# Questions tagged [speed-of-light]

The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.

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### Is it possible for information to be transmitted faster than light by using a rigid pole?

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 ...
7answers
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### If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?

Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm ...
8answers
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### What really causes light/photons to appear slower in media?

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 ...
8answers
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### Would time freeze if you could travel at the speed of light?

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 ...
4answers
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### If a mass moves close to the speed of light, does it turn into a black hole?

I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever ...
16answers
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### How does gravity escape a black hole?

My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
12answers
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### Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
5answers
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### Does a photon in vacuum have a rest frame?

Quite a few of the questions given on this site mention a photon in vacuum having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at ...
4answers
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### What is the mechanism behind the slowdown of light/photons in a transparent medium?

So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the "...
5answers
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### Why is a black hole black?

In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a ...
6answers
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Let's say we have $2$ particles facing each other and each traveling (almost) at speed of light. Let's say I'm sitting on #$1$ particle so in my point of view #$2$ particle's speed is (almost) $c+c=... 5answers 7k views ### What is so special about speed of light in vacuum? I will try to be as explanatory as possible with my question. Please also note that I have done my share of googling and I am looking for simple language preferable with some example so that I can get ... 3answers 94k views ### How is light affected by gravity? Light is clearly affected by gravity, just think about a black hole, but light supposedly has no mass and gravity only affects objects with mass. On the other hand, if light does have mass then doesn'... 9answers 44k views ### How can a photon have no mass and still travel at the speed of light? I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make ... 7answers 11k views ### Why is there no absolute maximum temperature? If temperature makes particles vibrate faster, and movement is limited by the speed of light, then temperature must be limited as well I would assume. Why there is no limits? 8answers 6k views ### What is the proof that the universal constants ($G$,$\hbar$,$\ldots$) are really constant in time and space? Cavendish measured the gravitation constant$G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain ... 13answers 126k views ### Why does the (relativistic) mass of an object increase when its speed approaches that of light? I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy. However, the equation breaks down when the ... 3answers 15k views ### How does light speed up after coming out of a glass slab? As I learned today in school, my teacher told me that when light enters a glass slab it slows down due to the change in density and it speeds up as it goes out of the glass slab. This causes a lateral ... 5answers 14k views ### Why are objects at rest in motion through spacetime at the speed of light? I read that an object at rest has such a stupendous amount of energy,$E=mc^2$because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ... 4answers 6k views ### Speed of light in a gravitational field? How do I solve the speed of light in gravitational field? Should I just add gravitational acceleration in speed of light? $$c'=c_0+g(r)t~?$$ 6answers 11k views ### Do photons have acceleration? Photons travel at the fastest speed in our universe, the speed of light. Do photons have acceleration? 4answers 4k views ### How does a photon experience space and time? To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere. ... 9answers 27k views ### Accelerating particles to speeds infinitesimally close to the speed of light? I'm in a freshmen level physics class now, so I don't know much, but something I heard today intrigued me. My TA was talking about how at the research facility he worked at, they were able to ... 6answers 9k views ### If I am travelling on a car at around 60 km/h, and I shine a light, does that mean that the light is travelling faster than the speed of light? The title says it all. If I was on a bus at 60 km/h, and I started walking on the bus at a steady pace of 5 km/h, then I'd technically be moving at 65 km/h, right? So my son posed me an interesting ... 3answers 11k views ### Is it really possible to break the speed of light by flicking your wrist with a laser pointer? Minutephysics has a popular YouTube video 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 ... 6answers 2k views ### Does a photon instantaneously gain$c$speed when emitted from an electron? An excited electron looses energy in the form of radiations. The radiation constitutes photons which move at a speed$c$. But, is the process of conversion of the energy of the electron into the ... 