1
$\begingroup$

Consider the situation below. I’m at a point in space along with a bunch of detector away from any gravitational field. I see a star moving away from me at a relative speed of $$v$$by measuring the velocity I can know the frequency shift of the light. That is the redshift. $$f=F*\sqrt{\frac{1-(v/c)}{1+(v/c)}}$$

Here f is the observed frequency and F is the source frequency.

Now consider this. For a photon of light which is near to the source, the energy can be given by $$E=h*F$$ But the same photon near me or my detector( which is close to me) will have the energy $$E’=h*f$$ Since $$F>f$$ Therefore $$E>E’$$ *from the above Doppler shift equation

Since this energy is lost where exactly is it lost ?

Does it get absorbed by the ‘fabric of space time’?

But if energy is conserved and $$E=E’$$ Then what am I missing from the above equations?

Improve this question
$\endgroup$
2
  • 1
    $\begingroup$ Energy is not invariant under Lorentz transformations. I.e. different observer disagree. For each observer energy is conserved though (if you forget about an expanding universe) $\endgroup$
    – lalala
    Mar 18, 2018 at 20:11
  • $\begingroup$ @lalala: That should be an answer. $\endgroup$
    – user4552
    Mar 18, 2018 at 20:16

1 Answer 1

Reset to default
1
$\begingroup$

Energy is not invariant under Lorentz transformations. I.e. different observer disagree. For each observer energy is conserved though (if you forget about an expanding universe). For GR see: Is the law of conservation of energy still valid?

Improve this answer
$\endgroup$
1
  • $\begingroup$ Thanks for the answer . But what if the universe is expanding? Does that mean that$$E=E’$$ or does it mean that energy is lost? $\endgroup$
    – physics2000
    Mar 19, 2018 at 4:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.