A bit confused with the massless nature of photons

[YT2095]

and Again, What is the Mechanism that causes the forwards momentum in a solar sail ?

 

 

all these posts from me, and yet not ONE answer!?

[Dror]

I`de still like a description of the mechanism of photon interaction with a material that creates a forwards push.

 

As a photon hits an atom it excites it and causes it to accelerate.

Isn't that enough?

ALL of the matter around you including you is being accelerated by the light that hits it. It's just not enough to beat earth's gravity. And even if it was the effect would have been negligable.

However, if you put a book in deep space, and aim a flashlight at it, it'll slowly accelerate due to the energy that hits it.

If you attach a huge sail to the book, and shine the sun on it, it'll accelerate faster. A lot faster.

[YT2095]

by changing the electron orbit(als)?

[swansont]

swansont:

 

can YOU explain the mechanism behind HOW a solar sail works?

the only thing I can think of as a guess' date=' is that Photons "hit it" and eject Electrons inversely to the hits and thus provide motion with electrons having a known mass.[/quote']

 

A photon has momentum of p = E/c. If that photon is absorbed, that momentum is transferred to the system that absorbed it - it will recoil.

 

Better yet is to have the photon reflect: now the photon has momentum of E/c, but in the opposite direction. The change in momentum (it's a vector) is 2E/c if it's reflected back along the same direction (i.e. all of this is perpendicular to the surface). Any change in momentum is due to a force - if you do this with a bunch of photons, the force will be F = dp/dt = d(2E/c)/dt.

 

but dE/dt is the power of the laser, so F = 2P/c for reflection, and P/c for absorption

 

For absorption, it doesn't matter what transition is made - whether it's an electronic excitation, or a change in a vibrational or rotational state or whatever. Any emitted photon will go in a symmetric distribution, and the average momentum of these photons will be zero.

[swansont]

I agree they fall at the same rate' date=' that's not what I said.

All I said is that the rock will hit first. Due to the moon activly participating and getting closer. The bigger the gravitational field the stronger pull on the moon, hence differnt hit times.

This effect will not reveal itself if you throw both the rock and the feather at the same time. The gravitational pull on the moon in that case will be rock+feather, and as they both have the same velocity, it's only logical that the will hit at exactly the same time.[/quote']

 

OK, I misread what you were saying.

[YT2095]

Swansont Thanks, I`m partway there in understanding, I think you`re saying that the photon is pure energy, and that energy MUST go somewhere when absorbed (hope I`m right so far).

 

I`m still confused as to WHY/HOW it moves though? why doesn`t it just get HOT and stay still, or start making electric sparks and corrona discharges and stay still, or become magnetic as a result of the current and stay still?

 

if it`d the orbitals in the atoms that the sail`s comprised of, basicly that linear momentum, accelerating existing Angular momentum (on an atomic scale) being converted back to Linear momentum?

 

I still don`t understand (feel free to "Dumb it down" if you like, I`ll NOT be offended!)

[alt_f13]

Throw a ball at someone's face. The kinetic energy in the ball is transferred to the face. The resulting effect on the ball and photon would be different, but the kinetic energy is transferred all the same.

 

The ball may bounce (maybe giving twice the force as explained earlier) and the photon may bounce. Same mechanism, same result.

 

[edit]

 

To the face, the photon does have mass because it travels at c, so it does have momentum to act on the face.

 

Going in to the mechanism on the subatomic level would be as pointless (in this case) as going into the interaction of the ball and face at the subatomic level. The net effect is the same.

 

The point is, the kinetic energy is transfered from the light to the thing it's hitting.

[YT2095]

but what does it HIT?

 

and what happens to the photon afterwards if not reflected backwards?

 

and why don`t the ones that get absorbed just make the material Hotter?

 

if it has no mass, how can it "pack a punch"?

 

 

all these and Many Many more, coming to a cinema near you!

[swansont]

but what does it HIT?

 

and what happens to the photon afterwards if not reflected backwards?

 

and why don`t the ones that get absorbed just make the material Hotter?

 

if it has no mass' date=' how can it "pack a punch"?

 

 

all these and Many Many more, coming to a cinema near you![/quote']

 

It hits the atoms - there's some transition available that causes them to be absorbed, but it doesn't have to be a change in an electron orbital. In a solid, many different types of transitions are possible*. It can make them hotter, but the hit causes a recoil as well. (Since p=E/c, there's a lot of energy for the amount of momentum available, so for absorption it's pretty certain that it will make things hotter)

 

As far as why a photon packs a punch with no rest mass, that's one of the weird things of QM and relativity. Things are not as classical physics tells us.

 

*in a gas, it does have to be a change in the orbital, but there are still ways of arranging things that let the gas cool down rather than heat up. In this regard the simpler atomic system works in your favor.

