Timeline for Why is gravity only an attractive force?
Current License: CC BY-SA 3.0
19 events
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| Jan 27, 2018 at 0:07 | history | edited | zephyr | CC BY-SA 3.0 |
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| Aug 8, 2017 at 13:57 | comment | added | Mea Culpa Nay | @zephyr, well, thanks. I am sorry, I could not express myself in a better manner... what I meant in my earlier posts are: +ve = positive, -ve = negative. | |
| Aug 8, 2017 at 13:53 | comment | added | zephyr | @MeaCulpaNay I still don't understand what '-ve' is supposed to be. What is 'v' and what is 'e'? Is this an equation? Is it a suffix? And I still don't understand your doubt. I basically showed F depends on three terms, all of which must be positive, implying F necessarily must be positive as well. | |
| Aug 8, 2017 at 13:47 | comment | added | Mea Culpa Nay | @zephyr, well, my earlier comments were based on entirely a single-sided argument which considers what are all possible quantities can go '-ve' in the illustrated equation...and there by counter-arguing ...that neither mass/nor distance/nor 'G' can become negative ...and there by proving that 'F' must be a positive quantity. Till this it is fine. But then comes the crux - how nature of F is linked with the force being attractive or something else ? That is my basic doubt. Hope this clarifies. | |
| Aug 8, 2017 at 13:39 | comment | added | zephyr | @MeaCulpaNay What is '+ve' supposed to be? I don't know what you mean by a force of that type. I make no assumptions about anything, I just take the normal laws of physics and say what happens if mass can be negative. | |
| Aug 8, 2017 at 13:38 | comment | added | zephyr | @barrycarter Yes, you're correct that the functional forms of graivty and electromagnetism diverge when $r\rightarrow 0$, however that's not possible in reality. Not to mention that has nothing to do with my answer anyway, so I don't see why you mentioned it. | |
| Aug 3, 2017 at 1:48 | comment | added | Jack R. Woods | I don't see a whole lot of talk about General Relativity here. If gravity is just thought of as mass causing 3d "divots" in space time, I don't see why some unknown entity wouldn't cause "bumps", other than the fact that we don't observe them. Or, maybe all of space-time in our universe is one big "bump" that we call "dark energy". | |
| Aug 1, 2017 at 7:14 | comment | added | Mea Culpa Nay | @zephyr, well, thanks for providing a sort of explanation ....but it seems there is an assumption that a force being '+ve' would mean attraction...which does not make sense ...as it could not explain why two masses should attract each other ... | |
| Jul 29, 2017 at 5:04 | comment | added | user21 | "suddenly infinite forces are being applied"... OK, but when a proton attracts an electron, doesn't the same thing happen? The two get closer and closer and the attractive force increases to infinity? In reality, at some point, the weak and strong nuclear forces take over. | |
| Jul 28, 2017 at 19:23 | comment | added | zephyr | @Octopus I didn't contradict myself. I said mass can't be negative. But, if it could, I don't see why you believe you couldn't push it with your hand. And what does anti-matter have to do with any of this? That is not pertinent to this discussion. | |
| Jul 28, 2017 at 18:06 | comment | added | Octopus | @zephyr, well you just said "mass can't be negative" and then "I don't see why I wouldnt be able to push it if it were negative." you've contradicted yourself. There are theories on antimatter. You wouldn't push antimatter with matter. | |
| Jul 28, 2017 at 17:46 | comment | added | zephyr | @Octopus There are many reasons why mass can't be negative. The one I outlined is just one reason using one particular case. And I don't see why you wouldn't be able to push the mass with your hand if it were negative (aside from the runaway motion problem I described). | |
| Jul 28, 2017 at 17:19 | comment | added | Octopus | The "why mass can't be negative" doesn't seem very convincing to me. F in your example could just as well be a force caused by gravity and there would be nothing in the way. Presumably, if the mass were negative you wouldn't be able to push it with your hand. | |
| Jul 28, 2017 at 16:55 | comment | added | zephyr | Interestingly, the force of gravity would be negative if the distance was imaginary! So just imagine some mass at a particular distance from you and it will repel you. | |
| Jul 28, 2017 at 16:32 | comment | added | zephyr | @adrianmcmenamin But all evidence suggests the two are equivalent. In fact, their equivalence is a major component of GR and there has been no evidence so far showing this part of GR is wrong. I described the answer for the universe we appear to live in (aside from the potentiality for negative mass). If you want to throw in all sorts of other complications, that's outside the scope of my answer. | |
| Jul 28, 2017 at 16:24 | comment | added | adrianmcmenamin | The second law describes inertial mass, not (necessarily) gravitational mass though. | |
| Jul 28, 2017 at 15:48 | comment | added | zephyr | @barrycarter I thought about addressing this in my answer. I guess I should have. The catch here is that Newton's second law is not $F=ea$, it's $F=ma$. You can't apply the argument above to negative electric charges for that reason. The reason mass can't behave in the same way is for the reason I outlined above. It isn't a mystery. If Newton's second law instead was $F=ea$, then electric charge could not be negative. | |
| Jul 28, 2017 at 15:33 | comment | added | user21 | As convincing as this explanation seems, electrical charge follows the inverse square law and charge can be positive or negative. I see no reason why mass couldn't theoretically behave the same way. I believe it's actually a "fundamental mystery" as to why gravity is the only one of the four known forces that acts only to attract and never to repel. The other 3 fundamental forces can do either. | |
| Jul 28, 2017 at 13:36 | history | answered | zephyr | CC BY-SA 3.0 |