# Comparision of gravity with other forces

1. Dec 4, 2014

### rajeshmarndi

If gravity is not a force, but a result due to curvature of spacetime. Then why does science trying to compare gravity with other forces.

2. Dec 4, 2014

### Staff: Mentor

Can you expand on this question a bit? It's not clear to me exactly what you're asking.

3. Dec 4, 2014

### Staff: Mentor

Do you have a reference to "science trying to compare..."? It's not clear exactly what you're asking, and without that it's hard to sensibly answer your question.

You may want to search this forum for some of the previous discussions on this topic. Also, take a look at the "Similar Threads" section at the bottom of this page.

4. Dec 4, 2014

### stevendaryl

Staff Emeritus
I think that what he means is comparisons of the relative strengths of fundamental forces, such as shown on this page
http://scienceworld.wolfram.com/physics/FundamentalForces.html

5. Dec 4, 2014

### rajeshmarndi

Yes, thats right. Isn't gravity considered ,one of the four fundamental forces.

6. Dec 4, 2014

### harrylin

The opinions may differ about how to call the cause of the unopposed motion of an object in a gravitational field; however, the force that is indirectly shown on a scale is not a matter of opinion, and it closely follows Hooke's law.
As analysed in the "ECI frame", that force is the net reaction force resulting from gravitational acceleration minus centripetal acceleration. Simplifying by measuring the same on the North Pole, it's then quantitatively equal to the force from gravitation.

7. Dec 4, 2014

### nitsuj

That's the thing with terminology. You're right in the curiosity of Gravity being under the umbrella of "Fundamental Forces", note however this grouping may also be called, and from your perspective is more accurate to be refered to as "Fundamental Interactions".

Now the question should still stand, why is gravity descriptively distinct from the other forces...errr...interactions. :)

Last edited: Dec 4, 2014
8. Dec 4, 2014

### pervect

Staff Emeritus
That part is (I think) easy - no other "force" causes time dilation, but gravity does.

9. Dec 4, 2014

### Mentz114

What about free-fall ? No other interaction can produce it.

10. Dec 5, 2014

### harrylin

I don't think so. Instead, I think that energy causes attraction towards energy containing bodies (called gravity) and time dilation. Isn't electric field energy, in addition to a source of electric attraction, also a source of gravity and time dilation?

11. Dec 5, 2014

### nitsuj

Yes, gravity is the very clear example. It's the other three which I think the interaction interpretation is "messed" up with force carrier concept, which is being put to gravity. Gravity; to our point clearly is a manipulation of the geometry, where when in "free fall" we are time dilated. The other three are "nothing" like that from our perspective.

Moreless I am questioning if the virtual particle concept instead describes the passing of manipulated geometry (that's specific to the recipient elementary particle of course) where it is absorbed/received changing it's geometry resulting in motion..in the same sense that gravity does this to everything with it's field (to everything is so key, in that even an accelerometer is reading zero, despite every observer seeing the free fall body accelerating towards the massive body and seeing them time dilated as well)

Gravity is the stripped down interaction, imo from it the other three should be described; not the other way around.

12. Dec 5, 2014

### stevendaryl

Staff Emeritus
Well, Kaluza-Klein theory describes electromagnetism as an effect of gravity. However, that kind of unification goes in the wrong direction, from the point of view of quantum physics, because we have a quantum theory of electromagnetism, but not a quantum theory of gravity.

13. Dec 5, 2014

### nitsuj

The scientific method is very sound. Regardless of the path taken (i.e. interpretation / description / model) the "end of the day" results are the same. In other words the end result isn't dependent on how we decide to go about "modelling" physics, though it may effect the amount of time to build the complete model.

Pesky QM and it's ridicules accuracy and predictability is just so sound, that the entire complication of QM is deemed more fundamental than gravity's transparent effect on geometry (and in turn motion)...weird.

14. Dec 5, 2014

### pervect

Staff Emeritus
Yes, the electric field is also generates gravity. But the electric field doesn't produce time dilation directly, it can cause time dilation only insofar as it produces gravity which then produces the time dilation.

15. Dec 6, 2014

### harrylin

The electric field energy causes gravity. Gravity is experienced by a clock on Earth as a force; how could a force affect the frequency of atomic clocks?
But of course, since energy and gravitation are inseparable, it can probably not be decided by experiments who is right - we enter in the realm of pure philosophy :)

Last edited: Dec 6, 2014
16. Dec 6, 2014

### pervect

Staff Emeritus
It's not quite philosophy. But the answer to this problem might suggest a different answer than the one I originally gave. You can tell an electric field from a "gravity field" because an electric field only affects charged objects.

One of the problems with defining a 'gravity field' is the fact that it affects everything, so you don't have any objects without "gravitational charge". Attempts to define a "gravity field" in the same manner as we do an electric field fail :(.

However, you can use the definition of an electric field as something that accelerates only charged objects to separate out the gravity generated by an electric field from the electric field itself. Using this separation, you can then say that the electric field is just a force, you don't get any time dilation effects no matter how large the electric force is, you can trace all the effects you get from the unavoidable inclusion of generated gravity to the gravity you've generated via the electric field.

17. Dec 6, 2014

### nitsuj

18. Dec 6, 2014

### rajeshmarndi

Just a thought. Isn't gravity too exist between two charged body along with electrical force. And the strong nuclear force is actually the gravity between nucleons. So can gravity become, that much stronger, when distances reduces to the extreme.

19. Dec 6, 2014

### nitsuj

I read more about the standard model & QFT & the development / trend / direction of QM, and wow did I misunderstood gravitons.

I understand them to be a "quanta" of gravity, which I understand to be "distorted" geometry and "induces" motion. My sci-fi imagination wonders if we will someday have such a fundamental way to do "work" that precedes thermodynamics, via gravitons. So I presume the "opposites" are length/time = proton/electron

I wonder what a graviton does to the body that absorbs it?

Anyways here's a Wiki from the Gravitational Waves Entry:
Energy, momentum, and angular momentum carried by gravitational waves
Waves familiar from other areas of physics such as water waves, sound waves, and electromagnetic waves are able to carry energy, momentum, and angular momentum. By carrying these away from a source, waves are able to rob that source of its energy as well as its linear and angular momentum. Gravitational waves perform the same function. Thus, for example, a binary system loses angular momentum as the two orbiting objects spiral towards each other—the angular momentum is radiated away by gravitational waves.

The waves can also carry off linear momentum, a possibility that has some interesting implications for astrophysics.[31] After two supermassive black holes coalesce, emission of linear momentum can produce a "kick" with amplitude as large as 4000 km/s. This is fast enough to eject the coalesced black hole completely from its host galaxy. Even if the kick is too small to eject the black hole completely, it can remove it temporarily from the nucleus of the galaxy, after which it will oscillate about the center, eventually coming to rest.[32] A kicked black hole can also carry a star cluster with it, forming a hyper-compact stellar system.[33] Or it may carry gas, allowing the recoiling black hole to appear temporarily as a "naked quasar". The quasar SDSS J092712.65+294344.0 is believed to contain a recoiling supermassive black hole.[34]

20. Dec 7, 2014

### harrylin

Once more, I did not suggest otherwise; electric field energy is not an electric field! ;)