On a website regarding the physics of solar sailing, I read that "At Earth's distance from the sun, the solar flux, Ss, in space is about 1.4 kilowatts per square meter. This is enough power to run a hair dryer continuously, but not enough to power a car." I would like to know on the surface of the earth what an average range for the number of photons per cubic meter is? I know this is not the typical units of measurement for quantifying light in space. Any help is appreciated.

The light from the sun has a spectrum of wavelengths, so it's not a simple matter. But if we had a monochromatic source, the numbers are easier, and would serve as an approximation. The energy of a green photon is about 2.25 eV, which means that there are about 2.8 x 10^18 photons per Joule of energy. You have 1.4 kW traversing 1 meter, which takes 3.33 nanoseconds, so in a cubic meter there will be 4.7 microJoules. That gives a little more than 10^13 photons per cubic meter under that assumption, which means this is probably correct to within an order of magnitude.

Given that photons have a momentum, is there an analogy to atmospheric pressure that can be made - but in this case for light instead of for atmospheric atoms? If so, how strong or weak compared to atmospheric pressure would this "light pressure" be?

Given that photons have a momentum, is there an analogy to atmospheric pressure that can be made - but in this case for light instead of for atmospheric atoms? If so, how strong or weak compared to atmospheric pressure would this "light pressure" be?

 

Yes, and there are a number of phenomena that use the effect. With a perfect reflector, to accelerate 1 kg at 1g requires about 1.21 GW of optical power.

Yes, and there are a number of phenomena that use the effect. With a perfect reflector, to accelerate 1 kg at 1g requires about 1.21 GW of optical power.

 

1.21 Gigawatts? That happens to be exactly the amount required to power my time machine.

In the nomenclature of physics, is there a formal name for "light pressure" or "photon density"?

 

losfomot, I don't know what you are referring to?

 

Also is there a word for a vacuum that is not just that absence of atoms, but also the absence of photons?

 

And does the technology exist to create such a vacuum?

 

swansont, What are the names of the phenomena that use this effect?

Radiation pressure:

 

http://en.wikipedia.org/wiki/Radiation_pressure

 

Solar sails use it to fly:

 

http://en.wikipedia.org/wiki/Solar_sail

losfomot, I don't know what you are referring to?

He is joking, the DeLorean time machine in the movie "Back to the Future" needs exactly 1.21 GW to work.

 

Back to the Future is a 1985 American science-fiction comedy film. It was directed by Robert Zemeckis, written by Zemeckis and Bob Gale, produced by Steven Spielberg, and starred Michael J. Fox, Christopher Lloyd, Lea Thompson, Crispin Glover and Thomas F. Wilson. The film tells the story of Marty McFly, a teenager who is accidentally sent back in time from 1985 to 1955.

...

On the morning of October 25, 1985, Marty meets his friend, scientist Dr. Emmett "Doc" Brown, at Twin Pines Mall at 1:15 am at Doc's request. Doc reveals a DeLorean DMC-12 which he has modified into a time machine; the vehicle's time displacement is powered by nuclear fission and using plutonium as fuel, which generates 1.21 gigawatts of power into a device he calls the "flux capacitor".

http://en.wikipedia.org/wiki/Back_to_the_future