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The Gravity of a Photon

jwkennington.com

71–80 of 90 posts

Re: The Gravity of a Photon

#71
post #62
post #14

Earlier quoted context omitted.

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? :)

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.

Re: The Gravity of a Photon

#72

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...)

If you haven't seen the brief lecture from Admiral Grace Hopper regarding nanoseconds it's well worth a watch. https://youtu.be/JEpsKnWZrJ8

Re: The Gravity of a Photon

#73
post #38

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…

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.

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.

Re: The Gravity of a Photon

#74

Earlier quoted context omitted.

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 o…

[...] the diameter of the observable universe is ~42 billion light-years [...]

That's the radius.

Re: The Gravity of a Photon

#75

Earlier quoted context omitted.

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...

They are also just in the models and heads of physicists.

Re: The Gravity of a Photon

#76

Earlier quoted context omitted.

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 i…

> 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.

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.

Re: The Gravity of a Photon

#77
post #40
post #39

Earlier quoted context omitted.

> 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_Theo…

That links says: “Unsolved problem in physics: Are the three forces of the Standard Model unified at high energies?” But we may be close, for some values of close :-)

> That links says: “Unsolved problem in physics: Are the three forces of the Standard Model unified at high energies?”

Indeed, that's why I said only that there were theories about it.

Re: The Gravity of a Photon

#78
post #24

Earlier quoted context omitted.

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.

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.

Re: The Gravity of a Photon

#79
post #38

Earlier quoted context omitted.

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.

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.

Re: The Gravity of a Photon

#80

Earlier quoted context omitted.

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)

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