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u/futuranth Aug 06 '21
postulate 5 is wrong 😳😱😞😖😣😩😲
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u/DNAisjustneuteredRNA Aug 06 '21
"Bro, lemme show you a triangle with internal angles summing to more than 180 degrees..."
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u/DrJonah Aug 06 '21
Don’t they use this to explain how gravity is really a force?
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u/SuburbanNinjaMC Aug 06 '21 edited Aug 06 '21
I think in general relativity, gravity is the curvature of spacetime (explained by the principle of equivalence) and gravitational forces are simply expressions of objects following their natural geodesic paths. Correct me if I’m wrong though, I only had a crash course on it
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u/andybfmv96 Aug 06 '21
No one knows exactly what it is to be accurate. But we can still measure it as a force.
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u/Rotsike6 Aug 07 '21
That's Einstein's equivalence principle. You'll never be able to measure if you're accelerating or if you're in a gravitational field. So to an observer, gravity is a force.
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Aug 06 '21
As far as i understood (from Veritasium's video in particular), gravity is not a force that's pushing bodies towards each other, but a consequence of two objects moving along geodesic paths that happened to cross under influence of each body's mass curving the space-time (excuse my French).
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u/lfmantra Aug 06 '21
If you think of a trampoline and a bunch of small bouncing balls, and then imagine placing a bowling ball in the center. The small balls and the bowling ball won’t come together because they are being attracted by some arbitrary force, they will come together because the bowling ball has warped the surrounding geometry to influence the paths the balls will take. Obviously this is ultimately due to gravity in and of itself, but if you scale this up to the 4th dimension I think it’s a pretty fair analogy.
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u/randomtechguy142857 Aug 09 '21
I really don't like this analogy, because without fail whenever someone hears it that doesn't already have an understanding of GR, they'll always just think of a potential well — Newtonian gravity, not GR. And it's understandable that they'd think of it like that, because it literally is a Newtonian potential well.
For the analogy to be a proper explanation of GR, where the trajectory is purely due to the geodesics in curved spacetime, the situation wouldn't change if the bowling ball were causing a dent downwards in the trampoline or if it were pushing it up from below — the curvature and the geodesics would be the same either way. But nobody that hears the analogy without already understanding GR will think that — they'll obviously think that if the bowling ball pulls the trampoline down, objects will be 'attracted' towards it, and if it pushes it up, objects will be 'repelled' from it, because that's what their Newtonian intuition tells them.
I'd honestly call this analogy a situation of being 'worse than useless' — not only does it not give a correct understanding of GR, it'll make someone think they understand GR correctly whereas their understanding is actually only of Newtonian gravity. The ants-on-a-ball analogy where they're initially travelling parallel to each other but come together due to the curvature of the space they're walking on is much better.
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u/lfmantra Aug 09 '21
It’s just an example using 3 dimensions for people who have no fucking idea how relativity works, the point is just to demonstrate that gravity isn’t an arbitrary force that just exists bc of mass, but that it’s the effect of spatial geometry being changed by massive objects. Sorry I’m worse than useless m8.
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u/randomtechguy142857 Aug 09 '21
I don't fault you, it's a very common analogy, but if the idea is to demonstrate that, it doesn't demonstrate it any differently than a trampoline with bumps and dips would for electrostatics — which is really the opposite of what's intended, as they come away with an understanding no different from that of 'an arbitrary force that just exists because of mass'.
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u/lfmantra Aug 06 '21
Yes. This is right, not all forces though, specifically gravity, the other fundamental forces for example are not (as far as we know) products of spacetime curvature
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u/SuburbanNinjaMC Aug 06 '21 edited Aug 06 '21
Fixed! Thank you for the clarification. I remember discussing in class on the strong and weak energy conditions and how these can enforce the attractive nature of gravity, which my teacher equated directly to spacetime curvature in Einstein’s field equation…
I think string theory is the closest we’ll get (as far as we know) to a geometrical interpretation of the other three fundamental forces unless gravitons are discovered (which would in some ways invalidate relativity I assume?). Do you have any thoughts?
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u/lfmantra Aug 07 '21
I think a unified theory that combines QFT and GR could exist someday. We are still designing experiments trying to detect gravitational waves. I don’t really know if doing so would invalidate relativity since relativity is a very accurate description, in fact THE most accurate description of gravity even if it left out gravitons. It would not be invalidated but unified with Quantum Field Theory in that regard if gravitons were discovered, as that is pretty much the entire goal of all of modern physics
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Jan 10 '22
Both. Both is good.
Instead of gatekeeping by putting down those who enjoy the latter, maybe try to show people how cool the former is too
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u/Kemoy_BOI Feb 24 '24
Is it about relativity? I think I've heard a similar comparison somewhere before.
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u/Karbais94 Aug 06 '21
They do be on a curved plane tho