# Questions about lights constant and photons

1. Sep 8, 2010

### snackster17

Hi I am curious about a few things i have read about light and would appreciate some help.

First of all to my knowledge the speed of light is 186,000 mps and of course its impossible to exceed or reach the barrier of light for any other object other than light. But im curious as to why lights limit is only 186,000 mps and not faster, aren't photons weightless and would not have any resistance while traveling through space? Is there some sort of universal constant that cannot allow anything to achieve speeds higher? Help would be greatly appreciated.

2. Sep 8, 2010

### HallsofIvy

??? Well, yes, there is such a "universal constant"- it is the speed of light! The constancy of the speed of light is, like all fundamental ideas in physics, the result of experimentation.

And the constancy of light may be much deeper than you are thinking. It is not just that light always moves at a specific constant speed- it moves at that constant speed relative to any frame of reference. If you were standing still (relative to the ground around you) light would come toward you at 186,000 mps and if you then moved toward the light source at, say, 100,000 mps, you would still see that same light coming toward you at 186000 mps. Newtonian theory would say you would see it coming toward you at 186000+ 100000= 286000 mps.

3. Sep 8, 2010

### bcrowell

Staff Emeritus
The c in relativity isn't really the speed of light. The modern way of looking at this is that c is the maximum speed of cause and effect. Einstein originally worked out special relativity from a set of postulates that assumed a constant speed of light, but from a modern point of view that isn't the most logical foundation, because light is just one particular classical field -- it just happened to be the only classical field theory that was known at the time. For derivations of the Lorentz transformation that don't take a constant c as a postulate, see, e.g., Morin or Rindler.

One way of seeing that it's not fundamentally right to think of relativity's c as the speed of light is that we don't even know for sure that light travels at c. We used to think that neutrinos traveled at c, but then we found out that they had nonvanishing rest masses, so they must travel at less than c. The same could happen with the photon; see Lakes (1998).

Photons do have momentum. (For example, you can have solar sails.) If you add more momentum to a photon, it just gets brighter, not faster.

Morin, Introduction to Classical Mechanics, Cambridge, 1st ed., 2008

Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html

4. Sep 9, 2010

### Saw

Do you mean that the experimentally determined speed of light (a little less than 300,000 km/s) might theoretically be lower than c, c meaning that invariant speed that (following Rindler and others) you usually claim that flows from the 1st postulate alone and that you sometimes also call the maximum speed of cause and effect?

If so, I wonder why you call that the maximum speed of cause and effect... Ah..., thinking aloud, is it because you deduce that from another postulate, apart from the 1st postulate, i.e. no causality violation?