From: Ronald Wong (ronwong@inreach.com)
Date: Mon Apr 22 2002 - 13:37:58 PDT
Message-Id: <l03102801b741fec7b512@[209.209.20.154]> Date: Mon, 22 Apr 2002 13:37:58 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: pinhole relativity
This response of mine is long in coming. Somewhere along the way it got
buried in one of my folders.
My apologies to Gary Horne.
>On Wed, 16 May 2001 14:18:26 -0700, Paul wrote:
>
>
>>
>> Objects which have mass when they are at rest cannot travel at the speed
>of
>> light. They are ssaid to have "rest mass." These are things like
>electrons
>> and protons and people.
>> Light does not have "rest mass" so it can travel the speed of light.
>> Since light is not at rest it can and does have mass.
>>
To which Gary Horne replied on Wed, 16 May 2001 17:12:53:
>
>So what does it mean, to a physicist, to be "at rest"? I'm seem to be at
>rest relative to my computer, but not to the traffic outside.
You're right, Gary. The rest mass is the mass the object has in the frame
of reference in which it is "at rest". If you were to determine the mass
of your computer while you sat in front of it, you would be determining its
"rest mass". If the traffic moving outside your house traveled at a speed
that was a significant fraction of the speed of light relative to you, the
computer's mass would appear to be greater to those traveling in the
traffic than the "rest mass".
>I was going to guess that it had to do with the fact that light can't be at
>rest relative to anything, but Ron described a scenario where particles could
>move faster than light, which means they could also move the same speed as
>light.
The fact that light can't be at rest relative to anything is unrelated to
the fact that particles can move faster than light IN A GIVEN MEDIUM.
If point A and point B are in a region devoid of matter then any
electromagnetic wave - including light - will travel at 299 792 458 m/s as
it moves from point A to B. Nothing else can equal this speed. There isn't
an object in the universe that will beat an electromagnetic wave in a race
between these two points.
If we now place a chunk of transparent material between these two points,
we will discover that it will take light longer to get from point A to
point B than before. In this region, it will be possible for a particle to
travel faster than light.
The reason for this is that the light interacts with the molecules of the
transparent material differently than the particle as they move through the
transparent medium.
******************************************
Both Paul and I addressed this issue in detail in response to Regan's
inquiry regarding "rainbows/prism and why light bends" see:
http://www.exo.net/ti/pinhole/hypermail/0484.html
or:
http://www.exo.net/ti/pinhole/hypermail/0501.html
******************************************
This process delays the light ray's progress as it moves from A to B so
that it takes more time to travel between these two points than before. It
is in this sense that its speed has been reduced.
Unlike light, the particle sees a lot of electrically neutral objects
separated by great distances (in the case of glass, the distance is about
10 times the diameter of the glass molecule). So it goes whizzing through
at whatever velocity it has until it gets close enough to interact with one
of the neutral objects. This makes it possible for a particle to travel
through the medium at a speed greater than that of light.
In crown glass, light travels with an AVERAGE speed of around 200 000 000
m/s as it moves from point A to point B.
Although a particle's maximum speed can never be equal or greater than 299
792 458 m/s, it doesn't violate any laws of physics if it is greater than
the average speed of light IN THIS MEDIUM. So, a particle moving through
crown glass at speeds greater than 200 000 000 m/s is indeed moving faster
than the speed of light in this medium.
>... where particles could move faster than light, which means they could also
>move the same speed as light.
Uh, not quite. As the beam of light goes from point A to point B, it's
average speed is 200 000 000 m/s but, as it travels in the transparent
medium from the molecule that has just emitted it to the one that will
absorb it, it does so in a vacuum and thus is moving with a speed of 299
792 458 m/s. So, as far as the photon of light is concerned, while
traveling from one molecule to the next it is traveling with a speed
considerably faster than that of the particle.
When a jet plane travels through the air at speeds greater than the speed
with which sound waves travel through the air, it will create a conical
shell of compressed air called a shock wave and the solid angle of the cone
has a direct relationship to the speed of the plane. In a similar fashion,
when a boat moves through the water at speeds greater than the speed with
which water waves travel through the water it will create a two dimensional
"cone" of "compressed" water called a bow wave and the angle of the bow
wave has a direct relationship to the speed of the boat.
In exactly the same fashion, particles of matter moving through transparent
material at speeds greater than the speed of a light wave in that material
create a conical shell of compressed electromagnetic wave called Cerenkov
radiation and, just like the plane and the boat, the angle of the cone has
a direct relationship with the speed of the particle. One of the ways to
determine the speed of the sub-atomic particles produced by the nuclear
changes that occur when scientists probe the structure of the atom's
nucleus is to measure the angle of the Cerenkov radiation produced as the
sub-atomic particles race through the transparent region of their detectors.
>...and of course, light could be at rest relative to an equal and
>parallel light beam.
Moving with uniform velocity is dynamically equivalent to being at rest in
classical physics. For this reason, two objects with identical velocities
can be considered to be perfectly at rest with respect to one another - in
classical physics.
As Paul pointed out, light NEVER rests (in our frame of reference). So this
principle of classical physics does not apply to light. Being "light-like"
involves a frame of reference that is very different from ours. Parallel
beams of light have no need to communicate with each other regarding their
motion (how could they?). It isn't that they are or are not moving in their
frame of reference. It's just that our concept of motion doesn't exist in
light's frame of reference.
>Ouch, my head hurts!
That's the problem with trying to look at the world from different frames
of reference. It's easier said than done (well, for most of us it's easier
said than done).
Einstein's theory of relativity was presented to the world a little less
than 100 years ago. Like Copernicus' model of the universe, it offered a
worldview based on a perspective that was different from that of his peers.
For Einstein it involved looking at nature in terms of spacetime instead of
the prevailing view of space AND time. The transformation from one to the
other, like that from the Ptolemaic to the Copernican, is fairly straight
forward - in principle. In reality, getting educated people (like thee and
me) to the point where we can comfortably view the world from this new
perspective has proven to be a daunting task. But then we have to remember
that the concept of zero, the meaning of negative numbers, and a lot of
other things we take for granted nowadays didn't go down very well when
they were first introduced either.
Sorry about the extraordinary delay in responding to your comments Gary.
Hope you are feeling better.
ron
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