The limits of red/blu shift?

[mzatanoskas]

Was watching some astronomy program which was talking about red shift and it got me wondering how much red/blue shift is possible.

 

Now I don't have the knowledge to do the calculations but according to wikipedia, in order to get a yellowish light to appear orange or green (a change in wavelength of 10nm or so), the light source would have to be travelling at about 5200 km/s away or towards you.

 

So first question:

 

1. If we discounted the limit of the speed of light, theoretically radio waves broadcast by some object traveling at a gazillion miles an hour towards earth might start microwaving us and at yet faster speeds become visible?

 

Second question has a bit to do with special relativity but my understanding of the subject is basically non-existent.

 

2. Say you have a light source traveling as close as possible to the speed of light, and a receiver traveling towards it also at a speed close to that of light. Would the amount of blue shift measured by the receiver reflect the relative velocity that is greater than the speed of light?

[timo]

1) When the wavelength is shifted into the visible spectrum, then the result can be visible. I see no reason why this should be different here. Perhaps someone even knows a real-world example of appropriately-shifted light (emissions from fast atoms or something like that).

 

2) No. For the observer (or the sender) the relative velocity is not greater than the speed of light.

 

EDIT: Perhaps it should be noted here that there's other sources of shifts in wavelength that can fit to astronomy. Gravitational shift and the shift caused by the expansion of the universe as a whole. My comments above refer to shifts caused by relative velocities since it is what you seemed to ask about. It does not necessarily have to be what the show talked about.

Edited October 26, 2009 by timo

[mzatanoskas]

Thanks for your answers Atheist.

 

2) No. For the observer (or the sender) the relative velocity is not greater than the speed of light.

 

This was the question that was getting to me given the special relativity speed o' lite don't change thingy... So can I ask in the above situation, what speed would the blu shift represent?

 

If I stick in some numbers: say the light is moving towards the observer at 200,000 km/s and the observer is moving towards the light at 200,000 km/s, how much blu shift would there be?

 

Not 400,000 km/s because that is more than c, so does blu/red shift max out at c, or is there some funky equation which shows that it gradually gets closer to but never reaches c?

 

1) When the wavelength is shifted into the visible spectrum, then the result can be visible. I see no reason why this should be different here. Perhaps someone even knows a real-world example of appropriately-shifted light (emissions from fast atoms or something like that).

 

Given your answer to 2), I guess though the amount of shift is very limited. Say from infrared to red, or violet to UV only given the speeds it takes to shift 10 nm of wavelength.

 

EDIT: Perhaps it should be noted here that there's other sources of shifts in wavelength that can fit to astronomy. Gravitational shift and the shift caused by the expansion of the universe as a whole. My comments above refer to shifts caused by relative velocities since it is what you seemed to ask about. It does not necessarily have to be what the show talked about.

 

hmm, I know nothing of these shifts I'm afraid! I thought the red shift was to do with relative velocities and was the evidence for the expansion of the universe... I may be using lots of terms incorrectly and confusing the issue.

[swansont]

Thanks for your answers Atheist.

 

 

 

This was the question that was getting to me given the special relativity speed o' lite don't change thingy... So can I ask in the above situation, what speed would the blu shift represent?

 

If I stick in some numbers: say the light is moving towards the observer at 200,000 km/s and the observer is moving towards the light at 200,000 km/s, how much blu shift would there be?

 

Not 400,000 km/s because that is more than c, so does blu/red shift max out at c, or is there some funky equation which shows that it gradually gets closer to but never reaches c?

 

Yes, there is a funky equation. the relative speed is [math]\frac{u-v}{1-\frac{uv}{c^2}}[/math]

 

http://hyperphysics.phy-astr.gsu.edu/HBASE/relativ/einvel2.html#c2

[mzatanoskas]

Thanks swansont, this is just special relativity 101 isn't it. Trying to get my head round its most basic tenets almost feels like a religious experience it's so counter-intuitive.

 

Just to check I've got it right:

 

In the wonderful world of special relativity then the following situation is just normal:

 

There are 3 light emitting objects in a line: A, B and C.

 

A__________>>>B__________C<<<__________

 

1. A is "stationary"

 

2. B has a velocity of 0.6c measured by red shift from A (going away from A towards C)

 

3. C has a velocity of -0.6c measured by blu shift from A (going towards A and B)

 

4. So B has a velocity of **funky equation**c as measured by blu shift from C? (0.8823529411764707c according to the link you gave me)

[swansont]

Looks right.

[Mr Skeptic]

Thanks swansont, this is just special relativity 101 isn't it. Trying to get my head round its most basic tenets almost feels like a religious experience it's so counter-intuitive.

 

The reason for all of that is that the principle of relativity requires that all the laws of physics work the same regardless of your velocity, ie you can't tell how fast you are going without looking out the window. Meanwhile, Maxwell's equations say light must travel at c. Putting these together means that all observers have to see light traveling at c, which requires the distortions mentioned.

[mzatanoskas]

Looks right.

 

Good, good!

 

The reason for all of that is that the principle of relativity requires that all the laws of physics work the same regardless of your velocity, ie you can't tell how fast you are going without looking out the window. Meanwhile, Maxwell's equations say light must travel at c. Putting these together means that all observers have to see light traveling at c, which requires the distortions mentioned.

 

thanks Mr Skeptic, that's a nice little summary. I'm plagued by millions of niggly little thought experiments that I'd like to straighten out in my head, but I think I should take a look at a special relativity primer first.