The Gravity of a Photon • Hacker News Board

Re: The Gravity of a Photon

#51

Post by 100ideas » Wed, Jul 10, 2019, 2:09 PM UTC

radford-neal wrote: post #47

obastani wrote: post #45

Earlier quoted context omitted.

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…

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 energ…

“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?

Re: The Gravity of a Photon

#52

Post by 100ideas » Wed, Jul 10, 2019, 2:16 PM UTC

Xcelerate wrote: post #12

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 othe…

Is 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.

Re: The Gravity of a Photon

#53

Post by 100ideas » Wed, Jul 10, 2019, 2:21 PM UTC

danbruc wrote: post #37

thomaswang wrote: post #29

Earlier quoted context omitted.

Is there a line between where light neither escape or get swallowed. Simply just stands still in perfect balance between being pulled and escaping?

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…

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

Re: The Gravity of a Photon

#54

Post by JWKennington » Wed, Jul 10, 2019, 2:22 PM UTC

typothrowaway wrote: post #8

JWKennington wrote: post #3

Earlier quoted context omitted.

Hence 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)

That 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.

Photons have momentum. momentum is analogous to mass in special relativity (specifically, mass is the 0 component of the four-momentum)

Re: The Gravity of a Photon

#55

Post by JWKennington » Wed, Jul 10, 2019, 2:23 PM UTC

kgwgk wrote: post #7

JWKennington wrote: post #3

Earlier quoted context omitted.

Hence 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)

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 ;-)

It was weird - I've reparaphrased that to be "momentum" now for improved clarity. thanks for feedback

Re: The Gravity of a Photon

#56

Post by JWKennington » Wed, Jul 10, 2019, 2:24 PM UTC

filmor wrote: post #34

JWKennington wrote: post #3

Earlier quoted context omitted.

Hence 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)

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…

I've cleaned up the last paragraph, and replaced my friend's reasoning with the term "momentum" to avoid confusion. thanks for feedback

Re: The Gravity of a Photon

#57

Post by 100ideas » Wed, Jul 10, 2019, 2:26 PM UTC

Causality1 wrote: post #32

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…

A handy rule of... thumb I like to keep in mind is that it takes about 1 ns for light to travel from your hand to your eye...

(... and then 10,000,000 ns for your brain to process it...)

Re: The Gravity of a Photon

#58

Post by 100ideas » Wed, Jul 10, 2019, 2:32 PM UTC

vectorEQ wrote: post #41

dschuetz wrote: post #6

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.

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...

Re: The Gravity of a Photon

#59

Post by [deleted] » Wed, Jul 10, 2019, 2:56 PM UTC

zaarn wrote: post #24

dschuetz wrote: post #16

Earlier quoted context omitted.

"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 le…

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…

[deleted]

Re: The Gravity of a Photon

#60

Post by jessriedel » Wed, Jul 10, 2019, 3:12 PM UTC

100ideas wrote: post #53

danbruc wrote: post #37

Earlier quoted context omitted.

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…

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.