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# Where does the energy come from? Watch

1. In a reaction such as nuclear fusion in a star, nuclei fuse and photons are released. If this star is moving towards an observer, the wavelength of the photons decreases due to the Doppler effect. Thus, their frequency is higher, so the photons have more energy than if the star were stationary, as E = hf. Where does this energy come from? Surely there is no difference in the energy released from stationary nuclei than from moving ones - so the energy appears to come from nowhere...

Can anybody shed a light on this issue?

Note - I considered the possibility that photons are released in opposite directions, but this still wouldn't explain the hypothetical situation in which a reaction releases only one photon.
2. (Original post by DavidH20)
In a reaction such as nuclear fusion in a star, nuclei fuse and photons are released. If this star is moving towards an observer, the wavelength of the photons decreases due to the Doppler effect. Thus, their frequency is higher, so the photons have more energy than if the star were stationary, as E = hf. Where does this energy come from? Surely there is no difference in the energy released from stationary nuclei than from moving ones - so the energy appears to come from nowhere...

Can anybody shed a light on this issue?

Note - I considered the possibility that photons are released in opposite directions, but this still wouldn't explain the hypothetical situation in which a reaction releases only one photon.
Have you considered the wavelength as E=hc/wavelength? A lower wavelength results in a higher energy level. I'm not too sure about this, my quantum physics and wave-particle duality ain't so great.
3. But is equivalent to E = hf...both of these suggest that the photons have higher energy, yet they are from the same reaction as if the star were stationary...
4. (Original post by DavidH20)
In a reaction such as nuclear fusion in a star, nuclei fuse and photons are released. If this star is moving towards an observer, the wavelength of the photons decreases due to the Doppler effect. Thus, their frequency is higher, so the photons have more energy than if the star were stationary, as E = hf.
Aren't the photons OBSERVED to have more energy? I'm not fully sure though as I haven't covered the nuclear stuff in A level physics yet, but just Doppler effect in AS.
5. (Original post by krisshP)
Aren't the photons OBSERVED to have more energy? I'm not fully sure though as I haven't covered the nuclear stuff in A level physics yet, but just Doppler effect in AS.
What do you mean? They are observed to have more energy insofar as they can be measured to have a higher frequency; surely this is only possible if they genuinely did have more energy? They would, for example, give an electron freed due to the photoelectric effect a greater KE.
6. You haven't understood the Doppler Effect correctly It is not the frequency of the photon, it is the observed frequency of the photon that you measure. There are good simulations to get the idea of this effect. Of course the actual frequency at which the E field and B field of the electromagnetic wave oscillates does not change...
7. (Original post by DavidH20)
In a reaction such as nuclear fusion in a star, nuclei fuse and photons are released. If this star is moving towards an observer, the wavelength of the photons decreases due to the Doppler effect. Thus, their frequency is higher, so the photons have more energy than if the star were stationary, as E = hf. Where does this energy come from? Surely there is no difference in the energy released from stationary nuclei than from moving ones - so the energy appears to come from nowhere...

Can anybody shed a light on this issue?

Note - I considered the possibility that photons are released in opposite directions, but this still wouldn't explain the hypothetical situation in which a reaction releases only one photon.
The star is in a different inertial frame to the observer, so the photon energy in the rest frame will not be the same as the photon energy we observe.
This is just a consequence of special relativity. You probably know that quantities such as time and spatial intervals are not conserved between inertial frames, the same is true of energy.
Within the same inertial frame, energy will be conserved (as far as you and the star are concerned, there is no violation of the laws of physics, but your measurements do not equate with each other).
8. (Original post by JayTeeKay)
The star is in a different inertial frame to the observer, so the photon energy in the rest frame will not be the same as the photon energy we observe.
This is just a consequence of special relativity. You probably know that quantities such as time and spatial intervals are not conserved between inertial frames, the same is true of energy.
Within the same inertial frame, energy will be conserved (as far as you and the star are concerned, there is no violation of the laws of physics, but your measurements do not equate with each other).
Ah, thank you - I thought it was something to do with special relativity, but didn't know that energy was also relative to one's inertial frame

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