# Why is the Speed of Light what it is

1. Aug 26, 2010

### roy_e17

Can anyone explain what restricts the speed of light to about 186,000 miles per second? Why can't it travel faster?

2. Aug 27, 2010

### diazona

Fun fact: it actually can't travel slower either. Light always goes at the same speed. Einstein's theory of special relativity says that there is one, and only one, special speed in the universe which is "seen" to be the same by all (inertial) observers. And there are several ways of calculating that light must travel at that speed. (To think of it one way, photons are massless particles, and all massless particles have to move at that one special speed at all times. I don't remember the reasoning behind that offhand, but perhaps someone else can elaborate, or I could look up the details.)

For an alternative explanation, consider this: back in the 1700's and 1800's, physicists did experiments with charged particles and magnets and discovered two fundamental constants of nature, the electric constant $\epsilon_0$ and the magnetic constant $\mu_0$. And shortly afterwards, James Maxwell developed a theory that neatly explained the results of those experiments and also predicted that electromagnetic waves should travel at a particular speed, namely $1/\sqrt{\epsilon_0 \mu_0}$. If you plug in the numerical values, you get the speed of light. (In fact, at the time, nobody knew that light was an electromagnetic wave, so you can imagine that Maxwell was rather surprised when he calculated the wave speed and found that it matched the speed of light, which had previously been measured.) So you could say that light travels at the speed it does because of the "electrical" and "magnetic" properties of empty space.

Of course, that begs the question of why the electric and magnetic constants have the values they do. We don't really have a good answer for that.

3. Aug 27, 2010

### phywjc

because the “t”>=0

4. Aug 27, 2010

### gk007

It is to do with the curvature of the spacetime predicted by Einstein, which curves like hyperbolic space (in order to protect causality). If you draw a diagram of hyperbolic space with one time dimension and one space dimension:

Then no single event should be allowed to cross the "space" axis, this means restricting the speed of light to 186,000mi/s (which equates to a 45 deg line going between two hyperbolics), otherwise the information the light carried would cross the "space" axis and reach an observer before an event happened.

(an event can be veiwed as happening anywhere along a hyperbolic line depending on the observers location and velocity, but it cannot jump from one line to the next.)

5. Aug 27, 2010

### phywjc

I got it!Thank you so much!