# The Naked Scientists Forum

### Author Topic: How fast does photon spin?  (Read 12952 times)

#### yor_on

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##### How fast does photon spin?
« Reply #25 on: 17/12/2010 16:40:25 »
The double turn, and I was joking of course.
Still it would be interesting to see a explanation of how to see it.

#### jartza

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##### How fast does photon spin?
« Reply #26 on: 18/12/2010 16:09:35 »
Sometimes these problems can be tricky--absorbing angular momentum from a photon is a quantum effect--but as long as the photon direction is aligned with the axis of the rotating disk, every one that gets absorbed should impart a tiny bit of angular momentum to the disk.  Neglecting friction, air resistance, etc., the disk will continue to rotate faster and faster without need to adjust the direction of the light beam so long as it keeps absorbing photons.

The reason for this is simply angular momentum conservation.  The photon has angular momentum, which, when absorbed, has to transfer to the disk.  The only way for the disk to gain angular momentum is to spin faster.

Do you want to know where the angular momentum goes?
It is radiated away with the radiation that is radiated away from the wheel.

Or mass of wheel increases.

#### jartza

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##### How fast does photon spin?
« Reply #27 on: 20/12/2010 06:01:09 »
Let's say we are traveling through a galaxy very fast. In our frame of reference most photons are moving approximately along our line of motion. So most photon's spins are aligned approximately along our line of motion. So if these photons give us some net angular momentum, that angular momentum's direction is along the line of our motion.

#### Geezer

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##### How fast does photon spin?
« Reply #28 on: 20/12/2010 06:14:46 »
Or mass of wheel increases.

Can you explain how that might happen?

#### jartza

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##### How fast does photon spin?
« Reply #29 on: 20/12/2010 06:21:34 »
Mass increase happens when the wheel absorbs photons but does not radiate photons.

#### JP

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##### How fast does photon spin?
« Reply #30 on: 20/12/2010 06:28:42 »
Well basically if the wheel starts rotating, and you assume E=mc2, then the wheel gains mass.  But the energy we're discussing here is rotational energy, which is also explained in terms of angular momentum transfer.

#### Geezer

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##### How fast does photon spin?
« Reply #31 on: 20/12/2010 06:41:22 »
Well basically if the wheel starts rotating, and you assume E=mc2, then the wheel gains mass.  But the energy we're discussing here is rotational energy, which is also explained in terms of angular momentum transfer.

So, is it safe to assume that, typically, wheels don't gain mass?

#### jartza

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##### How fast does photon spin?
« Reply #32 on: 20/12/2010 07:54:55 »
JP, When a photon hits a black object, the mass of the object increases.

#### JP

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##### How fast does photon spin?
« Reply #33 on: 20/12/2010 07:57:42 »
To Geezer: Generally, no, unless you're in a case where relativistic effects are important.  Even then, defining mass from E=mc2 makes the assumption that you're dealing with relativistic mass rather than invariant mass (you have two kinds of mass in relativity).

To jartza:  The object gains energy.  If you define mass from E=mc2, then the object gains mass.  You've posted a few times about a law of conservation of mass.  This law simply doesn't exist.  It's conservation of energy and you've chosen to define mass as proportional to energy.
« Last Edit: 20/12/2010 08:01:22 by JP »

#### jartza

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##### How fast does photon spin?
« Reply #34 on: 20/12/2010 08:14:41 »
JP, I remember conservation of mass discussion. It ended when you said: OK I admit such law exists.

#### JP

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##### How fast does photon spin?
« Reply #35 on: 20/12/2010 08:48:27 »
It went pretty much the way this discussion is going.  I said you were talking about conservation of energy, but that if you wanted to call it conservation of mass, using E=mc2, you could, since energy is proportional to mass if you define it that way.

That's also an answer to Geezer's question.  The mass gained in this definition is proportional to the energy gained divided by the speed of light squared.  The speed of light is a huge number, so the mass gained is negligible in most cases.

#### peppercorn

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##### How fast does photon spin?
« Reply #36 on: 20/12/2010 09:43:56 »
JP, I remember conservation of mass discussion. It ended when you said: OK I admit such law exists[, but by scientific convention is called 'conservation of energy']

Then why are you going over the same ground again? Can't you just move on to pastures new?

Post by jartza click to view.

#### jartza

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##### How fast does photon spin?
« Reply #37 on: 21/12/2010 00:39:34 »
Shrunk
Let's call the "law of conservation of mass" "alphagammasuperduperlaw".
Let's call the "mass" that is conserved in alphagammasuperduperlaw "Crazy Jartza's Silly Mass"

When a photon hits a black object the Crazy Jartza's Silly Mass of the object increases.
When the Crazy Jarza's Silly Mass of an object increases the moment of inertia of the object increases.

A photon has a constant amount of angular momentum, called "spin".

