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  #1  
Old 04-26-2012, 08:10 PM
Leo Bloom Leo Bloom is offline
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How many photons are there in the Universe?

See subject. Or is this a nonsensical question?

I was trying to be clever in another thread and throwing terms about.

Thanks,
Leo
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  #2  
Old 04-26-2012, 08:36 PM
sevenwood sevenwood is offline
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According to this wiki article:

Approximately 6.42 x 10**61 (rounded).

(I have no idea how they came up with that number.)

Last edited by sevenwood; 04-26-2012 at 08:37 PM.. Reason: spelling
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Old 04-26-2012, 08:41 PM
cmyk cmyk is offline
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Quote:
Originally Posted by sevenwood View Post
According to this wiki article:

Approximately 6.42 x 10**61 (rounded).

(I have no idea how they came up with that number.)
Just brute force counting. I'm already up to 14.

15... 16... 17...
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Old 04-26-2012, 08:53 PM
njtt njtt is offline
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For some reason, at the bottom of this thread, there are two Google Ads for brain injury lawyers.
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Old 04-26-2012, 08:55 PM
Giles Giles is offline
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Quote:
Originally Posted by cmyk View Post
Just brute force counting. I'm already up to 14.

15... 16... 17...
But the first 12 have already gone out of existence. Start again, and count more quickly next time.
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Old 04-26-2012, 09:05 PM
cmyk cmyk is offline
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Dammit, now I lost my spot. Ugh.

1... 2... 3...
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  #7  
Old 04-26-2012, 09:41 PM
running coach running coach is offline
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There's only one and it just gets passed around. Just like fruitcake.
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Old 04-27-2012, 06:47 PM
cmyk cmyk is offline
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Except fruitcake has infinite mass.
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Old 04-27-2012, 09:17 PM
Qadgop the Mercotan Qadgop the Mercotan is offline
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I recall reading somewhere: a new hypothesis posits that there's only one photon, but it's able to travel back and forth in time
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Old 04-27-2012, 09:35 PM
Exapno Mapcase Exapno Mapcase is offline
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Quote:
Originally Posted by Qadgop the Mercotan View Post
I recall reading somewhere: a new hypothesis posits that there's only one photon, but it's able to travel back and forth in time
Not sure about about a new hypothesis, but the old one is from Feynman, who made the joke[?] about an electron.
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Old 04-28-2012, 02:45 AM
Chronos Chronos is offline
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That argument just about almost works for electrons, but not for photons. Photons aren't associated with any conserved quantum numbers, and thus can be and are routinely created and destroyed.

As for how it's determined, you can calculate the flux of photons due to a blackbody of a given temperature. Plug in the temperature of the CMB, and you've got a good approximation of our Universe (the CMB accounts for the lion's share of photons). Divide by c, and you turn the flux into a number density. Multiply by the volume of the observable Universe, and you get the total number.
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  #12  
Old 04-28-2012, 09:22 AM
Qadgop the Mercotan Qadgop the Mercotan is offline
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Originally Posted by Chronos View Post
That argument just about almost works for electrons, but not for photons. Photons aren't associated with any conserved quantum numbers, and thus can be and are routinely created and destroyed.
Another beautiful hypothesis slain by an ugly fact.

Thanks for the enlightenment about photons, you two.
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