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Author Topic: Does light have mass?  (Read 55850 times)

paul.fr

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Does light have mass?
« on: 09/06/2007 21:57:00 »
does it?
« Last Edit: 09/03/2008 14:47:01 by ukmicky »

ukmicky

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does light have mass?
« Reply #1 on: 09/06/2007 22:59:22 »
NO

syhprum

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does light have mass?
« Reply #2 on: 10/06/2007 04:29:10 »
According to my calculations a cubic meter of sunlight in the vicinity of the earth has a mass of 0.5*10^-22 Kg or at least that is the mass equivalent of the energy it contains

lightarrow

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does light have mass?
« Reply #3 on: 10/06/2007 11:27:56 »
does it?
If it moves in one direction only no.

Bored chemist

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does light have mass?
« Reply #4 on: 10/06/2007 12:28:51 »
Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?

ukmicky

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does light have mass?
« Reply #5 on: 10/06/2007 13:48:11 »
As far as rest mass goes the answer is no as nothing with rest mass can ever travel at C
« Last Edit: 10/06/2007 14:09:36 by ukmicky »

ukmicky

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does light have mass?
« Reply #6 on: 10/06/2007 14:14:27 »
Just found this which explains it much better than i could.

http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

Bored chemist

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« Reply #7 on: 10/06/2007 14:22:11 »
Since photons are never at rest, the rest mass of a photon probably isn't what was being asked about.

ukmicky

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does light have mass?
« Reply #8 on: 10/06/2007 14:58:27 »
Since photons are never at rest, the rest mass of a photon probably isn't what was being asked about.
But the only other type of mass is relativistic Mass but that isn't really Mass in the correct sense of the word so therefore a photon has 0 Mass.
« Last Edit: 19/06/2007 17:22:58 by ukmicky »

syhprum

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does light have mass?
« Reply #9 on: 10/06/2007 21:23:05 »
I have read the article quoted by Ukmiky and can now to be consided a convert, I realise that I have to capture my cubic meter of sunlight in a perfectly mirrored box before its mass becomes apparent.

lightarrow

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« Reply #10 on: 11/06/2007 11:58:10 »
This page also explain very well what is mass and its strangeness:
http://www.mathpages.com/home/kmath232/kmath232.htm
Quote
A photon has no rest mass, which implies that the Minkowskian norm of its energy-momentum vector is zero. However, it does not follow that the components of its energy-momentum vector are all zero, because the Minkowskian norm is not positive-definite. For a photon we have E2 - px2 - py2 - pz2 = 0 (where E = hn), so the energy-momentum vectors of two photons, one moving in the positive x direction and the other moving in the negative x direction, are of the form [E, E, 0, 0] and [E, -E, 0, 0] respectively. The Minkowski norms of each of these vectors individually are zero, but the sum of these two vectors is [2E, 0, 0, 0], which has a Minkowski norm of 2E. This shows that the rest mass of two identical photons moving in opposite directions is m0 = 2E = 2hn, even though the individual photons have no rest mass.

syhprum

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« Reply #11 on: 11/06/2007 14:56:17 »
I have read the article quoted by Lightarrow and my head is in a whorl, I am not going in future even to read any questions concerning mass, energy etc.

lightarrow

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« Reply #12 on: 11/06/2007 22:13:04 »
I have read the article quoted by Lightarrow and my head is in a whorl, I am not going in future even to read any questions concerning mass, energy etc.
The concept of mass is indeed less obvious than what we thougth; we should "digest" it a little at a time.
(My compliments to have read all the paper!)

ghh

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« Reply #13 on: 13/06/2007 21:01:51 »
The principle of equivalence comes from the equation
      E^2 = m^2c^4    +    p^2c^2                
In which m is mass, p is defined as momentum, and c is an absolute velocity:   
It is generally accepted that electromagnetic phenomena have “momentum, but no mass”, the proof being
E2 = 02c4    +    p2c2
therefore    √E2 =  √( p2c2)
and so       E = pc                        (1.0.1)

and similarly “rest” mass has no momentum, therefore
      E2 = m2c4    +   02c2
      E2 = m2c4   
      E  = mc2                         (1.0.2)               
However, the definition of p, momentum, is mass x velocity, albeit in this case the velocity has the absolute value “c”.

