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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4 answers
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How to prove that time stops at the speed of light using general relativity in a flat spacetime?

I know from special relativity that $t'=t/\sqrt{1-(v/c)^2}$, where t is proper time. I see from this equation that for each second of t, t' grows bigger and eventually reaches infinity as v goes to $...
13 votes
5 answers
5k views

Can we measure the speed of light in one direction? [duplicate]

I'm sorry if this is a dumb question. Recently I was learning that it's impossible for us to measure the speed of light in one direction. We can only measure it in two directions and we assume that it'...
0 votes
1 answer
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Surpassing 70%c of spinning black holes - projection [closed]

And in some cases, the holes are spinning at an extremely rapid pace. The hole in one quasar, Einstein Cross, was spinning about as fast as theoretically possible -- 70 percent the speed of light In ...
2 votes
1 answer
216 views

"Black hole spins at $X$% of the speed of light", what does that mean?

I've seen a few news stories recently (example, example) about some black holes spinning at X% of the speed of light. What does that mean? What exactly is moving at that speed, and with respect to ...
-3 votes
2 answers
83 views

What is wrong with these ways of measuring one-way speed of light? [duplicate]

I thought of 2 ways of measuring one-way speed of light. Can somebody explain why these won't work please? Create a machine that emits a pulse of light and a sound from far away. Have a clock ready ...
1 vote
3 answers
143 views

Do objects outside the Hubble sphere violate special relativity?

According to special relativity, objects can move with speed at most c. However, objects outside the Hubble sphere recede from us faster than the speed of light. How can these be reconciled?
0 votes
2 answers
131 views

Is Feynman right? Does mass change with speed? [duplicate]

In chapter one of Feynman's famous Lectures on Physics, he writes the following. Mass is found to increase with velocity, but appreciable increases require velocities near that of light. A true law ...
1 vote
2 answers
203 views

How high can a light-beam (or apple) travel when pointed (thrown) out from the event horizon?

I am a bit confused at the idea that information cannot be sent out from behind the event horizon of a black-hole. Consider the following figure: (source This link) An apple thrown up from the ...
0 votes
2 answers
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If a proton travels exactly at 99% of speed of light are the individual velocities of the quarks inside it also exactly 99% of $c$?

If a proton travels exactly at 99% of speed of light are the individual velocities of the quarks inside it also exactly 99% of $c$? So can we think that they all travel linearly as the center of mass ...
-3 votes
1 answer
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Measuring the speed of light with a black hole

is it possible to use the grabity a black hole has to bend the light back to the observer and measure it that way? also saying i made the light go from the right and bend back coming at my left and ...
3 votes
3 answers
702 views

Why does the Fizeau measurement of the speed of light use a sawtoothed gear instead of a single gap?

In his measurement of the speed of light, why did Fizeau use a sawtooth gear with $n$ teeth and $n$ gaps? Why not use only one gap, so when light passes through the gap we will know that it is that ...
2 votes
1 answer
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Relativistic ship and base on Earth talking

Let say humans make it possible to travel so close to the speed of light that time dilation for the ship can achieve 100 times the time of a standing still object. So one of that ships starts its ...
0 votes
0 answers
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Explain this para on electric field? [closed]

Suppose we consider the force between two distant charges $q_1,q_2$ in accelerated motion. Now the greatest speed with which a signal or information can go from one point to another is $c$, the speed ...
-2 votes
1 answer
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At which fraction of light speed could I expand ten times, and 100 times, my clock's ticking? [closed]

I face this problem in special relativity: for dilating the intervals observed in an at rest, foreign frame of reference by ten times, and also by one hundred times, how much should I reduce light ...
0 votes
3 answers
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How can gravity change the wavelength of a photon without the two peaks or troughs to accelerate one from the other?

How can gravity change the wavelength of a photon without the two peaks or troughs to accelerate one from the other? So is it possible that gravity accelerates light? Can the a certain wavelength ...
17 votes
3 answers
4k views

What exactly would it take to show (hypothetically) that the speed of light (in vacuum) is not constant?

