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The Gravity of a Photon
- kgwgk 7y ago> because photons have rest energy that can be viewed as mass in special relativity What?
- JWKennington 7y agoHence I said my friend was being a bit “interpretive”. He really meant that though photons have zero rest mass, they nonzero inertial mass (E=mc^2)
- kgwgk 7y agoMaybe you could write “because photons have energy that can be viewed as (relativistic) mass” then. Or maybe he did really refer to the “rest energy” of photons? That seems weird even for an experimentalist ;-)
- JWKennington 7y agoIt was weird - I've reparaphrased that to be "momentum" now for improved clarity. thanks for feedback
- typothrowaway 7y agoThat still does not make any sense. I'm sorry to be so direct, but I'm really worried laypeople here are going to think that this is correct.
- dschuetz 7y agoA layperson here. Don't.
- JWKennington 7y agoPhotons have momentum. momentum is analogous to mass in special relativity (specifically, mass is the 0 component of the four-momentum)
- deleted 7y ago[deleted]
- filmor 7y agoThat's not how it's written in the post though. If you just swap out the "inertial mass" at the end and the "rest energy" somewhere in the middle it makes more sense: > because photons have rest energy (=> kinetic energy) that can be viewed as mass in special relativity > [...] > impact the gravitational field despite having no inertial mass (=> rest mass)! Speaking of "mass" in context of relativity is always a bit tricky. The term "inertial mass" for example is really saying something about how an object behaves under the influence of a force (F = ma), not how it behaves relativistically (that would be "relativistic mass", E = mc2).
- JWKennington 7y agoI've cleaned up the last paragraph, and replaced my friend's reasoning with the term "momentum" to avoid confusion. thanks for feedback
- xupybd 7y agoWow I didn't expect that! For the layperson that's really interesting. "the conventional electromagnetic fields, and the photons that constitute them, impact the gravitational field despite having no inertial mass!" Does that mean the build up of the photon sphere around a black hole would have a significant impact on space time?
- jessriedel 7y agoThe photon sphere is not a place where photons collect because the orbits along the sphere are unstable fixed points. Small perturbations from these orbits either fall into the black hole or escape to infinity.
- thomaswang 7y agoIs there a line between where light neither escape or get swallowed. Simply just stands still in perfect balance between being pulled and escaping?
- danbruc 7y agoThat is what the parent comments were talking about - on the photon sphere - in case of a non-rotating black hole - photons could theoretically orbit the black hole but as pointed out by your parent comment this orbit is not stable and any tiny perturbation will either send the photons into the black hole or make them escape. And the photons would of course not stand still - they are massless particles and therefore can only move at the speed of light, neither slower nor faster - but orbit the black hole at the speed of light.
- 100ideas 7y agoIf you were further in the black hole’s gravity well than the photons in the photon sphere, you would see them redshift and eventually freeze, right? I.e. from inside the photons don’t appear to escape
- 7y ago
- dschuetz 7y agoThis is really interesting. Perhaps there is also a chance that electromagnetic fields are indeed entirely responsible for gravity, in some complicated yet unknown manner.
- danbruc 7y agoNo, energy curves the gravitational field and there are other forms of energy than the energy of the electromagnetic field. For example, if you start walking, you gain kinetic energy and this will affect the gravitational field, not much but it will.
- kgwgk 7y agoWhat would that mean? As far as we know there is gravity in the absence of EM fields and EM energy produce gravitational effects just like other forms of energy (and in the realm of direct human experience the contribution of EM is negligible).
- dschuetz 7y ago"there is gravity in the absence of EM fields" That just cannot be true. EM fields are just about everywhere. They permeate space almost as universally as gravity, but in form of photons (to name higher energy EM fields here) or other low energy waves. Gravity accumulates with great masses, mass is also energy (in rest). The article explores the idea that pure kinetic energy in form of photons indeed might have at least gravity-like properties. My presumption is that atoms are somehow made of really high energy EM fields so that they appear to be massive. Consequently, atoms having mass or photons having inertia are responsible for gravitation, and that could mean that EM fields are the cause of gravity entirely. The problem with that presumption is that perhaps there is no way to check.
- danbruc 7y agoEM fields are just about everywhere. Fields are mathematical tools, they are not - without wanting to go down the philosophical rabbit hole - real things out there in the universe. And the thing about everything just being electromagnetic fields is nonsense, physicists understand physics way better than you do. Sorry to sound condescending, but the internet already has enough obviously wrong ideas by random people without a clue what they are talking but.
- noobermin 7y agoThe important thing to remember is it's called the stress-energy tensor for a reason, it's not just mass distribution that causes curvature. Thought experiment: which will curve spacetime more, a 1kg box of gas at 1K or the same box after the gas inside is raised to 293K?
- danbruc 7y agoThe warm box will of course »heavier« because it contains additional energy. Fundamentally there is no mass, it is just a convenient way to talk about the energy hidden in the structure of matter.
- noobermin 7y agoBingo
- filmor 7y agoThe internal structure of leptons being? :)
- danbruc 7y agoNote that I wrote structure and not internal structure and I did so intentionally. As far as we know elementary fermions - quarks and laptons - have no internal structure but - and I am totally not a physicist - there are the Higgs mechanism, the seesaw mechanism, selfinteraction due to vacuum polarization, and other things I have never heard of. As far as I know, this is still an area of active research with unresolved questions, but I think it is also generally believed among physicists that all mass has a dynamical origin.
