The Gravity of a Photon
jwkennington.com
jwkennington.com
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.
If that's not interesting enough, do some reading on the relative nature of spontaneity. Two things that happen at exactly the same time in one reference frame don't happen at the same time in another. Crazy.
So the only thing I can work from right now is the elevator thought experiment and it seems to me that I can have a perfectly straight rod in a 100 km wide elevator accelerating at 9.81 m/s² while a light ray would bend downwards by 0.5 µm. So ignoring the fact that Earth's gravitational field is not uniform, this seems to suggest that even a rod made out of some common material would not be bend or appear bend or whatever in Earth's gravitational field. Maybe this analogy just leaves the realm of validity of the thought experiment, but I honestly don't know.
I am also not convinced by your analogy with the triangle on a sphere, gravity is curvature of spacetime but the analogy involves only curvature of space. If you tell me that you are a physicist specialized in general relativity, I will take your word for it, otherwise I will keep some doubt about getting my nice rod bend in the same way the light path gets bend, after all they are two quite different things, an extended object versus a path through spacetime.
(... and then 10,000,000 ns for your brain to process it...)
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?
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).
If so it resolves the mysterious central relationship this field has for all the others.
What?
> 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).
Can anyone with with real knowledge of gravity comment on whether these two intuitions are correct?
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.
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...
Are gravity waves similarly observer-reference-frame-dependent? I.e. the gravitation of a given object is not intrinsic but rather dependent on the energy difference between the object and a given observer’s frame?
"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?
I think what I find confusing is that I thought outside observers would never see the infalling observer reach or cross the EH due to time dilation.
I’ve read that an observer falling into a black hole would notice extreme time compression in the external universe (observing millions or billions of years pass in the external reference frame), and conversely external observers would notice extreme time dilation of people and redshifting of photons falling into the black hole. Infalling particles from their external perspective appear frozen and smeared into a blur outside the EH, fading away but never optically appearing to “enter” into the BH, even though physically these particles indeed have/will.
So I think I had it backwards, the external observers would see infalling objects redshift, and those falling in would see the universe blueshift (I guess getting fried by high energy photons before being torn up by tidal forces).
It depends what you mean by "see". An outside observer will certainly not see this this in the literal sense of seeing photons that image the horizon-crossing event. However, there are sets of space and time coordinates on the manifold where the infalling observer crosses the horizon at the same "time" as external events occur.
> frozen and smeared into a blur outside the EH, fading away but never optically appearing to “enter” into the BH
Yes, but note that the brightness of the image gets exponentially suppressed as the infalling object approaches the horizon, so it really just looks more like it's vanishing than freezing (although it's doing both). Importantly, a finite amount of electromagnetic energy is emitted/reflected by the infalling object before it crosses the horizon so, for any given minimum-energy threshold of your detection equipment on the outside, you will see no more than a certain finite number of photons no matter how long you wait.
> and those falling in would see the universe blueshift (I guess getting fried by high energy photons before being torn up by tidal forces).
No, I don't think much blueshifting happens. If you hover above the event horizon of a black hole, the outside world will look blueshifted, to a stronger and stronger degree as you get closer to the horizon. However, the amount of force necessary to maintain a stationary position above the horizon goes to infinity as you get close, so you can't get arbitrarily close and experience arbitrarily large blueshift; indeed, the blueshift is in general quite modest without extraordianry materials or fuels. And once your support fails, so that you start falling into the black hole, the blueshifting goes away.
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.
Considering noone died from waving iron rods through the air by way of electrocution (other than lightning) I'd say that means gravity is not an EM field.
There is also a different test: EM fields are either positive or negative in charge. If you have three objects, in an EM field they cannot all three attract eachother, one object must be pushed away from one of the other two.
Considering we don't see that behaviour in our solar system, gravity cannot be an EM field.
Quarks have -1/3 or +2/3 charge in the standard model and presumably(?) at high enough energies they separate out into a quark-gluon plasma thus exposing the universe to their unholy fractional charges. Above 2 trillion Kelvin according to wikipedia.
Saying “explore”, “idea”, “might” makes it sound as if this was not a well-established 100-years-old theory.
> My presumption is that atoms are somehow made of really high energy EM fields so that they appear to be massive.
Somehow... The easiest way to proceed in that program of research may be to rename the standard model as ElectroMagnetism and try to get a unified theory of the strong, weak and the-force-previously-known-as-em forces. Then you unify that with GR and you’re done :-)
Certainly the unification of electromagnetism + the weak force is understood, and there is a theory (or several theories) about electroweak + strong unification: https://en.wikipedia.org/wiki/Grand_Unified_Theory .
But we may be close, for some values of close :-)
Indeed, that's why I said only that there were theories about it.
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.
So these fields can’t interact with everything everywhere, only the portion of the universe In the field’s lightcone. if the universe keeps this up there’s a chance some day Everything will be so stretched out that the fields will be basically redshifted to null, so not interacting with anything anywhere. That’d be lame.
That's the radius.
I've seen this dualism claimed pretty recently by Leonard Susskind.
Isn't dark matter most likely some kind of material that has mass, but no EM interaction?
We also know that there are interactions inside atoms (the strong and weak forces) that have no known relation to electro-magnetism, so I'm not sure what you are getting at.
The next guess here is that dark energy has a much much higher rest energy so that no photon in this universe is able to interact with it via EM fields. The search for dark matter was unsuccessful so far, and the LHC is still trying to achieve higher energies to be able to detect dark matter. So, dark matter is just a guess.
The interactions inside atoms seem to have no known relations to EM, because, and this is my guess, there are much higher energies at work than we know. So, if dark matter is hiding behind high energies so that EM interaction seems impossible, why should it be different for EM interactions inside atoms. There still is a slim chance that it might be EM, just at much higher energies.
The electromagnetic and the weak interaction are known to be different aspects of the unified electroweak interaction. There are several grand unified theories and a few hints that at even higher energies the electroweak and strong interaction also unifiy into the electronuclear interaction. Due to the high energies involved, only indirect experimental evidence seems accessable for the forseeable future but there are various groups looking for hints. I am not aware of any uncontroversial results but there are a few non-reproducable experiments, for example observations of magnetic monopoles, which might be due to experimental error but also due to the rarity of magnetic monopoles.
Edit: ok so really there is only theoretical evidence of an electroweak field at the beginning (electromagnetic field and weak field are not independent at high energy) and everyone is currently hard at work trying to 1) figure out the “Grand Unified Theory” that will putatively integrate the strong force with the electroweak force, and 2) figure out how to rethink either gravity/spacetime in a quantum context to integrate it with the quantum GUT, or unquantize the GUT (not likely) so it can be combined with spacetime...