Re: Why is the Speed Of Light Constant?


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Bill Hobba wrote:
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PD wrote:
bornto@xxxxxxxx wrote:
There's too many topic's here for me to read to try to find a
"right" answer to this question so here is my post:

I've been trying to research the properties which define the speed
of light for a spacetime frame. Anyways I can't find any
explanation. Yet all I keep seeing is that light being this constant
speed. Interestingly gravity is also that speed. Since I can't find
any information that backs up <http://forum.physorg.com/index.php?
act=ST&f=16&t=5134&st=0> these theories I am going to guess that the
thread of space can only allocate traversing matter at
186,000mi/sec.

What dictates the speed of light are the laws of electrodynamics.
These
are most compactly represented in what's called "Maxwell's Equations",
which is usually a set of four equations that describe electric and
magnetic fields. There are two features of these equations that are
most noteworthy:

The equations model what happens, they do not dictate to nature. If
nature had chosen to behave differently we would have come up with
different equations.

That's true. But a detailed analysis of the equations show it is
basically
equivalent to local charge conservation (ie gauge symmetry) Is that
something you would seriously care to doubt? I think it is reasonable
that
it may be violated by an extremely small amount but beyond that everyday
experience tells us it is conserved.

Thanks
Bill

I don't doubt the equations are correct, I was just pointing out that
they are not the cause of light having a constant speed. They describe
what happens, they don't cause it to happen.

Bruce

No one claims theories are the cause of anything. But think about good old
Euclidian geometry - why do right triangles obey Pythagoras? Virtually
everyone exposed to it says because of Euclidian geometry and the axioms of
Euclidian geometry are known to be true to high accuracy - it is understood
from context exactly what is meant. Or do you assert exactly the same about
that as well? If you do then I ask you to think back to grade 8 when you
learnt Euclidian geometry and ask yourself why you did not raise the issue
then? And if you did raise it think about why a teacher would have probably
sent you to remedial class to understand the basics of context.

Bill



1. They are coupled. That is, the electric field appears in the
equation governing the magnetic field, and vice versa. This is
precisely what is required to imply the existence of a traveling
disturbance of both electric and magnetic fields, where the change in
one causes a change in the other, and vice versa.

2. There are constants in those equations that apply *even in a
vacuum*. The constants are the k and k' that appear in Coulomb's Law
and the Biot-Savart Law (or the Gauss's Law and Ampere's Law versions
of the same thing). These constants are related to what's called the
"permittivity of free space" and "permeability of free space". It
turns
out that in the coupled equations referred to above, the ratio of
these
two vacuum constants is related to the speed of the traveling
disturbance.

What's unusual is that the laws of electrodynamics give this
*particular* value of a velocity without reference to a particular
reference frame. The vacuum is the vacuum regardless of the reference
frame. The laws of electrodynamics then imply this particular value
applies in any reference frame. This is in stark contrast with other
laws that include mention of a velocity (such as the law of
conservation of mechanical energy or conservation of momentum), where
the law requires no *particular* value of velocity, only the velocity
relative to a given reference frame.

Relativity doesn't insist on its own that the speed of light is
constant. It only notes that the laws of electrodynamics insist that,
and then explores the consequences of that. It so happens that the
constancy of the speed of light and the consequences alluded to also
agree with experimental observations.

PD




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Relevant Pages

  • Re: Why is the Speed Of Light Constant?
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