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*From*: Jack Uretsky <jlu@hep.anl.gov>*Date*: Wed, 27 Oct 2010 21:20:02 -0500 (CDT)

"Instaneous" has meaning only if you specify the frame in which two events occur at the same time. The passage time of a photon is zero only in a frame traveling with the photon, which, traveling at the speed of light with respect to anybody's lab is an excluded limit of the group of realizable L-transformations. In other words, nobody who ain't a photon (or massless neutrino) can travel at c.

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, LaMontagne, Bob wrote:

Maybe I'm missing the point of the original posting, but it appears

to me that the "frequency" seen to the photon is an unanswerable question. Light is created, travels to a receiver (maybe close - maybe light years away), and ceases to exist. The proper time elapsed for the photon is zero, i.e. the journey is basically instantaneous. This would seem to make counting oscillations a mute point.

Bob at PC

-----Original Message-----_______________________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-

bounces@carnot.physics.buffalo.edu] On Behalf Of Ken Caviness

Sent: Wednesday, October 27, 2010 11:18 AM

To: Forum for Physics Educators

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-----light "see"

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

during the journey? None? I know that what looks like a B field toone looks

like an E field to another, but what about the oscillating fields ofa light wave -

what do they look like to the light?_______________________________________________

Thanks,

Josh

_______________________________________________

Forum for Physics Educators

Phys-l@carnot.physics.buffalo.edu

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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

**Follow-Ups**:**Re: [Phys-l] Starlight***From:*"LaMontagne, Bob" <RLAMONT@providence.edu>

**References**:**[Phys-l] Starlight***From:*"Josh Gates" <jgates@tatnall.org>

**Re: [Phys-l] Starlight***From:*Ken Caviness <caviness@southern.edu>

**Re: [Phys-l] Starlight***From:*"LaMontagne, Bob" <RLAMONT@providence.edu>

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