APOD: Split the Universe (2017 Apr 01)

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Re: APOD: Split the Universe (2017 Apr 01)

Postby Guest » Mon Apr 17, 2017 1:29 pm

Chris Peterson wrote:Keep in mind that photons are not really massless. They have energy, and therefore a mass equivalent. They also have momentum, which means a mass equivalent. The confusion likely comes from the understanding that a photon has zero rest mass. But that's a theoretical concept, given that a photon is never at rest.

While Art is correct that the kinetic energy of CMB photons is decreasing (because of redshift), this does not violate the law of conservation of energy, because kinetic energy is an observer dependent property- the kinetic energy we measure depends upon our frame of reference.


Can you blame us non scientists for thinking that conservation of energy is a law when scientists commonly call it such? (Just an amusing irony :wink: )

The frame of reference energy dependence is interesting. Would it be correct to then assume that the energy of these CMB photons hasn't really decreased, it's just that they look that way due to the expansion of the universe? Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

Bruce

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Re: APOD: Split the Universe (2017 Apr 01)

Postby Chris Peterson » Mon Apr 17, 2017 1:38 pm

Guest wrote:
Chris Peterson wrote:Keep in mind that photons are not really massless. They have energy, and therefore a mass equivalent. They also have momentum, which means a mass equivalent. The confusion likely comes from the understanding that a photon has zero rest mass. But that's a theoretical concept, given that a photon is never at rest.

While Art is correct that the kinetic energy of CMB photons is decreasing (because of redshift), this does not violate the law of conservation of energy, because kinetic energy is an observer dependent property- the kinetic energy we measure depends upon our frame of reference.

Can you blame us non scientists for thinking that conservation of energy is a law when scientists commonly call it such? (Just an amusing irony :wink: )

Well, many "laws" are like that. In science, the word itself generally applies to just the description of some observed behavior, not to an explanation (which would be a theory).

The frame of reference energy dependence is interesting. Would it be correct to then assume that the energy of these CMB photons hasn't really decreased, it's just that they look that way due to the expansion of the universe?

Right. The energy of a photon doesn't change, although the energy we measure depends upon our frame of reference. Photons can be red or blue shifted by cosmological expansion or by changing their position in a gravitational field, but these cases reflect different frames.

Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

I don't think it's normally suggested that the overall energy of the Universe is changing.
Chris

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Re: APOD: Split the Universe (2017 Apr 01

Postby BDanielMayfield » Mon Apr 17, 2017 2:10 pm

Chris Peterson wrote:
Bruce wrote:Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

I don't think it's normally suggested that the overall energy of the Universe is changing.

My worldview is substantially less rocked, so thanks. But that's not counting Dark Energy, right?

Bruce
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Re: APOD: Split the Universe (2017 Apr 01

Postby Chris Peterson » Mon Apr 17, 2017 2:18 pm

BDanielMayfield wrote:
Chris Peterson wrote:
Bruce wrote:Wouldn't that mean that the overall energy of the whole Universe is unchanged due to cooling?

I don't think it's normally suggested that the overall energy of the Universe is changing.

My worldview is substantially less rocked, so thanks. But that's not counting Dark Energy, right?

It depends who you talk to. Dark energy is not generally seen as violating conservation of energy, but in some models it does. Because there is little fundamental basis for conservation of energy, that doesn't bother too many cosmologists.
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