A bit confused with the massless nature of photons

[timo]

Surely i will not argue with this. You obviously know what your saying.

But it still strikes me as odd..

Can you maybe try and explain it non mathematically?

This message was meant for atheist.

Actually the non-mathematical explanation was included and very simple: A particle´s movement in a gravitational field is independent of it´s mass. So when you negelct air resistance a feather falls down exactly as fast as a stone. This in fact is odd and contradicts everyday´s experience. I didn´t believe it myself until I saw the experiment demonstrating this.

 

In Newtonian Gravity this is because the mass cancels out in the equation. In General Relativity the mass doesn´t even appear in the movement equation.

 

Mass creates gravitational fields (in GR even photons do, but that´s a different story) but the movement of a particle that´s influenced by a gravitational field is independent of it´s mass. So if a comet´s path is curved by a sun so must be a photon´s.

[swansont]

So what exactly is relativistic mass?

 

E/c², where E is the total energy

[Deified]

E/c^{2[/sup'], where E is the total energy}

^{So thats independent of rest mass?}

[J.C.MacSwell]

 

 

 

So what exactly is relativistic mass?

 

I look at it as the "effective" mass if you wanted to accelerate it further and still use F=ma which would be incorrect otherwise at relativistic speeds.

[Dror]

So if a comet´s path is curved by a sun so must be a photon´s.

 

But each one's path will not be identical.. This kind of comes as a given logically, and is why it takes mucho mass to trap light into orbit.

If u think of 2-3 identical mass planets orbiting each other, you'll come to understand that those effects are relevant even to planet/feather.

[timo]

The paths will not be the same for two reasons:

1) Both comet and photon come in with a different velocity

2) The comet attracts the sun differently than a photon does.

 

Point two becomes increasingly important as the mass of the object being bent on the sun is increased. This is however not because the movement of a comet is a gravitational field would be dependent on it´s mass. It´s because the comet due to it´s own gravtiational field does attract the sun and causes it to move. With the sun in a different position you have changed your gravitational field and thus the two gravitational field (the one that the comet moves in and the one the photon moves in) are not the same.

 

For particles much smaller than the sun you can simply forget about their attraction on the sun because it´s so tiny. You certainly cannot neglect this attraction when you have identical masses. I´m not completely sure if this attraction on the sun must be considered for comets (I doubt it) but you can also take a neutron if you like. My statement that both paths are curved remains.

 

btw.: If you completely ignore quantum mechanical effects and threat a photon as a point size particle it doesn´t even take much mass to trap light in an orbit. The (theoretical) radius of the orbit will just be so small that it would be inside the attracting object (it´s 3 km for our sun, for example). That´s because of the high velocity light has.

[swansont]

The paths will not be the same for two reasons:

1) Both comet and photon come in with a different velocity

2) The comet attracts the sun differently than a photon does.

 

I agree with 1, but not 2. The attraction of the comet is immaterial - its acceleration is only dependent on the sun's gravity, just like the photon. AFAIK the potential well is completely determined by the sun. Classically, the mass of the object never enters into the problem - it's the velocity only.

[YT2095]

if a photon doesn`t have mass (and I mostly accept this) can someone explain how a "Solar Sail" is suposed to work?

 

I`ve seen Lab experiments done using a very fine material suspended in an evacuated chamber in the path of a Very powerfull Laser, the "gossamer" was "Pushed" by the power of Photons alone, HOW!?

[Dror]

HOW!?

 

We already dealt with this..

Photons have 0 rest mass, and 0 energy as a result.

but as they go at c (They don't know different), they DO have a mass. (very tiny).

So bassicaly if you'll throw an imaginary number of objects (reagrdless of how small) at a huge sail, that is set to accelerate a substantionaly smaller vessel, it'll work.

 

If I made any nasty mistakes correct me.

[timo]

The attraction of the comet is immaterial - its acceleration is only dependent on the sun's gravity, just like the photon. AFAIK the potential well is completely determined by the sun. Classically, the mass of the object never enters into the problem - it's the velocity only.

