# Why light can't go faster then 300 000 km/s? What prevents it? [duplicate]

Why can't the speed of light be faster then approx 300 000 km/s? What prevents it?

## marked as duplicate by tpg2114♦, Abhimanyu Pallavi Sudhir, jinawee, Qmechanic♦Feb 19 '14 at 6:55

• – Brandon Enright Feb 18 '14 at 23:49
• I really want to say this is a duplicate of something - but I can't seem to find a well-worded question (or even many well-worded answers) on the site that deals with precisely the same thing. Everything just keeps linking back to that same addition-of-velocities on a bus thing. – user10851 Feb 18 '14 at 23:49
• mwweb, to be honest, from the way you've written this question it appears that you don't have the appropriate level of background knowledge to clearly formulate your question or understand an answer. It also doesn't appear that you've done any research on your own. My guess is that has something to do with why you're getting downvotes. – David Z Feb 18 '14 at 23:57
• Of course the answer is "the same physics that allows light to travel at all sets the speed". I repeat: exactly the same thing that lets light travel at all establishes how fast it goes. That means nothing is "preventing" it from going faster because it simple has a speed and isn't "trying" to do anything other than what it does. The question could be better phrased as "why that speed", but the answer is then "because $\epsilon_0$ and $\mu_0$ have the measured values" and no knows why that is. Somethings are just the way the world works. This is one of them. – dmckee Feb 19 '14 at 0:29
• [insert snarky comment about definition of SI units] – Brian Moths Feb 19 '14 at 4:08

Lightspeed can be anything non-zero and finite for special and general relativity to work. Any medium, including vacuum, has a permittivity (electric field) and permeability (magnetic field). The permittivity ε and permeability µ of a medium together determine the phase velocity v = c/n of electromagnetic radiation through that medium,

$\varepsilon \mu = \frac{1}{v^2}$

If you can change one independently, you alter ligthspeed. This can be done on paper, and trivially so. Alas, it does not reduce to practice given the finite size of atoms and the direction in which the diddle operates - the Scharnhorst effect,

http://www.npl.washington.edu/AV/altvw43.html
http://arXiv.org/abs/gr-qc/0107091
http://arXiv.org/abs/quant-ph/0010055
Phys. Lett. B236 354 (1990)
Phys. Lett. B250 133 (1990)
J Phys A26 2037 (1993)

• This is easy to find if you know where to look, and for what. It's that first step that hurts. Now, run the race. – Uncle Al Feb 19 '14 at 0:05
• Change one independently, but can't magnetism be viewed as electric fields viewed through relativity, which is tied back to c? Seems to me maybe you can't separate them for that reason. – Michael Aug 30 '14 at 5:41