Consider Minkowski space, which is determined by the spacetime line element $$ ds^{2} = -c^{2}dt^{2} + dx^{2} + dy^{2} + dz^{2}. $$ Now suppose we modify this so that the $c$ constant is not a ...
2 votes
5 answers
553 views

Problems with understanding the speed of light when emitted from a moving object

I'm having some problems understanding how if a moving object emits light, the speed of that light is still the constant, $c$, speed of light. According to Einstein's theory of relativity, light ...
0 votes
3 answers
175 views

Light moving toward a moving observer's reference frame

The speed of light is supposed to be the same for all observers in an inertial frame, no matter the observer's speed. As a result, time slows down for observers moving quickly, and this explains why ...
-1 votes
1 answer
56 views

Calculating the one-direction speed of light [duplicate]

According to Derek Muller from Veritasium, measuring the one-direction speed of light requires synchronised clocks, and that's a problem because moving one of the clocks will slow down its time. In ...
-2 votes
0 answers
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One-way speed of light determined by measuring Time Dilation in two directions? [duplicate]

I know it’s theoretically impossible to measure the one-way speed of light by measuring the time it takes to pass by two points. The issue is with synchronizing two clocks and Time Dilation that is ...
0 votes
1 answer
47 views

If during the resistance of a resistor against an electric current particles are colliding why not say that they produce a lot of gamma rays?

During the collision of particles in the LHC we say lots of gamma rays produce but if during the resistance of a wire or a resistor against an electric current particles are colliding why not say that ...
3 votes
3 answers
804 views

If an electric charge suddenly appears will an electric field be present everywhere instantaneously? [duplicate]

This question has been asked many times in conjunction with a suddenly disappearing sun. The common reaction to this is that it takes a while for the spacetime curvature to be changed around it. In ...
-2 votes
1 answer
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Why would FTL travel break causality, as oppose to the relative time reconciling as in the Twin Paradox? [duplicate]

I have heard that if faster than light travel were ever made possible, such as by an Alcubierre drive, the user would be taken backwards in time, violating causality. This is odd to me, as it already ...
2 votes
1 answer
243 views

Is Stellar Aberration a one-way speed of light measurement?

There is a discussion ongoing about the feasibility to measure the one-way speed of light, ever since a Veritasium video on the subject. There is also a wikipedia page about the subject. But ...
24 votes
6 answers
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Why can't light escape from a classical black hole?

Photons do not have (rest) mass (that's why they can move at the speed of "light"). So my question is: how can the gravity of a classical$^1$ black hole stop light from escaping? -- $^1$ We ...
-2 votes
0 answers
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Can we really prove that $E_0 = mc^2$ for a photon? [duplicate]

Let $E$ be the totale energy of a particle, $E_0$ the energy when its speed is nil, $m$ its rest mass, $\vec{p}$ the momentum vector, $\vec{v}$ the velocity vector, $c$ the speed of light, $\gamma$ a ...
3 votes
2 answers
67 views

What exactly is "dense" in Optical Density?

My book states that: When passing from one medium to another, if light slows down, the second medium is said to be optically denser than the first medium, and if light speeds up, then the second ...
0 votes
1 answer
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Can X-rays travel quicker than visible light in a medium?

I have seen while reading a few books that the refractive index depends on lambda as follows by the Cauchy equation. $${\displaystyle \eta(\lambda )=A+{\frac {B}{\lambda ^{2}}}}$$ From this, the book ...
1 vote
2 answers
110 views

Time and speed of light in Relativity

Time running slower near a massive object, but the speed of light does not really change near a massive object, according to Relativity - it just curves. Is not time directly related to the speed of ...
1 vote
2 answers
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Does anything "break" in physics if you allow for instant (faster-than-light) communication?

Assume that two entities, Alice and Bob, can communicate thoughts literally instantly. If Bob thinks "Alice, I want you to jump", then Alice (many, many light-years away) will know that ...
-3 votes
3 answers
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Measuring the one-way speed of light?

I know many people (including me) are bombarding this site with questions about the recent Veritasium video about the one-way speed of light, but I think I found a way. Here it is: (You can see sort ...
4 votes
2 answers
114 views

Invariant upper bound for velocity of a particle

In the book Relativity book by Wolfgang Rindler, he explains that the Galilean and Lorentz transformations exhaust all the possible transformations. And explaining that he says, "Now, either there is, ...
2 votes
1 answer
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Does Loop Quantum Gravity predict that high-energy photons or light waves will travel faster or slower than low-energy (longer-wavelength) ones?

