Does light have a mass?

I strongly suggest getting to know the basics before pondering more in-depth questions. Going too far in-depth without a strong foundation is like jumping into the deep end of the pool, before you learn to tread water. Feel free to ask more questions about any of the stuff already posted. :m:

 

Log in or Sign up to hide all adverts.

one_raven: What is the assumption that c is the universal speed limit based on (other than relativity math)?
GW: It's not an assumption -- it's a proven fact.

I wouldn't go so far as to say that this is proven fact. You can not prove that something is not possible. That being said, it appears to be a valid limit due to the mathatical proofs.

 

Log in or Sign up to hide all adverts.

You're absolutely right. I misread the exact statement I was replying to. What IS proven is that 'c' is constant for a given medium.

Please Register or Log in to view the hidden image!

my speculation:
However, from this I assume it logically follows that you cannot 'beat' the speed of propagation that the medium will 'support', without somehow 'detaching' yourself from the medium. This would mean that if you did go 'faster' than 'c' in a free vacuum, you are 'slipping past' the medium itself.... I think.

 

Log in or Sign up to hide all adverts.

GundamWing,

Well, I guess rocks don't have mass either.

You see, if I drop a rock, it won't fall to Earth because the rock has mass, it falls because it "follows" the curvature of space-time. And when it hits the ground with a force, that's not due to it's mass either, it is because the rock has "momentum" just like a photon. And the force that you feel against your hand when you through a rock isn't a result of the rock's mass either, it simply a result of the law of conservation of momentum.

Please Register or Log in to view the hidden image!

Tom

 

Momentum for a particle is calculated using p=mv, where m is mass and v is velocity. So, rocks do have mass. The rock has 'mass', 'momentum' and 'follows' (or 'falls' through) a curvilinear trajectory along spacetime.

Please Register or Log in to view the hidden image!

 

GundamWing,

Don't jump to conclusions. You, yourself, stated that photons have momentum, but don't have mass. Maybe, rocks also have momentum without mass.

Please Register or Log in to view the hidden image!

Tom

 

I'm not jumping to conclusions. You calculate momentum in two ways:

>> for photons as p = hf / c
>> for particles as p=mv. *
(*the relativistic correction would be m=m0/(1-(v/c)^2)^(1/2) for particles moving at appreciable velocities)

p - momentum
h - planck's constant
f - frequency of photon
c - speed of light
m - mass
v - velocity

from my understanding --
If particles didn't have mass, then there would be no 'gravitational force' between large objects (a collective of a large number of particles).

Massless things do not experience force or inertia, they only take part in momentum transfers. If massless things are insensitive to 'force' then it may be ok to say that they don't experience friction either. Since they cannot experience force -- there can be no nonconservative forces (friction) acting upon massless objects, then I guess both energy and momentum would always be conserved for massless objects.

Please Register or Log in to view the hidden image!

... anyway, i'm just speculating.

Please Register or Log in to view the hidden image!

 

the final word

I believe thed posted this earlier, but this makes for a good 'final word' on whether photons have mass or not.

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

enjoy.

Please Register or Log in to view the hidden image!

 

As of yet, there are no models or theories to prove that light has mass. However, recent developments DO show that neutrinos have mass; a very small amount, but still some. And the fact that neutrinos travel near the speed of light is quite interesting. Will we eventually find out that light DOES have mass? Quantum theorists are working on that one. There may be a new branch of physics (beyond QM) that astounds us in the future, but for now we're stuck with light being without mass.

 

This is not verified as yet but a fellow named E. Skyler claims to have tested laser beams projected to a distant target that shows sideral motion around the bulls eye. Calculations of the offset during certain hours mathematical compute to be the earths motion during the transit of the beam from the source to the target.

That is the light beam does not carry forward momentum as claimed by SRT diagrams which are the based (explanation for spatial contraction and time dilation in Relativity).

It does make sense that IF a photon had mass it would carry forward momentum of its source but if it is massless then we should expect to see the beam once fired remain in a straight trajectory without the forward momentum.

The crux of the issue is it nows seems a photon is indeed massless and does not carry the forward momentum of the projecting source.

 

The prediction and supporting observations support and perhaps prove the theory... The theory does not prove the prediction or observation.

General relativity is a successful geometric description of experience and obsevation, not a proof of experience. It does not explain how matter and spacetime interact only that they do.

 

In the last paragraph of the first section, of his 1905 paper, ON THE ELECTRODYNAMICS OF MOVING BODIES, Einstein is be quoted,

"In agreement with experience we further assume the quantity \(\frac{2AB}{t'_A - t_A} = c\) to be a universal constant--the velocity of light in empty space."​

From what I have read of the historical perspective, the experience that Einstein was referring to was the work of Fizeau, measuring the velocity of light in a variety of ways and in a variety of mediums.., air, liquid and vacuum, and Maxwell's work on electromagnetism, which also predicted \(c\) as a constant velocity for electromagnetic waves.

Extending those experimental results and experience — to the status of a universal constant, became one of the foundations of the theory of special relative, first published in that paper, where the laws of physics are shown to be the same for all inertial observers... All observers who are moving with a constant velocity relative to one another, measure the velocity of light to be the same, reguardless of thier own velocity relative to the source of the light.

This holds for the flat spacetime of special relativity, but becomes somewhat more complicated within the context of general relativity where the curvature of space involves both length and time dilations due to an observer's location within a gravitational field.

Where the luminiferous aether and the Michelson and Morley experiments are concerned, it is commonly assumed today that Micheson and Morley proved the aether did not exist. This is not entirely accurate. Their experiments and those that have followed have continually returned what are called "null" results, meaning that they were unable to detect an ether medium. This is not quite the same as proving that the ether does not exist.

The luminiferous aether itself, has been dismissed as a valid approximation of experience, primarily because it is inconsistent with the theories of special and general relativity, both of which have been very successful descriptions of the world. The aether of the 1800s was a fixed and static medium, closely associated with the ridgedly defined space of Newtonian dynamics. With the introduction of the theories of special and general relativity Einstein showed that space was not fixed and static. Instead it became a dynamic counter part to matter. The ridgedly fix ether was and is inconsistent with a dynamic space/spacetime. Einstein's SR and GR are more accurate and successful descriptions of what we know now of physics and the universe, than the ideas of Newton that they replaced.

The curved spacetime of general relativity replaced the ether of the 1800s.

 

Hold thread resurrection batman! This is a 10 year old thread - if you want to discuss something on a similar topic, start a new one.