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The Gravity of a Photon

jwkennington.com

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Re: The Gravity of a Photon

#31

Post by danbruc »

kgwgk wrote: post #28
danbruc wrote: post #27

Earlier quoted context omitted.

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.

Other people may say that photons are just mathematical tools that we use to describe fields. :-)

The fact that gauge fields have redundant degrees of freedom should at least be a strong hint that the fields are not the fundamental description. ;)

Re: The Gravity of a Photon

#32

Post by Causality1 »

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.

Re: The Gravity of a Photon

#33

Post by carlob »

danbruc wrote: post #31
kgwgk wrote: post #28

Earlier quoted context omitted.

Other people may say that photons are just mathematical tools that we use to describe fields. :-)

The fact that gauge fields have redundant degrees of freedom should at least be a strong hint that the fields are not the fundamental description. ;)

I don't think you are correct: you can view the extra degrees of freedom as just a symmetry

Re: The Gravity of a Photon

#34

Post by filmor »

JWKennington wrote: post #3
kgwgk wrote: post #2

> because photons have rest energy that can be viewed as mass in special relativity What?

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 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).

Re: The Gravity of a Photon

#36

Post by danbruc »

carlob wrote: post #33
danbruc wrote: post #31

Earlier quoted context omitted.

The fact that gauge fields have redundant degrees of freedom should at least be a strong hint that the fields are not the fundamental description. ;)

I don't think you are correct: you can view the extra degrees of freedom as just a symmetry

It is usually called a gauge symmetry but it is kind of a misnomer and should better be called a gauge redundancy. It is artifical and only due to the mathematical formalism we are using. As an analogy, probably not a really good one, is that you could decide to describe temperatures with a complex number and just say temperature is invariant under translation along the imaginary axis. Sure, the symmetry is in your mathematics but it doesn't really tell you anything new or interesting. Besides of course that the imaginary part is actually redundant and you better described temperature with a real number instead.

Re: The Gravity of a Photon

#37

Post by danbruc »

thomaswang wrote: post #29
jessriedel wrote: post #5

Earlier quoted context omitted.

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.

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 can only move at the speed of light, neither slower nor faster - but orbit the black hole at the speed of light.

Re: The Gravity of a Photon

#38

Post by danbruc »

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…

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.

Re: The Gravity of a Photon

#39

Post by JadeNB »

kgwgk wrote: post #23
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…

> 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 ma…

> 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 .

Re: The Gravity of a Photon

#40

Post by kgwgk »

JadeNB wrote: post #39
kgwgk wrote: post #23

Earlier quoted context omitted.

> 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 ma…

> 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 :-)

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