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OT: thought for the day
BenB - 31/1/12 at 09:55 AM

Ben's random shizzle of the day.

Do photons have mass?

My theory goes like this- photons have energy in them (otherwise what's powering the solar panels on my roof). We all know e=mc^2. For e to not be zero m must have a value (even a small one).

Then again, the photon moving at the speed of light in a vacuum would suggest it is truly massless.

Confusing eh?

Bloody duality of photon theory.

Are they a particle? Are they a waveform? Do they weight anything? Or nothing? Who cares?

jabbahutt - 31/1/12 at 10:03 AM

you lost me after thought for the day

roadrunner - 31/1/12 at 10:04 AM

Which came first, the chicken or the egg.

fesycresy - 31/1/12 at 10:19 AM

Easy.....


No, photons do not have mass, but they do have momentum.

The proper, general equation to use is E2 = m2c4 + p2c2

So in the case of a photon, m=0 so E = pc or p = E/c.

On the other hand, for a particle with mass m at rest (i.e., p = 0), you get back the famous E = mc2.

This equation often enters theoretical work in X-ray and Gamma-ray astrophysics, for example in Compton scattering where photons are treated as particles colliding with electrons.


I can copy and paste with the best of them

JoelP - 31/1/12 at 10:27 AM

A similar issue applies to gravitational lensing. Popular belief is that mass deforms space, hence why things can orbit around bodies with mass. This means photons think they are travelling in a straight line when they get bent around a heavy object, as it is the space that is bent. However, it could be they have mass and are attracted to it like any otehr mass.

Also, to have momentum, surely you need mass, since momentum is mass times velocity?

I do appreciate that what we are discussing was probably regarded as simple 30 years ago, and i doubt its even up for debate nowadays, but its always made me wonder.

whitestu - 31/1/12 at 10:33 AM

quote:

Do photons have mass?


I didn't even know they believed in God!

BenB - 31/1/12 at 10:34 AM

quote:
Originally posted by roadrunner
Which came first, the chicken or the egg.


That's easy.
Species are slowly evolving from one species into the next. Now there has to be a cut-off when we say a certain genotype is a new species and we shall call that new species "chicken". Now the DNA that subsequently makes that chicken was first present in it's entirety in the egg.
therefore the (species chicken) egg came first.

Unless it's a trick question and the "egg" in the question isn't a chicken egg but infact a dinosaur egg in which case the egg (as a method of gestation and birth) came about first.

but that's really cheating because everyone assumes we're talking about a chicken egg.

BenB - 31/1/12 at 10:35 AM

quote:
Originally posted by whitestu
quote:

Do photons have mass?


I didn't even know they believed in God!


LOL! Quality.

BenB - 31/1/12 at 10:41 AM

quote:
Originally posted by JoelP
A similar issue applies to gravitational lensing. Popular belief is that mass deforms space, hence why things can orbit around bodies with mass. This means photons think they are travelling in a straight line when they get bent around a heavy object, as it is the space that is bent. However, it could be they have mass and are attracted to it like any otehr mass.

Also, to have momentum, surely you need mass, since momentum is mass times velocity?

I do appreciate that what we are discussing was probably regarded as simple 30 years ago, and i doubt its even up for debate nowadays, but its always made me wonder.


This is where it gets confusing huh? Photons don't have mass but they have momentum. So either Newton was barking up the wrong tree and momentum isn't a function of mass or solar sails don't work due to the impulse imparted by photons.

jabbahutt - 31/1/12 at 11:11 AM

So far I've only understood that chickens come from eggs

balidey - 31/1/12 at 11:31 AM

My mate had one and it weighed just over a tonne.

owelly - 31/1/12 at 11:42 AM

Fanny batter.

matt_gsxr - 31/1/12 at 12:04 PM

of course they have mass!

Otherwise black holes wouldn't be black.

JoelP - 31/1/12 at 12:08 PM

quote:
Originally posted by matt_gsxr
of course they have mass!

Otherwise black holes wouldn't be black.


Thats the same concept as gravitational lensing, so techincally, i agree with you!

roadrunner - 31/1/12 at 12:47 PM

quote:
Originally posted by balidey
My mate had one and it weighed just over a tonne.

