How can light exert pressure?

[jaredkzr]

So we know solar sails are a real thing and can be used to propel a craft. My question is how can light exert pressure on an object. Light consists of photons which do not have mass yet somehow have momentum. How is this possible?

[Dynamo]

This one is a little tough. My own intution says that it has not to do with the collision of a proton with matter itself, but rather with its field of influence, the electron clouds. I'm no quantum physicist, but i think i remember that photons can collide with electrons, and raise them to a higher energy state. The emission of electrons, or repulsion of electron on electron as the excited electrons jump around, may generate the tiny forces associated with solar shields.

tldr: Light energy excites the atoms into pushing each other.

Will come back after a Wikipedia reading!

[lajoswinkler]

Read all about it. It's not simple.

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

[sparty48]

I don't think we REALLY know exactly why, we just know that it does.

Bad news is a much better propellant.

[thescientist]

I don't think we REALLY know exactly why, we just know that it does.

Nobody knows everything

Nobody is perfect

Nobody really knows what "truth" mean

Nobody, except he himself, is god

Once said that, we know very well something till someone discover that we are wrong.

Human aren't perfect, so you're probably right, but presently we know how and why it does. Maybe tomorrow things will change. Maybe

[Mr Shifty]

Surprisingly hard to find good technical info about this mission, but IKAROS was a pretty cool demonstration of this technology:

http://www.jaxa.jp/press/2010/06/20100628_ikaros_e.html

[Kryten]

Well, try thinking of it like this. Thanks to Einstein, we know that a change in the energy content of a body means a change in it's mass; E=MC² and all that. So the body emitting the photon must have a lower mass, and so a change in momentum. Momentum is conserved, so the photon must have some momentum of it's that it's carried off. Change in momentum=impulse on the body.

[vetrox]

The way i had it explained to me was.

Tennis balls = photons

saloon door = solar sail.

As you throw the tennis balls at the door the door gets pushed open slightly.

I think theres a theory that this effect is what causes some asteroids and meteors to lose their orbit in the asteroid belt. (some science guy on tv when that big ass meteroite went of russia)

thats the simple version anyway. the great thing behind the theory of solar sails is that momentum would constantly increase infinately. So a solar sail craft will just get faster and faster.

[Ralathon]

I always like to explain it with the Lorentz force. (I know this is a simplified version of reality and I skim over the vector calculations. Don't hate on me )

Lets start by looking at a simple lightwave. Monochromatic and polarized:

As you can see the wave consists of 2 parts: an electric wave and perpendicular to that a magnetic wave.

Now lets suppose this wave hits a particle with a charge. The electric field will start to accelerate the particle so it starts wiggling in the same direction. But, we also have a magnetic field. As you might remember from physics class a charged particle moving in a magnetic field is deflected with a force perpendicular to both the velocity and the magnetic field. Drawing out the situation it looks like this:

(The magnetic field is pointing into the screen as indicated by the crosses)

This is the situation for half of the wave period. In the other half both the electric and the magnetic field are reversed. This means the velocity vector is also reversed. And as we know -1*-1 = 1. So the force experienced by the particle is in the exact same direction. So the lightwave will continuously push the particle in a certain direction. It even compensates for negative charges, a negatively charged particle will wiggle in the opposite direction while the lorentz force is also in the opposite direction. Those 2 will again cancel out. In other words, everything that contains charged particles will be pushed by a lightwave. Pretty much anything in existence contains charged particles so we can move stuff around by shining lights at it.

[MBobrik]

photons do have mass, it's just their rest mass that's zero but this is irrelevant as they always move at c. E=mc^2 after all.

.

and the radiation pressure can be directly measured in the lab http://en.wikipedia.org/wiki/Nichols_radiometer

[Comrade Jenkens]

I always thought that the photons had momentum and so could impart energy to an object which they hit.

[Vilheim]

Photons do not have mass at all. E = mc^2 is actually an incomplete equation which describes only slow moving particles. Since light is massless and moves very fast (at the speed of light) This special case does not apply. The full equation, from Einstein's special relativity, is E^2 = (mc^2)^2 + (pc)^2 where p is momentum. Since photons have no mass, this equation simplifies to E = pc or p = E/c. Thus the momentum a photon carries is equal to its energy divided by the speed of light. When a photon strikes a particle and is absorbed, its momentum must be transferred to the particle, and so the particles momentum will change.

[DaveofDefeat]

But then again light is not just a particle but a wave!

[lajoswinkler]

It's a particle for some purposes, and a wave for another purposes. Don't try to understand why. It just is.

[DaveofDefeat]

It's a particle for some purposes, and a wave for another purposes. Don't try to understand why. It just is.

Not a very good attitude for science doncha think?

WHY IS THE SKY BLUE

BECAUSE REASONSSSSSSSSSS

[Vilheim]

Not a very good attitude for science doncha think?

WHY IS THE SKY BLUE

BECAUSE REASONSSSSSSSSSS

The why, so far as I understand, is quantum mechanics, in terms of light acting both like a wave and a particle (see uncertainty principle). As a matter of fact, ALL particles exhibit this wave-particle duality, though the more massive a particle gets, the less wavelike it acts and the more particle-like it acts.

Oh, and the sky is blue because of Raleigh scattering. (info: http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html )

[drakesdoom]

The really funky bits are when it acts like a wave till you look at it then it decide to be a particle stream.

[lajoswinkler]

Not a very good attitude for science doncha think?

