Light Pressure

Asking duplicate questions usually doesn't get different answers.

How to Demonstrate Light Pressure?

To use that formula, one would also have to know basic Newtonian kinematics. I don't see this happening no matter how much light one puts on this stage. (Pun intended)

The photon is a one of a kind particle in the Big Picture - It is massless - meaning that for it to impart kinetic energy to a mass the mechanism for doing so must involve causing the mass to emit a portion of its mass in some net vector/acceleration direction, with the movement of the mass being the F=ma response. Momentum implies mass.

Photons most certainly do have momentum. Planck's relationship ρ=h/λ

Momentum without mass.

It is a derived momentum when expressed in terms of that (momentum) of electons, which due have mass. Photons are energy particles, pure and simple, one of kind guys yet to be found in the universe. As such they can not impart momentum to mass, only impart energy to cause the mass (or constituent mass particles thereof) to react.

All measurements of momentum are derived measurements.

Sure in the macro world, everything that collides has mass and velocity. But at some point, in the quantum world, there is only energy and how it interacts with matter to get it to move. How does massless energy move a mass? Planck had an equation relating energy to frequency E=hf. Einstein took this and used it to explain the photoelectric effect where the energy coming from the frequency of a photon equals the velocity an electron is ejected in an unpredictable direction from the atom. Since an electron has mass, mv was found to equal hf.

The math gets kind of screwy here because energy is not the same as momentum. Also if you work from Einstein's energy mass equivalence where E=mc2 you get the mass of a photon to be m= E/c2. Multiply both sides by v, mv=Ev/c2. Substitute E=hf and you get momentum p=hfv/c2. The answer should be p=hf so I'm missing some math tricks.

The point is the photon bumps out the electron and the momentum of the electron bumps the atom it's ejected from. So now these atoms are in motion crashing against each other, taking up more room than they should. They're also swollen by photons which have bumped internal electrons to higher orbits. All this swelling and crashing makes the entire mass expand. If that expansion is set by an external device such as a piston to move in one direction, then big things begin to move. The photon exciting the electron is the source of all our mechanical energy which gets directed to crash billiard balls together confusing us that this is how momentum is supposed to work at every scale.

I know, I put a 100 watt bulb in my refrigerator and now when I close the refrigerator door, the freezer door pops opens.

Do you have that problem too?

Yes there are, but why are you asking these questions here and in other threads, is this a homework project or something?

Try an internet search for the basics (you knew who Peter Nikolaievich Lebedev was so you should know enough to find out the basics). Come back if you have specific questions that an internet search and some personal research on your behalf cannot answer.

With a laser light on an object DOES the object move at all?

It comes to mind that friction might still be a big issue.

Have fun he said, set his sun sail and went away!

The initial question has no meaning. The distance depends on the friction.

1) If the friction is zero and the laser shots just once on the object then the object will obtain a momentum (transfer of momentum from the laser's photons) and, hence, a velocity. Then the object will keep on moving forever having this specific, steady velocity (due to zero friction). If the friction is not zero then the object will be decelerated and eventually will stop.

2) If the friction is zero and the laser shots continuously on the object then a continuous power will be applied on the object and it will be accelerated forever. However, this acceleration will be gradually dicreased in relation with the distance due to the attenuation of the laser beam. (After a very long distance maybe no laser's photons will fall on the object at all, and the object will end up moving with a steady velocity forever.) If the friction is not zero then: a) if power>friction → accelerated movement b) if power=friction → steady velocity and c) if power<friction → decelerated movement.

okay, if you say no because I do not believe you,
Just tell me that you are using laser (wavelength), horsepower, and the coefficient of friction of the table, or if the object to which you shoot him, this under vacuum on a bed of air as a closed system .. .

WHO don't you believe? It is customary to make that clear, if you are going to call into question someone's veracity. I see many good, useful, substantive answers here, though personally I don't believe we have a laser with enough "grunt" to actually impart motion to anything of substantive mass.

Notwithstanding, don't call all of us liars. Be specific. Otherwise you'll never be able to figure out why EVERYONE here attacks you. In print, of course.

micahd02 - Another way of seeing things: The impact of the energy released for repulsion

Do you really think that you have made things more clear? You should eschew obfuscation by answering direct questions.