[YT2095]

can you provide some kind of textual illustration/analogy to liken it to?

 

I`ve now got fragmented snippets of understanding (I think), any chance you can bring them together for me?

 

p=e/c doesn`t mean Jack to me for example, F = dp/dt = d(2E/c)/dt.

means considerably LESS! and so on

 

when I said "I don`t mind if you "Dumb it down"" for me, that was a HINT! )

[5614]

YT:

 

there are different types of masses... when you say 'mass' in a converstation such as this one about photons its not really specific enough, although everyone still says it!

 

photons have 0 rest mass... its quite a common fact, just as common as photons have energy and the fact that e=mc^2

 

now if e=mc^2 and the m=mass=0 then there's no energy, which is obviously wrong.

 

what we have here is different types of mass. when we say e=mc^2 we are referring to a photons relativistic mass. when we say photons have 0 mass we are referring to a photon's rest mass.

 

[edit] oh yeah, the conclusion being that photons can have a mass, it depends on how you define the word 'mass'.... (damn definitions!)

[YT2095]

Mass to ME, is weight at 1 G.

 

as in Atomic mass or Molar mass.

 

I loosely understand Mass as that of being Inertial, seperate from that involving gravity.

[swansont]

Conceptually it's really like any other collision. Throw a pebble at a large ball and the large ball will recoil a tiny bit, regardless of whether the pebble bounces off or sticks. Throw a lot of pebbles and it recoils more.

 

The only detail that is different is that a photon generally has a lot less momentum for how much energy it has.

[The Rebel]

WOW!!!

 

I'm away at the weekends but what a thread to come back to, very good reading. Just the sort of thing I'm trying to get into and understand.

 

In my opinion, a photon is not really a particule but a packet of energy that forms a communication between two "talking" particles. For example a reaction in the sun causes electrons to react in such a way that is resonant with the receptors in our eye. Just like electrons vibrating in an aerial are resonant with a tuned radio circuit. With light there maybe interceptors along the way, such as when the energy is absorbed into particles in the sky, and consequently "new" energy sent on to our eyes. I don't think light actually exists per say, its just an energetic interaction. This leads me to appreciate the idea of photons not having mass, except in abstract form.

 

With the sail thing, I think the high energy in the laser causes the electron configuration to take the equivalent of an electric shock.

 

To the original question photons travel at c therefore don't have mass. No problem with that, the only thing that bothers me is where the value of C come from, i.e. why is it 300000000ms-1 all the time. A question that could be asked of all constants.

 

And why are photons effected by black holes despite not having mass. This I believe to be the distortion of the EM energy between the "communicating" particles. I fail to consider matter without the presence of some of electromagenetic energy in the particles that make a black hole. And I think its these property of the black hole that distorts the energy path like a magnet distorts the tv screen.

 

Please fault find my opinions, I'd really like to get to the bottom of this,

[5614]

a photon is not really a particule but a packet of energy that forms a communication between two "talking" particles.

Photons acts as particle and waves, aka QM's wave-particle duality, although I suppose your definition works on a basic level.

 

I don't think light actually exists per say, its just an energetic interaction.

I don't quite understand what you meant by that.

 

To the original question photons travel at c therefore don't have mass

You can't say that! read my post #61, it depends on your definition of 'mass'

 

And why are photons effected by black holes despite not having mass.

Photons do not travel in straight lines, they take the shortest distance between two points. Near black holes space time is bent, the shortest distance for light to travel is to bend with space-time, hence photons seem to bend through space, when in fact they are just taking the shortest route and following the lines of space-time.

[The Rebel]

5614,

 

The interaction thing. What I'm getting at is we don't sense something until we interact with it. When the receptors in our eyes are stimulated, its from sensing the disturbance we are "looking" at. Light is what describes the way in which the disturbance travels to our eyes. Its hard to explain. It's a bit like electricity in a way, electricity itself does not exist its just what we call the interaction between disturbances in the electroncs . . .

 

Anyway, definition of mass. I mean't rest mass, or more basically matter. I was trying to quote the orignal post which queried that mass could not be accelerated to speeds of c.

 

I have a bit of a problem understanding space-time, may need to do some more research. You say space-time is bent but then what is space, is it the particles within space? whenever I try to understnad it in forms of a bowling ball on a sheet bending the sheet such as the ball bearings fall it, I think well that's because gravity is drawing the bearings down against the sheet. If photons have no rest mass, then they would not be "roll" towards the black hole. What's wrong with the idea that EM energies in the black hole matter are causing it instead?

[KennyC]

sayonara' date=' the only thing I didn't understand was how come a massless particle is affected by gravity.

swanston cleared it out by saying that gravity affects both matter and energy.

