change in phase velocity of light. if we consider light travelling into a slab of negative refractive index non-dispersive material, let’s say the light is directed perpendicular to the slab. The phase velocity’s direction will be flipped, but group velocity’s direction in the material will not change. Only the second explanation can explain the flipped phase velocity direction. I guess that the velocity that we get in the first explanation is actually belongs to the group velocity. It makes sense to me that the front most of the photon stream determines the first information that the light delivers. So the question is What really cause the phase velocity of light to be decreased?  &quot;drift velocity&quot; of photons (they aren't the same photons, they are re-emitted all the time) phase difference between absorbed and emitted light something else  And also, I still don't really understand detailed explanation of the absorption-emission process for small light's wavelength (for large lambda compare to the atoms spacing, the photons will be absorbed by the phonons). The dispersion relation that we know is continuous and also some material is non-dispersive, therefore the absorption process must occur in all frequency for a certain range. So definitely it doesn't involve the atomic transition, otherwise it will be quantized. My guess is that the relevant absorption process gets smooth out by the dipole moment. What makes the spectrum continuous? EDIT: link for dispersion relation:
7705
I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy.     However, the equation breaks down when the electron velocity approaches the speed of light as mass increases. At such velocities, one needs to do relativistic correction to the mass so that it becomes[...]   We all know about the famous theory of relativity, but I couldn't quite grasp the "why" of its concepts yet. This might shed new light on what I already know about time slowing down for me if I move faster.  Why does the (relativistic) mass of an object increase when its speed approaches that of light?
55552
I know that as the velocity increases, the mass of the object also increases so it becomes tougher and tougher to move the object which ultimately leads to a requirement of infinite energy to accelerate an object to the speed of light. But I have a doubt.  As far as I know only the observable mass (Relativistic mass) increases but not the Proper Mass or Intrinsic Mass, right? The actual mass of the object will remain the same. So if the actual mass ($m_0)$ remains the same and only the observable mass increases, why is more and more energy required? I know that even if the object exceeds the speed of light, we will not be able to say that it is moving faster than the speed of light but is it possible to make it move faster than light?  If rest mass does not change with $v$ then why is infinite energy required to accelerate an object to the speed of light?  I quote Igor Ivanov (a stackexchange user from this question ) to give an insight into my question.     The mass (the true mass which physicists actually deal with when they calculate something concerning relativistic particles) does not change with velocity. The mass (the true mass!) is an intrinsic property of a body, and it does not depends on the observer's frame of reference. I strongly suggest to  popular article by Lev Okun, where he calls the concept of relativistic mass a "pedagogical virus".   EDIT  After reading the answers I have a doubt.  SO can I say that only the overall energy of the system increases while the mass remains constant? But then if mass remains constant, then why is more and more energy gradually required? I mean there should be a reason for that requirement of infinite energy.
2041
I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit.    So, if I were to travel in a spacecraft at (practically) the speed of light, would I freeze and stop moving?  Would the universe around me freeze and stop moving?  Who would the time stop for?
86783
I was reading this article this morning:    Scientists have discovered a gamma-ray burst whose light has taken 13.14 billion years to reach Earth. This much is cool and interesting. However, the assumption is then stated that this is "the most distant single object yet spied by a telescope".  But hold on a minute. It is also known that galaxies are moving away from each other at incredible speeds, most faster than the speed of light, because the space/time between them is expanding (or something like that... I'm no scientist!)  So these so-called amazingly distant objects, well for starters, they don't exist any more... They are things that happened billions of years ago. But not only that, they are probably objects that were only a few million light years away from us when they actually took place. So surely then, the objects themselves aren't the most distant ones, but the light from them has been distorted such that the light has taken that long to reach us?  Furthermore, if the galaxies are spreading out faster than the speed of light, who is to say this explosion actually happened 13.1 billion years ago? Isn't it possible that the light was created say 5 billion years ago, but has taken much longer to reach us because of the expanding space between the galaxies?  I'm sure this stuff has already been considered by scientists, but I find it weird the way news articles always assume that just because light travels at a specific speed, that it's always going to take the same amount of time to reach us.  Or am I getting it wrong? I'd love to know!  EDIT: For people discussing the whole faster-than-light-speed thing, I came across :     As you look at galaxies further and further away, they appear to be moving faster and faster away from us. And it is possible that they could eventually appear to be moving away from us faster than light. At that point, light leaving the distant galaxy would never reach us.   When that happens, the distant galaxy would just fade away as the last of the photons reached Earth, and then we would never know it was ever there.   And :      That mysterious dark energy force, which is accelerating the expansion of the Universe is making the most distant galaxies move faster and faster away from us. Eventually, they will cross an event horizon and appear to be moving away from us faster than the speed of light. At this point, any light emitted by the galaxy will cease to reach us. Any galaxy that crosses this horizon will fade away from view, until its last photon reaches us. All galaxies will disappear from view forever.
