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 $299792458\frac{\mathrm{m}}{\mathrm{s}}$.
2,819
questions
0
votes
0
answers
73
views
What would happen if $c$ wasn't constant, but the quotient of $c$ with the scale factor $a(t)$ was? [closed]
I'm supposing there are strong reasons to disregard this scenario but I can't understand exactly what would it imply, and why it is not at all considered in cosmology. What I'm talking about is: we ...
0
votes
0
answers
40
views
What is a simple example that contradicts the theory that light (photons) could have a super small mass? [duplicate]
The obvious example is that, to bring something that has mass to the speed $c$ requires infinite energy due to special relativity. But what if a kid asked me "What if light/photos had a super ...
- 1,090
2
votes
5
answers
465
views
Is a photon truly massless? [duplicate]
First of all, I am not a physicist or mathematician, not even a hobbyist but the following statements have always puzzled me:
$E=mc^2$
A photon is a desecrated particle of energy.
A photon is ...
- 61
2
votes
1
answer
113
views
Physical meaning of $vx/c^2$ in Lorentz transformation
In Lorentz transformation, this is the formula of time dilation
$$ t' = \gamma \left( t - \frac{vx}{c^2} \right) $$
It can be derivated from the formula of length contraction and be proved ...
- 33
-1
votes
0
answers
46
views
Is there a good quantum explanation of refraction? [duplicate]
I'm aware of the classical explanation of refraction which deals with light being a wave that gets "slowed" down while passing from a medium to another. One problem that I have with this ...
- 99
-2
votes
0
answers
37
views
Black hole production via light speed [duplicate]
As a test particle with rest mass approaches $c$ relativistic mass increases to infinity, does this mean that the energy required to continue the acceleration will create a black hole (as time also ...
- 31
2
votes
1
answer
161
views
When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?
An observer A is close to a black hole and an observer B one light year away. They are both remaining at constant radial distance from the black hole. A is at 2 Rs away from the center of the black ...
- 171
-2
votes
1
answer
87
views
Second postulate of Special Relativity - Finite Invariante Speed?
@JohnRennie, the most decorated physics stack exchange user of all time, answered the question whether the first postulate of special relativity implies the second (Einstein's first postulate ...
- 71
1
vote
2
answers
126
views
If the speed of causality changes, could you go FTL?
In the middle of some research, I reached a sort of confusion that I’d like to sort out. In flat space FTL is impossible, because in a Minkowski metric,
$$\mathrm{d}s^2=c^2 \mathrm{d}t^2-\mathrm{d}x^2-...
- 689
0
votes
2
answers
83
views
How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?
If you have a "perfect mirror" and a "perfect trampoline" at some constant distance outside a black hole's event horizon:
a) How would a shell observer at some distance farther ...
- 171
5
votes
3
answers
2k
views
Is it possible to get an electromagnetic wave equation if the speed of light were infinite?
In an old question: How would night sky look like if the speed of light was infinite? the best answer was voted down to negative credits.
I cannot understand why. From Maxwell's equations, we derive ...
- 59
0
votes
1
answer
85
views
Questions about speed of gravity [duplicate]
If gravity "travels" at $c$, and the sun is travelling "forward", does it mean the planets are actually orbiting various points "behind" the center of the sun? Does it ...
0
votes
1
answer
27
views
Beam displacement at focal point
I'm working on a project that uses galvanometers with mirrors. They're configured to have a focal point of 175mm. The motors rotate at 1 mechanical degree per 500mV, that will be 2 degrees optical per ...
- 11
0
votes
4
answers
168
views
The speed of light in vacuum (but which speed?)
Since there are several different definitions of speed describing light propagation like Group velocity, Front velocity and Phase velocity.
Which speed is meant when the phrase "The speed of ...
- 253
-1
votes
1
answer
78
views
Inconvenience of speed of light in optic fiber
As far as I'm concerned, optic fiber is great in order to transport information quickly using light. Since light needs to undergo total internal reflection every single time it hits the walls of the ...
