# The Naked Scientists Forum

### Author Topic: where does the energy go from a redshift?  (Read 5802 times)

#### thebrain13

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##### where does the energy go from a redshift?
« on: 28/06/2012 23:01:16 »
when light redshifts due to gravity where does the energy from the lost photon go?

#### acecharly

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##### Re: where does the energy go from a redshift?
« Reply #1 on: 29/06/2012 12:11:38 »
The energy doesnt change so much as the wavelength stretches out changing its colour. If the source was heading towards you the waves would bunch up, its all due to the doplar effect. A passing car is a good example, you hear the sound change as the car passes. Sound at sea level travels at approximately 340 M/s or about 770 Mph, so if a car is traveling toward you at 30 Mph then  the sound is travelling at 770 + 30 Mph = 800 Mph after it passes the sound you hear is traveling at 770 - 30 Mph = 740 Mph so the energy does not particularly change, turbulence has a small effect but the pitch goes from higher to lower just like the way starlight red shift's

#### thebrain13

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##### Re: where does the energy go from a redshift?
« Reply #2 on: 29/06/2012 14:58:42 »
But a blue photon has more energy than a red one does. The photon itself changes it's wavelength, it's not the same as sound.

#### JP

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##### Re: where does the energy go from a redshift?
« Reply #3 on: 29/06/2012 15:15:55 »
Energy in gravitational fields is a complicated topic, but very roughly it's the same as throwing a ball up in the air.  The ball is traveling with high speed (high kinetic energy) when it leaves your hand, but it slows down and loses energy as it travels up.  The kinetic energy energy changes to potential energy which is energy due to the positions of the earth and the ball, and this potential energy has the potential to turn into other forms of energy (for example, when the ball falls back down, it changes back into kinetic energy).

Similarly, a photon traveling out of a gravitational field loses energy.  It can't change speed (due to the speed of light being constant), but it does lose energy.  When photons lose energy, they redshift.  This lost energy is somehow stored in the relationship of the gravitational field/gravitating object to the photon, but that's not my area of expertise.

However, I can tell you that if you consider the total energy of a photon + a planet, both when the photon is near the planet's surface and when the photon is in deep space, there is no energy loss.

#### Geezer

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##### Re: where does the energy go from a redshift?
« Reply #4 on: 29/06/2012 17:26:59 »
I think the excess energy is stored in the aeth spacetime.

#### thebrain13

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##### Re: where does the energy go from a redshift?
« Reply #5 on: 29/06/2012 21:35:57 »
Jp. when the ball travels up in the air, the gravitational pull that slows down the ball is matched by the gravitional pull from the ball on the earth. I'm not positive the photon can do that, I'm also not sure what potential energy means to a photon either. I agree with your last statement about the energy of the photon planet system being the same, it must be true. Of course in order for it to be true the photon has to transfer energy somehow. And geezer can you extrapolate on the stored energy in spacetime idea?

#### JP

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##### Re: where does the energy go from a redshift?
« Reply #6 on: 30/06/2012 15:41:50 »
Why shouldn't a photon exert a (tiny) gravitational pull on the earth?  It contains energy, which curves space-time.  And curved space-time effects the motion of the earth.  It's obviously a tiny effect, but so is the ball you throw up from the earth's surface.

Potential energy is tough in the GR case, mostly because gravity isn't a force in the same way it is in Newtonian gravity.  This means that energy lost to gravity is somehow stored in the geometry of space-time as Geezer said.  In other words, the photon + earth + spacetime configuration always has to maintain constant energy.  As the photon flies up and loses energy, the way spacetime is curved changes by virtue of the moved photon, and this change accounts for the energy lost by the photon.  If you reflected it back to the earth's surface, it would regain energy and the space-time configuration would lose energy, I suppose.  Again, I'm not an expert, but in a very handwaving way, this is how it works (at least in my understanding of it).