WHY IS THE SKY BLUE

BECAUSE REASONSSSSSSSSSS

No. You can explain the sky because there's an intricate mechanism behind it.

Quantum stuff is the fundament below which we can not go. You can't explain the fundamental stuff of the universe because you're a human being, you see and feel the macroscopic world and the only way you can depict stuff is to make analogues at your level of existence.

Wave-particle duality is not even microscopic level. It's quantum level. Everything you know and love breaks down at it. It's a world of math with a tiny pinch of physics. Pure equations. No balls, no springs, no levers, no switches. Every picture, every sketch is a grossly disfigured representation of the actual stuff that will forever be hidden from us.

It's a world we can try to understand only through equations about various fields. And even they are approximations. Get it now?

The really funky bits are when it acts like a wave till you look at it then it decide to be a particle stream.

No. That does not happen.

[GregoryNeal]

Okay, I'll give it a go:

Imagine a photon hitting a wall of area A and the wall absorbing the photon. According to Einsteins relativistic invariant

E²= (pc)² + (mc²)²

Where E is the energy of an object, c is the speed of light (299,792,458m/s) and p is the momentum. For a photon, whose rest mass m = 0, you get

E² = (pc)²

or more simply,

E = pc

A good thing to know is that in relativity, the total energy and total momentum of an inelastic collision (ie, the two objects stick together after they collide) is conserved. This lets us write the following equation

p_{system,initial} = p_system,final

when the photon is absorbed by the wall, the total momentum is just the initial momentum of the wall plus the photons momentum. The net change in momentum is 0 since we are not considering any other actions (we are in space, remember?) acting on the system. So the net change in momentum of the wall is just what was added to it after the collision, namely, the photons momentum.

ÃŽâ€p_wall = p_photon = E_photon/c

Now we get to step 3 of solving inelastic relativistic collision problems, my professor calls it Algebraic Wizardry, I agree. From regular mechanics, you know that power is energy per unit time, or

ÃŽâ€Power = E/ÃŽâ€t, so E = ÃŽâ€(Power)ÃŽâ€t

now we can say that

ÃŽâ€p_wall = (power)ÃŽâ€t/c

well, you could now solve for ÃŽâ€p/ÃŽâ€t

ÃŽâ€p/ÃŽâ€t = Power/c

And since the change in momentum over time is just the force on that object (Newtons Second Law),

F = Power/c

since pressure is just force per unit area,

Pressure = F/A ---> F = (Pressure)A = Power/c

now we solve for the pressure exerted by the photon, it's just

Pressure = Power/(Ac)

Incidentally, for light, the power per unit area is the intensity I, so the pressure exerted on the wall by the photon is just

Pressure = I/c

Now if you have a reflecting source, the pressure is doubled because the change in momentum is doubled because now after the collision, the light is traveling away from the wall at speed -c, so the momentum of the light had to double in the opposite direction, this causes the force to be doubled, which causes the pressure to be doubled.

Now to answer you question, why? Because energy does not only depend on the mass of a particle, but rather its momentum, for a photon, that comes from it's frequency and wavelength. If I'm wrong someone please correct me so I don't act like an idiot on more than one forum .

Oh, by the way, hello everyone, this is my first post

[Holo]

No. That does not happen.

What's the double slit experiment about then?

[Kryten]

What's the double slit experiment about then?

In order to make a measurement of a particle, you have to physically interact with it. You can't just 'look at' something on that scale.

[lajoswinkler]

What's the double slit experiment about then?

It's about "ruining" properties of a particle/wave in the moment of measuring it, because measuring induces changes.

Imagine if you're blindfolded in a room filled with stuff you have to touch to determine where are they and what are they made from. If you encounter a large cast iron statue, your fiddling won't change it much. Fingerprints and local weak elevation of temperature, that's it.

If you encounter a tower made of playing cards, you'll destroy it. You tried to measure it, you destroyed it in the process. You determined its position, but the rest of the data is lost.

"You" are irrelevant. That could be a robot in the room, it would be the same.

Particles are not changing their properties because a scientist is watching them. Nothing is exiting our eyes and interacting with them. It's the act of measuring that destroys some of the data, and it's machines that do it.

Lots of quacks have been exploiting the incorrect explanation of this experiment to push forward their lunatic ideas and to make money.

[Fractal_UK]

No you are quite wrong, quantum mechanics tells us that measuring a system absolutely does affect its properties, this is the concept of wave function collapse and one of the interesting things that the double slit experiment demonstrates. That is why the interference pattern disappears if you measure which slit the particle passes through but is very visible if you don't.

A quantum mechanical wave function can contain superpositions of many fundamental states if you don't measure the system but the moment you perform the measurement you cause it to collapse into one of these discrete states or another. This can profoundly change the behaviour of the quantum system.

[Kryten]

Measuring a system profoundly changes it's properties because of what he just said. You haven't refuted his statement, just couched it in buzzwords.

[lajoswinkler]

No you are quite wrong, quantum mechanics tells us that measuring a system absolutely does affect its properties, this is the concept of wave function collapse and one of the interesting things that the double slit experiment demonstrates. That is why the interference pattern disappears if you measure which slit the particle passes through but is very visible if you don't.

A quantum mechanical wave function can contain superpositions of many fundamental states if you don't measure the system but the moment you perform the measurement you cause it to collapse into one of these discrete states or another. This can profoundly change the behaviour of the quantum system.

... which is basically what I've said.