I was thinking about something and I wish to share this thought with you in order to understand better something that bothers me.

Relativity theory says that the speed of light is the highest possible speed that can be developed. My question is: what is physical (and simplified) explanation for this? In other words, what is physical explanation beneath higher limit of speed? Taking classical example for a definition of speed as path divided by time (although aware it would be more appropriate to speak about waves) , what is possible explanation that something cannot travel more than 3*10^8 m in a second? Why not 3,1*10^8? What is physical constraint for that ?

  • $\begingroup$ This is a "feature" of the universe that we currently have to accept. We do not have an underlying theory as to why the speed of light is what we expermentally find it to be. In particle physics, there are around 20 different parameters that we use experimentally established values for, rather than any theoretical understanding. physics.stackexchange.com/q/172846 $\endgroup$ – user154420 Apr 28 '17 at 19:54
  • $\begingroup$ So there is no deductive explanation for what would happen if a particle/wave would travel with a speed >c ? $\endgroup$ – user406046 Apr 28 '17 at 19:56
  • $\begingroup$ There are lots of ideas, (speculations rather), based on various assumptions, but there is no experimental evidence for anything moving at a velocity beyond c. $\endgroup$ – user154420 Apr 28 '17 at 20:01
  • $\begingroup$ Why c? Don't know. However, a logical analysis of spatial motion does tell you that there has to be a finite limit to make motion possible. $\endgroup$ – Sean Apr 28 '17 at 20:11

actually special relativity does not say anything about the maximum speed, it is based on the assumption that the speed of light $c$ is the same for all inertial observers. This leads to things such as

$$ x' = \gamma(x -vt) ~~~\mbox{and}~~~ t' = \gamma(t-vx/c^2) $$


$$ \gamma = \frac{1}{\sqrt{1 - v^2/c^2}} $$

This clearly breaks when $v>c$.

  • $\begingroup$ I know these relationships but this says nothing about physical breaking of things :) . It is telling us about mathematical logic behind. What I am trying to ask is ''what will happen with a particle if it had a speed that goes beyond c?'' The answer I am looking should have a form ''it would ... and this is impossible'' . In other word I am looking for absurd that would happen in case of reaching this speed which is of physical nature. $\endgroup$ – user406046 Apr 28 '17 at 20:27
  • $\begingroup$ @user406046 SR doesn't prohibit a particle from traveling faster than $c$, but it does prohibit a particle with $v<c$ from being accelerated to $c$: you can keep adding kinetic energy to such a particle and its speed will remain below $c$. Similarly, if a hypothetical particle did have $v>c$ you couldn't slow it down to $c$. But if you plug $v>c$ into the equations in caverac's answer you get distances & times involving the square root of negative numbers, and it's difficult to see how such values could be applied to real objects. $\endgroup$ – PM 2Ring Apr 29 '17 at 13:53

Relativity theory says that the speed of light is the highest possible speed that can be developed.

  1. The constance of the speed of light is an empirical fact, means it is a phenomenon of nature that was stated after a long series of experiments on our planet.
  2. That is not right for an observer that observes two positions in space with different gravitational potential. In Einsteins General Relativity Theory he is deriving this.
  3. Once emitted photons are going with c in vacuum. Matter is moving with different velocities and their velocity depends from the impulse they got from what?

Wrapping your head about this question it would be clear why matter isn't able to move with c. If you bounce a billiard ball with another billiard ball on the atomic level the outer electrons from both surfaces are interacting with their electric fields and it is stated that this happens by the exchange of virtual photons. So at the end every interaction between matter - be this a hand clapping or our stabile standing on earth - is accompanied by the electromagnetic interaction. The highest velocity of matter we realize for electrons in linear and ring accelerators, which are driven by electric fields (and magnetic fields too).

... what is physical explanation beneath higher limit of speed?

Our macroscopic world is driven by the gravitational influence of masses and by EM interactions. As explained above the gravitational potential could not be ignored for the locality of the speed of light (in higher gravitational potential c is smaller for an observer located in a point with smaller gravitational potential). Others than this phenomenons we don't observe until now and the law about the local constantly of the speed of light is a consequence from this observations.


Some people think that the reason behind the upper limit of light speed is the inner properties of the matter.

So – called force carriers or messenger particles or intermediate particles are particles that give rise to forces between other particles.




Explanation of limit of the speed of light implies that these partricles move inside material bodies with the same velocity as they move outside material bodies.

Now, let‘s think whether we can send a massless particle back and forth immediatelly. What „immsediatelly“ means? In the physical sense, “instantaneously” means that no processes and variations occurred between these times – even at the microlevel. The dispatch time and the return time must merge together in this instance. After all, if any processes and variations occurred in a body between these times, then by occurring in time, the processes and variations required a specific amount of time. This means that the times are separated by a time interval to which the arbitrarily accurately running clock must react by a change in their readings.

One young man was even awarded (400 K bucks) for the idea that sounds like that!

As I understand, if you measure velocity of light with a lightclock, you will always measure the same velocity, because accumulated time (number of oscillations) only depends on travelled by lightpulse distance L and distance between lightclock‘s mirrors l.

And in simple words all the matter (on microlevel) consists of small lightclocks.




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