# Tagged Questions

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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### Will relative velocity of light exceed $c$? [duplicate]

Suppose I'm standing a few light minutes away from a light emitting object, as soon as the light is turned on, I project myself towards the object with some speed. Will the relative velocity of my ...
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### How does the propagation of gravity work for photons?

As explained in the answers to this post, photons apparently exert a gravitational pull on other objects. It has also been explained on this site, that gravity propagates at the speed of light. I'm ...
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I have tried asking this question a number of different ways but I seem to keep messing it up. Please let me try another way. I know that the number of times a photon oscillates per second (it's ...
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### Two “devil's advocate” questions related to LIGO measurement results interpretation

"If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things." Rene Descartes Laymen like me typically refers to Wikipedia ...
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### How can a photon oscillate when it experiences no time? [duplicate]

How can light experience change if it is motionless in time?
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### Why is there a cap on the speed at which we can attain? [duplicate]

Also, If you are traveling 1 mph under the speed of light on a train and throw a baseball in front of you at 20mph what would a viewer outside the train see?
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### How does light change directions? [duplicate]

To the best of my knowledge, light will always travel at the speed of light. How then does it get reflected and change directions? Wouldn't it have to decelerate, come to a stop, and then accelerate ...
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### Does anything exist between singularity and the event horizon of a black hole

It is said that anything that falls into the event horizon, quickly reaches the singularity. That would imply - there should be nothing between event horizon, and the singularity. Is that true? If not,...
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### Speed of light and infinite energy [duplicate]

While browsing some physics websites, I saw that to make an object reach the speed of light, it requires infinite energy and talked about its relation with Einstein's famous equation $E=mc²$. However, ...
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### Newton's gravity vs Einstein's gravity

Hello so if i were to simplify the difference between gravity as seen from Newton and gravity as seen from Einstein, could i say that Einstein's gravity is just adding the fact that gravity does not ...
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### Event horizon, and Hawking radiation [duplicate]

When the escape velocity from EH is c, then how can Hawking radiation escapes it? Does it travel faster than c? Or, it escape from outside the EH? If so, how a BH can evaporate beyond EH?
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### Relative velocity of light in a medium

The speed of light in a medium is independent of motion of source relative to medium but it depends on the motion of observer relative to the medium. I don't understand why it is so.
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### Beyond the speed of light [closed]

Suppose that I can walk/run/travel (not sure if this matters) faster than the speed of light. What would I experience? I know speed of light is a constant and cannot be reached. If someone were to ...
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### Does light accelerate as it nears a black hole? [duplicate]

As light is affected by gravity ( gravitational lending and black holes), it would seem that gravity causes acceleration. Acceleration has two parts: direction and magnitude. It is clearly evident ...
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### Is the maximum possible measurable velocity = 2c

What is the maximum velocity that can be measured between two objects? Is 2C the correct answer? Two photons (A & B) are emitted simultaneously from my position; photon A going north and photon ...
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### If light slows down while passing through a material, does it speed back up while in a vacuum?

Light slows down while passing through, say, water. After passing through the water, does the light speed back up to the "speed of light"? If so, how and why?
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### Does light in vacuum actually travel at the speed of light? [duplicate]

I know my question sounds like a joke (and I suppose on some level it is) but I'm confounded by the following: As the thought experiment goes, if I'm in a spaceship flying rapidly the people on earth ...
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### Change in momentum in a photon collision?

Let's say I drop a ball on the ground and it bounces up. According to classical physics, once the ball hits the ground it goes trough a short phase of deceleration until the velocity reaches zero, ...
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### What happens to light when an optical cavity path length doubles instantaneously?

Let's say we have light contained in a cavity (of initial length $L$) and then instantaneously remove the second mirror, thus removing the barrier between the first and a third mirror which is some ...
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### Is it possible to travel faster than the speed of light under extreme warping of spacetime?

The event horizon of a black-hole is defined as the point at which light itself cannot escape, supposedly due to a black-hole's immense gravity and effect on spacetime. However, would this not require ...
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### Neutrinos are slower than gravitational waves according to their mass?

Neutrino bursts are observed before the visible light of supernovas, most famously SN 1987A. Astronomers likewise expect gravitational waves. Gravitational waves are supposed to travel at exactly the ...
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### Velocity of propagation of an EM field in vacuum

I am asked to find the speed of propagation $\vec{v}$ of an electromagnetic field, in a region of empty space. The magnetic field is described by $\vec{B} = B_0e^{ax}\sin{(ky-\omega t)} \hat{z}$. I ...
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### Light at the Speed of Light [duplicate]

Since light particles travel at the speed of light, we know it's possible for something to travel at that speed. However, since all things are relative, and if time does stop at the speed of light, ...
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### If the speed of light was infinite would elementary particles still have a property called spin?

From what I understand the spin of elementary particles comes from combining relativity with quantum mechanics so if the speed of light was infinite would elementary particles still have spin?
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### How is it that we can send signals faster than the speed of light?

