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Author Topic: Do photons have momentum?  (Read 1611 times)

waytogo

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Do photons have momentum?
« on: 23/09/2012 11:17:52 »
Here we go....

lightarrow

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Re: Do photons have momentum?
« Reply #1 on: 23/09/2012 13:27:57 »
Of course: p = hf/c   f is the photon's frequency. Anyway, even classical light have momentum. Before someone could think that this means it has mass, momentum IS NOT m*v, this formula is FALSE for photons or fast elementary particles, it is only approximately valid for objects with mass at low speeds.

evan_au

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Re: Do photons have momentum?
« Reply #2 on: 23/09/2012 13:34:23 »
The momentum of light is the basis for the Solar Sail concept - a way to navigate around the solar system without using fuel (as long as you aren't in a hurry).

If you reflect the light, you impart twice the momentum to the spacecraft than if you absorb the light.

Phractality

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Re: Do photons have momentum?
« Reply #3 on: 23/09/2012 18:22:04 »
You could propel a spaceship with a laser. Each photon has momentum; an equal and opposite momentum is imparted to the ship. The force imparted to the ship is the momentum of one photon times the number of photons emitted per seconds.

If the laser is 100% efficient, the force imparted to the ship will be the power consumed divided by the speed of light. The power required to get a certain force is the force times the speed of light. For one newton of force, you need to supply 300,000,000 watt of power to the laser.

syhprum

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Re: Do photons have momentum?
« Reply #4 on: 23/09/2012 22:12:14 »
Do more energetic i.e higher frequency photons impart more momentum or is only a matter of the total power.
The SpectraPhysics lasers I used to work with produced 10mW output for 1Kw in !

Phractality

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Re: Do photons have momentum?
« Reply #5 on: 24/09/2012 01:43:21 »
Do more energetic i.e higher frequency photons impart more momentum or is only a matter of the total power.
Yes and yes. Each higher energy photon has more momentum; you get fewer photons per joule of energy, and fewer photons per second per watt. The momentum of one photon is its energy divided by the speed of light. Total power is the energy of one photon times the number of photons per second. The total force is the momentum of one photon times the number of photons per second (assuming they are all emitted in the same direction).
The SpectraPhysics lasers I used to work with produced 10mW output for 1Kw in !
At 1% efficiency, you would need to supply 30 Gigawatt to get a 300 Megawatt laser beam with a reaction force of 1 newton. That ten times the output of the world's largest nuke plant. The other 29.7 Gigawatt would be need to be dissipated somehow. Perhaps you could convert most of the waste heat to black body radiation. If you could reflect all the radiant heat in the same general direction as the laser, you still would get pretty high efficiency. Radiant heat would also have a reaction force of power/c multiplied by the dispersion factor. Light/heat radiated 60 off center would contribute half as much force; perpendicular would contribute nothing to the thrust.

Keep your seat belt fastened, and please be careful where you aim your laser when departing from Earth!

The Naked Scientists Forum

Re: Do photons have momentum?
« Reply #5 on: 24/09/2012 01:43:21 »