 

That´s because you neglect the attraction the comet exerts on the sun because it´s so small. I also said this in my post you were referring to. Point two was added because Dror started to talk about "2-3 identical masses".

I´d be surprised if the mass really canelled out when you consider both movement equation and field equation for both particles involved. The center of mass is a different one if you change the mass of the comet.

 

if a photon doesn`t have mass (and I mostly accept this) can someone explain how a "Solar Sail" is suposed to work?

 

I`ve seen Lab experiments done using a very fine material suspended in an evacuated chamber in the path of a Very powerfull Laser' date=' the "gossamer" was "Pushed" by the power of Photons alone, HOW!? [/quote']

While a photon does have no rest mass it does have energy (relativistic mass) and momentum. This energy and momentum is transferred to the particle causing it to move.

[YT2095]

it`s not been dealt with at all, at least not in anyway I can discern.

 

and

 

"they DO have a mass. (very tiny)."

runs contrary to what I`ve heard from many sources incl those outside SFN.

 

 

the only Logical reason I can see at the moment, is that it must be some form of ablative product being ejected from the film material, creating "thrust".

[Dror]

As photons hit matter they translate to kinetic energy. As simple as that.

Kinetic energy isn't new, and thats the mistery behind those solar sails.

The trick is, if you want any non negligable effect on the vessel, you gotta have huuuuuge sails.

 

As to the paths of different masses along a gravitational field:

Let's conduct a thought experiment.

Let go of a feather 1 meter from the surface of the moon. Count the exact time it takes it to hit from your perspective.

Stay in your perspetive.

Now, let go of a rock from the same distance, and count the exact time it takes it to hit the surface.

You'll come to see that the rock took less time to get there.

This is because it's gravitational pull on the moon is more powerful then the feathers.

[YT2095]

As photons hit matter they translate to kinetic energy. As simple as that.

Kinetic energy isn't new' date=' and thats the mistery behind those solar sails.

The trick is, if you want any non negligable effect on the vessel, you gotta have huuuuuge sails.[/quote']

 

assuming I take Your word for it then.

 

by what Mechanism does this take place?

 

 

as for Kinetics being "New" erm.. Thanks for that, I never would have guessed!

same for the Sail size, I`m sure it mentioned something about that too!

[Dror]

by what Mechanism does this take place?

 

By the same mechanism that enables you to see.

Energy accelerates things.. That's not new.

Sorry for being a smartass, have no bad intension.

[YT2095]

I see because the chems in the rod/cone receptors in my eye are photo sensitive and create a tiny electrical signal, passed down the optic nerve to my brain which then interprets these as an image.

 

Photon to Electricy I have no problem in understanding, the photon is an energy packet that can be converted to electrical current.

 

my issue is HOW a photon can be converted DIRECTLY to a "push" if it`s without Mass?

[swansont]

We already dealt with this..

Photons have 0 rest mass' date=' and 0 energy as a result.

but as they go at c (They don't know different), they DO have a mass. (very tiny).

So bassicaly if you'll throw an imaginary number of objects (reagrdless of how small) at a huge sail, that is set to accelerate a substantionaly smaller vessel, it'll work.

 

If I made any nasty mistakes correct me.[/quote']

 

The photon's energy is h^{[math]\nu[/math]}, not zero. It's momentum is E/c

 

They have no rest mass, and so no mass energy. All of their energy is kinetic, and they have momentum as a result. This is why a solar sail, not to mention laser cooling, works.

[swansont]

As to the paths of different masses along a gravitational field:

Let's conduct a thought experiment.

Let go of a feather 1 meter from the surface of the moon. Count the exact time it takes it to hit from your perspective.

Stay in your perspetive.

Now' date=' let go of a rock from the same distance, and count the exact time it takes it to hit the surface.

[b']You'll come to see that the rock took less time to get there.[/b]

This is because it's gravitational pull on the moon is more powerful then the feathers.

(emphasis added)

 

Absolutely not. Galileo disproved this ages ago. All objects fall at the same rate due to gravity, regardless of their mass.

