(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

(my own translation)

Light with a wavelength of 360 nm emitted from a quasar is observed as light with a wavelength of 120 nm on earth.

a) Calculate the speed of the quasar relative to the earth.

The universe is approx. 16 * 10^9 yrs old in the earth system. Assume that all the material exploded into the universe with the Big Bang. Throughout the years the separate pieces of material from the quasar merged to formed the quasar. Assume that all these separate pieces have been travelling with the same speed as the quasar, relative to the earth. Assume that light from the quasar has just reached the earth.

b) Calculate when the quasar was formed in the earth system.

Assume that the quasar has a lifetime of about 1.0 * 10^6 yrs in the system of the quasar.

c) Calculate how many years light from the quasar will be visible on earth.

2. Relevant equations

It's my first attempt at solving a more complex special relativity problem and I have a very basic knowledge of it. But I know the basic equations of time dilation, length contraction, doppler effect, relativistic speed that seem relevant.

3. The attempt at a solution

a) is simple and can easily be solved using the Doppler equation.

I'm getting stuck at b). I have drawn some diagrams and tried to understand the situation, but I can't make head nor tail out of it. Can you express the time expired since the BB in the quasar system using time dilation? If so, how can you possibly find out when the quasar was formed?

I only have a slight idea for c).

Using time dilation one can calculate the lifespan of the quasar in the earth system. Then just add that to the time when the quasar was formed. You'll need the answer from b). The time it takes the light from the quasar to reach the earth is the time between the formation of the quasar and now. So if you count that up you get simply the lifetime of the quasar in the earth system?

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# Homework Help: S. Relativity question

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