Ask any question you want about Physics

originally posted by: Arbitrageur

originally posted by: Phantom423
So how does a photon transition from having no mass to having some mass?

It doesn't. Bedlam is right as usual of course but here's the dumbed down explanation, where he claims we are all "lied to" in school which is probably true depending on your point of view, but it's not a very big lie because in normal circumstances the gravitational effect of a photon is negligible meaning not exactly zero but it's so close you can't typically measure it as being different from zero:

Common Physics Misconceptions

Even though we can't confirm in experiment the mass of a photon is EXACTLY zero, it probably is, so it probably travels at the speed of light in a vacuum.

Why aren't you asking about neutrinos instead, which probably do have a tiny mass, yet we seem to be able to measure them traveling at the speed of light, which is supposed to be impossible for a particle with mass because it would need infinite energy to do that, right? At least now we know the experiment at CERN was wrong which measured them going slightly faster than light.

So you would expect photons to respond in a gravitational field? I was curious why it was a significant element in the NASA research project. I assume then that the effect is very small but still measurable.

Well, its hard to say that is accurate, but we can say that we are able to test against it more and more every day as previous tests give us more and more data.

I look forward to a day when I can "spooky action at a distance" myself to Hawaii every weekend.

originally posted by: Phantom423

So you would expect photons to respond in a gravitational field?

Oh, you betcha. It's one of the ways quantum theory was established - the 1914 eclipse.

The observation of starlight being bent by the Sun's gravitational field matched the amount predicted by the tensor. Voila! It's the little things that get you noticed in theoretical physics, I always say.

originally posted by: Bedlam

originally posted by: Phantom423

So you would expect photons to respond in a gravitational field?

Oh, you betcha. It's one of the ways quantum theory was established - the 1914 eclipse.

The observation of starlight being bent by the Sun's gravitational field matched the amount predicted by the tensor. Voila! It's the little things that get you noticed in theoretical physics, I always say.

Good analogy. Time to revamp my definitions of the photon. Thanks.

originally posted by: Phantom423
Good analogy. Time to revamp my definitions of the photon. Thanks.

This bit of behavior gets into the esoteric, though. There's an engineering way of looking at it that would send mbkennel here shrieking in horror at the total inaccuracy and misleading nature of it, even if it gets decent approximations, but in his defense, it actually IS wrong, it just happens to work.

From my perspective, a photon is actually a sort of pushmi-pullyu mashup of an electric field and a magnetic field, each making the other as it merrily goes along, goes along. Or, as I also say, the wheels on the fields go round and round.

Tensors. Ouch.

I tried reading up on landau-lifsh!tz psuedotensors about a year ago. All my eyes kept wanting to read is ...so you can make gravity go away? Doesnt mean thats what i was actually reading though.

I put it down realizing that if i dont get the first paragraph i shouldnt dismiss that i dont understand the foundation and keep reading further getting more lost.

Ready to pick it up again.

Any laymans explanation you can give regarding landau-lifsh!tz psuedotensors?

originally posted by: Bedlam
Oh, you betcha. It's one of the ways quantum theory was established - the 1914 eclipse.

The observation of starlight being bent by the Sun's gravitational field matched the amount predicted by the tensor. Voila! It's the little things that get you noticed in theoretical physics, I always say.

originally posted by: Phantom423
So you would expect photons to respond in a gravitational field? I was curious why it was a significant element in the NASA research project. I assume then that the effect is very small but still measurable.

The video in the post you replied to was supposed to show that it does if you had a chance to watch it. He used some artistic license to show light traveling in a very curvy path past a star, planet and black hole, which I have no problem with, I think the video is brilliantly done.

He more or less has to use some artistic license because if he didn't, the amount of bending that happens to light going past the sun would look like a straight line on the scale of his video. In fact it's bent so little by the sun that the eclipse to which bedlam refers (I'm pretty sure he meant the 1919 eclipse) wasn't really as definitive a confirmation of light bending due to general relativity predictions as we are led to believe in much of the popular literature. In fact, Eddington's main equipment showed results consistent with Newton's predictions, not Einstein's (see below). He didn't trust those results because of focus issues so he threw them out relying instead on his backup equipment, which some could say is a scientific experimental sin of sorts to throw out the data you don't like and just use the data you do like. Later in 1919 a group of scientists met to review the eclipse results and there was some skepticism:

