[paul cotter]

Interesting material concerning the math behind the derivation of the uncertainty principle, Thank you, ES.

[alancalverd]

the nature of the Hydrogen atom may follow just from one (or a few) simple principles of Quantum Mechanics

Not in my book! The nature of the hydrogen atom (and indeed everything else) is a given. Quantum mechanics, like any other scientific principle, gives us a model for predicting the behavior of stuff. You might find a better model, but it won't change the diameter of an atom. Hence my closing sentence: quantum theory is the predictive interpretation of everyday observations.

However, the Hydrogen atom is the way it is and has the line spectra it has because of.... the details you can only get out of the full solution to the Schrodinger equation for the Hydrogen atom

which is exactly what I said. It just happens (fortunately) that the solution for the ground state of hydrogen has spherical symmetry.

[evan_au]

Another random thought....
- All current interpretations of quantum theory are incomplete, because nobody has (yet) managed to include gravity near a black hole in quantum calculations. Every attempt (so far) has ended up with awkward infinities that can't be resolved. https://en.wikipedia.org/wiki/Renormalization#Renormalizability
- Gravity interacts with all other particles, so some future quantum theory incorporating gravity would include interactions between the hypothetical gravitons and other quantum states
- Does this mean that if we want quantum computers to work reliably, we would need to put them into "free fall", to reduce quantum decoherence due to the hypothetical gravitons?

[alancalverd]

There's something unsettling about gravitons.

The emission of any other particle results in a change of the emitter, and no other particle (including the photon) transfers negative momentum to the target.

As you say, we don't like infinities, so the idea that a mass can emit an infinite number of gravitons at a constant rate is, to say the least, intellectually challenging. But if it is the case, then the emission of negative momentum must imply an indefinite increase of mass. Now that is very handy because if we allow a nonzero amount of quantum fluctuation (Heisenberg again!), the creation and distribution of mass ex nihilo is inevitable and the observable universe is being created everywhere, always.

[paul cotter]

I would have expected gravitons to be exchanged between, for example two masses rather than one-way emission. The case of a single mass in an otherwise empty infinite universe conflicts with this notion.

[alancalverd]

It makes sense for all masses to radiate gravitons but that still doesn't explain why gravity sucks whilst photon (or any other) radiation blows!

[Eternal Student]

Hi,

Another random thought....
.....   ...  ....
- Does this mean that if we want quantum computers to work reliably, we would need to put them into "free fall", to reduce quantum decoherence due to the hypothetical gravitons?

We're not ignoring your post.   We (or at least I) just don't have a lot to say that is usefull or much more than just speculation.

    There's a lot of things that can cause decoherence in a quantum computer.    Gravity is only a weak force, generally considered the weakest of the fundamental forces.   Ignoring interactions with gravitons, a full working theory for which doesn't even exist yet (exactly as you stated),  probably isn't the greatest issue to worry about.   At the time of writing, none of the objects claimed to be working quantum computers  (e.g. The machines built by IBM or available on cloud services from Google) actually work well.    Google's own AI search assistant summarises the situation as follows  (with the input "is there a working quantum computer" as your search term):
   ..... they are not yet practical for general use and are still in their infancy. These existing machines are experimental and require significant advances in error correction and qubit stability to become commercially relevant for complex, real-world problems....  
   - which I am going to paraphrase as  "they don't work".

Best Wishes.

[paul cotter]

"They don't work", that's the sort of waffle-free incisive answer I like. Too many inherent instabilities to be of practical value.

[Halc]

- Gravity interacts with all other particles, so some future quantum theory incorporating gravity would include interactions between the hypothetical gravitons and other quantum states

Careful since gravitons carry gravitational waves, but do not carry gravity, which isn't anything that 'travels'.

- Does this mean that if we want quantum computers to work reliably, we would need to put them into "free fall", to reduce quantum decoherence due to the hypothetical gravitons?

Putting a computer in freefall just removes an EM force otherwise being applied to the device.  It in no way removes it from whatever gravitational field it is in.  To do the latter, you'd need to find a place with relatively flat spacetime, such as in a hollow shell, which helps flatten local curvature but doesn't completely eliminate it like it does under Newton.

The emission of any other particle results in a change of the emitter, and no other particle (including the photon) transfers negative momentum to the target.

It makes sense for all masses to radiate gravitons but that still doesn't explain why gravity sucks whilst photon (or any other) radiation blows!

Gravity doesn't suck. A particle in freefall traces a straight line through spacetime, just like anything else without a force being applied to it.
All three forces are quite capable of both attraction and repulsion, so I don't agree with 'radiation blows'.

I would have expected gravitons to be exchanged between, for example two masses rather than one-way emission. The case of a single mass in an otherwise empty infinite universe conflicts with this notion.

Gravity is not modeled as a force, so gravitons don't transfer momentum.

Still, your model seems similar to protons in a nucleus furiously exchanging gluons or whatever it is that holds these positively charged things in close proximity despite the EM repulsion.

As you say, we don't like infinities, so the idea that a mass can emit an infinite number of gravitons at a constant rate is, to say the least, intellectually challenging.

