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Re: [Phys-l] Starlight



Hi all-
see my (and John Ise)
paper in AJP, 26 (1958) 431
Regards,
Jack
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley




On Wed, 27 Oct 2010, Moses Fayngold wrote:

I agree 100% with everything Ken Caviness has said, except for the very last
statement - regarding the oscillation frequency in the limit as the observer
speeds up after the light wave. Both - frequency and amplitude - in this limit
can be determined from the Lorentz transformation for EM field. Applying this
transformation between a frame K0 where the monochromatic EM wave has a certain
frequency f0 and amplitude E0 and another frame K moving relative to K0 in the
direction of the wave, taking account of the fact that B0 = E0/c, as well as of
the mutual orientation of E and B vectors in EM wave, one gets an interesting
result - that the field strength decreases at exactly the same rate as
frequency, that is,
E = E0 sqrt {(1 - b)/(1 + b)}, where b = v/c, and the same for B and f. This
expression has a decent, well-behaved limit for all 3 characteristics, namely E
--> 0, B --> 0, f --> 0 at v --> c. So the observer speeding up after the light
wave, will record its speed as the same constant c, but with ever decreasing
frequency and amplitude. In the limit the frequency is zero, which does not
contradict anything, since the amplitudes go to zero as well, and the wavelength
goes to infinity, according to lambda = c/f. Moreover, this result has to be
the way just described in order to be consistent with QM: when you describe
light as photons whizzing by, and start rushing after them to catch one, you
fail not only because you find them whizzing by as fast as before, but also
because their energy hf goes to zero as you speed up.

Moses Fayngold,
NJIT



________________________________
From: Ken Caviness <caviness@southern.edu>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Wed, October 27, 2010 11:18:15 AM
Subject: Re: [Phys-l] Starlight

More precisely: What looks like a B field to one observer looks like a
combination of E & B fields to another observer (one in motion with respect to
first), and what looks like an E field to the first observer looks like a
combination of E & B fields to the second observer.

What looks like _only_ a B field to one observer does _not_ look like only an E
field to any other observer.

This is closely related to the spacetime interval in relativity. If one
observer sees an interval as entirely space, no observer will see it as entirely
time, and vice versa.

This is not a coincidence, since Maxwell's equations are invariant under the
Lorentz transformation, the transformation of coordinates in special
relativity. They have the same form for all observers, although each observer
uses her own distance & time scales, etc. Electromagnetic waves, oscillating E-
& B-fields moving at the speed c, are valid solutions for Maxwell's equations.
All inertial observers see electromagnetic waves as oscillating E-&B-fields,
moving at the speed of light.

At first glance it seems like a copout to say, "No-one can travel at the speed
of light, so it makes no sense to ask what light looks like to the light
itself." But it may be the best answer possible. We _can_ say that no matter
what your speed is (with respect to any convenient reference frame), 0.9c,
0.99c, 0.999c, 0.9999c,..., you will _still_ see all light as composed of
oscillating E-&B-fields, moving at the speed of light -- with respect to _you_.
No matter how closely you approach the speed of light (again, with respect to
any convenient reference frame), you will never see these E-&B-fields stop or
even travel with a different speed.

Of course, the frequency of the oscillation _is_ different according to
different observers (Doppler shift). So the color of the light may be different
(red/blueshifted). An observer speeding along the path of the light, starting
at the distant star and heading for earth, will still see the light ray moving
much faster than him (at speed c, of course), and because of length contraction
may see the distance traveled as very, very small, so that the trip might only
take a fraction of a nanosecond (according to his clocks), maybe not even one
complete period of the oscillation of the light. (According to earth observers
the trip time was long, but the moving observer's clocks were running slow
because of time dilation.)


In any case, it is not fair to consider the E- & B-fields as stationary even in
the limit, they always oscillate, for all inertial observers.

All the best,

Ken Caviness
Physics
Southern Adventist University

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Josh Gates
Sent: Wednesday, October 27, 2010 7:27 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] Starlight

http://www.xkcd.com/811/

So... how oscillations of the magnetic and electric fields does the light
"see"
during the journey? None? I know that what looks like a B field to one looks
like an E field to another, but what about the oscillating fields of a light
wave -
what do they look like to the light?

Thanks,
Josh
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l