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View Full Version : Would a photon be emitted, if its fate was only to infinitely redshift?



Frog march
2009-Aug-24, 07:29 AM
If the Universe expands in such a way that a photon's fate was to end up infinitely red shifted would it be emitted in the first place?

As light experiences zero time for its transit, such a photon would, to its self, be instantly infinitely red shifted.

So if it was emitted, where would the energy have gone?


If a hot body was unable to emit photons in all directions, would it be a bit hotter, or would it just emit more photons in the direction that they were received, or both?


Maybe it photons had to be received somewhere then that should mean that matter everywhere would be a bit hotter than if lost photons were just infinitely red shifted...

novaderrik
2009-Aug-24, 09:31 AM
is this one of those" if a tree falls in the forest" types of questions?

Frog march
2009-Aug-24, 11:25 AM
is this one of those" if a tree falls in the forest" types of questions?


well, if it is one of those questions, then the answer, either way, would be metaphysical, I suppose.

Jeff Root
2009-Aug-24, 12:17 PM
If the Universe expands in such a way that a photon's fate was to end
up infinitely red shifted would it be emitted in the first place?
Of course. The charged particle emitting the photon has no knowledge
of what will happen to the photons it emits; It just emits them.



As light experiences zero time for its transit, such a photon would, to
its self, be instantly infinitely red shifted.
No photon is ever anything to itself. In particular, no photon is ever
redshifted according to its own measurement of itself. A photon has
no wavelength, no frequency, no amplitude, no energy, no nothing in
its own view of itself. A photon's view of itself is that it never exists.

Poor photon. :cry:



So if it was emitted, where would the energy have gone?
It stays with the photon. The photon doesn't know that it has any
energy, though. Only someone who runs into the photon detects it.
The harder they run into it, the greater the energy and frequency
and the shorter the wavelength they detect. It's relative.

If a photon travels through space for ten billion years, it will be in a
place where the matter is moving at high speed away from the matter
that emitted the photon. That means an observer in that place sees
the photon redshifted compared to how it was when it was emitted.
But if the observer matches speed with the distant matter that
emitted the photon ten billion years earlier, he will see the photon in
all its original glory, with the energy, frequency, and wavelength it
had when it was emitted. Because the photon hasn't changed at all
in those ten billion years. Not a smidgin.



If a hot body was unable to emit photons in all directions, would it be
a bit hotter, or would it just emit more photons in the direction that
they were received, or both?
You could make the body out of different materials with different
emissivities. Like Space Shuttles have black tiles for high emissivity
but low reflectance, and white tiles for high reflectance but low
emissivity. The white tiles will radiate less at a given temperature.
So a body which has black and white areas will cool by radiation in
the black areas faster than in the white areas, and the black areas
will be cooler than the white areas as a result.

-- Jeff, in Minneapolis

DrWho
2009-Aug-24, 12:41 PM
Frog march, you really need to ditch this notion of a photon needing a 'receiver' before it can be emitted (which you already asked about in the other thread).

Incomplete
2009-Aug-24, 03:19 PM
Frog march, you really need to ditch this notion of a photon needing a 'receiver' before it can be emitted (which you already asked about in the other thread).

Why is it wrong?

Single photon emission is a quantum event. If the photon isn't detected and doesn't interact with anything macroscopic, the universe will be in a quantum superposition in which the photon both has and hasn't been emitted.

So the answer to the OP is: yes and no. :)

mugaliens
2009-Aug-24, 04:48 PM
If the Universe expands in such a way that a photon's fate was to end up infinitely red shifted would it be emitted in the first place?

Yes.


As light experiences zero time for its transit, such a photon would, to its self, be instantly infinitely red shifted.

Whether or not it happens instantly depends on the mechanism of red-shifiting. For Hawking radiation (more properly, Bekenstein-Hawking radiation), vacuum fluctuations create a particle-antiparticle pair just beyond the event horizon. This event actually occurs everywhere, continuously, but the particles annihilate. Near the event horizon, however, the intense gravitational sheer is able to pull the particles apart, with one falling into the BH and the other one escaping.

The one that escapes must still climb out of the BH's incredibly strong gravitational well, resulting in it's red-shift to near infinity.


So if it was emitted, where would the energy have gone?

It is lost as the photon moves from the higher gravitational density near the event horizon to the much lower gravitational density of interstellar space.

Please note, however, that there is nowhere in the universe where the gravitational density is zero. Thus, while the red-shift may approach infinity, it never actually gets there.


If a hot body was unable to emit photons in all directions, would it be a bit hotter, or would it just emit more photons in the direction that they were received, or both?

Provided the external source of the photons remained lopsided (on one side only), and barring other processes (rotation, conduction, convection), then yes - re-emitted photons would be greater on the side receiving the photons.


Maybe it photons had to be received somewhere then that should mean that matter everywhere would be a bit hotter than if lost photons were just infinitely red shifted...

I'm not sure I understand where you're going with this last one. Could you re-phrase it? Thanks!

speedfreek
2009-Aug-24, 05:41 PM
From Expanding Space: the Root of all evil? (http://arxiv.org/abs/0707.0380)


The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied. The difference between frames relates to a changing background metric rather than a differing velocity. Page 367 of Hobson, Efstathiou, & Lasenby (2005) as well as innumerable other texts shows how redshift can be derived very simply by considering the change in the orthonormal basis of observers with different scale factors in their background metrics. This process is discreet, occurring at the point of reception of the photon, rather than being continuous, which would require an integral. If we consider a series of comoving observers, then they effectively see the wave as being stretched with the scale factor.

The photon never changes - the universe changes around it!

DrRocket
2009-Aug-24, 07:26 PM
If the Universe expands in such a way that a photon's fate was to end up infinitely red shifted would it be emitted in the first place?

As light experiences zero time for its transit, such a photon would, to its self, be instantly infinitely red shifted.

So if it was emitted, where would the energy have gone?


If a hot body was unable to emit photons in all directions, would it be a bit hotter, or would it just emit more photons in the direction that they were received, or both?


Maybe it photons had to be received somewhere then that should mean that matter everywhere would be a bit hotter than if lost photons were just infinitely red shifted...

This makes no sense.

A photon is a quantum lof energy, the energy being related to the frequency. An infinitely red-shifted photon would have zero frequency, therefore zero energy and therefore not exist. So oyur question is basically "is something that does not exist emitted" ?

How to you stop a hot body from radiating in all directions? You are rather stuck with the 3 spatial dimensions that we seem to have. If you are simply talking about insulating a body on one side, then that is a standard text-book heat transfer problem. It radiates where it can radiate and eventually reaches equilibrium with its surroundiings.