Red shift

[Brianr]

When a photon from a distant rapidly receeding star is observed on earth it appears red shifted. I understand that red photons have less energy than blue ones. ( E = hf h is Planck's constant )).

What happens to the change in energy ?

I have discussed this problem with some other physics teachers and the proposed explanations have included, not in any order of merit.

1. The energy is used to drive the expansion of the universe

2. Conservation of energy does not apply in quantum physics

3. It is all to do with frames of reference

Can anyone cast some light on this paradox ?

[kashi]

I wasn't sure about your question initally so I had to think about it. This is what I came up with.

There is less energy in the red-shifted waves because the energy is reaching us more gradually as we are moving away from the source. No energy is lost at all.

The doppler effect which causes the red-shifting of many objects in the universe is not really in the realm of quantum mechanics to my knowledge, and the very concept of a photon doesn't have to be introduced to explain it (the frequency of any form of energy exhibiting wave-like properties will be altered by the observer's velocity relative to it in the direction of the line of sight/sound etc.). Also, the idea that the energy is used in the expansion of the universe is ridiculous, because this apparent energy loss would also occur in any experiment on a much smaller scale.

Kashi

[PeterG]

I believe that the red shift is actually caused by the conservation of energy.

Imagine two space ships, each of which send a photon to you. One space ship (A) is not moving (relative to you), and the other space ship (B) is speeding toward you very fast.

First imagine that, instead of photons, the each space ship fired a bullet at you. The bullet from B should have more energy than the bullet from A. This is because the bullet from B started off with a lot of energy behind it. This would be represented by the fact that the bullet from B would come at you with its fireing velocity plus its space ship velocity. The B bullet would therefor hit you with more energy than the A bullet.

The funny thing is that no matter how much energy you put into it, light always travels at the same speed. However, the photon from B should have more energy in it than the photon from A. Again, because it started out from a point (B) that had more energy to begin with. However, it can't express that increased energy with an increase in speed. Instead, it does it with a decrease in wavelength. Given the strange fact that it can not increase its speed, it is a clever way to preserve the conservation of energy.

The classical physics phenomenon of conservation of energy is therefore preserved in the relativistic world of traveling at the speed of light.

This is not that strange, however, because sound behaves the same way. It will always go at a fixed speed through a given media. The Doppler shift of a approaching train whistle sounding high (and a receeding one sounding low) is usually explained as a timing thing. But that can be explained as a conservation of energy as well.

[kashi]

I don't agree with you PeterG. Those same photons could appear any colour of the rainbow if you were travelling at different velocities in their direction of travel. This is why bullets are a bad analogy, because their speed is variable, and thus their momentum is variable etc. Sound pulses of the same volume are a better analogy.

Someone is sending sound pulses each separated by one second towards you (ignore diffraction etc. in this example please, as it does not apply to light in space). You start moving away at a constant speed. The sound pulses will sound further than one second apart because they are successively taking longer and longer to reach you. You are therefore receiving less soundpulses per minute, and thus less energy! The nature and timing of the sound pulses did not change, they merely took longer and longer to travel so the amount of energy being received per time decreases, but the amount of energy being transmitted per time stays the same. No energy is lost, but more and more exists in the form of sound pulses in between the sound source and the observer (as the distance is getting wider).

Light is red shifted because successive wave crests take longer and longer to reach the observer. Wave crests are therefore observed less frequently and energy is received at a lower rate. As you move further from the object emitting the light, more and more energy is in transit, and we receive less as less. No loss of energy.

The timing thing as you mentioned is indeed a much better analogy, as the packets of energy sent stay the same (which is the case for photons too&#33.

Kashi

[kashi]

This topic was posted ages ago before there were many users on the forum. I'd like some peace of mind. Am I correct?

[Matthew]

I don't believe you are correct.

All light in a vacuum moves at ~300000km/sec. For the energy to take longer to reach you would constitute that the energy was moving slower than speed of light, and seeing how the energy travels at c, your statement is flawed. Less energy is in the photons, originally these photons are being emitted from a star/galaxy while going away from us, so the wavelengths increase. In a 'blue shift' (when things are coming toward us), the photons have more energy and so have a shorter wavelength. They have more energy because they are essentially being pushed toward us.

[kashi]

No! It takes progressively longer to reach you because you are further and further away. Hence less energy is received per time.

[VanderL]

Take care not to mix up the different redshift; the Doppler redshift is different from the cosmological redshift. And then there's the people who try to find how objects can have intrinsic redshift.
Cheers.

[kashi]

Initial post:

"When a photon from a distant rapidly receeding star...."

We are talking about doppler redshift.

[Menikmati]

As the universe expands, its volume grows larger, and the number density of photons declines. But the wavelength of the photons grows larger as well and hence the energy of the each photon decreases. Because of this additional property of wavelength strecthing , also called redshifting, photons lose energy density faster thean ordinary massive particles in the face of an expanding universe.

Fred Adams, Greg Laughlin " The five Ages of the Universe" 1999

[starrman]

Initial post:

"When a photon from a distant rapidly receeding star...."

We are talking about doppler redshift.

No, actually that is cosmological redshift. The Doppler metaphor for cosmological redshift has led to some unfortunate misunderstandings. Doppler's experiments all had to do with wave propogation in a carrier medium, i.e. sound waves in air. In those instances, the increase or decrease in wavelength was proportional to an increase or decrease in energy. This is not the case in the instance of cosmological redshift, where the change in wavelength is the result of an inherent change in the geometry of spacetime. There is no carrier to affect the energy of the photons. Their perceived wavelength is the result of their travel through an expanded and expanding spacetime geometry which imposes the wavelength by its inherent characteristics.

Clear skies

[Edwin]

I may be barking up the wrong tree but let me put my thoughts.

Speed of light is constant and as such the energy is constant. This is measured in relation to the object that is emitting the light (photons).
The change is because we (the receivers) are either moving away or toward the emitter.

Therefore surely any change of energy is down to us and not the actual photon changing. If we are getting closer then we add energy and the photon shifts toward the blue and if we are travelling away then the energy is lost and the shift is to the red end of the spectrum.


Does that make sense?

[Matthew]

Speed of light is constant and as such the energy is constant. This is measured in relation to the object that is emitting the light (photons).
The change is because we (the receivers) are either moving away or toward the emitter.

The speed of light is not a contant value. 'c' is contant, and 'c' stands for the speed of light in a vacuum. In a medium the speed of light changes.

If light is travelling at 'c' it can have varying amounts of energy. For an extreme example some radio waves have a wavelength of longer than 1 km, while some X-Rays have a wavelength shorter than 1 nanometre. The shorter the wavelength the high its frequency and the higher its energy. So X-Rays have more energy than radio waves, though both travel at the same speeds.

[VanderL]

Thanks matthew,

And now the cosmological redshift part, what causes photons to be redshifted when we measure them?
And "C" is constant in vacuum, but vacuum is not an empty void, it contains all kinds of fields (electrical/magnetic/gravitational) and an enormous amount of neutrino's. So maybe the speed of light can be even higher that 300.000 km/sec?
Cheers.