5answers 4k views ### Will an object always fall at an infinite speed in a black hole? [duplicate] Most of you if not everybody will agree that the stronger the gravitational pull, the faster an object will fall. For example, on a planet with 50 times the gravity of Earth, any object will hit the ... 9answers 4k views ### Does the (relativistic) mass change? Why? I learned recently that when an object moves with a velocity comparable to the velocity of light the (relativistic) mass changes. How does this alteration take place? 4answers 4k views ### Will free-fall object into black hole exceed speed of light$c$before hitting black hole surface? In Newtonian mechanics, if we throw an object in against direction of gravity with speed$v$and it achieve max height of$h$. Now if we allow object to fall from that height$h$, it will eventually ... 3answers 8k views ### Is a photon “fixed in spacetime”? From what I've read, according to relativity, a photon does not "experience" the passage of time. (Can we say there is no past/present/future for a photon?) Would it be better to say a photon is "... 19answers 4k views ### The origin of the value of speed of light in vacuum Meaning, why is it the exact number that it is? Why not 2x10^8 m/s instead of 3? Does it have something to do with the mass, size or behavior of a photon? To be clear, I'm not asking "how we ... 3answers 10k views ### Does$E = mc^2$apply to photons? Photons are massless, but if$m = 0$and$E=mc^2$, then$E = 0c^2 = 0$. This would say that photons have no energy, which is not true. However, given the formula$E = ℎf$, a photon does have energy ... 8answers 13k views ### What happens when a photon hits a mirror? When a photon of light hits a mirror does the exact same photon of light bounce back or is it absorbed then one with the same properties emitted? If the same one is bounced back does it's velocity ... 3answers 3k views ### Extended Rigid Bodies in Special Relativity I was reading Landau & Lifshitz's Classical Theory of Fields and I noticed that they mention that an extended rigid body isn't "relativistically correct". For example, if you consider a rigid ... 4answers 8k views ### Do all massless particles (e.g. photon, graviton, gluon) necessarily have the same speed$c$? I suppose there was a discussion already on speed-of-gravity-and-speed-of-light. But I silly wonder whether all the massless mediators of four fundamental forces, i.e. Graviton:$g_{\mu\nu}$(... 3answers 1k views ### GR. Einstein's 1911 Paper: On the Influence of Gravitation on the Propagation of Light Regarding the paper, what does Einstein means when he says: "If we call the velocity of light at the origin of co-ordinates$c_0$, then the velocity of light$c$at a location with the gravitation ... 4answers 715 views ### Speed of light travel I have just started special theory of relativity. The limiting speed known as speed of light fascinated me most. I asked my teacher: Consider two massless objects moving in the same direction at ... 3answers 166k views ### Why does wavelength change as light enters a different medium? When light waves enter a medium of higher refractive index than the previous, why is it that: Its wavelength decreases? The frequency of it has to stay the same? 4answers 2k views ### “Speed” of Gravity and Speed of Light Some threads here touching speed of gravity made me think about that. This lead to some questions. The speed of gravity was not measured until today (at least there are no undebated papers to that ... 2answers 4k views ### Has the speed of light changed over time? Could someone judge my (stoner) hypothesis that the speed of light has changed over time -- i.e. as the universe has expanded in volume light has slowed down, perhaps going so far as back to the big ... 4answers 5k views ### What's so special about the speed of light? [duplicate] What's so special about the speed of light? Why do many equations in physics include the speed of light in vacuum$c$? Why do so many thing depend upon it? Why can't it be the speed of sound? ... 31answers 40k views ### Do we know why there is a speed limit in our universe? This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is? I'm not asking why the speed limit is equal to$c$... 5answers 2k views ### Special Relativity Second Postulate That the speed of light is constant for all inertial frames is the second postulate of special relativity but this does not means that nothing can travel faster than light. so is it possible the ... 4answers 1k views ### At what speed does our universe expand? Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand? 2answers 10k views ### Does the speed of light vary in non-inertial frames? The speed of light is the same in all inertial frames. Does it change from a non-inertial frame to another? Can it be zero? If it is not constant in non-inertial frames, is it still bounded from ... 3answers 6k views ### In superluminal phase velocities, what is it that is traveling faster than light? 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 ... 11answers 30k views ### If all motion is relative, how does light have a finite speed? I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds ... 3answers 2k views ### The reference frame of$c\$

I don't have a lot of knowledge of special relativity and associated topics; some of the few things I know are that "all motion is relative" (that is, there is no 'stationary reference frame'), and ...
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### Does gravity slow the speed that light travels?

Does gravity slow the speed that light travels? Can we actual measure the time it takes light from the sun to reach us? Is that light delayed as it climbs out of the sun's gravity well?
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### Would it be possible to develop special relativity without knowing about light?

My understanding of special relativity is that it is fundamentally based on the constancy of the speed of electromagnetic radiation - that this speed is a physical law (or derivable from physical laws ...