Author Topic: Do Photons have mass?  (Read 7230 times)

Offline Amy

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Re: Do Photons have mass?
« Reply #15 on: June 25, 2013, 05:48:21 PM »
Wouldn't such a small black hole evaporate in a burst of Hawking radiation and a bit of infra-red though? O_o

Offline bn

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Re: Do Photons have mass?
« Reply #16 on: June 25, 2013, 05:56:41 PM »
that's the theory!
that's what people were looking for at CERN when they were looking for black holes which *might* have been created by the LHC.
they didn't find any.

the theories which said it might happen were pretty... close to the edge of the table anyway.
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Offline Grawk1

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Re: Do Photons have mass?
« Reply #17 on: June 27, 2013, 08:24:42 AM »
So I read over this and noticed that I was still confused... If a sufficient density of photons can make a black hole, and black holes are points of extreme curvature in the universe, then photons must be curving space (please correct me if I have made an error here.) Why do we not call this property of having the effect of curving space "mass"?

Also, presumably photons occupy finite a volume if they are able to pass through small gaps and so on: if wavelength is inversely proportional to energy, is there a wavelength beyond which a photon becomes a black hole travelling at the speed of light?

Offline bn

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Re: Do Photons have mass?
« Reply #18 on: June 27, 2013, 11:13:16 AM »
well,
it's tricky to describe.
the short answer is that it shows up in the math.

the "mass" which we are talking about when we say "a shot-put has mass" is a thing which shows up in the momentum of the object. In the four-dimensional mathematics of relativity, an observer will measure his own energy to be equal to his rest-mass.

a photon would describe it's own energy as being zero. (do photons have energy? yes. but they don't have any self-esteem, so they think they don't... HARD TO EXPLAIN)

on the other hand, there's the "mass" which causes spacetime to curve... which is another type of mathematical object called a "stress-energy tensor". photons have non-zero stress energy tensors. so they cause spacetime to curve.


hmmm
maybe i should work out a more intuitive answer to this question
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Offline Grawk1

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Re: Do Photons have mass?
« Reply #19 on: June 27, 2013, 07:10:01 PM »
Okay, I eagerly await your analogy.  :)

Would the black-hole photon work? Let me clarify: say I have a very short straw that I can shine a light through which is 50mm long and has a 3 mm radius. I can shine a light through and then (in the style of the amazing "rocket barn" thought experiment) seal and open both ends, completely enclosing the photon, then open them up and release it before it reaches the end of the straw. This sets a finite maximum volume of the photon (pi x 3^2 x 50 = ~1400mm.)

A sphere of the same volume has a radius of 10mm, and according to the black hole calculator http://xaonon.dyndns.org/hawking/ a black hole of this size has a mass of ~7 x 10^21 tonnes which converts to ~6 x 10^40 J.

1) could you have a photon of that energy

2) by creating a photon of that energy (or of the energy necessary to collapse whatever the max size of a photon is) would you make a black hole travelling at light speed?

Offline bn

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Re: Do Photons have mass?
« Reply #20 on: June 27, 2013, 07:46:29 PM »
good questions.

1) yes. maybe you could make a photon that short. but like... it would be way past the planck scale. so... maybe you couldn't?

2) good question. I think the answer is "no". it would probably move like something with mass. but it would have the same momentum as the photons.
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Offline Lynx Cat

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Re: Do Photons have mass?
« Reply #21 on: August 06, 2014, 08:37:45 AM »
well,
it's tricky to describe.
the short answer is that it shows up in the math.

the "mass" which we are talking about when we say "a shot-put has mass" is a thing which shows up in the momentum of the object. In the four-dimensional mathematics of relativity, an observer will measure his own energy to be equal to his rest-mass.

a photon would describe it's own energy as being zero. (do photons have energy? yes. but they don't have any self-esteem, so they think they don't... HARD TO EXPLAIN)

on the other hand, there's the "mass" which causes spacetime to curve... which is another type of mathematical object called a "stress-energy tensor". photons have non-zero stress energy tensors. so they cause spacetime to curve.


hmmm
maybe i should work out a more intuitive answer to this question

That's very interesting! IIRC, in the pre-Einstein days, there was this big debate in physics regarding the duality between inertial mass (the 'm' in Newton's F=ma) and gravitational mass (the m's in gravity's F=Gm1m2/r2. Like, the two have always turned up to be exactly the same in every measurement in every object under any circumstances, but there was no theoretical reason why they should be the same, or even related.

Now, I know (or think I know, correct me if I'm wrong) that general relativity united those two concepts into a coherent whole. But, going back to that duality for a moment - is what you're saying more or less the same as saying that electrons have zero inertial mass, but nonzero gravitational mass?
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