24412
The question is the well-known      If there are so many/infinite stars, in every direction, why is it dark at night? Where goes all the light we don't see?    Discussing with a friend we found different answers.   The first (my friend's answer) is that given the fact that in a finite amount of time (in this case, the age of the universe), light can travel only a certain, finite, distance, we cannot see, if they actually exist, stars farther than such distance (about 13 billions of years, I guess). Then he states (or better, I got him notice he implicitly does this)  that within this radius there aren't enough stars to fill the night sky, and that's why we have darkness without sun's light.   The second answer (mine, or anyway the one I heard from a minutephysics YouTube video) is that despite of the big size of the observable universe, cosmological expansion red shifts most of the light out of the visible radiation range, so that the majority of light can't be seen by our eyes at night, while if we use, for example, an infrared camera, we should see a brighter sky, and lenghtening more and more the wavelength at which we observe the sky, we come to the uniform background radiation.  So my question is, what is the right answer? Is it one of the above-mentioned or even a third one?  EDIT: I've seen both answer may be acceptable, but what is the main contributing phenomenon?
104305
I understand that information cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but the actual content is carried in the modulated wave, the sidebands $\omega_c\pm\omega_m$. The modulation envelop has its group velocity and this is the speed at which information is being transmitted. We also know, for example, that x-rays in glass have a phase velocity which is greater than the speed of light in vacuum.   My question is, what exactly is it that is travelling faster than the speed of light?    EDIT: I know it couldn't be anything physical as its superluminal. My problem is, what is it that has absolutely no information content yet we associate a velocity with it.
120079
It is a well known fact, that nothing can travel faster than light. I have discussed that fun fact numerous times with my 8 year old son. One day he asked "why?", and I realized that I didn't know the answer myself. So... Why is the speed of light the upper speed limit of everything?  Can it be explained in a way that an 8 year old can understand?
90480
The light we detect today in our telescopes from distant stars is really old.  If we could travel fast (lets say, 90% the speed of light) towards a star a million lightyears away, all the time looking straight at it, would we see that star's entire history unfolds in fast-forward ▶▶ until we reach it? (if it's still there, of course).   It seems reasonable that "newer" photons emitted from that star would hit our eyes at a faster rate while we approach it.  Thanks.
103943
I just learned that, according to Einstein's relativity theory, time reaches zero for an observer (light) when traveling at the speed of light, so everything is supposed to be at the same place in the universe for light. But why does it take 8 minutes for light to travel from the Sun to Earth? Is it because we are observing it from earth? At 300.000 km/s distances in the universe are hardly zero? I can't seem to grasp this.
6830
Does it make sense to say, "The speed of light varies?"  Some may say right off the bat "Yes, it changes as a wave passes through a different medium."  However, I'd like to say no, because when I hear someone say the speed of light, I always think of the constant $c$ (unless the medium is specified to not be a vacuum, but then it isn't $c$ anymore), not the speed of a particular wave.  To me, it makes more sense to say something like, "The speed of a particular wave varies." What is the correct way to state this in the professional world? And in general, when professional physicists say "the speed of light," are they referring to the constant or the actual speed of the wave?
106628
I've been trying to learn about the speed of light and time dilation, but I'm at an impasse.  The presented facts say that if I travel around the solar system at 50% the speed of light and then come back to earth I will have experienced less local passage of time than them. I will effectively have traveled to their future. I've also read that gravity causes time dilation too.  I understand that space-time is a sort of unified thing and that affecting space affects time. It makes sense then that gravitational forces bending space will also bend time. But velocity? I can't wrap my head around it and I can't find a good explanation for it.  People cite orbiting craft and planes as proof of time dilation, since their clocks will go slower than those in the surface (and vice-versa). This certainly explains gravitational time-dilation, but not necessarily velocity. Can't the change in local time passage be caused solely by the gravitational bending of space-time?   Trying to find an answer, I came to a very recurring and frustrating example in texts that seek to explain time dilation. A man on a moving train throws a ball forward. Since he's moving with the train and the train is his point of reference, the ball to him only moves at the speed he threw it. But to a woman on the station the ball is moving at the speed of the train plus the speed it was thrown with. To some authors, this seems to open the mind to time-dilation understanding. To me it only explains the relative nature of motion. It says nothing of time.  Another example I've found: if person A speeds away from person B very quickly, A's clock will seem to advance slower from B's point of view. How is this time dilation, though? The difference can be explained by the longer time it takes light to get to B, can't it?  I realize I can't be right against the fine physicists out there, so I was hoping someone here could enlighten me. Where does the notion that velocity causes time-dilation come from?
64486
Consider the scenario where you measure the time it takes for light to travel to the left 10 meters and to the right 10 meters. Both measurements will take the same time, even though we are moving through space at crazy speeds. This must mean that light is not moving relative to "space" as a whole. What does it move relative to? The light emitter? If so, try shooting two beams of light 10 meters from the wall. The first time the emitter is stationary; the second time it is moving at 100 m/s. Am I mistaken in thinking that it would hit the wall slightly faster? Wouldn't this light be moving faster than the light emitted from the stationary source?