I came across this while reading The Feynmann Lectures on Physics (Vol.1,Ch.7)- According to Newton, the gravitational effect is instantaneous, that is, if we were to move a mass, we would at once ...
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### Do/Would Gravitational Waves have these properties? [closed]

I haven't done a ton of research on Gravitational Waves, but I have done some theorizing. Are these properties confirmed (by theory or observation) characteristics of GW? Gravitational Waves travel ...
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### Boldly taken postulates

Einstein's Special theory of relativity was a landmark in physics, which totally changed our viewpoint of observing physical phenomena. While proposing his new theory, Einstein assumed two simple but ...
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### Why can't we use this method to achieve extreme speeds?

Turn an axle with diameter 1 inch at 1 mile per hour, and an attached wheel with diameter 5 inches will spin at 5 miles per hour. Why can't we apply this principle on a much bigger scale? Say you put ...
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### Can the value of $c$ be calculated from Relativity? [duplicate]

I have three related questions about what theories or equations can calculate and what is an input parameter/constant to the theories. This question is not about parameter units (as another similar ...
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### Are there any known non-zero mass objects travelling at almost $c$ observed in the universe so far?

Have we ever observed a body or particle with non zero mass traveling at almost $c$? I'm asking this because according to relativity as an object approaches light speed it's mass approaches infinity, ...
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### Can speed of light be $c$ in air or other medium?

I know that the speed of light in a vacuum is $c\sim 3\ 10^8\ \mathrm{m/s}$, but I also know that speed of light in a medium (e.g., air) is less than that in vacuum. Special relativity says that ...
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### Does the speed of light change? [duplicate]

I know that there is a similar questions, but I think mine is a bit different. I wonder if with the expansion of the universe the speed of light changes. It seems that the speed of light is very ...
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### How is it that distortions in space can be measured as distances? [duplicate]

Imagine yourself and anything you're able to perceive to be drawn onto a rubber band. Any means of determining distances would be affected by the stretching/contracting of the rubber band. The ...
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### Special Relativity and the Light Clock and which direction was the laser pointing? [closed]

If I were to make a light clock, I would have some sort of laser pen attached to the top of one of two mirrors. That laser pen would be perpendicular to the surface of either/both mirrors. I would ...
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### Why is the speed of light exactly $299,792.458$ m/s and not faster or slower? [duplicate]

Why is the speed of light $299,792,458$ metres per second, and not faster or slower. Why not $500$ trillion kilometers per second or $120$ miles per hour? This has been 'bothering' me for a while. ...
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### A classical explanation of why there is a speed limit in the universe [closed]

The question is - is there any flaw in this explanation, or it is the right explanation in general language? Let me break the explanation into two parts - Why there is a certain speed of ...
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### When travelling in the direction of light, what is my relative velocity?

If suppose I am traveling in the same direction as that of light, the relative velocity I am traveling with is speed of light + my speed in the direction of light. Is this correct? Does this have ...
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### Explanation about the speed of light according to Maxwell from the book : “ A brief History of Time”

I just started reading Hawking's book: " A brief history of time" and there is a section that I don't understand referring to the speed of light moving through the "ether". In the second chapter, ...
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### Cold Solid Matter at the End of a Black Hole's Acceleration Disk?

Is it possible that black holes magnetic field are rapidly cooling matter using a type of magneto caloric effect and cold fusion creating new matter only possible with a black hole's gravity? A type ...
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### Is light relative? [duplicate]

I was wondering, if two cars (A and B) are going at different velocities let's say A=90km/h, B=80km/h then if we add the two speeds we get 170km/h meaning they are relative to each other. But if one ...
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### How can you find the $u$ using time dilation if you only know $\Delta t_o$? [closed]

I've come across two problems dealing with time dilation now that ask how fast should an astronaut go in order to travel EG: 400 LYears in 10 Years. What I can not wrap my head around is how this is ...
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### Why does time stop its flow? [duplicate]

According to the "list verse" time gets to a pause when someone or something travels at the speed of light. But a question still lies in my mind, if time always travels forward and does not stop at ...
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### Interpretation of time dilation

Is there any interpretiation of time dilation for photons? I mean, how does time work for them? Do photons have any distinction between their own and observer's time? Or is it something like ultimate ...
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### Help understanding why can't we measure absolute velocity?

(This is probably a clone of some other question, but I haven't been able to find a satisfactory answer, so I hope this is okay.) So, I know that in special relativity, there's no such thing as a ...
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### Why isn't a Photon justified in concluding that it is at rest and everything else is moving past it at c? [duplicate]

Why isn't a photon moving at c not justified in concluding that it is at rest and everything else is moving past it at c since relativity postulates that the laws of physics are invariant (i.e. ...
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### How fast can video/ transmission waves travel through space? [duplicate]

Something that has been on my mind for quite a while now and has sprouted lots of thought, is whether we can create a machine to put into space, far away from the earth, and use it to see back in time....
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### light in black holes and space time

I watched videos, read articles, learned a lot about black holes and got confused: Lets say that a light is doing its way inside a black hole toward the singularity. If it's inside the event horizon,...
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### Implication of Relativity [duplicate]

According to relativity, no useful matter can travel faster than light. Suppose that your are a light beam travelling. Now you have a parallel light beam travelling beside you. Suppose you look at the ...