Linked Questions

6 votes
5 answers
6k views

Group and phase velocity - why can the latter be faster than light? [duplicate]

The phase velocity can be faster than light. Some argue that the phase velocity doesn't convey information, but this doesn't convince me. We can emit a wave of a single one frequency. Then it will ...
marmistrz's user avatar
  • 567
3 votes
2 answers
3k views

What does phase velocity physically represent, and why can it be superluminal? [duplicate]

Phase velocity is defined as $v_p=\frac{\omega}{k}$ and is described in various textbooks as being the speed at which the phase of a wave propagates. If you have a wave train that is modulated by an ...
Pancake_Senpai's user avatar
0 votes
1 answer
92 views

The phase velocity of a massive field is greater than $c$ [duplicate]

Assuming $c=1$, $v=\frac{\omega}{k}=\frac{\sqrt{k^2+m^2}}{k}>1$, for $m \neq 0$. Why is it not an issue that this $v$ is greater than the speed of light?
Ryder Rude's user avatar
  • 6,332
42 votes
3 answers
16k 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 ...
miguel.martin's user avatar
0 votes
1 answer
23k views

Could the shadow move with faster-than-light speed? [duplicate]

If I make a huge laser with a figure for shadow in front of the laser, and I shine it on to the moon, will I see the light from the laser AND the shadow moving the same speed? (I read somewhere the ...
static's user avatar
  • 181
5 votes
2 answers
4k views

Can a dot of light travel faster than the speed of light? [duplicate]

Say I have a laser. If I spin the laser so that the beam sweeps in an arc along a very distant object, could that dot travel faster than the speed of light? In Diagram form:
damned truths's user avatar
9 votes
1 answer
482 views

Seeming violation-wave travelling faster than speed of light

Consider the basic relation $$E=\sqrt{(pc)^2+(mc^2)^2}.$$ Every particle possesses a wave nature and it depends on the situation in which one among the two is perceptible... Consider a particle with ...
Schwarz Kugelblitz's user avatar
3 votes
5 answers
721 views

How can I explain this fallacy of conveying information faster than Light?

First statement of this question: In superluminal phase velocities, what is it that is traveling faster than light? says that information cannot be transmitted at a velocity greater than speed of ...
gpuguy's user avatar
  • 823
3 votes
2 answers
2k views

Klein-Gordon equation and wave velocity

It looks like solutions of the KG eqn travel faster than light, because if $$\omega^2 - k^2 = m^2$$ then $$\mid\ \omega\mid \ > \ \mid k\ \mid$$ and I thought the wave velocity was $\omega / k$. ...
Adrian May's user avatar
1 vote
1 answer
852 views

Why doesn't anomalous dispersion allow faster-than-light propagation?

It seems that the phase velocity of light could be greater than $c$, if $\sqrt{\epsilon \mu} < 1/c$, i.e. for anomalous dispersion. Are there examples of such media? For diamagnetics it seems ...
weisskreuz's user avatar
5 votes
1 answer
161 views

anticausal group velocity?

https://en.wikipedia.org/wiki/Anomalous_dispersion claims: Recently, it has become possible to create gases in which the group velocity is not only larger than the speed of light, but even ...
Arnold Neumaier's user avatar
1 vote
2 answers
96 views

Help understanding Rindler's lighthouse

Context I am studying special relativity using [1]. In [1], Gray writes that, The Minkowski diagram can help us see what is going on in a given special relativity problem: if we plot the relevant ...
Michael Levy's user avatar
0 votes
2 answers
114 views

Information can't travel faster than $c$

When we say that information cant travel faster than light, we mean that phase velocity can go beyond $c$ but the group velocity can't. Hence, we say that a plane wave carries no information. What I ...
Sara Johanson's user avatar
2 votes
0 answers
156 views

Is the phase velocity of plane wave solutions of the Klein-Gordon equation larger than $c$?

The phase velocity is given by $$ v= \frac{\omega}{k} \, .$$ Using the usual dispersion relation $$ E^2 = p^2c^2+ m^2c^4 \leftrightarrow \omega^2 \hbar^2= k^2\hbar^2 c^2 + m^2c^4$$ yields $$ v= \frac{\...
jak's user avatar
  • 9,937