Infinite red shift and photon energy

• September 8th, 2009, 10:56 AM
KALSTER
Infinite red shift and photon energy
Due to the expansion of the universe, light that originated from a point far enough away will become infinitely red shifted before reaching us. But what does this actually mean?

Let's for illustrative purposes use a wave packet as a model for a propegating photon:

or maybe the tip of a honey spoon:

http://image.ecplaza.com/offer/w/wyxggs/4954426_s.jpg

As the photon moves through space, the expansion will keep stretching it and stretching it until.......? Where does the energy of the photon go?
• September 8th, 2009, 02:14 PM
thyristor
The energy is just spread out over a greater distance. This results in a longer wave length, as les energy hits your detector per unit of time.
• September 8th, 2009, 06:53 PM
KALSTER
Quote:

Originally Posted by thyristor
The energy is just spread out over a greater distance. This results in a longer wave length, as les energy hits your detector per unit of time.

Right, but eventually you won't be able to detect anything. We can only see light that has been emitted up to a maximum distance from us, so where does the energy of the rest of it go? It is like light trying to escape a gravity well. A photon that is emitted right at the event horizon can never be detected.
• September 9th, 2009, 03:23 PM
thyristor
Well, we can still detect it, or well detect might be the wrong word. What I mean is that the photon energy is always present in the noise. When the enrgy is less than that of the noise, we cannot detect it but the enrgy contributes to the noise.
• September 9th, 2009, 06:21 PM
KALSTER
Ok, so a photon can simply be stretched out towards infinity, but never reach it? Surely there is only so much it can be stretched before it becomes a line or something? To take the analogy of the wave packet again (I am assuming it is applicable, which you can correct me on if it is not), then there is a limited amount you can stretch it before it is gone, no?
• September 14th, 2009, 11:11 AM
thyristor
I'm sorry it's taken me so long to reply.
Ok, so really I'm not an expert in this field so maybe someone else ought to partcipate in the discussion.
However I can tell you the way I think it works:
Ok, there is a formula where f' is the frequency that is observed, the velocity of the light source with respect to the observer and v is the speed of the waves (in this case c). So now we se that which yields that f' >0. Thus, it will not be stretched to infinity, but it will get closer and closer.
I don't know if this was satisfactory.
• September 14th, 2009, 12:04 PM
KALSTER
That certainly has shed some light on the subject, thanks! The thing is, we measure the receding speed of the furthest galaxies, whose light we are only now receiving, to near C. That would imply that the light of galaxies yet further away would have an effective receding speed of > C, which does not violate relativity because those galaxies are moving with space, as opposed to through space.

Another thing is that the light does not red shift because of a Doppler effect, but because of the expanding space it is travelling through.
• September 14th, 2009, 12:56 PM
Scifor Refugee
It's kind of like asking about the kinetic energy of a ball being thrown toward you from a car that's driving away from you. If the ball is thrown at X m/sec and the car is traveling away from you at X m/sec, then from your perspective the ball doesn't have any kinetic energy.
• September 14th, 2009, 01:18 PM
KALSTER
Quote:

Originally Posted by Scifor Refugee
It's kind of like asking about the kinetic energy of a ball being thrown toward you from a car that's driving away from you. If the ball is thrown at X m/sec and the car is traveling away from you at X m/sec, then from your perspective the ball doesn't have any kinetic energy.

That would make sense if the galaxies were simply moving away from us in a normal way, but in this case the red shift occurs because of the expansion of the space it is moving through. At some point the amplitude (if it applies to the situation) has to start to move very close to zero and (I am shooting in the dark here) start to blend into the quantum soup or something.

Another thought: one of the reasons we don't see the light is certainly because it isn't moving fast enough to ever reach us, but how much of it is actually because it is red shifted "into infinity"? In other words, how much is the light from the furthest observable galaxies red shifted and might it be that our current estimates of the diameter of the universe isn't big enough for light to be red shifted "into infinity"?

(On the other hand, if the local big bang universe is, say, trillions of light years across in an infinite expanse, then there might be enough space for the red shifting into oblivion to happen, but I digress.)

Like I said, I am shooting in the dark here.