# Is gravity really weak?

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1. Apr 18, 2015

### Saffat Rafsan

Today i was just randomly browsing the internet. Then i found this image:

What if gravity is not the weakest fundamental force at all? Like the picture above if we remove the space among all atoms the earth will occupy relatively a very small space. Then we know the gravity of that tiny earth will be much greater. What if the space among atoms is causing gravity seem weaker?
Make me clear if i'm getting it wrong.

2. Apr 18, 2015

### Orodruin

Staff Emeritus
The reason you cannot remove all the empty space between atoms is that gravity is weaker than the electromagnetic forces between atoms ...

3. Apr 18, 2015

### rootone

If it actually were possible to compress the Earth, it's gravity would NOT change.
It would be very much denser, but would still have the same mass.

4. Apr 18, 2015

### SteamKing

Staff Emeritus
The first page of the article linked below gives the relative strengths of the fundamental forces of nature, taking the force of gravity = 1:

http://web.mit.edu/sahughes/www/8.022/lec01.pdf

As you can see, gravity is not just a little weaker than the other forces, but is weaker by many orders of magnitude.

5. Apr 18, 2015

### Saffat Rafsan

i get it. thanks

Last edited: Apr 18, 2015
6. Apr 20, 2015

### SpiderET

It depends what gravity do you mean. If we are talking surface gravity, then it would change. Compressed Earth would have significantly higher surface gravity. And when we would compress Earth to 8,7 mm, then it would turn into black hole.

7. Apr 20, 2015

### rootone

Yeah I was thinking of it's total gravity - the moon would remain in orbit as if nothing had changed.

8. Apr 21, 2015

### ChrisVer

weak at what energy scales ?

9. Apr 21, 2015

### nitsuj

any, it's comparatively weak...always

10. Apr 21, 2015

### ChrisVer

It's supposed to overpower the rest forces at some point.

11. Apr 21, 2015

### nitsuj

Nah it doesn't.

12. Apr 21, 2015

### Staff: Mentor

It's not really that simple. We don't have a quantum theory of gravity, so we don't actually know how its strength, considered as an interaction, will scale at increasing particle energies. The obvious expectation is that its strength will increase, since energy is a source of gravity; but we don't know for sure that that's the case, and we certainly don't have any theory that tells us how its strength changes, quantitatively, with increasing particle energies, the way we do for the other forces. There are speculations that all the forces should have the same strength at around the Planck energy, but there is no real theory backing that up, it's just a heuristic guess.

13. Apr 21, 2015

### nitsuj

Ah so the retort
weak at what energy scales ? by Chrisver went over my head....but suppose it goes beyond our model (lets find the mediating partical 1st)

That said I thought Chrisver's example was a blackhole "over powering" the other 3 fundamental forces.

14. Apr 21, 2015

### ChrisVer

What do you mean by heuristic guess?
The need to make gravity equal to the rest forces is essential if you wish to apply quantum gravity, otherwise I don't find any reason to try and look for quantum gravity if it's always negligible compared to other interactions.
The dimensionfull coupling can mean a lot in a theory, in this case obviously: that gravity as we know it is not applicable in the whole energy spectrum without problems ....

15. Apr 21, 2015

### Staff: Mentor

It's a guess because, as I said, there is no real theory backing it up. It's heuristic because the Planck scale quantities are the ones that pop out if you combine Planck's constant with other fundamental constants to get quantities with the appropriate units (mass, distance, time, etc.); in other words, it seems plausible given what we currently know.

Why so?

16. Apr 21, 2015

### ChrisVer

I gave some point for that in the same sentence. For example we are able to apply the Standard Model in such great extends because gravity is negligible. If it was not the SM would not be so predictive but you'd need a quantum theory of gravity.
Maybe not equal but at least you need to have gravity comparable to the rest forces.

17. Apr 21, 2015

### Staff: Mentor

Ah, I see. I'm still not sure I agree. There are two reasons I can see why we need a theory of quantum gravity:

(1) Our current classical theory of gravity, GR, predicts spacetime singularities: points at which spacetime curvature becomes infinite. But all that really means is that GR breaks down at those points. In other words, when spacetime curvature gets strong enough, GR is no longer the correct theory of gravity, so we need one that covers that regime. The only real candidate we have for that is a quantum theory of gravity.

(2) Our current quantum theory of everything else but gravity allows superpositions of states. But that includes superpositions of states that have different properties when considered as sources of gravity. If gravity is not quantized, it is hard to see how to handle such superpositions.

Neither of these reasons requires gravity to be as strong as the other forces, as far as I can see.

18. Apr 21, 2015

### phinds

Try jumping 20 feet in the air and see what you think.

19. Apr 21, 2015

### ChrisVer

Well, and try jumping from 20 feet height to see whether EM force is able to crush your legs or Gravity will take you to the center of the earth... hahahaha

20. Feb 26, 2016

### HammerQ

I had this idea just now and I did some calculations according to this theory but still it was not strong enough. but maybe my collocations are wrong.(I am no physicist)
• The Earth radius after taking out all empty space = 184 m Source
• Earth surface gravity after scaling it up will = 36001 m/s^2 which = 34673 G
• compared to other forces it's still weak. Weak force = 10^25 G

Last edited: Feb 26, 2016
21. Feb 26, 2016

### Staff: Mentor

That's not what this actually means. It means the radius of a neutron star with the mass of the Earth and a density which is within the range of assumed "reasonable" densities for neutron stars. We don't actually understand the equation of state of neutron stars very well, so there is considerable uncertainty about calculations like this. But in any case, to say that the neutron star is what you would get if you took out all the "empty space" is not correct; it is indeed meaningless, since there is no rigorous definition of what counts as "empty space" anyway. You could just as well argue that an atomic nucleus or a neutron star is mostly "empty space" because we can shoot high energy electrons into it and see them bouncing off quarks as though the quarks were free particles in a mostly empty space (look up "deep inelastic scattering"). Either way the argument is based on a misunderstanding of the actual quantum mechanics involved.

Your conclusion that gravity is really weak compared to other forces is valid, though.

22. Feb 28, 2016

### haushofer

23. Feb 28, 2016

### stevendaryl

Staff Emeritus
This might be too obvious to mention, but I will go ahead and do it: To talk about the relative strength of gravity or electromagnetism, you really have to consider typical masses and charges. In particle physics, typical masses might range from that of the electron, to that of the top quark, which is around 300,000 times as massive. Typical charges are around the charge of an electron. So a measure of the relative strength of gravity versus electromagnetism might be:

Force of gravity between two objects of typical mass divided by Force of electrostatic repulsion of two objects of typical charge

$\dfrac{G M^2}{k Q^2}$

where $G$ is Newton's gravitational constant (defined through Newton's gravitational law: $F = \dfrac{G M_1 M_2}{r^2}$), $M$ is a typical mass, $Q$ is a typical charge, and $k$ is Coulomb's constant (defined via Coulomb's law: $F = \dfrac{k Q_1 Q_1}{r^2}$). So the claim that "gravity is very weak compared with electromagnetism" is equivalent to the claim:

Typical masses in particle physics are very small, compared to typical charges.