0

I saw the exact same question here but the answer is beyond my limited knowledge. Gravity is not a force as general relativity explained. then why an object always has to move along a geodesic? I mean if were to make an artificial vacuum space little bit higher from the ground top of a pole, can we keep and object stationary inside of that vacuum floating? If cant is it cos its rotating along with the earth? Do geodesics also have some wave like motion?

Qmechanic
  • 201,751
Hermes Trismegistus
  • 67
  • Just because gravity is well described by the geometrical theory of general relativity does not mean that it is not a force. – Koschi Oct 13 '21 at 12:30
  • Because "following a geodesic" is conceptually equivalent to "being somewhere". See my anwer here: https://physics.stackexchange.com/a/456550/109928 – Stéphane Rollandin Oct 13 '21 at 15:07
  • 1
    Does this answer your question? Why do objects "fall" along spacetime geodesic lines? – Stéphane Rollandin Oct 13 '21 at 15:09
  • @Koschi its not a force at all. well described https://youtu.be/XRr1kaXKBsU – Hermes Trismegistus Oct 14 '21 at 05:24
  • 1
    @HermesTrismegistus As much as I like Veritasium, the title and video in general is very misleading. It just confuses people who did not study physics i.e. GR. Of course gravity is still a force. That energy and masses bend space-time and so other objects do not move on straight lines if a mass is present is a description of this force. Search my comment under the veritasium video if you like. By the way: If you look at the mathematical description of the other three forces, the description does not look THAT different. To see that you need to know at least a bit of differential geometry. – Koschi Oct 14 '21 at 07:17
  • @Koschi Thank you for your answer but honestly i m really confused. if gravity is a force think something like a blackhole, all the charged particles attracts towards it must give up their energy. i didnt see there are talking about are Em radiation detected from the objects attract towards blockholes. and the acceleration is also constant .not a=f/m . – Hermes Trismegistus Oct 17 '21 at 17:43
  • 1
    @HermesTrismegistus To be honest, I do not understand what you are asking about black holes and charged particles. – Koschi Oct 17 '21 at 18:58
  • i meant if gravity is a force ,if a charged particle is falling towards a black hole should emit photons. so vibration has noting to do with acceleration and do not make charged particles emit energy or gravity is not a force at all as veritasium explained . assuming the first is true i actually thought when objects attract, there has to be some energy loss as i showed above else acceleration has nothing to do with making EM waves. so simpley if you claim gravity is still a force do charged particle has to give up enerry ? – Hermes Trismegistus Oct 18 '21 at 18:29

2 Answers2

1

Gravity is not a force as general relativity explained.

This is true for large masses and energies, not for your example.

For large masses and energies a new frame is needed, General Relativity, but this does not destroy the low mass and energy theory of Newtonian Mechanics. At the region of low mass and energy GR converges mathematically to Newtonian mechanics , so the modeling of gravity as a force still holds for your example. (this is what the mathematics of the answer you link to is saying).

For GR to be needed one has to go to planetary dimensions, as with the GPS numbers GR corrections are needed.

anna v
  • 233,453
  • 4
    But GR is still applicable for weak fields as well, it's just unnecessarily complex. So, I think, OP asks a valid question. – Photon Oct 13 '21 at 09:50
  • @Photon the examples the OP is asking at the end of the question are meaningless if one knows that Newtonian physics has to hold, i.e. force of gravity , because of the consistency of the mathematics. – anna v Oct 13 '21 at 10:36
1

A geodesic in this context is a path in spacetime, that is, essentially, a location changing over time. In effect, saying this means that objects in a particular place will accelerate in a particular way. Which is not that much different in terms of what you see than the idea that gravity is a force. Making a vacuum chamber does not remove the curvature of spacetime. In effect - you cannot shield gravity. The curvature is still pretty close to that associated with the Earth. The idea that gravity is not a force is more related to 1) that every object accelerates the same way, so it is not a material property (unlike electric forces) and 2) that you can ascribe this behaviour to the shape of spacetime - the geometrization of mechanics. In many ways, it is a mathematical trick. (I say that with not intention of deriding the idea, it is a good trick that has many advantages).

Bruce
  • 164
  • Thank for your answer . vacuum space is not tot prevent the effects of gravity but to eliminate any environmental effects such as wind . so is it fairer to say we have to assume that all the objects in the universe are constantly moving, in order to make sense that objects must move along geocide's ? like a postulate ? – Hermes Trismegistus Oct 17 '21 at 17:49
  • 1
    @HermesTrismegistus I think that is not such a bad way of putting it. I can think of some quibbles that others might state. But, I think in the sense you mean it it is a reasonable phrasing. – Bruce Oct 18 '21 at 00:58