# Gravitational attraction between two atoms

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1. Jun 13, 2015

### Dmitry Tyurev

The question: Is there a gravitational attraction between two atoms if they are located at a distance of several light years of each other? Or physics does not have the answer to this question yet? )
(Sorry if this question has already been discussed on the forum. Please send a link to the topic then.)

2. Jun 13, 2015

Staff Emeritus
Sure. But it's too small to measure with just two atoms.

3. Jun 13, 2015

### Staff: Mentor

It would be a good exercise to calculate the predicted strength of the gravitational force between two atoms several light years apart. The classical Newtonian formula $F_g=Gm_1m_2/r^2$ will work just fine; google will give you $G$ and good ballpark numbers for the masses.

4. Jun 13, 2015

### Dmitry Tyurev

Correct me if I'm wrong, but this formula can be used only if gravity is not quantized (so the gravitational force can be infinitely small). But if gravity is quantized, then this formula cannot be used. In this case the gravitational attraction of the first atom will act on the second atom only if it will absorb a graviton emitted by first atom. But at such a great distance, none of gravitons emitted by the first atom is unlikely to be absorbed by the second atom. So will not be gravitational interaction between them at all.
Is this right or not?

5. Jun 13, 2015

Staff Emeritus
1. No, it's not right.
2. Classical gravitation is adequate to describe the situation.

6. Jun 13, 2015

### Dmitry Tyurev

Question A: Whether gravity is quantized or not - it's not known yet. Right?
Question B: If gravity is quantized then my previous post is correct. Right?

7. Jun 13, 2015

### Topolfractal

Picture this gravitons are waves of space time curvature. Propagation of the waves through objects in between the atoms doesn't deter them, it just bends the objects. The graviton waves will reach the other atom. For an example picture wave particle duality as in the earlier days of quantum mechanics.

8. Jun 13, 2015

### Topolfractal

And quantum gravity as it is currently formulated requires the existence of a graviton and therefore a quantization of gravity.

9. Jun 13, 2015

### Topolfractal

And for question B no quantization has nothing to do with the calculation due to the g-force between the atoms being more than that of the force of one graviton.

10. Jun 13, 2015

Staff Emeritus
"Gravity is quantized" is a string of scientific-sounding words strung together. It is meaningless - it has so many possible interpretations that there is no way to pick one. If you mean "is there a complete quantum theory of gravity", the answer is that is that there is not, but one does not need it to answer this question. And no, your description of quantum gravity is not correct, and I can say that without reservation. Quantum gravity is a quantum field theory, and what you wrote is incorrect for any quantum field theory.

It sounds a lot like you are promoting your own theory here - I would recommend you look at the PF Rules before going any farther in this direction.

11. Jun 13, 2015

### rumborak

I don't understand why you are coming down so hard on the OP just now. While he may not have chosen the most precise terminology, it's rather clear what was meant: He assumes, and not without reason, that just like light, gravity is mediated with quantized particles. So, he makes the reasonable conjecture that unless an object is interacting with a graviton, it will not experience a gravitational pull.
The final *conclusion* is sketchy, but his question is perfectly fine.

His other assertion, that we simply don't know, is not too far off either. I remember reading that at least up until a few years ago, the validity of the regular gravitational law had not been experimentally verified for distances less than a few centimeters.

Last edited: Jun 13, 2015
12. Jun 13, 2015

### Topolfractal

But the law is being applied across light years.

13. Jun 13, 2015

### Mark Sloan

Like Dmitry, I am interested in the answer to his question.

I might expand on what I understand to be the intent of his question by adding:

“By general relativity, the mass of the first atom will distort space-time so there will be an apparent super tiny force acting on the second atom after that distortion has time (several years) to propagate to the second atom – so there is no limit on how small a gravitational force can be. But if graviton particle exchange (a quantum mechanical idea) is the mechanism that produces gravity, then could the momentum of a single graviton be so large (or so unlikely to be in the area) that no gravitational force at all is felt?”

Dmitry, correct me if I have misunderstood your question.

I also wonder about another possibility, if the gravitational force from gravitons from the other atom is felt only sporadically and only averages over time to the same as the general relativity value – so there is no “on average” limit on how small a gravitational force can be even if gravitons are real and have some quantized minimum momentum.

My question doubtless reveals my ignorance on the subject, but that is why I am asking.

14. Jun 13, 2015

### Staff: Mentor

The proper analogy to make with the gravitational force between two objects, is the electrostatic force between two charges, not the electromagnetic radiation produced by oscillating charges.

People often describe the electrostatic force as being mediated by virtual photons, especially in the popular literature and maybe low-level introductory textbooks. However, virtual photons are not in fact necessary for this. One can analyze electrostatic interactions in QED without using virtual photons at all. A former poster here named Tom Stoer, and probably some other people, have discussed this repeatedly on PF, although I can't lay my fingers on a specific post just now.

15. Jun 13, 2015

Staff Emeritus
Yes, and? There is no asterisk after either gravitational or electrostatic force laws saying "except when things are separated by light-years".

16. Jun 13, 2015

### rumborak

I think that exact limit is what he's interested in, and whether it still holds. I mean, duh, of course he can plug just a large distance value into the usual gravity formula and just get a very small value out.
The question is no different than asking "A radio tower is sending a regular radio wave from Earth; at what distance does one need to consider singular photons and whether they hit my receiver on Alpha Centauri?"

17. Jun 13, 2015

### Topolfractal

18. Jun 13, 2015

### Topolfractal

The Rumborack post is what I meant to quote before.

19. Jun 13, 2015