Matthew O'Dowd, the Australian-born host of PBS Space-Time, insists that high-energy (short-wavelength) photons (or waves) of light will travel slightly more slowly through space than lower-energy, ...
0 votes
1 answer
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How do we know that the gluon travels at $c$? [duplicate]

How do we know gluons travel at the speed of light? can this be derived or is it just an assumption based on the similarity to photons?
3 votes
2 answers
2k views

How do we know that gluons travel at the speed of light?

Since gluons are located within nucleons and immediately outside of them, how do experiments determine parameters like their speed? Is it possible we could be assuming they travel at the speed of ...
-8 votes
2 answers
181 views

Can this experiment successfully be used to find out one-way speed of light? [duplicate]

As with almost every question on this topic, this question is also inspired by the Vertiasium video Why No One Has Measured The Speed Of Light. This experiment tries to measure the one way speed of ...
3 votes
2 answers
102 views

Time differential between two signals sent at two different near-relativistic speeds

There is something I don't quite get about relativistic velocities, which I am hoping to clarify with this question. Suppose you have an emitter and receiver both located somewhere far away from one ...
3 votes
1 answer
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Proper time for a light particle

For a mass point in a local intertial system on which no forces act, we have that $$ \frac{\partial^2\xi}{d\tau^2}=0$$ where $\tau$, the proper time, is defined through $ds = c d\tau$ and the $\xi$ ...
0 votes
5 answers
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A question about relativity and time dilation from a lay-person

Observer A is in a spacecraft flying toward earth Observer B is standing on earth A is traveling at near light-speed and will reach the earth in 1 week. When B looks at A's clock, he sees it moving ...
-4 votes
1 answer
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Self-coupling of gravity and gravitation escaping a black hole - contradiction?

The field equations are non-linear, that can be interpreted as gravity is coupling with itself, see for example here: Non-linearity and self-coupling of gravity I'm trying to understand what that ...
3 votes
3 answers
414 views

Can a spaceship accelerate at $1g$ forever?

Let's say a spaceship is designed to accelerate continuously at 9.8m/s/s, and thereby create an earth-like environment on one surface for its inhabitants to live. From the perspective of the ...
0 votes
1 answer
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What happens to the time period when refraction occurs? [duplicate]

I was studying light and had a doubt that when refraction occurs (rarer to denser just assuming) then frequency remains same but lambda (wavelength) and velocity decrease. But I cant quite figure out ...
7 votes
3 answers
335 views

How to understand $E=mc^2$?

I am trying to understand Einstein's mass-energy equation $E=mc^2$. Knowing about the atomic bomb, I am inclined to believe that $E$ is proportional to the mass: $E=Cm$. For this equation to be ...
6 votes
3 answers
188 views

Could the speed of causality be (significantly) faster than $c$?

The other day my son (13) asked me whether it was possible that light went very slightly slower than our best measured $c$, and at the same time had a very tiny mass, but we aren't able to measure ...
-1 votes
3 answers
108 views

Is light's momentum $0$?

From Paul Dirac's extended mass-energy equivalence equation, we know that, E2 = (mc2)2 + (pc)2 We also know that E = mc2 So, if we write mc instead of ...
1 vote
2 answers
121 views

Recordings of journey traveling near speed of light

If you had two identical video cameras and started recording synchronized atomic clocks recording at the same speed and you put one on the spaceship and sent it out traveling at 99% of the speed of ...
11 votes
3 answers
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Does the first postulate of special relativity imply constant speed of light?

The two postulates of special relativity are The laws of physics take the same form in all inertial frames of reference. The speed of light in free space has the same value $c$ in all inertial frames ...
2 votes
3 answers
147 views

Is there an exact constant value for the speed of light in a vacuum distorted by a gravitational wave?

In a vacuum, construct a cylinder of photons arranged as follows: A 'measuring apparatus' computes the speed of light as 299,792,458 m/s. However, suppose the vacuum is distorted by a gravitational ...
1 vote
0 answers
25 views

How long would it take to build a wormhole? [closed]

If it were possible to build wormholes, would the process of building them also be faster than light as seen outside the wormhole, or would it take at least as much time as it would light to traverse ...
3 votes
1 answer
164 views

Why is refractive index equals to speed of light upon phase velocity and not group velocity?

We have been using group velocity for most purposes as its the velocity with which the wave packet travels, they why do we use phase velocity for calculating refractive index.

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