End of story.[/quote']

 

It's not, but space is.

 

KAC

[J.C.MacSwell]

It's not' date=' but space is.

 

KAC[/quote']

 

Space is, so it is.

[5614]

What I'm getting at is we don't sense something until we interact with it.

Yeah, but that doesn't mean its not there.

 

You say space-time is bent but then what is space, is it the particles within space?

well matter tells space-time how to curve and space-time tells matter how to move. It it a four-dimensional space used to represent the universe in the theory of relativity, with three dimensions corresponding to ordinary space and the fourth to time. Also known as space-time continuum, its basically the world in 4D... the universe.

 

If photons have no rest mass, then they would not be "roll" towards the black hole.

As previously said this is assuming photons travel in straight lines, which they dont, they'll follow space-time which is bent by gravity and black holes... i dont think its related to mass at all, if it is then its to do with relativistic mass... its more about light following space-time which is bent by the black holes, you will notice a similar effect around the sun, although it'd be on a seriously smaller scale.

[Severian]

YT:

 

there are different types of masses... when you say 'mass' in a converstation such as this one about photons its not really specific enough' date=' although everyone still says it!

 

photons have 0 rest mass... its quite a common fact, just as common as photons have energy and the fact that e=mc^2

 

now if e=mc^2 and the m=mass=0 then there's no energy, which is obviously wrong.

 

what we have here is different types of mass. when we say e=mc^2 we are referring to a photons relativistic mass. when we say photons have 0 mass we are referring to a photon's rest mass.

 

[edit'] oh yeah, the conclusion being that photons can have a mass, it depends on how you define the word 'mass'.... (damn definitions!)

 

I'm sorry, but this is just silly. You can't just make up a new definition of mass to allow you to say that photons have mass. Any 'definition' of mass which is not frame independent is completely useless and just confuses the issue.

 

Although photons have no mass, they still have energy. As has already been pointed out numerous times

[math]E^2=m^2 c^4 + p^2c^2.[/math]

The 'm' in this equation should be (for the purposes of this thread at least) the definition of mass. For a photon m=0, so this becomes E=pc. Photons have energy corresponding to their momentum.

 

Also note that I didn't say that photons have no 'rest' mass. Since photons have no rest frame, the concept of rest mass for a photon is not defined.

[5614]

What's wrong with saying that in e=mc^2 mass is referring to the relativistic mass and that the relativistic mass for a photon is not zero?

[swansont]

What's wrong with saying that in e=mc^2 mass is referring to the relativistic mass and that the relativistic mass for a photon is not zero?

 

As just came up in another thread, you run into the problem of mass not being a scalar anymore. If m=F/a, then the mass will be different depending on the speed and the direction of the applied force - it will be harder to accelerate the object in the direction of motion than in the perpendicular direction.

[Severian]

What's wrong with saying that in e=mc^2 mass is referring to the relativistic mass and that the relativistic mass for a photon is not zero?

 

It's just wrong. That is not the definition of mass. You are trying to generalise the rest frame energy mass relation to something it isn't applicable for, because the photon has no rest frame.

 

Let me put it this way, for a massive particle you are saying that E=mc^2, therefore why not regard this as the definition of mass and thus the mass of the photon is E/c^2. Right?

 

But this is not the correct expression - you should really write [math]E^2=m^2c^4 +p^2c^2[/math]. Then we have

 

[math]m=\sqrt{E^2/c^4 - p^2/c^2}[/math]

 

Now, it is OK to use this as a definition of mass, but then you will find for the photon that E=pc and m=0. The photon has no mass.

[Johnny5]

It's just wrong. That is not the definition of mass. You are trying to generalise the rest frame energy mass relation to something it isn't applicable for' date=' because the photon has no rest frame.

 

Let me put it this way, for a massive particle you are saying that E=mc^2, therefore why not regard this as the definition of mass and thus the mass of the photon is E/c^2. Right?

 

But this is not the correct expression - you should really write [math']E^2=m^2c^4 +p^2c^2[/math]. Then we have

 

[math]m=\sqrt{E^2/c^4 - p^2/c^2}[/math]

 

Now, it is OK to use this as a definition of mass, but then you will find for the photon that E=pc and m=0. The photon has no mass.

 

Severian, photons do have rest frames. I suggest you try to derive the equation for total energy yourself. See the other thread.

 

Regards

[Severian]

Severian' date=' photons do have rest frames. I suggest you try to derive the equation for total energy yourself. See the other thread.

[/quote']

 

No they don't. Light speed © is a constant in all frames. It doesn't matter which frame you sit in, light always travels at c and is therefore not at rest. Thus there can be no rest frame for light. (Assuming they don't have mass.)

 

I suggest you read my post before commenting on it.