A large mass just sitting there doesn't emit gravitons. Earth does (at the rate of about 200 Watts), but only because it is changing position relative to other masses like the sun, and those 200 Watts cost the solar system mass/energy over time.  Absent all other effects, this gravitational wave drain would eventually spin all the planets into the sun.

   - which I am going to paraphrase as  "they don't work".

Excellent summary.  Those optimists among the researchers might add "... yet'.

I think I read somewhere that photosynthesis cannot work without quantum computing. If true, then we have evidence that it can be done, and the plants have already figured it out.

[evan_au]

gravitons carry gravitational waves

In my primitive understanding, Gravitational Waves* would be zillions (technical term) of coherent gravitons traveling "to infinity" at the speed of light
- Generated, for example, by colliding Black Holes or Neutron Stars
- The gravitons we can detect with LIGO have frequencies around 50Hz to 4000Hz
- ie a wavelength of around 750 to 6000km
- These are "real" gravitons, which carry real momentum and energy away from the orbiting masses

However, the gravitational attraction between "stationary" lead spheres as measured by Cavendish would be gravitons which do not propagate "to infinity"
- They would be "virtual" gravitons, as permitted by the Heisenberg Uncertainty Principle. They do not carry away real energy or momentum.

Does this sound like a real distinction?

*We can calculate how many real photons there are in a coherent laser beam, because we know the energy and momentum of an individual photon, given the wavelength of the laser beam.
- However, at present, we don't have a way to measure the energy and momentum of an individual real graviton, except to say that it is very small.

[paul cotter]

Engineering department here, looking for an answer: in my abysmal understanding of particle physics I thought that gravitons were the hypothetical particle assumed to mediate gravitation in the efforts to attach a particle to all phenomena, including chronons for temporal effects?

[Eternal Student]

Hi,

     About the graviton debate in the last few posts:

I'm going to start by sympathising or reminiscing with @Halc.....

I do seem to remember reading something about 5-10 years ago where the term "graviton" was used to describe something more like a pseudo-particle  or   quasi-particle.     

    Examples of other quasi-particles:     Phonons  (a quantum of a sound wave, this is most like that quasi-particle notion of a graviton.   It's something that exists only in a wave but can be assigned properties as if it is a particle);    a "hole"  (the absence of an electron in a state);   an electron quasiparticle  (although electrons are real particles, in a solid they can acquire properties as if they have a different mass, called the effective mass).

Then I'm going to move to supporting @paul cotter 's comments in these modern times

   However, I haven't seen the term "graviton" being used in that way for... maybe 10 years.    The term "graviton" is now widely used to describe the force carrying particle of gravity.    It's a real elementary particle that should exist, although we haven't found much evidence of it in any particle collider.    In a Quantum Field Theory corresponding to the Standard Model of particle physics in which gravity has been sucessfully included, the graviton would be a boson responsible for mediating the force of gravity,  similar to the photon as the boson that mediates the electromagnetic force.   As already mentioned in several earlier posts, we don't have a QFT that includes gravity and works well in every place yet.   So "gravitons" remain essentially hypothetical and it's only that the meaning of the term has become reasonably standardised now.   It's the force carrying particle for gravity.

Best Wishes.

[alancalverd]

All three forces are quite capable of both attraction and repulsion, so I don't agree with 'radiation blows'.

I'm not aware of any instance of bodies moving apart under the influence of their mutual gravitational warping of spacetime. I am aware of radiation pressure tending to move objects apart, but not  of objects being mutually attracted by the exchage of photons.

Or am I being exceptionally dense?

[Eternal Student]

Hi,

...I'm not aware of any instance of bodies moving apart under the influence of their mutual gravitational warping of spacetime....

...yes you are....   
   You've spoken about the Hubble expansion of space in other forum posts.   

Best Wishes.

[evan_au]

I'm not aware of any instance of... objects being mutually attracted by the exchange of photons.

the photon is the boson that mediates the electromagnetic force

Magnetism is one component of electromagnetism, and so is mediated by photons.

I think most people will have experienced the attraction between the North and South pole of a bar magnet
- Or the attraction between a fridge and a fridge magnet
- These are mediated by (virtual?) photons

[alancalverd]

Hi,

...I'm not aware of any instance of bodies moving apart under the influence of their mutual gravitational warping of spacetime....

...yes you are....   
   You've spoken about the Hubble expansion of space in other forum posts.   

Best Wishes.

So why didn't Cavendish or Maskeleyne measure the force between massive objects being repelled by each other?

[alancalverd]

Magnetism is one component of electromagnetism, and so is mediated by photons.

I think most people will have experienced the attraction between the North and South pole of a bar magnet
- Or the attraction between a fridge and a fridge magnet
- These are mediated by (virtual?) photons

The real photons I work with emanate from a source (sun, LED, x-ray tube...) and apply a force (radiation pressure) to objects that absorb or reflect them. The force vector is always directed away from the source.

The problem with a particle model of gravitation is that, in order to explain the everyday observation, the target has to be able to work out where the source of the particle was when the particle was emitted. This requires a particle that carries a "birth certificate" quantum with absolute spatial coordinates - not something we have observed or would care to think about too deeply.

[paul cotter]

Surely the expansion is mediated by dark energy and not gravity? That is, if one believes in dark energy- I am not a fan of dark "anything" but as I don't have a better explanation I have to reluctantly follow this path.