Light, doppler shifts, relativity.

[cwes99_03]

FIRST, let me say this is theoretical.

SECOND

If you were capable of traveling very near the speed of light, what would radio waves from a source behind you look like?

 

The real question is, how far can you redshift a certain frequency of light (i.e. what speed would you have to travel to reduce this wave to nearly infinite wavelength?) ? I've never come across this question in my studies of doppler shift, and was wondering if anyone else had given it much thought.

[Jay-qu]

well I have heard of people saying that light leaving a black hole is infinitly red-shifted so that it effectivly cant escape the black hole... so I guess that light from 'behind' you would be linearly red-shifted in proportion to your speed and if you where to reach c it would mean infinite red-shift and wave would not be detectable

[cwes99_03]

Yes, but I'm not asking if I were to reach c. I'm asking about reaching a near c velocity. Maybe 3/4 c. Would that be sufficient to redshift some low frequency light to near infinite wavelength?

[Jay-qu]

well what I was saying was that it would not become infinite untill c was reached - anything below c would produce some red-shifting but I dont know how much

[Bo]

things don't get infinite in SR as long as speeds don't go to c. (see http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html )

 

the black hole story has nothing to do with the relativistic red shift, but with the so called gravitational redshift (an effect produced in general relativity)

 

bo

[cwes99_03]

so a velocity of 0.99999999999c causes a redshift of about 4 orders of magnitude according to that site. Guess that answers the question. It doesn't matter what frequency you look at. You'd have to be traveling so fast that you'd effectively be going close enough to c to negate the underlying question I posted.

[UncleAl]

You can redshift a source to an arbitrarily low frequency. Cosmic red shfits move hydrogen UV emission lines deep into the IR.

 

Relativistic doppler shift,

http://www.mathpages.com/rr/s2-04/2-04.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html

http://www.phys.ufl.edu/~rfield/PHY2061/images/chp39_2.pdf

[cwes99_03]

Yes, but I was hoping to find that for some lower frequency wave, that at some value v less than c, you could red shift that frequency by like 20 orders of magnitude. I see that this is not possible.