[YT2095]

swansont:

 

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

the only thing I can think of as a guess, is that Photons "hit it" and eject Electrons inversely to the hits and thus provide motion with electrons having a known mass.

[Dror]

I see because the chems in the rod/cone receptors in my eye are photo sensitive and create a tiny electrical signal, passed down the optic nerve to my brain which then interprets these as an image.

 

Photon to Electricy I have no problem in understanding, the photon is an energy packet that can be converted to electrical current.

 

my issue is HOW a photon can be converted DIRECTLY to a "push" if it`s without Mass?

 

Oh so your eye "knows" when an electron changed orbit? Do you realize what your saying?

The photo sensitive cells in your eyes go into action when excited by energy. The cells on your hand also are excited when hit by energy, but they could care less.

Ofcourse, the underlieing mechanism is electrons switching orbits, but its really irrelevent. Even a pure reductionist will tell you that.

 

As for the photon translated into electricity that go in your brain. No.

The energy for the signal comes from mcdonalds (probably) . This signal passes the value recived by the photo sensitive cell.

 

The photons that hit the sail excite the atoms, causing an increase in kinetic energy. This is equivilent to acceleration. If the sail is big enough in respect to the vessel, it'll make it accelerate. In fact even 1 photon will make it accelerate but no one will notice.

[YT2095]

Oh so your eye "knows" when an electron changed orbit? Do you realize what your saying?

The photo sensitive cells in your eyes go into action when excited by energy. The cells on your hand also are excited when hit by energy' date=' but they could care less.

Ofcourse, the underlieing mechanism is electrons switching orbits, but its really irrelevent. Even a pure reductionist will tell you that.

 

As for the photon translated into electricity that go in your brain. No.

The energy for the signal comes from mcdonalds (probably) . This signal passes the value recived by the photo sensitive cell.

 

The photons that hit the sail excite the atoms, causing an increase in kinetic energy. This is equivilent to acceleration. If the sail is big enough in respect to the vessel, it'll make it accelerate. In fact even 1 photon will make it accelerate but no one will notice.[/quote']

 

WTF? I`ll have some of what you`re smoking please!

 

it`s nothing to do with changing orbit, have you actualy read up on photo chemical reactions at all?

or indeed the Photoelectric effect?

 

as for the mcdonalds thing, Well! I think you`ve alot of learning to do with regards to decorum and behaviour here, it`s only YOU you`re letting down.

[Dror]

Absolutely not. Galileo disproved this ages ago. All objects fall at the same rate due to gravity, regardless of their mass.

 

I agree they fall at the same rate, 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.

[YT2095]

oh dear, you forgot to factor in the attraction of the rock to the feather in that also!

 

I mean if you Wanna get all technical and sh!t

[Dror]

it`s nothing to do with changing orbit, have you actualy read up on photo chemical reactions at all?

or indeed the Photoelectric effect?

 

as for the mcdonalds thing, Well! I think you`ve alot of learning to do with regards to decorum and behaviour here, it`s only YOU you`re letting down.

 

By changing orbit, I mean changing orbit inside the atom.

As an electron orbiting an atom is hit by enough energy it takes a further orbit around the nucleus. This in turn translates into the atom moving faster. This is the kinetic energy I was talking about.

The photo senseitive cells in your eyes need something like 5-6 photons in order to pick up on it and send the signal. They notice the heat cause by this - the kinetic energy.

 

As for the mcdonalds thing, all I meant is that the energy for the signal to your brain comes from the food you eat.

 

The photoelectric effect that accures with certein materials has absolutly nothing to do with this.

[YT2095]

I appreciate the clarification, yes the food we eat does get converted to a variety of neuro-transmitters for receptor sites.

 

seriously though, you sound genuinely interested in this, look up the photo-electric effect and photo-chemical actions, if for nothing else, it`s quite fascinating

 

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

 

 

edit: Damn this keyboard! esp the T and R keys!

[Dror]

I thought the photoeletric effect accurs only with certein metals.. Guess I was wrong.

 

But, I'm 100% sure that it has nothing to do with how we see..

 

By the way I know what the photoelectric effect is..