MadSci Network: Physics

Re: 'Vacuum' itself constraining 'actual' Light velocity?

Date: Fri Feb 9 12:50:25 2007
Posted By: Jim Guinn, Staff, Science, Georgia Perimeter College
Area of science: Physics
ID: 1167760145.Ph

Dear Ray,

This is an interesting question, and it is becoming more interesting the 
more I think about it.  You are correct that even a vacuum seems to be 
filled with a “quantum soup” of subatomic particles constantly being 
created and annihilated.  But the next question is how these particles 
affect the speed of light, or the measurement of the speed of light.

There does seem to be something about “space” that limits the speed of 
light.  If you look at the equations that describe electric and magnetic 
fields (these equations are called Maxwell’s Equations) and you solve them 
for the motion of an electromagnetic wave, out pops the speed of light!  
This speed ultimately depends on two quantities, the “permittivity of free 
space” (“free space” means a vacuum) and the “permeability of free 
space”.  These two quantities describe how strong the electric and 
magnetic fields are, respectively, when created by charges.  If these 
quantities had different values than what they have now, light would 
travel at a different speed.  Apparently there is something about space 
that determines these numbers.  Does it have to do with this 
background “quantum soup”?  That is not so clear.  Light is not the only 
thing that is limited to 300,000km/s; gravity waves also travel at that 
speed.  Perhaps we will really understand from where this number comes 
only when we have a complete unified theory that combines quantum 
electrodynamics with gravity, that is, with Einstein’s Theory of General 
Relativity.  Your idea does bring up a very interesting analogy, though.  
When light enters a medium, water or glass for example, the light slows 
down and moves at a rate that depends on the permittivity and permeability 
of that material.  Perhaps the existence of the “quantum soup” is what 
determines the values for the permittivity and permeability of “free 
space”, which is really the permittivity and permeability of “quantum 

One more aspect about your question that I would like to address is 
whether the speed of light is additive or not.  Back when physicists were 
looking for confirmation for the existence of the aether, two physicists, 
Michelson and Morley, performed a famous experiment where they measured 
the speed of light in different directions relative to the direction of 
the Earth’s motion.  If speeds were somehow additive, Michelson and Morley 
should have detected a difference in the speed of light that depended on 
the direction of the light.  Why?  Since the Earth is going around the 
sun, the sun is going around the galaxy, the galaxy is traveling through 
space, etc, etc, and if there is an additive component to the way light 
travels relative to the motion of the source or detector, the Michelson 
and Morley experiment should have measured it.  They didn’t find any 
difference that depended on direction.  Another way of looking at this is 
to say that there is no preferred reference frame, relative to which the 
motions of other frames are measured.  Any frame is just as good as 
another.  No frame is any more special than another.  The “quantum soup” 
looks exactly the same in one frame as it does in another that is moving 
relative to the first.

To sum up, there does seem to be something about space that determines the 
limiting speed of light.  Would it be different if we could remove 
the “quantum soup” that seems to fill space?  We won’t know that until we 
have a more complete understanding of space-time.

Well, Ray, I hope that answers your question.  Please let us know if you 
would like any more information.

Thank you for your interest.


Jim Guinn
Georgia Perimeter College

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