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# Re: [Phys-l] Starlight

• From: Jack Uretsky <jlu@hep.anl.gov>
• Date: Wed, 27 Oct 2010 21:07:06 -0500 (CDT)

Hi all-
I certainly would not say it like that. While I agree with:
"This is not a coincidence, since Maxwell's equations are invariant under the Lorentz transformation, the transformation of coordinates in special
relativity", there is no transformation that transforms the time coordinate of a system at rest into a space coordinate of a system at rest. 'Nuff said.
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, Ken Caviness wrote:

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

-----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
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