When a photon hits a spinning black object it increases the angular momentum and the moment of inertia of the object.

Oh yes, when a photon hits a black object the increase of Crazy Jartsa's Silly Mass is proportional to the frequency of the photon.

So, when a very low frequency photon hits a spinning black object, the increase of moment of inertia can be ignored as very small, and the spinning rate of the object increases.

When a very high frequency photon hits a spinning black object, the increase of angular momentum can be ignored as very small, and the spinning rate of the object decreases.[whitespace removed]
« Last Edit: 21/12/2010 01:36:29 by peppercorn »

Post by peppercorn click to view.

#### peppercorn

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##### How fast does photon spin?
« Reply #38 on: 21/12/2010 01:43:43 »
Shrunk
[-take your pick from any of it-]
Silly new words, same old waffle.

#### JP

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##### How fast does photon spin?
« Reply #39 on: 21/12/2010 08:03:06 »
If you put a disk on a frictionless axle through its center and bombard it with photons carrying angular momentum then it will get contributions to its energy from rotation.  It will also get pushed onto its axis, but since this axis is presumably bolted to the earth somehow, this push gets transferred into the earth.  This contribution to energy is lost.  Therefore if you just look at the disk itself, energy isn't conserved.  Angular momentum is.

#### jartza

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##### How fast does photon spin?
« Reply #40 on: 21/12/2010 10:41:49 »
Hey JP that went wrong

But I'll fix it for you.

If you just look at the disk, neither energy nor angular momentum is conserved. (both increase)
If you look at the disk and the photon, both energy and angular momentum are conserved.
If you look at the disk and the photon, some linear momentum disappears, it goes to the continental plate, through the bolts.

OK?

(if you are looking from a frame where disk and continental plate that the disk is bolted on are moving, then the continental plate receives an energy force times distance, where force is radiation pressure and distance is the distance that the continental plate moved while the force was pushing )

#### yor_on

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##### How fast does photon spin?
« Reply #41 on: 21/12/2010 20:32:00 »
I don't agree there Jartza. Energy and mass is not the exact same. If we want to keep a 'transforming' universe we need a law of conservation of energy, that as energy is the smallest ah, 'property' we know of :)

But matter is different, exactly how it is we don't really know, but there is none that have made a lasting piece of matter (out of light) that I can hold in my hand yet. And very few particles of spontaneous, or not, pair-production stay without transforming almost instantly, well, as I know. Then there are other arguments too for stating the same.
« Last Edit: 21/12/2010 20:33:45 by yor_on »

#### JP

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##### How fast does photon spin?
« Reply #42 on: 22/12/2010 01:20:45 »
If you look at the disk and the photon, both energy and angular momentum are conserved.

That's not true.  Angular momentum is conserved in the special case that your light beam strikes the coin in the exact center.  Energy isn't conserved since some is transferred into the earth.  Momentum conservation still holds, as long as you don't consider the components directed into the earth.  You can do this because momentum has a direction.

#### jartza

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##### How fast does photon spin?
« Reply #43 on: 22/12/2010 01:47:34 »
JP, when a photon hits a solar panel, it's not the Earth that gains energy, it's the panel.

#### JP

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##### How fast does photon spin?
« Reply #44 on: 22/12/2010 03:32:06 »
JP, when a photon hits a solar panel, it's not the Earth that gains energy, it's the panel.

Not if that panel's connected to the earth by an axle, which is what we've been assuming in this problem.

You're making some very basic mistakes

#### jartza

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##### How fast does photon spin?
« Reply #45 on: 22/12/2010 08:16:24 »
What's the matter JP, it's an odd claim that a car wheel does not get hot in the sun, if the  bearing of the wheel is very good, isn't it?.

#### JP

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##### How fast does photon spin?
« Reply #46 on: 22/12/2010 15:48:45 »
What's the matter JP, it's an odd claim that a car wheel does not get hot in the sun, if the  bearing of the wheel is very good, isn't it?.

What does this have to do with my claim that energy is transferred into the car's axle and into the earth?  It still is.

#### jartza

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##### How fast does photon spin?
« Reply #47 on: 22/12/2010 20:38:02 »
I think I need some more info.

A continuous one watt laser beam hits a wheel with frictionless bearing, and the axle is bolted to the ground.
How many Joules does the Earth receive in a second? Is it closer to 0 Joules or 1 Joules?

#### Geezer

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##### How fast does photon spin?
« Reply #48 on: 22/12/2010 20:43:04 »
0 J

(Not Simpson)

#### peppercorn

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##### How fast does photon spin?
« Reply #49 on: 22/12/2010 23:58:57 »
I think I need some more info.
I think you need some help (mathematical obviously)

#### The Naked Scientists Forum

##### How fast does photon spin?
« Reply #49 on: 22/12/2010 23:58:57 »