Light has mass?

My physics teacher taught me that waves can exhibit particle properties, and particles can exhibit wave properties. But I have a hard time grasping one concept... that light has momentum.
I recently saw a show that had a "solar sail" as a form of propulsion. I understand how that worked and all... but what I just don't get is, that in order for something to have momentum, it must have mass and velocity (p=mv) and I know light has all the velocity it needs. But light doesn't have mass. If it did, wouldn't the mass become infinite? and at the same time infinitly small? due to the mass and length dialiation from relativistic effects (time would also stop too, but does that have anything to do with this too?).

That is just something I can't seem to understand, if someone could explain this to me better, It would be greatly appreciated

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The best answer I can give is "yes and no; mostly no". Photons have energy (e=hf, where h is plank's constant and f is the frequency of the photon). Energy is equivalent to mass (e=mc^2 (sort of)). So photons have relativistic mass, which I suppose could be considered potential mass.

Note that I'm not a genuine physicist, so that could all be inaccurate. But I don't think it is.

 

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light

Oh this is going to get some one going.

I beleve light does has a little bit of mass. Not much but none the less there. Gravity effects it and can cause it to be bent (gravity lensing). Gravity normally can only effect that which has mass.

 

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The photonmass is still to be discovered...

Hi all,

The problem with light is that it has some very special properties (it moves faster than any real matter could). Therefor the relation p = mv is not well-definied for light (since that only applies to particles with mass).

The momentum of light is defined with its wave-vector k:
p = (h_bar)*k
where h_bar is Planck's constant (divided by 2*Pi)

The reason momentum is used for light is that experiments suggest that particles can accelerate when they collide with light (for example: an electron gets accelerated when it collides with a light-"particle", the photon). Since conservation of momentum is a very useful relation in physics, physicists prefer to solve that problem by giving light some momentum so conservation of momentum can still be used for the description of collision processes.

RDE, for the E=mc^2 formule: The full version of the formula reads: E^2 = (mc^2)^2 + (p*c)^2. So if light has no mass (m=0), the first term disappears, but you are left with the second (which relates momentum with energy). This would give: E = p*c . Using the p = h_bar * k definition for momentum you would get:
E = k * c * h_bar.
By then using that the wavevector equals 2*Pi / L with L the wavelength, you get:
E = (c / L ) * h
And since c / L = f (the frequency), you get the E = h*f formula you mentioned.

Bye!

Crisp

 

Re: light

hmm, your wrong.

Gravity lensing isn't caused by the attraction from a black hole or a galaxy on the photons.
The space around a black hole is bent...the well known dents in space-time (mention that I use a literally translation from dutch for this.)
So it is space what is bent, and because of light is going through that space light doesn't come to us in a 'straight' line.

And so we get the gravity lensing.
And so we can conclude photons have no mass, else the lensing effect would be a lot bigger.

(btw, I'm glad I can finally add something to your post, instead of you answering my questions, wet1)

 

Unless, of course, Gravity is just Inertia in a fourth dimension... (see my thread of the same name)

BTW - If photons do somehow have a small amount of mass, just think of the implications. Two-thirds of the mass in the universe is supposedly made of an unknown "dark matter" (could be brown dwarves, black holes, neutrinos, or even quintessence) but if photons have mass...

Dark matter could actually be light!!!

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I'm not saying this is neccessarily true, just bouncing some wild thoughts off the wall.

~Caleb

 

About light having mass...

Light cannot have mass. If light, or (photons) have any mass whatsoever then they couldn't travel at light speed. Einstein proved this in the special theory of relitivity, any object with mass, no matter how small as it approaches the speed of light would become infinatly heavy. I.e. it woulud take more matter than is in the entire universe, as fuel, to push a single hydrogen atom to the speed of light.
The reason gravity can effect light is because gravity puts "dents", in space-time. Light flys along the fabric of space and falls into these dents. Just like a pot hole in a road. The gravity of the object creating the depression in space-time is pulling in light because of the space-time curve, not because photons have gravity.

A word from the wise to the equally wise.