- 1,616
0
votes
0
answers
36
views
Plausible explanation for anomalous plasma antenna time difference of arrival?
Consider the below-pictured time difference of arrival system consisting of two plasma antennas* with opposing DC bias.
Samples of time difference of a 100MHz signal are taken as the TDOA system is ...
- 1,357
4
votes
3
answers
1k
views
Is the meter relative to time?
Is the meter relative when we are near the speed of light?
I was reading a physics book and I found that the meter is the length that light travels for an amount of time, so since time is relative ...
1
vote
1
answer
104
views
Understanding the math of the Michelson-Morley experiment
I'm having some conceptual misunderstandings of the Michelson-Morley experiment. The time for the beam going perpendicular ($t_{across}$) of the aether wind I am getting:
$$(ct)^{2}=d^{2}+(vt)^{2}\\
(...
- 431
2
votes
3
answers
193
views
Why is the speed limit of special relativity $C$ equal to the speed of light $c$ and why must there be only one?
DISCLAIMER: this question is different from other questions posted here previously despite accidentally similar titles. This question is not a duplicate of the one linked bc the linked question is ...
2
votes
3
answers
127
views
Is a Lorentz transformation allowing an infinite value $c$ still a proper Lorentz transformation?
Is it correct to say that inertial systems are related by Lorentz transformations even if we do not know if the "invariant speed" is finite or infinite? To me, this is incorrect because $c$ ...
- 71
1
vote
3
answers
138
views
What are relativistic particles? [duplicate]
What are relativistic particles? I got it in a question of mechanics.
So, what is it about a particle that makes it "relativistic"?
0
votes
1
answer
108
views
Can the velocity of an image produced by a plane mirror be greater than that of light?
We are currently studying ray optics in school and it made me wonder if the velocity of an image produced by a plane mirror can theoretically be greater than that of light.
Using the relation $v_i = 2 ...
0
votes
1
answer
57
views
Non-homogenous Helmholtz equation in 3+1D: Green's function and solution
I've been reading Jackson's Chapter 8.10 and trying to find the Green's function for a non-homogenous Helmholtz equation. The problem is in cylindrical coordinates.
I first made a Fourier transform to ...
7
votes
2
answers
1k
views
Fermat principle: how did they know the speed of light in different mediums?
Pierre de Fermat derived his principle of least time by considering the path that light would take when traveling between two points. He postulated that light would follow the path that minimized the ...
- 81
0
votes
1
answer
54
views
Proof of the invariance of $c$ using the Lorentz group
Apologies if this question was already asked a few times but i could only find proofs of the invariance of $ ds^2 $.
Is there any way of proving the 2nd postulate (that $c$ is invariant in all ...
- 309
1
vote
2
answers
59
views
Speed of light along $y$ and $z$ axes when there is relative motion
There are two coordinate systems. One is stationary and the other moves in the positive $x$-direction with a speed of $v$. I don't know how to interpret the $\sqrt{V^2 - v^2}$ term since light always ...
- 331
0
votes
0
answers
26
views
Why the photons are deflected during the refraction? [duplicate]
From what I learnt, when photons are passing throw a denser environnement with an positive angle (from the atmosphere to water for exemple), they are slow down. But I can't understand how this ...
0
votes
0
answers
32
views
What is the speed of light during reflection? [duplicate]
What is the speed of light in a vacuum when the light reflects off of a mirror?
- 331
0
votes
2
answers
177
views
Rindler coordinates and objects possibly exceeding the speed of light
Assume $c=1$
So I've been on a bit of a journey trying to derive special relativity by myself and in doing so was analyzing the viewpoint of a uniformly accelerating observer.
I was particularly ...
- 312
-8
votes
3
answers
217
views
How Taylor series is compatible with special relativity?
In mathematics, an analytic function is defined by its possession of a Taylor series with a positive radius of convergence ($R_c>0$). Notably, certain analytic functions—such as holomorphic ...
- 342