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Comment by exabrial | Hacker News Reader

I always find it interesting we have the photon for light, electron for electricity, but no particle for gravity.

Gravity waves definitely exist, and have photon-like properties, but I believe the problem is designing an experiment which would prove that gravity is quantised and therefore carried by particles.

(As with electrons and photons, in a lot of cases their individuality isn't meaningful, so we talk of light wavefronts and bandgaps and so on)

Not a physicist.

Assuming gravitons exist, then that would mean that they are constantly being exchanged between objects of mass. This would imply that objects containing mass at a fixed distance would spontaneously become entangled due to their gravitational interaction.

A bit of googling reveals [0], which I believe describes such an experiment.

[0] https://arxiv.org/pdf/1707.06050.pdf

There is a particle for gravity, it's the graviton (massless, spin-2). It is not observed experimentally because it's extremely far from our technological capabilities.

In fact, mathematically, General Relativity can be understood as the classical field theory of a massless, self-interacting, spin-2 field. In the same way that electromagnetism is the classical field theory of a massless, non-interacting, spin-1 field.

We haven't observed the graviton, but it's something theorists routinely discuss. Basically, any quantum theory of gravity that reduces to general relativity in its classical limit will pass through a semi-classical regime where it's well-approximated by gravitons.

My understanding is that from General Relativity (GR) mass carrying particles (and other energy-momentum) curves space-time, and that curvature is what we understand as gravity. I'm aware of there needing to be an exchange particle due to quantum mechanical (QM) representations, but have not been convinced how they work.

I wonder if the Higgs particle could be a candidate for the force carrier in the QM representation of GR (e.g. the resulting higgs field interacts with and causes space-time curvature instead of energy-momentum doing it directly). That is speculation on my part from a lay-person perspective, so could be completely wrong.

> I wonder if the Higgs particle could be a candidate for the force carrier in the QM representation of GR

It couldn't. The Higgs boson is a scalar particle (spin 0), the graviton (if it exists) must have spin 2.

I have no knowledge beyond first-year university physics i.e General Relativity and my understanding of Gravity was that two masses are attracted to one another, which is due to the "curvature" of spacetime.

However, this sort of stuff i.e. the above article, makes me so curious about the universe and fills me with joy just reading about it. Would you possibly have any suggestions as to what resources one can read preferably books as it allows a journey or at the very least a concrete thing to study. ( I get distracted with wikipedia like websites becuase I jump from link to link and then get completely lost ).

How much time do you have on your hands? And how long-term a commitment are you willing to make?

I'm asking because you don't learn these things over night. Gravity & geometry in particular take quite some time to digest and then there's quantum field theory which, in my opinion, takes even more time and is even harder to digest.

Also, how mathematically inclined are you? Does it bother you when things are not clearly and precisely defined?

Technically, the photon is also the particle for electricity, not the electron.

What? Give me a link so I can learn.

Trick question (or trick answer): depends what you mean by "electricity", which is one of those questions that's simple to ask but collapses into a pile of details on closer inspection.

The electric field and magnetic field are tied together in the manner described by Maxwell's equations. This results in "electromagnetic waves", which are indeed carried by photons in free space.

It is often helpful to look at the fields as being the real "electricity", or at least use the lumped approximation we call "current", and this brings various charge carriers along with it. Electrons are the common charge carrier, but ions can carry charge (e.g. in batteries), and in semiconductors we use "holes", which are the absence of an electron. Not an anti-electron, just the space where it's supposed to be.

So the first paragraph on the photon wiki page might give you a start: https://en.wikipedia.org/wiki/Photon

https://en.wikipedia.org/wiki/Photon

I don't think that's correct. Electricity is the electron potential across a medium. That's not the same as an electromagnetic wave.

The reason there is a potential is because of electromagnetic forces, which are mediated by photons. It’s a bit complicated.

Yeah, it is correct. The electromagnetic interaction is mediated by the photon. An electron is just one of the fundamental particles that interacts electromagnetically. We use free electrons in metals for the conduction of electricity, but in the end all the interactions between the electrons, protons, etc are governed by photons.

If you want to learn more, something to google would be quantum electrodynamics. There are some nice introductory talks about it, but be warned. In order to really understand this, you'll need to study physics, more specifically quantum field theory.

we don't have particle for gravity, but we also don't have particle for temperature. we just define it as statistical value of multiple particles and everybody is happy and don't search for some tempon particle. Maybe mass is also some statistical property of group of particles.

Couple of weeks ago there was post on HN about it.

Temperature is usually exchanged through electromagnetic interactions.

> usually

What's the temperature of the cosmic neutrino background now and at decoupling?

infrared radiation is just photons...

"In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravity."

https://en.wikipedia.org/wiki/Graviton

There is a theoretical graviton particle. But reconciling quantum world with general relatively is not easy. https://en.wikipedia.org/wiki/Graviton

There is a hypothetical particle named Graviton that would be the force carrier for Gravity.

https://en.wikipedia.org/wiki/Graviton

It should be the graviton.

No experimental detection and none expected any time soon.

To be specific, from Wikipedia [1]: "a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions"

[1] https://en.wikipedia.org/wiki/Graviton#Experimental_observat...

It's like the neutrino of neutrinos.

Interesting.... I wonder if it is just a matter of time and focus.

Or really difficult math.

https://en.wikipedia.org/wiki/Graviton

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