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Physicists discover that gravity can create light

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Re: Physicists discover that gravity can create light

#81

Post by upsidesinclude »

stared wrote: post #14

All quantum fields have their vacuum energy fluctuations. When we change the parameters of the space (e.g. stretch or compress it), the current state is no longer the ground state but becomes a so-called squeezed vacuum state. This effect is used in laser physics to "split photons" in spontaneous parametric downconversion. That is, an intense laser changes the refractive index of a medium periodically. These oscillat…

You seem educated on some matters, so please reply here. My physics education stopped at modern physics (optics, general relativity) so I'm missing the core mathematics of quantum mechanics and deeper dives into particle physics.

In the actual summation, does the calculation of vacuum energy take into account a cross section of neutrinos and extended "particles". I've always found the concept of vacuum energy lacking in that most(even physicists) do not fundamentally conceptualize a permanent neutrino and elementary particle flux in all but the most astoundingly shielded vacuum.

If there is a medium in which to effect the refractive index, then isnt this an interaction of the molecular structures therein?

Re: Physicists discover that gravity can create light

#82

Post by fallingknife »

XorNot wrote: post #73
fallingknife wrote: post #61

Earlier quoted context omitted.

That doesn't make sense. Everywhere is the edge of the observable universe to someone.

Yes, but because space is expanding - i.e. there's a value of meters per meter per second for the expansion of space, then every point in space has a sphere around it of locations which are sufficiently far away that they are growing more distant at the speed of light. Any virtual particle pair which appears along that horizon can potentially be caught on the wrong side of it - i.e. one virtual particle is in a part…

Ok, but we are at such a horizon right now. Why can't we observe the hawking radiation from it?

Re: Physicists discover that gravity can create light

#84

Post by XorNot »

fallingknife wrote: post #82
XorNot wrote: post #73

Earlier quoted context omitted.

Yes, but because space is expanding - i.e. there's a value of meters per meter per second for the expansion of space, then every point in space has a sphere around it of locations which are sufficiently far away that they are growing more distant at the speed of light. Any virtual particle pair which appears along that horizon can potentially be caught on the wrong side of it - i.e. one virtual particle is in a part…

Ok, but we are at such a horizon right now. Why can't we observe the hawking radiation from it?

It's a very large horizon, so the Hawking radiation is very weak (it's always a weak effect). We can't see it because the cosmic microwave drowns it out.

Broadly Hawking radiation intensity goes with curvature - i.e. a more curved surface has a better chance of separating a particle pair then a flatter one. This is because a sharper curve means more vectors which carry you away from the event horizon.

This is also why small black holes evaporate faster then big ones - as the black hole shrinks, the Hawking radiation intensity increases because it's curving more and more (hence why microscopic black holes don't devour everything).

Re: Physicists discover that gravity can create light

#85

Post by computerfriend »

Maursault wrote: post #39
TheOtherHobbes wrote: post #35

Earlier quoted context omitted.

Now make it work with QFT.

It already works. Contrary to vacuous and uninformed popular belief, there is no fundamental incompatibility between QFT and GR.

What's the gravitational field around a photon?

Re: Physicists discover that gravity can create light

#86

Post by computerfriend »

Maursault wrote: post #13
_8j50 wrote: post #10

Isn't gravity just a fundamental force? Could it be matter reacting to gravity? Light is energy but gravity is not energy and has no mass so how can it create something out of nothing? How is hawking radiation even possible unless something else was involved?

> Isn't gravity just a fundamental force? Not really. Gravity is most accurately described by the general theory of relativity... which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.[1] [1] https://en.wikipedia.org/wiki/Gravity

Electromagnetism is also curvature though.

Re: Physicists discover that gravity can create light

#87

Post by adastra22 »

neatze wrote: post #66
adastra22 wrote: post #63

Earlier quoted context omitted.

Dark energy is adding energy to the universe (mechanism unknown).

How does expansion of universe increases total energy ? If anything seems like due to dark energy total energy dissipation per area is increasing until death of universe.

It takes work to separate two masses from each other (increasing gravitational potential energy). The universe is not just expanding, which might possibly be understood in an energy conserving way if the rate of expansion is slowing, but it is expanding at an ever increasing rate. Ergo enter guy is being dumped into the universe from some repository/field/whatever that we don’t understand. That’s what Dark Energy is.

Re: Physicists discover that gravity can create light

#88

Post by adastra22 »

neatze wrote: post #65
eep_social wrote: post #60

Earlier quoted context omitted.

and (depending how time actually works) transformed also may be wrong, rather, everything simply is and time is our hallucination of reality

This does not compute in my mind, energy transfer (in some form) it is observed phenomena (my understanding (primitive) of time is energy transfer), even on quantum level we can measure it, what am I missing ?

You aren’t missing anything. He’s playing definitional word games, a common pastime of philosophers.

Re: Physicists discover that gravity can create light

#89

Post by mitthrowaway2 »

raattgift wrote: post #58
mitthrowaway2 wrote: post #53

Earlier quoted context omitted.

It's a direct consequence of general relativity, because photons carry energy and momentum. Some discussion here: https://physics.stackexchange.com/questions/22876/does-a-pho...

Maybe better as "momentum-energy" than as "energy and momentum". "Stress-energy" is probably better still. I don't think we have to consider the photons as such because large-photon-number beams sent from, to, and between spacecraft have been shown to deflect around masses in our solar system, sunlight generates measurable radiation pressure (and greybody radiation contributes to the Yarkovsky effect), and because yo…

> "Stress-energy" is probably better still.

There's better in the sense of correctness, and there's better in the sense of "words that will be useful to someone who has never heard that light can create gravity". My inference is the GGP does not already have a deep technical background in general relativity. This guides the words I choose.

But I'll always upvote for technical correctness!

Re: Physicists discover that gravity can create light

#90

Post by pa7x1 »

_8j50 wrote: post #71
_8j50 wrote: post #10

Isn't gravity just a fundamental force? Could it be matter reacting to gravity? Light is energy but gravity is not energy and has no mass so how can it create something out of nothing? How is hawking radiation even possible unless something else was involved?

Ok, why all the downvotes even after starting a healthy discussion thread? And can someone help me understand in layman terms how current physics posits that gravitation is generating light or even hawking radiation?

Gravity couples with anything that carries energy (in more precise terms with the energy-momentum tensor). Photons carry energy. Therefore there is interaction between photons and gravity. In particular, you can write Feynman diagrams for the interaction between a quanta of gravity (a graviton) and a photon. Some of the simplest ones look like this:

https://www.researchgate.net/figure/Tree-level-Feynman-diagr...

There you can see different ways in which a pair of gravitons can interact leading to a pair of photons. If you want to dig deeper in the math behind it: https://physics.stackexchange.com/questions/335917/how-do-i-...

Regarding Hawking radiation, it's not gravity itself that causes it. It's a consequence of the existence of an event horizon. In fact, you can have a similar effect in flat spacetime (i.e. no gravity), an accelerating observer will see an event horizon and it will feel itself immersed in a thermal bath of particles at certain temperature that has to do with its acceleration. This is called the Unruh effect. Further reading: https://en.wikipedia.org/wiki/Unruh_effect

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