# Half the spped of light

1. Oct 4, 2005

### golith

Hi,
If you were to slow down the speed of light (299,792,458 m/s)
to half then what is the mass of a photon of light?, this is using the premis a photon has no mass at the speed of light, or it is losing mass as it approaches the speed of light. Therefor a photon must assume mass as it slows down.
P.s what would its relative temperature be at 1/2 of C.

Thanx

2. Oct 4, 2005

### RandallB

Problem is in trying to find a way, even in a thought experiment to
1) put the brakes on a photon and slow it down OR
2) find a ‘slow photon’ to speed up to light speed
It be nice to find a slow one actually measure its mass then watch it lose mass to zero as we ‘allow’ it or force it up to light speed.
But like a Teenage kid in a hotrod, light doesn’t know the meaning of slow, so you aren’t going to find one at anything but full speed.

We can find a crystal that will take a single photon and split into two. Due to conservation of energy both can only have a MAX of 1/2 of the original energy! That should translate to half the speed. But this is the deal; if you start with BLUE light and split a photon in two new photons. They would indeed have half the energy needed to be able to go at full speed. However they are so insistent on top speed you will find they just change color, frequency, and wavelength to RED. And by measurement we know that each red photon has half the energy of the blue. The energy needed for red to keep going at the speed of C. Note the original mass is also cut in half. You already had that a zero right.

3. Oct 4, 2005

### pervect

Staff Emeritus
When you say "slow down the speed of light", I'm not sure what you mean. If I were saying it, I would assume that I was changing either epsilon-0 (the permitivity of free space), or mu-0 (the permeability of free space) somehow. Note that these two changes are not equivalent, though they both would slow down the propagation of light, so I would specify which one.

In any case, if you cange the speed of light in the sense I'm talking about, light still travels at the speed of light, and it still has no mass, it's simply that that speed at which light travels is slower because of the difference in physical constants.

I really can't make any sense out of what you are trying to do at all.

4. Oct 24, 2005

### professor

to revive this old thread. i see what your attempting to say, though it does not seem to have a a great deal of rationality about it- if you were to measure the mass of light with an instrument that would have the same means of movement (essentialy the same thing as having a stopped photon) the mass would again still be zero, i do not se any possible way that the "slowing down" of a photon could increase its mass rather the change of this enegry to a more "particle like" form would do this job, this could be better defined when describing your (slowing down) change more exactly.
-i hope this was of some help, and you are still keeping tabs on this thread