The Gravity of a Photon • Hacker News Board

Re: The Gravity of a Photon

#61

Post by 100ideas » Wed, Jul 10, 2019, 3:14 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…

I like your elegant thought experiment!

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.

Re: The Gravity of a Photon

#62

Post by filmor » Wed, Jul 10, 2019, 3:19 PM UTC

danbruc wrote: post #14

noobermin wrote: post #10

The 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?

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.

The internal structure of leptons being? :)

Re: The Gravity of a Photon

#63

Post by liberte82 » Wed, Jul 10, 2019, 3:29 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…

How does one particle know that is here know that there's another one over there?

Re: The Gravity of a Photon

#64

Post by liberte82 » Wed, Jul 10, 2019, 3:31 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…

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.

Re: The Gravity of a Photon

#65

Post by 100ideas » Wed, Jul 10, 2019, 3:39 PM UTC

jessriedel wrote: post #60

100ideas wrote: post #53

Earlier quoted context omitted.

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.

Ok, thanks for bringing precision to my comment. It makes sense that any photons reaching an observer who is inside a BH event horizon must also already be trapped inside the EH.

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).

Re: The Gravity of a Photon

#66

Post by 100ideas » Wed, Jul 10, 2019, 3:47 PM UTC

liberte82 wrote: post #63

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…

How does one particle know that is here know that there's another one over there?

How does spacetime know to curve?

Re: The Gravity of a Photon

#67

Post by 100ideas » Wed, Jul 10, 2019, 3:56 PM UTC

cycomanic wrote: post #44

danbruc wrote: post #18

Earlier quoted context omitted.

EM 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 wi…

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?

What, you don’t think the Hylaean Theoric World is realer that reality?

Re: The Gravity of a Photon

#68

Post by radford-neal » Wed, Jul 10, 2019, 4:01 PM UTC

100ideas wrote: post #51

radford-neal wrote: post #47

Earlier quoted context omitted.

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 wo…

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.

Re: The Gravity of a Photon

#69

Post by 100ideas » Wed, Jul 10, 2019, 4:05 PM UTC

dschuetz wrote: post #20

danbruc wrote: post #18

Earlier quoted context omitted.

EM 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 wi…

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.

Re: The Gravity of a Photon

#70

Post by 100ideas » Wed, Jul 10, 2019, 4:08 PM UTC

dschuetz wrote: post #26

tsimionescu wrote: post #22

Earlier quoted context omitted.

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.

Let’s talk virtual particles...