#### yor_on

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##### Re: where does the energy go from a redshift?
« Reply #7 on: 05/07/2012 14:08:40 »
One way is to think of it as a jello inside a 'box'. That is the quantity of 'SpaceTime' inside it, that makes up our universe. Assuming the box to be tight, there should be no energy loss, like for example using 'perfectly reflective' walls. But there are several ways to create that phenomena, the most interesting to me involves ideas of dimensions linking together so that when you go out to the right you simultaneously come in from the left. That would create a warped room time/ universe in where there is no 'ends' and probably no 'walls' either. It's a weird thought though :)

But assuming a 'closed' universe, then you can't have anything disappearing. Transforming is okay, disappearing is not. If it would then the universe can't be closed.

#### evan_au

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##### Re: where does the energy go from a redshift?
« Reply #8 on: 07/07/2012 13:45:09 »
There are two different sources of red-shift in the above discussion:

Energy is being generated at the source at a faster rate than it is being received by us:
• Doppler shift: For an object moving away from us at (say) 1% of the speed of light, the light energy generated in 1 second will be received over 1.01 seconds. This reduction in power is reflected in a reduction of the photon energy, which is seen as a reduction in frequency = red-shift (or blue-shift, if the source is moving towards you).
• For objects traveling at closer to the speed of light, an additional relativistic correction must be made for time going slower on the moving object.
• Einstein Shift: For sources in a strong gravitational field, time goes more slowly in the gravitational field. The reduced rate of emission is revealed as a reduction in the frequency of the photons = red-shift (or blue-shift, if the observer is in the more intense gravitational field)

So it seems that energy is not gained or lost, but is being received over a longer (or shorter) time period than it was transmitted in the source's frame of reference.

#### yor_on

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##### Re: where does the energy go from a redshift?
« Reply #9 on: 16/07/2012 15:23:34 »
Nope :)

What you say there is actually that you have different 'spheres' or whatever of 'slow time' and 'fast time'.
Can you prove that?

#### yor_on

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##### Re: where does the energy go from a redshift?
« Reply #10 on: 16/07/2012 15:24:34 »
If you on the other hand mean that it is observer dependent?
Was that what you meant?

#### evan_au

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##### Re: where does the energy go from a redshift?
« Reply #11 on: 20/07/2012 13:53:09 »
yor_on: The degree of red (or blue) shift is dependent on the relative motion of the source and the observer.
So different observers moving at different speeds relative to the same source will see a different red-shift.

#### yor_on

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##### Re: where does the energy go from a redshift?
« Reply #12 on: 29/07/2012 12:15:27 »
Yep, we agree, and that counts for 'time dilations' too, as I think. That a 'twin experiment' can present us twins of different biological age does not proof that there are pockets of 'slower time' existing. What it do proofs though, to me that is, is that all descriptions of SpaceTime has be observer dependent.

If it was differently those 'slow time pockets' would have defined static positions, but they don't, they are always observer dependent.

#### yor_on

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##### Re: where does the energy go from a redshift?
« Reply #13 on: 29/07/2012 19:30:09 »
If you think of it that way it becomes quite easy to see why Einstein (in vain) in his later years searched for a mathematical description of a fifth 'dimension', as in a new degree of freedom for all to move in, something at a acute angle to the reality we see, that would allow all those observer dependencies to coexist in some other reality as 'one coherent description' of the, eh, 'true reality' :) What we have is mathematical translations from one observer to another, but not the explanation for why it can be this way.
« Last Edit: 29/07/2012 19:34:46 by yor_on »

#### CZARCAR

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##### Re: where does the energy go from a redshift?
« Reply #14 on: 30/07/2012 16:28:02 »
imagine a foton machine gun shooting 100 photons at a target. If the machine gun is moving away from the target the photos will hit the target at a lower frequency.whether the gun is moving or not ,100 fotons will deliver the same amount of energy to the target

#### The Naked Scientists Forum

##### Re: where does the energy go from a redshift?
« Reply #14 on: 30/07/2012 16:28:02 »