Thats one fat chicken or egg.

roadrunner - 31/1/12 at 12:48 PM

quote:
Originally posted by matt_gsxr
of course they have mass!

Otherwise black holes wouldn't be black.

I thought Black holes where black because the gravitational pull is so strong that light can not escape from them.

RK - 31/1/12 at 12:55 PM

and here I was wondering what I was going to have for breakfast.

blakep82 - 31/1/12 at 12:58 PM

quote:
Originally posted by roadrunner
quote:
Originally posted by matt_gsxr
of course they have mass!

Otherwise black holes wouldn't be black.

I thought Black holes where black because the gravitational pull is so strong that light can not escape from them.


but then wouldn't the light particles have to have a mass to be affected by the gravity pull?

otherwise, if they didn't have mass, and weren't affected, the light could then escape, and they wouldn't be black

MikeRJ - 31/1/12 at 03:33 PM

They have relativistic mass i.e. their resting mass is zero, but their mass increases as their velocity increases.

JoelP - 31/1/12 at 06:37 PM

quote:
Originally posted by MikeRJ
They have relativistic mass i.e. their resting mass is zero, but their mass increases as their velocity increases.


Thats hypothesis! All that is certain is the maximum limit on their mass (which is indeed tiny), and that massless fits in with some models.

And yes, i did google that earlier, but i suspect you did too mike

Marcus - 31/1/12 at 08:27 PM

quote:
Originally posted by balidey
My mate had one and it weighed just over a tonne.


Yeah, that'd be a Proton mate (and not the positively charged sub atomic version)

deltron63 - 31/1/12 at 08:39 PM

So, if the universe is ever expanding, what's it expanding into ? Before the big bang there was nothing.
Just a thought

Ninehigh - 31/1/12 at 09:29 PM

quote:
Originally posted by whitestu
quote:

Do photons have mass?


I didn't even know they believed in God!


I believe that is the win.

I suppose if they must have a mass, and yet be regarded as massless then maybe the mass is so small that even with the mass exponentially increasing as they approach the speed of light they can still be mathematically (and practically) massless.

I.e if they weigh 2x10-500g (zero point five hundred zeroes and two grams) then you could multiply that by a british billion and it would still be in need of some pies to be noticed!

JoelP - 31/1/12 at 10:45 PM

A quick google revealed that, when physicists are discussing photons mass, they are on a totally different level to us. They arent on about gravity as we see it, they arent on about mass as we see it, and everything they are considering is tied up deep in general relativity, quantum mechanics, string theory, and a whole load of stuff that turns the whole debate on its head.

At that point i gave up reading, because no one can know everything, and there really is no point spending ages trying to understand something that has absolutely no value beyond baffling your mates in the pub.

MikeRJ - 31/1/12 at 11:14 PM

quote:
Originally posted by JoelP

And yes, i did google that earlier, but i suspect you did too mike


Actually I asked the same question to our chief scientist at work a few weeks ago

MakeEverything - 31/1/12 at 11:29 PM

quote:
Originally posted by MikeRJ
They have relativistic mass i.e. their resting mass is zero, but their mass increases as their velocity increases.



Exactly. Thats why the Photon Torpedos were so powerful on the Enterprise.

ashg - 1/2/12 at 08:36 AM

i feel like i should expand but fear people thinking im a complete weirdo rather than just a little bit weird

Ninehigh - 1/2/12 at 09:44 AM

quote:
Originally posted by ashg
i feel like i should expand but fear people thinking im a complete weirdo rather than just a little bit weird


And if you expand too much people will tell you to leave the pies alone

ashg - 1/2/12 at 11:29 AM

just not sure if i feel like having the wee taken out of me today

Liam - 1/2/12 at 12:27 PM

quote:
Originally posted by jabbahutt
you lost me after thought for the day


Photons have energy given by h.f (Planck's constant x frequency) rather than m.c^2

So if h.f = m.c^2, you can rearrange to give m = h.f / c^2, which you can then plug into the momentum equation p = mv to get p = h.f / c, which if I'm getting all this very old A-level physics correct is the right formula for the momentum of a photon, depending only on its frequency. This can be transfered to such things as solar sails.