# Thread: Question about the speed of light

1. ## Question about the speed of light

How many types of particles can travel at the speed of light? I think that only photons, gravitons and gluons all are permitted to travel at the speed of light, but we can't find gravitons and we can't look at gluons too closely because they are confined. Is there anything else that moves at the speed of light?

Second, is there any theory or law of physics that requires that the speed of light be a particular number of units per second? I was thinking that as long as you don't try to model reality or match observations with made up numbers, can you just plug anything in for the value of C?

2. Order of Kilopi
Join Date
Jan 2010
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4,270
Originally Posted by Solfe
How many types of particles can travel at the speed of light?
All massless particles must travel at the speed of light, and all massive particles must travel slower.

I think that only photons, gravitons and gluons all are permitted to travel at the speed of light, but we can't find gravitons and we can't look at gluons too closely because they are confined. Is there anything else that moves at the speed of light?
Those three are the main ones, but many particles are being thought of in many theories trying to extend the standard model.

Second, is there any theory or law of physics that requires that the speed of light be a particular number of units per second?
Maxwell's equations. Special relativity says that there is some maximum speed in the universe but it doesn't say what that speed actually is, just that there is some universal constant. It is when taking Maxwell's equations into account that it turns out that that maximum speed is actually the speed of light.

3. Originally Posted by Solfe
How many types of particles can travel at the speed of light? I think that only photons, gravitons and gluons all are permitted to travel at the speed of light, but we can't find gravitons and we can't look at gluons too closely because they are confined. Is there anything else that moves at the speed of light?
Originally Posted by Solfe
Second, is there any theory or law of physics that requires that the speed of light be a particular number of units per second? I was thinking that as long as you don't try to model reality or match observations with made up numbers, can you just plug anything in for the value of C?
? I'm not sure what you are asking here. Are you asking does everything still work if you made c = 10m/s? The number we get is Planck length divided by the Planck time. Slowing it down would change that. Something has to give. I'm not sure what the ramifications would be but I'd think there would have to be some...but then gut feels in science are often wrong especially at those scales.

4. It sounds like it changes to the speed of light changes changes how other terms work and that would impact many other things.

Why is c cubed in the Planck length and c^5 in Planck time and temperature but is "just" c in Planck mass and charge? I hope the answer is "dimensional analysis" because I have a vague understanding of that...

5. Established Member
Join Date
Aug 2007
Posts
137
Yes, it is. The Planck mass, length, and time are just various combinations of G, c, and h-bar arranged to give various units.

6. Sorry, I didn't mean to drop off like that but all of you gave me a lot of reading material.

Thank you,

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