Author Topic: Do Photons have mass?  (Read 6428 times)

Offline bn

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Do Photons have mass?
« on: January 10, 2013, 10:10:30 AM »
Quote
listening to the show has increased my conversations with my mechanical engineer buddy.
            I'm reminded of a conversation we had a few weeks ago in which we were discussing if photons have mass. My friend argued that photons do indeed have mass and his reasoning was because they bend around massive objects. To which I countered: photons do not bend, it is the path that is bent. Due to space-time warping and bending, the photon does not see itself as bending, but as following the path of least resistance. e.g. straight line.(due to frame dragging?)
            He countered that explanation with: Photons must have mass to push solar sails. I never really thought of that, and the only thing I could think of to argue was: it is not the photons that push the sails, but the excitation of gasses and plasma from a star that push the sails.


you are right and your friend is not. (congratulations!)

so photons DO travel in straight lines, it's just spacetime which is bent. Like oedipus or some other tragic figure, i guess.
your intuition is correct, but it's not called frame dragging. rather it's called... well, gravitational lensing.

now. photons do indeed push solar sails. they have momentum (without mass). a guy named Poynting (pronounced "pointing". i think i spelled his name right) figured out that wavey electromagnetic fields do indeed carry momentum. confusingly, the direction the momentum travels along in an EM wave is given by the Poynting vector. which, when you explain it aloud to someone they mistakenly proclaim "oh, it's called 'pointing' because it points in the direction of the momentum" and then you need to explain how there was a man named Poynting whose entire existence was a joke just to prove that CROM loves puns.

the calculation for how much momentum an electromagnetic wave has isn't the old P=mv  formula, since it doesn't have mass. Instead it's a thing which takes into account the wavelength and the intensity of the light.

when you do quantum mechanics to it (and talk about photons instead of waves) the momentum of the photon will therefore depend on its wavelength.

congratulations on being correct.
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Offline Grawk1

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Re: Do Photons have mass?
« Reply #1 on: May 12, 2013, 03:09:57 AM »
But if E=mc^2, don't photons have to retain mass equal to their matter equivalent?

Let me ask this differently: If have a pair of perfectly reflective ball of mirrors pointing inwardly and I release 9x10^16 joules of photons inside so they bounce back and forward forever, does this system now have a kilogram more mass than when it was empty?

Offline bn

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Re: Do Photons have mass?
« Reply #2 on: May 12, 2013, 11:14:37 AM »
yes and no.
if you packed that many photons in a mirror box, the mirror box would get heavier.

but photons still don't have mass.
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Offline Grawk1

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Re: Do Photons have mass?
« Reply #3 on: May 13, 2013, 07:50:49 AM »
Hahaha, now this is the kind of mind-bendy fun I like... How does this work?  ;D

I remember from your Higgs show that massless particles travel at the speed of light, and I know from special relativity that massive objects will have their mass go to infinity as they approach c, so how do these massless particles impart mass on a system they are part of?

If they do not have mass, where is the extra mass in this system?

Here's another question to try to untangle what is happening: What if you had a sphere of inward pointing lasers of incredibly high intensity that simultaneously fired a very short pulse of light at a central point. If the combined energy of all these laser bursts is equivalent to the mass-energy of the sun and for the instant they all intersect, all of the light pulses are contained within an (imaginary) sphere less than the schwarzschild radius of the sun, would this form a black hole?

(sorry, i know I'm like the child asking "why" to every answer, but this is really cool! :) )

Offline Lynx Cat

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Re: Do Photons have mass?
« Reply #4 on: May 13, 2013, 09:26:45 AM »
Here's another question to try to untangle what is happening: What if you had a sphere of inward pointing lasers of incredibly high intensity that simultaneously fired a very short pulse of light at a central point. If the combined energy of all these laser bursts is equivalent to the mass-energy of the sun and for the instant they all intersect, all of the light pulses are contained within an (imaginary) sphere less than the schwarzschild radius of the sun, would this form a black hole?

