# Does light have a mass?

Discussion in 'Physics & Math' started by GRO, Apr 6, 2002.

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1. ### GundamWingRegistered Senior Member

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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:

3. ### PersolI am the great and mighty Zo.Registered Senior Member

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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.

5. ### GundamWingRegistered Senior Member

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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.

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.

Last edited: Feb 23, 2003

7. ### James RJust this guy, you know?Staff Member

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Re: Some help?

one_raven:

<i>1.) The two slits experiment:
Can someone point me to more information/analysis of what happend in this scenario?</i>

Just search for "two-slit experiment" on the web. There are plenty of good sites.

<i>I don't believe that we have any apparatus that can "see" a single photon of light. let alone follow it's path and trace it's movements, therefore how is it we know that a sibgle photon of light followed two seperate paths at the same time?</i>

We know because even if we fire one photon at a time, an interference pattern still forms. Hence, photons must interfere with themselves.

<i>How do we know that photons are particles?</i>

From the behaviour of experiments on the photoelectric effect and the Compton effects, for a start.

<i>For example, are photons considered particles because the math of relativity says they are, or are they know to be particles and the math of relativity is supported by that fact?</i>

Relativity doesn't really care whether photons are particles or waves.

<i>What wave-like properties do they exhibit?</i>

Diffraction, interference.

<i>What particle-like properties do they exhibit?</i>

Momentum and energy are transferred in "chunks" or quanta.

<i>3.) What is the assumption that c is the universal speed limit based on (other than relativity math)?</i>

Nothing. We make the assumption and see where it takes the theory. We then test the theory against experiment. The agreement of the experimental results with the theory tells us the assumptions of the theory are correct within the limits imposed by experimental error.

<i>I understand the experiments that support tha c is constant, but what experiments have been done to verify teh validity of c being the limit for anything to travel?</i>

Particle accelerators routinely accelerate particles to over 99% of the speed of light. But no particle has ever been accelerated to a speed greater than the speed of light. The energy requirements match the predictions of relativity.

<i>4.) I have read in a couple of places that light used to be thought of as a wave the propagates through some medium (aether or some other medium) rather than particles actually traveling.
Why was that idea laid to the side?</i>

Because attempts were made to detect the medium, and the medium was not detected. If there had been a medium, the relevant experiments would have detected it.

<i>I read somewhere about an experiemnt with a beam of light that did not display the results of speed change that would have been expected if the aether theories were correct.</i>

It's called the Michelson-Morley experiment after the guys who did it. Again, detailed explanations are available on the web, with diagrams. (Or somebody might post an explanation here.)

<i>Were there any other experiments regarding this belief of light being a wave rather than sperate particles done?</i>

Any experiment showing interference or diffraction of light is evidence of the wave nature of light.

<i>5.) I read somewhere on this forum (perhaps this post, but I am not sure) plus I recall something in "The universe in a Nutshell" about one of the experiments that were done with the atamic clock/plane setup.
The plabe flew East the clock lost time.
BUT when it then flew West it gained time.
(or it may have been vice versa)
Does anyone know any more about this and why it is considered to support the claims of relativity rather than refute them?</i>

I'm not sure of the details, but two effects are important in that experiment: the height of the plane above the ground and the speed of the plane relative to the ground. When both factors are correctly taken into account, the experimental results support relativity. The Earth's rotation would also have a small effect, but I'm not sure how big that effect is or how relevant it was to the results.

Hope this helps.

8. ### ProsoothusRegistered Senior Member

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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.

Tom

9. ### GundamWingRegistered Senior Member

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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.

10. ### ProsoothusRegistered Senior Member

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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.

Tom

11. ### GundamWingRegistered Senior Member

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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.

... anyway, i'm just speculating.

Last edited: Feb 25, 2003

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13. ### Byron MerrittRegistered Member

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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.

14. ### MacMRegistered Senior Member

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10,104
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.

Last edited: Mar 6, 2005
15. ### OnlyMeValued Senior Member

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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.

16. ### OnlyMeValued Senior Member

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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.

17. ### prometheusviva voce!Registered Senior Member

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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.