What 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.
That's the fundamental idea behind a black hole. The concept of escape velocity is based on the curvature of spacetime and is represented by √2GM/R where M is mass and R is radius. Light travels along the curvature of spacetime. At the event horizon of a black hole, escape velocity equals c, so from that point down to the singularity, spacetime is curved so much that light simply isn't traveling fast enough to escape.
That 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.
Your comment reminded me of Gamow's Mr. Tompkins in Wonderland. I don't know if he mentioned that in particular, but he discussed what relativistic effects would look like if the speed of light was 10 MPH.
It'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.
The 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.
If you had a 100km long rod that was perfectly, impossibly straight and infinitely strong, placing it in a gravity well equal to that at the earth's surface would still make it look bent by half a micrometer to an outside observer, in the same way you can draw a triangle on a sphere using nothing but three right angles and three straight lines.
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.
I guess this is an autocorrect issue, but you certainly mean simultaneity. Anyway, I have been through several university courses on special and general relativity with all the math and have at least some basic understanding of the thing. However, as said before, I have no real intuition for it.
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.
This ties in well with the revelation many high school physics students have that a bullet fired from a gun parallel to the ground also falls to the earth at the same rate as any other object.
The 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.
Note 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.
This 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.
I 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/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).
That'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).
Maybe 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 ;-)
If 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?
I 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.
If 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...
“Electromagnetic waves” are the manifestation of relative motion between an electric charge and an observer’s reference frame. A detector inside a cryogenic ion trap would detect much lower energy EM radiation from the contained cloud of ions vs a sensor speeding towards the trap at 10% c. If this second detector slowed towards the inertial reference frame of the ions, the apparent EM radiation emanating from them would decay towards zero from the second detectors point of view as it became stationary relative to the charges.
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?
I think the charges need to be accelerating, not just moving, to emit radiation. Whether or not something is emitting radiation ought to be invariant to the motion of the observer.
The 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.
That 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.
If 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
There are no frames of reference where photons reverse direction. If everything is spherically symmetric, a photon emitted from any point in spacetime is either (1) already heading outward and escapes or (2) already heading inward and is consumed. Within the event horizon, only trajectories of type (2) exist. The picture of a photon struggling outward to escape and then reversing direction under gravity is incorrect.
I 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?
This is really interesting. Perhaps there is also a chance that electromagnetic fields are indeed entirely responsible for gravity, in some complicated yet unknown manner.
No, 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.
What 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).
"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.
Easy way to check, just wave a long piece of iron through the air. If there is truly a massive EM field, it should induce a decent voltage into said iron rod as it moves through the magnetic field lines produced by the "gravity EMism"
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.
Even with fraction, either you have repulsion or attraction, depending on the sign. -1/3 charge will be repulsed by +1 or +2/3 charge, just at different strengths.
> The article explores the idea that pure kinetic energy in form of photons indeed might have at least gravity-like properties.
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 :-)
> 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.
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 .
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.
Well by saying that fields are not "real things" you have just started going down the "philosophical rabbit hole". What is a real thing, is the gravitational force that we feel not a real thing, is the energy we feel in an EM wave (e.g. The sun on our skin) not a real thing?
Dude, fields are a tool physicists use to describe phenomena. It's okay to say EM fields exist because they interact with everything everywhere and we can actually observe that.
The universe appears to have expanded or be expanding faster than the speed of light: the diameter of the observable universe is ~42 billion light-years but best estimates of age of universe is ~14 billion years.
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.
Honest question: aren't radio waves "physical objects" that represent oscillations of an electro-magnetic field, to the best of our current understanding?
There is an agreement in physics that low energy EM fields are called waves, such as radio waves, and high energy EM fields are called photons or even particles. In any case they are actually both. If they are physical objects I don't know. I think, I wouldn't go that far.
Radio waves are streams of photons, the fields are just a mathematical tool we use to describe this. And there are several of them, the classical electromagnetic field but also an entire set of fields involved in the quantum version.
> EM fields are just about everywhere. They permeate space almost as universally as gravity
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.
As far as I understand, dark matter is an educated guess (which was made upon the discovery that galaxies rotate differently than expected). Something out there must be causing gravity, but we cannot see it, which means that there is no observable EM interaction.
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 interactions inside atoms seem to have no known relations to EM [...]
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.
there's a whole field of science who believes gravity is just electromagnetic effect... it's just particle physicists who struggle with such notions...
Well in the beginning there was just the one electro-strong-weak-gravity field... until inflation cooled everything down enough for the four “fundamental” forces to freeze out.
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...
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.