 

reply

i really think crisp is correct theoritically.but i believe that photons
have infinite relativistic mass.m=m0/91-v^2/c^2)....as vtends to c.. change in its mass starts..now,whenv=c,thenm=m0/0
which is nothing but infinitely high..high because:
that is the only way to travel at such high speeds.gain so much mass so that the "g"or acceleration due to gravity increaeses to such an extent that say if photons want to move forward,they will exert so much force that the cosmos moves backwards..
and since there is no preferred inertial frame of reference,therefore we can consider all the frames inside the universe stationary..and only the cosmos itself moves so as to provide reaction.say if i am moving a bucket of water,round and round,then we can say my bucket is stationary,and whole cosmos is moving.the idea of photons having energy infinitely may give solution to our time travel problem could have an easy solution.what do you think???

 

Ah, I told you that would get things going.

Kneed you are always welcome to add, I have no collective knowledge but the reading of thoughts does indeed help.

While a lot of the thoughts here are well documented and based in hard science there is still a lot of frontier science to be done. Even where we think that we understand the properties of light I would hazard the guess that there is still that to be found that we don't know enough to even ask the proper questions for much less formulate the answers yet. Science has that about it in that for every question it answers it raises many more because of the answer. One day a new door on this subject will open from the lest suspected feild. Where I can not say but it seems that is another property of science.

 

light

could photon get mass when it decreases its velocity ex. in a collision ?

 

The use of a 'Solar Sail' does not necessarily require that light has mass. There is a great deal more than light emanating from the Sun - the assorted particles in the solar wind include enough mass at high enough velocities to provide significant momentum

 

Hi all,

Lund,

Could photon get mass when it decreases its velocity ex. in a collision ?

Not according to our current theories (I mostly speak in terms of these). A photon has no restmass, and it cannot get any. A photon can also not change velocity, it always moves at the speed of light.

What can happen is that in a collision, the photon loses energy (and hence relativistic mass, because of the energy-relativistic mass equivalence). A well known example is the Compton effect, where an electron at rest gets hit by a photon. After the collision, the electron gains momentum, and hence, by conservation of energy, the photon must have lost energy. This is physically observable by measuring the change of frequency of the photon (the frequency is directly related to the energy of a photon).


tsq,

The use of a 'Solar Sail' does not necessarily require that light has mass. There is a great deal more than light emanating from the Sun - the assorted particles in the solar wind include enough mass at high enough velocities to provide significant momentum

Okay, this is true, but the principle of a solar sail is really to move by light. Photons also have (a tiny amount of) momentum, and all these tiny bits can be combined to push a macroscopic object, like a satelite, towards one direction (away from the sun for example).

Bye!

Crisp

 

if i may interject

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im pretty sure what you are looking for is "rest mass"

My high school physics teacher (phD nuclear physics) said things along the lines (perhaps not to confuse us too much) :

Light (photons and whatnot) have mass (i.e there is something tangible there, see solar sails) BUT no rest mass. I cant recall the formula offhand but if an object has mass as it tends towards the speed of light the formula will work out to infinite, however if mass = 0, the formula will be finite

my 2 cents

 

LIGHT ...HAS...MASS

Light has mass, the reason you say it cannot is becasue of the speed which it travels...but it isn't traveling fast, Time the 4th dim effects light. The photon has a small mass, and travels around the speed of sound2, its time which speeds up with the hyper matter in the proton that affects it. If we enhance this hyper matter, we too can travel at amazing speeds, but in reality time just speeds up. Take a torch at the side of our galaxy and beam it it the direction of the other side, it takes it, what 180,000 years to get to the other side, but if it had age it would be 180 times older.

If you dont get what im saying ( or more lightly to thinks its utter dribble) see my other posts

 

Umm, come on, folks.

Quantum Electrodynamics. Electrons, particles with mass, repond to the absorbtion of massless photons.

Imagine how massless particles can induce forcefully-induced behaviors of particles with mass.

Sheesh.

 

Cut'n'pastes should be properly attributed.

 

energy and mass go hand-in-hand...


E = mc^2 E-photon = hf


an object emitting photons is losing mass. How much? hf / c^2 WOAH not very much. VERY small number.. but nonetheless a number

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