Does the frequency of light change as I travel toward it?

[cubud]

I'm hoping someone can clear something up for me.

I was recently asked if someone traveling close to the speed of light toward a star would see that star's light as gamma rays. I argued that they would not due to their perception of time slowing down, whereas someone else argued that observing red shift in all stars moving away from us demonstrates that we would.

My impression as a complete layman is that we observe red shift in stars because space itself is expanding and therefore stretching out the wavelengths of the light, whereas we do not observe any difference in the speed of light as our perception of time alters as we travel through space close to the speed of light - and therefore we also do not observe a difference in the frequency of the light we observe traveling in the opposite direction.

It's left me somewhat confused, could someone tell me if I am wrong or not please, and if so then why I am wrong ?


Thanks!

 

[jtbell]

I suggest you read up on the relativistic Doppler effect:

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

 

[cubud]

The maths is beyond my current level.

 

[ghwellsjr]

Yes to both questions.

 

[cubud]

Yes to both questions.

So our perception of time would slow down, but not sufficiently to stop us seeing all light as gamma rays?

 

[jtbell]

The maths is beyond my current level.

Sorry, I accidentally edited your post instead of quoting it as I intended, and deleted most of the post!

If square roots are challenging you, try the calculator that Hyperphysics also provides:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop3.html#c3

One thing that may be confusing you about those pages is that it uses both the latin letter "v" for velocity and the Greek letter "$\nu$" ("nu") for frequency, which look very similar.

One of the calculators uses wavelength $\lambda$ instead of frequency. Try entering a typical wavelength for visible light (say 5 x 10^-7 meters), and try various velocities to see what gives you a typical gamma-ray wavelength of about 1 x 10^-12 meters.

 

[cubud]

jtbell

I'm sorry but even the input on that form is beyond me at this stage. It's not that I want to learn how to calculate the answer (yet) it's just that I want to get an idea of the basic "plain English" principles.

 

[jtbell]

The main effect here is that as you move towards the approaching light, you encounter the successive "peaks" in the wave faster (more often) simply because of your motion. It's like running along a beach with water waves moving parallel to the shore. If you run in the opposite direction as the waves are moving, you pass the peaks faster. This is the Doppler effect.

In relativity, time dilation does modify the result of the Doppler effect, but doesn't eliminate it.

 

[cubud]

In relativity, time dilation does modify the result of the Doppler effect, but doesn't eliminate it.

Thanks very much. If I recall correctly we don't really see massive time shifts until we are traveling very close to the speed of light, so even with time dilation it seems that everything we see would look like gamma rays, would this still be the case for light that had passed us and then reflected back on the exact same trajectory?

 

[ghwellsjr]

The difference between sound doppler and light doppler is that for sound, you have to take into account the absolute speeds of the source and the receiver with respect to the stationary air but we don't have to do that with light. Two observers with a constant relative speed that have been emitting the same color light will be appear to the other one with the same shift in color, it doesn't matter which one accelerated in the far distant past.

I don't understand your question about the light passing us and being reflected back on the same trajectory. Are you talking about a mirror that we carry with us that it reflects back to us or a mirror that is far away from us and may or may not be stationary with respect to us? Are suggesting that we get to observer the light when it went past us prior to its being reflected? Please provide specific details.