- noobermin 7y agoThis isn't quite the same thing as what adds mass to say hadrons like who you're replying to is, but at least according to the Standard Model, leptons as well as quarks start out massless and acquire an inertial mass due to interaction with the Higgs field. You wouldn't say it is "internal structure" but it arises due to interactions rather than being an a-priori property.
- Xcelerate 7y agoI find gravity interesting, because everything that exists interacts with everything via the gravitational field. This is not the case with any other sort of interaction; gravity is truly different. See this diagram showing the ways that different particles can interact in the Standard Model: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Elementary_particle_interactions.svg/400px-Elementary_particle_interactions.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/El... Photons, for instance, cannot interact directly with each other via the electromagnetic field. Gluons do self-interact via the strong interaction. But both photons and gluons interact with all types of particles via gravity (although sometimes this interaction is so small that we would never be able to measure it).
- danbruc 7y agoPhotons do interact electromagnetically [1], just not in the same direct way as gluons. [1] https://en.wikipedia.org/wiki/Two-photon_physics https://en.wikipedia.org/wiki/Two-photon_physics
- Xcelerate 7y agoYeah, I was more so speaking of direct coupling but didn’t want to get too technical. That interaction is due to virtual fermion-antifermion pairs.
- 100ideas 7y agoIs it ok to mentally replace the word/concept “gravitational field” with “energy density field”? If so it resolves the mysterious central relationship this field has for all the others.
- liberte82 7y agoHow does one particle know that is here know that there's another one over there?
- 100ideas 7y agoHow does spacetime know to curve?
- Causality1 7y agoWhat I've always found wild about gravity and photons is that gravity produces the same proportionate effect, i.e., acceleration, on them as it does normal matter and the only reason the sun and planets don't render light nearly unusable for sensing by utterly distorting the light's path is that it travels so quickly it doesn't spend any significant amount of time in a gravity well. I don't think I'll ever fully wrap my head around the kind of numbers that implies, that when light is at approximately sea level it is accelerating at 9.8m/s^2 toward the core of the earth and the only reason it doesn't all get dragged out of the air and to the ground is that it moves too fast to notice.
- danbruc 7y agoThat is an interesting thought that never occured to me. It would render the headlights of your car pretty much useless if the light just fell to the ground a few meters in front of the car. I wonder if one has or could measure the effect in the lab - it's about 0.5 μm over a distance of 100 km, so it seems not totally out of reach.
- Causality1 7y agoIt's a bit tricky because it isn't light taking a curved path through your lab; it's light taking a straight path through the curved spacetime in your lab.
- danbruc 7y agoThe elevator thought experiment suggests that the light will indeed also take a curved path through my lab, but I am way out of the territory where I have any good intuition. And I have no intuition at all what would happen to the matter the elevator is made of if one accelerated it hard enough to make the effect noticable, nor do I have any idea what Earth's gravity does to matter in my lab. Can I have a trully straight ruler out of some common material? Or at least not as bend as the light beam? And now I am not even sure anymore what straight would mean.
- 7y ago
- Tepix 7y agoThe low contrast on that web page is terrible.
- radford-neal 7y agoIf you think electrons and positrons have a gravitational influence, then it seems to me you should also think photons do. Otherwise, if an electron and positron annihilate, producing photons, there will be a discontinuous drop in the gravitational field, which doesn't seem right somehow. Furthermore, if you put all this in a black box, the gravitational influence of the box will change because of what's happening within it, which also doesn't seem right. Can anyone with with real knowledge of gravity comment on whether these two intuitions are correct?
- obastani 7y agoI don't have real knowledge of gravity, but I can speculate a bit. First, I don't think the blackbox argument necessarily makes sense. If the black box contains a massive point particle, and this particle moves from the left side of the box to the right, then the gravitational properties certainly change. For example, if there is a particle on top of the box, it will initially be pulled to the left, and subsequently be pulled to the right. Having said that, your intuition that photons should have gravitational influence makes sense, since according to special relativity, mass and energy are equivalent. Since total energy (mass + energy) is conserved, the gravitational influence of the photon should equal that of the electron-positron pair (assuming they are all located at the same position). I think the article is agreeing with this point of view.
- radford-neal 7y agoIf the black box is freely floating (which is what I was thinking of), then a particle within it can move to one side only if something else in it moves the other direction. But on further thought, I wouldn't be surprised if some changes in the internals of the box could produce gravitational waves, so maybe my intuition for this is wrong. Although if I further clarify that the box isn't supposed to be emitting energy (which will obviously reduce its gravitational influence as it loses energy), then maybe the intuition is correct after all...
- 100ideas 7y ago
- Ceezy 7y agoI don t understand. The title is gravity of photon. Then you start talking about electromagnetic field. I sure something is missing here. What would be the source of gravitation and who is subject to that field?
- Sharlin 7y agoPhotons are excitations of the electromagnetic field. The source of gravity is the energy in the EM field, as described by general relativity.