Huh, I was thinking about the exact same experiment this weekend, but as a (kinda supervillainy) thought experiment on how to actually create a black hole that won't evaporate before it gets to swallow enough matter to be dangerous. Only I was thinking (massive) particle beams instead of laser... like a supercollider except that it's got particles coming from like a couple dozen directions at once instead of two, and they're streams of matter rather than individual particles, but still somehow accelerated as in supercolliders. But, if it works, I think your idea's much better, since it sounds more feasible... so I'm waiting on the answer as well  :D
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Offline bn

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Re: Do Photons have mass?
« Reply #5 on: May 13, 2013, 11:01:07 AM »

Here's another question to try to untangle what is happening: What if you had a sphere of inward pointing lasers of incredibly high intensity that simultaneously fired a very short pulse of light at a central point. If the combined energy of all these laser bursts is equivalent to the mass-energy of the sun and for the instant they all intersect, all of the light pulses are contained within an (imaginary) sphere less than the schwarzschild radius of the sun, would this form a black hole?

(sorry, i know I'm like the child asking "why" to every answer, but this is really cool! :) )

you could INDEED make a black hole that way.

Shorthand: photons bend spacetime, but travel at the speed of light.
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Offline Lynx Cat

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Re: Do Photons have mass?
« Reply #6 on: May 13, 2013, 01:27:06 PM »
EXCELLENT! *rubs hands*
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Offline bn

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Re: Do Photons have mass?
« Reply #7 on: May 13, 2013, 03:12:50 PM »
No! NO!


NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!!!!
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Offline calura

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Re: Do Photons have mass?
« Reply #8 on: May 13, 2013, 08:01:12 PM »
Here's another question to try to untangle what is happening: What if you had a sphere of inward pointing lasers of incredibly high intensity that simultaneously fired a very short pulse of light at a central point. If the combined energy of all these laser bursts is equivalent to the mass-energy of the sun and for the instant they all intersect, all of the light pulses are contained within an (imaginary) sphere less than the schwarzschild radius of the sun, would this form a black hole?
I like the way you think.

Offline bn

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Re: Do Photons have mass?
« Reply #9 on: May 14, 2013, 02:54:32 AM »
yeah, the scenario you're asking about is actually a pretty fun and useful way to explore black hole growth. I wrote part of my thesis on a thing like it. it's super powerful and fun :3
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Offline darkfizzikcowboy

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Re: Do Photons have mass?
« Reply #10 on: May 14, 2013, 08:12:36 PM »
Continuing with the lasers focused in a sphere to create a small black hole. If it possible to create a small black hole for a long period of time couldn't it be possible to introduce hydrogen into the laser sphere to create sustained fusion?
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Offline bn

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Re: Do Photons have mass?
« Reply #11 on: May 15, 2013, 04:13:07 AM »
when you make a black hole, it last forever.
and when you pour stuff into it, you don't get it back.

(ish)
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Offline CthulhuKid

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Re: Do Photons have mass?
« Reply #12 on: May 15, 2013, 07:16:33 AM »
How about put some magnets into the accretion disc.  Surround it with a spool of wire, and BOOM, endless energy.

If we ignore how much energy the lasers need to make the black hole in the first place....

Offline scikopas

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Re: Do Photons have mass?
« Reply #13 on: May 15, 2013, 05:11:34 PM »
How about put some magnets into the accretion disc.  Surround it with a spool of wire, and BOOM, endless energy.

If we ignore how much energy the lasers need to make the black hole in the first place....
you might also have to take into account the energy used to continuously throw magnets into orbit around a black hole as the old ones get sucked in.   and the giant space magnet you'd need in the other axis to keep the orbiting magnets from orienting themselves in a direction not-perpendicular to our spool of wire... 


I like this idea very much.  its fun. But imagine trying to sell black-hole energy to people who are already  afraid of nuclear fission power plants!
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Offline bn

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Re: Do Photons have mass?
« Reply #14 on: May 15, 2013, 06:02:42 PM »
No, I mean, you can get energy out of a black hole pretty easily. we did a show on it. but any black hole you make with lasers will be pretty small... probably too small to have things orbit around them.
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