I have no doubt energy is released, but according to any calculation you care to name, light "pressure" from photonic impact is so miniscule that I have serious doubts it would impart any measurable momentum to any measurable mass.

It's there, but it's too small to be of any practical use. And yes, I've VERY familiar with the characteristics of coherent light, and the fact that since it IS coherent, out-of-phase (i.e., negative) reinforcement is for all practical purposes non-existent, so that nearly all of the launch energy is available at the terminus to do the work. Still, it would take a laser of considerably more SUSTAINED impulse power than any available to do the kind of acceleration work the OP is speaking of.

Don't condescend to me. I know the sciences, and particularly physics, as well as being able to work out the fairly elementary math required here.

I just don't see the kind of energy necessary to overcome the various forces of stiction, friction, and sheer mass/energy input required to move anything significant.

But my main point, since you missed it, either accidentally or on purpose, is that you said "I don't believe you" without bothering to say who it is you don't believe. That is rude, and makes you many unnecessary adversaries, since we are ALL subject to your implied "you are lying". Key your answer to the specific post, as I did to yours. Then you'll only annoy one person, and perhaps his friends, instead of all of us.

Thanks

The energy of the photon either imparts randomly directed momentum to electrons from atoms (as in the photoelectric effect by the formula given in this thread) or causes electrons to jump up a shell causing the atom to swell. When a lot of atoms swell, the entire mass swells. Movement of the mass is dependent on how you direct the swelling. For example, water is heated into steam by both atoms swelling and moving around due to Brownian motion. Then you take that swelling to drive a piston in a steam engine and suddenly you have a whole locomotive moving from the energy imparted by photons.

OK. You've lost me, so I'm going to bail on this one. How in the world did you get photons to jump up a shell and create steam to drive your locomotive?

Also google crookes radiometer.

Yeah. I know about Crookes Radiometers. I've seen them since I was a kid. Almost no mass, almost zero friction on the bearing, absolutely zero practical work being done.

Kind of makes my point for me, doesn't it?

I'm outta here. Wasting time is a waste of time.

Lo pongo en mi lengua natal, para evitar confusiones:

Está bien, lo que dices no tengo por qué no creerte. (Luego pido los datos del experimento)

Todos sabemos lo que es la presión de luz, yo la llevo al impacto de la energía liberando repulsión.

Nunca había recibido este tratamiento en CR4, era ameno, armonioso y amistoso, y no tan atemorizante.

Tradutor Google:
Okay, what you say does not have why not believe you. (Then I ask the experiment data)
We all know what is the pressure of light, I took her to the impact of energy releasing repulsion.
I never received this treatment in CR4, was pleasant, harmonious and friendly, and not so scary.

I am sure google translate will not help us here at all.

Imagine the question is translated, an answer is prepared and posted here (which there was) and nobody knows whats going on.

Imagine - this how it feels!

I'm in the dark on this one.

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Quantum mechanics prevents the object from moving in any direction except to randomly swell up. Atoms swell due to photons bumping up electrons to higher shells. Electrons are also randomly ejected by photons which cause their atoms to move and hit other atoms. High energy light comes in, work is created and heat is expelled from the object.

The crookes radiometer apparently looks like light is directly moving the vains but it is not. It's the low level heat that heats up the air near the vains and that air bounces against them moving them. In a vacuum, the veins would not move because the motion inside the metal totally cancels itself out. However if the random motion of the electrons ejected could be set in a direction inside a magnetic field or by a diode junction then a current could drive a motor that would give the object a ride. The object could not be moved by the light directly.

I've never heard of atoms ever swelling up like a sponge.A photon of the correct energy can move an orbital electron to a higher energy, vacant orbital but that does not make the atom any larger than it was before.

You are correct that a Crookes radiometer or light mill does not demonstrate the effect of light pressure. One must go to all of the trouble Pyotr Lebedev did in his experiment to show that light does indeed create a measurable pressure.

Wiki atomic radius and electron shell theory. I am surprised to find light does have a pressure but the articles seem to smear radiation pressure and light pressure. I must read further how this is physically possible.