So in SI units    pc = Kg.(m.sec-1)(m.sec-1), and mc2  =  (Kg.m2.sec-2)      (1.0.3)      
                           
These are dimensionally identical, so the solutions (1.0.1) and (1.0.2) do not seem to be legitimate, although they are claimed by experiment to be “proved”. If this is the case, and E = pc and E = mc2 are valid then either (pc)2 + (mc2) = E2 and-a-bit, or if equation (1.0) is to be upheld, the possibility has to be considered that  m has to be non-zero, and c has to be variable.

lightarrow

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« Reply #14 on: 14/06/2007 16:03:01 »
The principle of equivalence comes from the equation
      E^2 = m^2c^4    +    p^2c^2                
In which m is mass, p is defined as momentum, and c is an absolute velocity:   
It is generally accepted that electromagnetic phenomena have “momentum, but no mass”, the proof being
E2 = 02c4    +    p2c2
therefore    √E2 =  √( p2c2)
and so       E = pc                        (1.0.1)

and similarly “rest” mass has no momentum, therefore
      E2 = m2c4    +   02c2
      E2 = m2c4   
      E  = mc2                         (1.0.2)
It was good up to here.

Quote
However, the definition of p, momentum, is mass x velocity,
That's wrong.

Quote
albeit in this case the velocity has the absolute value “c”.

So in SI units    pc = Kg.(m.sec-1)(m.sec-1), and mc2  =  (Kg.m2.sec-2)      (1.0.3)      
                           
These are dimensionally identical, so the solutions (1.0.1) and (1.0.2) do not seem to be legitimate, although they are claimed by experiment to be “proved”. If this is the case, and E = pc and E = mc2 are valid then either (pc)2 + (mc2) = E2 and-a-bit, or if equation (1.0) is to be upheld, the possibility has to be considered that  m has to be non-zero, and c has to be variable.
I didn't understand anything of this.
« Last Edit: 22/07/2008 23:55:30 by lightarrow »

felixtheferret

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does light have mass?
« Reply #15 on: 18/06/2007 22:30:25 »
Slightly tangential, but interestingly mass still remains the one SI unit that cannot yet be defined in terms of something else:  'mass' is DEFINED by the kilogram sample of platinum alloy kept in a box in Paris !   No-one really knows, at a fundamental level, what mass 'is.' You may as well ask what a quark is made of , or what a magnetic field is made of, these questions have no answers in current physics.  Think of it like this:  as a particle (say an electron) speeds up, it slowly gains mass according to GR. just exactly how is this 'mass' added?  - little tiny bits of electron magicking themselves out of nothing and sticking to the surface of the moving electron?!!  Very unlikely.  'mass' at the particle level becomes a purely mathematical abstraction - don't let anyone kid you otherwise :-)  Therefore for all intents and purposes a photon DOES have mass, purely by virtue of it's motion, i.e. its energy.   

lightarrow

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« Reply #16 on: 19/06/2007 17:16:14 »
Slightly tangential, but interestingly mass still remains the one SI unit that cannot yet be defined in terms of something else:  'mass' is DEFINED by the kilogram sample of platinum alloy kept in a box in Paris !
Time is not also defined in such a way? Can you define time in terms of something else? And current intensity?
Quote
   No-one really knows, at a fundamental level, what mass 'is.' You may as well ask what a quark is made of , or what a magnetic field is made of, these questions have no answers in current physics.  Think of it like this:  as a particle (say an electron) speeds up, it slowly gains mass according to GR.
Sorry, that's wrong. Most physicists (Einstein too) agree on calling "mass" just the rest mass and not the relativistic mass (I have barred that name just because it's now considered wrong).
Quote
just exactly how is this 'mass' added?  - little tiny bits of electron magicking themselves out of nothing and sticking to the surface of the moving electron?!!  Very unlikely.  'mass' at the particle level becomes a purely mathematical abstraction - don't let anyone kid you otherwise :-)  Therefore for all intents and purposes a photon DOES have mass, purely by virtue of it's motion, i.e. its energy.
A photon is mass-less.
« Last Edit: 19/06/2007 17:17:52 by lightarrow »

JP

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« Reply #17 on: 19/06/2007 17:30:13 »
I don't think physicists yet understand what generates inertial (rest) mass.  They've proposed a "Higgs field" in high energy physics, which is the most promising theory (the field couples to fundamental particles and the interaction gives them mass), and I'm sure there are other models out there. 

On another note, I'm pretty sure there are cases where a photon can interact with things as if it has mass: via wierd quantum mechanical fluctuations in which the photon generates fleeting "virtual particles" which have mass. 

But on the original topic, a photon itself has no mass.

lightarrow

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« Reply #18 on: 19/06/2007 17:43:10 »
Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?

I have discovered not long ago how the equation E = mc2 is very often misunderstood (and it was also by me); it must be considered in this way:

If you have a stationary body or physical system with a total rest mass M, and you give it some energy E, so that the system remains stationary, then it gains a rest mass m equal to E/c2.
So the new mass is M + m = M + E/c2
.