Einstein, Eddington and the 1919 Eclipse

The reaction from scientists at this special meeting was ambivalent. Some questioned the reliability of statistical evidence from such a small number of stars. This skepticism seems in retrospect to be entirely justified. Although the results from Sobral were consistent with Einstein’s prediction, Eddington had been careful to remove from the analysis all measurements taken with the main equipment, the astrographic telescope and used only the results from the 4-inch. As I have explained, there were good grounds for this because of problems with the focus of the larger instrument. On the other hand, these plates yielded a value for the deflection of 0.93 seconds of arc, very close to the Newtonian prediction. Some suspected Eddington of cooking the books by leaving these measurements out

Eclipse measurements continued and in 1922 the results were more convincing, however still the errors were a relatively large amount of the effect that they were attempting to observe because the amount of bending they were trying to measure was so small. The point of all this isn't to cast any doubt on experimental confirmation of relativity which is definitely confirmed, rather it's to emphasize how very small was the GR bending of light by the sun, such that the bending was on the margin of what could be reliably measured using 1919 technology.

So yes the sun bends light, but not very much, is my point. As an aside, there's some interesting reading available about the eclipse confirmation expeditions in 1919. One team trying to make eclipse measurements 6 years earlier, closer to the year bedlam mentioned, even got arrested as spies so they were not able to complete their measurements.

Is it plausible, that Higgs Boson could interact with Dark Energy?

a reply to: Fingle
Our current model suggests that it's gravity on the scale of galaxies that tends to keep galaxies from expanding, while the spaces between galaxy clusters and superclusters are increasing at an accelerating rate due to dark energy.

So, to the extent that the Higgs field is responsible for mass it tends to counteract dark energy's tendency to make space expand at an accelerating rate. Most baryonic matter doesn't get its mass from the Higgs mechanism, and furthermore, we think most mass is likely to be non-baryonic (non-baryonic dark matter). Since we don't understand the nature of this non-baryonic matter we can't say if there's any Higgs interaction with it.

a reply to: Arbitrageur

Thanks for the detailed reply. I watched the video and I get it now. It's so true that you remember the easy stuff like - "photons have no mass" and forget the real stuff. Next time I use the Cary 5000 UV-Vis-NIR I'll remember this conversation!

Thanks again - I'm always learning something here.

originally posted by: Bedlam

The stress-energy tensor will also explain behavior like why two photons coming at each other head on will mutually deflect each other as if they both had actual mass and were deflecting each other due to gravity

I thought Photons cannot interact with one another? They cannot collide because pauli exclusion?

a reply to: Arbitrageur

Could the photons trajectory being bent around a massive body (such as the sun) be due to the bodies (suns) movement, rotation?

Like consider a playground merry go round, spinning, and then baseballs or tennis balls or marbles or water being tossed from behind it at the edges. Not to mention the fact the body itself is not only rotating but also moving other ways.

then also how do you tell a photon which came from behind the body was bent? The only photons that can ever be detected at the ones which land in the detector.

originally posted by: ImaFungi
a reply to: Arbitrageur

Could the photons trajectory being bent around a massive body (such as the sun) be due to the bodies (suns) movement, rotation?

Like consider a playground merry go round, spinning, and then baseballs or tennis balls or marbles or water being tossed from behind it at the edges. Not to mention the fact the body itself is not only rotating but also moving other ways.

The Lense–Thirring effect results from rotating bodies and can affect orbits by very tiny amounts but I don't see how it would affect eclipse measurements. Also there were some bright stars on all sides of the sun so if you wanted to look for some unknown effect of rotation you could compare the deflection on the side where the surface is rotating toward you with the deflection on the side where the surface is rotating away from you, or see if there's any difference in deflection near the equator versus near the poles.

The sun is moving through space at a fantastic speed due to the rotation of the Milky Way, however the closest, brightest stars would be moving along with us so relative motions will be small and over the time of an eclipse exposure probably insignificant. However comparison plates had to be taken of the same region of stars without the sun and sometimes those were months apart from the eclipse exposures so stellar motions over such a period of time would need to be considered.

then also how do you tell a photon which came from behind the body was bent? The only photons that can ever be detected at the ones which land in the detector.

You can compare the eclipse photo to another photo of the same region of space without the sun present, and the photons in the latter will not be bent by the sun. If the relative positions of the stars around the sun change when the sun is in the picture, then we can expect the sun has something to do with it, but as I mentioned, there are some relatively large sources of error, or were in 1919 at least. In the 1922 eclipse measurements, astronomers thought they had the errors down to about 20% of the expected effect of general relativity and it didn't get much better for the next few decades using that technique but we have more accurate techniques available now.

originally posted by: ImaFungi

originally posted by: Bedlam

The stress-energy tensor will also explain behavior like why two photons coming at each other head on will mutually deflect each other as if they both had actual mass and were deflecting each other due to gravity

I thought Photons cannot interact with one another? They cannot collide because pauli exclusion?