Examples:
1. You give heat to a stat. body.
2. You spin it.
3. You make a cavity with internal reflecting walls, then, through a tiny hole, you shoot a light beam inside of it.
4. You compress a spring and you let it stay compressed with a clamp. The mass of the system "compressed spring + clamp" is greater than that of the system "non-compressed spring + clamp".
« Last Edit: 19/06/2007 17:47:19 by lightarrow »

edward2007

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does light have mass?
« Reply #19 on: 20/06/2007 10:36:42 »
If light has no mass, how would you explain the working of a solar sail?

ghh

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« Reply #20 on: 20/06/2007 13:56:54 »
The “equivalence” depends upon “rest mass”. A moving mass has inertial momentum, or a total energy of mc^2 + mv, so the solution is not so much that a photon has no mass, but has no inertial momentum but through E = pc does have an intrinsic momentum but always with the velocity c. There is no definition of momentum which doesn’t contain “m”. And as Edward2007 points out photons still exhibit boson like properties.
Whilst we are digesting that, you might also refresh your reading on the convergence of the photon gas calculations of Maxwell-Boltzman, Bose-Einstein and Fermi-Dirac. And then look up Wikipedia on the subject of J-Band radars and the cavity magnetron, and the resolution of pinhole cameras. This suggests that for microwaves the energy quantum, as defined by Planck's constant may have a wave width equal to the wave length ie a volume, and before you say “but…” there is an interesting calculation. The discovery of the CMBR shows that space is not empty. There is a continuous flux of radiation, defined by the “black body temperature”. Since this is a steady state we can ignore the velocity and the number of quanta in the volume at any instant is constant. If we consider a volume of space, say 1 cubic metre, and calculate how many quanta (of a wavelength diameter) will fit in that space, and apply The equation of state for an ideal gas, which is PV/NkT, where P = pressure, V=volume, k is the Boltzman constant, T is the temperature (Kelvin) and N is the number of molecules (in this case quanta) of the gas. So pressure P = NkT/V. Since kT = E, and since E = hf , (the number of quanta per second) the number of quanta in the volume at any instant is kT/f. This returns a constant value 6.626069x 10-34, recognisably “h”. The Pressure is therefore Nh/V. Similarly the density can be obtained from the number (N) quanta contained in the volume at the mass equivalent of h
So density = N x 7.37249577722913E-51/V. Now the speed of sound in a gas is √pressure/density. If you do that calculation for any wavelength you get the same very interesting number.
Graham

lightarrow

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does light have mass?
« Reply #21 on: 20/06/2007 21:14:40 »
If light has no mass, how would you explain the working of a solar sail?

Light has momentum p even if it has no mass: p = E/c.
The electric and magnetic field of an EM radiation make a force which is orthogonal to the surface, to the material's electric charges. This force pushes the sail.

G-1 Theory

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does light have mass?
« Reply #22 on: 19/07/2007 12:16:29 »
Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?

Bored Chemist is RIGHT!
Even through we will never be able to mesure it as it allways moves @ C because if we say that it doesn't mass then we are saying than E=MC2 is totally wrong.
I donn't think anyone here is going to think that E=MC2 is  Totally Wrong !!!!!!

Ed

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« Reply #23 on: 19/07/2007 13:10:05 »
If light has no mass, how would you explain the working of a solar sail?

Light has momentum p even if it has no mass: p = E/c.
The electric and magnetic field of an EM radiation make a force which is orthogonal to the surface, to the material's electric charges. This force pushes the sail.

Sorry, I must dissagree, EM forces has nothing to do with solar sails.

It is the fact that the solar sail is highly reflective and it’s reflected

light counteracts the on coming light and the sail moves forwards.

Here at the University Texas’s laser lab we use lasers to push solar sails

in a vacuum chamber for testing.

Ed


maff

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« Reply #24 on: 19/07/2007 15:54:24 »
In order to decide if light is heavy you need to work out what kind of energy it's mass would inflict on this world.
It's not at rest so rest energy or potential energy is eliminated. Does it have kenetic or moving energy? It doesn't seem to cause damage while travelling at super high speeds when colliding with something. There's no fission or fusion involved so nuclear is eliminated. Electrical energy isn't a factor because there's no electrons involved. So if it has mass where is there a measurable result of that mass on this Universe.
On the other hand photons get pulled into large gravitational fields such has black holes and it can be slung shot around large stars. So it behaves sometimes as if it had mass. We all treat light as energy but light is the only form of energy that has a moving particle which remains constant throughout the delivery of that energy.
Rest, kenetic, nuclear and electric energy all have a net product involving mass while light seems generally not to do this.
It gives the human eye the ability to see all the other energies at work.  My thinking is that light is actually heavy but not in our relative Universe.
..maff

 

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