Pauli exclusion applies to two identical fermions. Photons are bosons, not fermions. Even with electrons which are fermions, Pauli exclusion doesn't prevent interaction of two electrons.

Photons have never been observed directly interacting with each other as far as I know, but if they have enough energy it's theoretically possible and some indirect effects have been observed. See two photon physics.

originally posted by: Arbitrageur

Pauli exclusion applies to two identical fermions. Photons are bosons, not fermions. Even with electrons which are fermions, Pauli exclusion doesn't prevent interaction of two electrons.

Photons have never been observed directly interacting with each other as far as I know, but if they have enough energy it's theoretically possible and some indirect effects have been observed. See two photon physics.

I used the term pauli exclusion to infer, my understanding that Photons do not obey the term. I said, I thought photons do not interact due to pauli exclusion; meaning, the concept of pauli exclusion, includes the believed, and believed to be interesting knowledge regarding bosons and fermions, I thought I could get away with just evoking the term, and trusting we knew that by using the term and referring to photons, I meant the ways in which photons relate to the term.

I know you will say; but thats muh virtual photons; but if photons cannot interact with each other how could magnetic repulsion be possible (in vacuum, anywhere) you take a N and an N and place them near, and in terms of the potential tinyness of spatial volume, there is quite a lot of distance between the repulsed N and N, that distance must be full of something, in a vacuum, that distance must be full of (muh virtual) photons;

The photons must be interacting with one another, touching one another, and holding, for one to not be able to touch the N directly to the N, the photons create a solid barrier.

a reply to: ImaFungi

What happened to you reaching out to the world's top physicists and them appreciating your beautiful mind, rewriting the laws of physics in your wake?

Did that not pan out or something?

a reply to: Phantom423

There are two distinct components: do photons respond to a gravitational field? And, do photons create a gravitational field?

According to general relativity, the answer to both is 'yes'. The first was experimentally demonstrated in 1919 by observations of star light deflection during a solar eclipse, and amplified many years later with the precise experimental demonstration of the gravitational redshift from a photon traveling up against the Earth's gravity.

The second is much harder to test experimentally. It is a quantitatively very tiny effect, and it is utterly impossible to test on any Earth-bound experiment, and could only remotely plausibly be observed in extreme astrophysical situations like black holes, and in those there would be too many model uncertainties to really tell for sure. However, the theoretical rationale is very strong, the more general form of mass-energy equivalence, and nobody doubts it.

originally posted by: ImaFungi

I know you will say; but thats muh virtual photons; but if photons cannot interact with each other how could magnetic repulsion be possible (in vacuum, anywhere) you take a N and an N and place them near,

Photons do not interact with one another, meaning that the electromagnetic field dynamics is linear.

Regarding your example, you're talking about matter interacting with the EM field which then interacts with other charged matter. Photons (in free space) do not mutually interact directly.

and in terms of the potential tinyness of spatial volume, there is quite a lot of distance between the repulsed N and N, that distance must be full of something, in a vacuum, that distance must be full of (muh virtual) photons;

The photons must be interacting with one another, touching one another, and holding, for one to not be able to touch the N directly to the N, the photons create a solid barrier.

It doesn't work like that. The elctromagnetic field is not like a fluid of material particles.

originally posted by: ImaFungi
a reply to: Arbitrageur

then also how do you tell a photon which came from behind the body was bent? The only photons that can ever be detected at the ones which land in the detector.

They knew the relative angular relation between multiple different stars. During the eclipse the star whose light is being bent appeared to be in a slightly different position temporarily.

originally posted by: GetHyped
a reply to: ImaFungi

What happened to you reaching out to the world's top physicists and them appreciating your beautiful mind, rewriting the laws of physics in your wake?

Did that not pan out or something?

It did

originally posted by: ImaFungi

I thought Photons cannot interact with one another?

The stress-energy tensor allows two photons to attract each other in a sort of gravitational way, but it's extremely small and it's dependent on the vectors of the photons. Maybe also the other bits like polarity. But definitely the vectors. Two photons running parallel to each other don't interact that way. Two coming exactly head-on maximize it.

They can also annihilate each other in certain circumstances, a photon is its own anti-particle.