Gravity is most accurately described by the general theory of relativity... which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.[1]
> Considering any disagreement with the idea that gravity is a force would end your academic career
Gravity is not a force. So thinking it is a force would end one’s academic career. See, even scientists can live in their own “bubbles” once things get complicated enough :-)
Gravity is the effect of masses moving along geodesic lines in curved space-time.
> I really don't understand the question "is gravity a force?"
The question is not "is gravity a force?". The question is "What is gravity?"
An accepted answer, what I was taught in school, and what I would answer if you asked me is "it's a force". And labeling something as "a force" has huge implications for people that know what that label means.
If a four year old asked me "What is gravity?", I'd know better than to give him a label, because he wouldn't know what a force is from school. Obviously.
If it turns out gravity is not really a force -- if modelling gravity as a force breaks down at some point -- that's a good fundamental result. It is a falsification of the theory "Gravity is a force". (So the line of thought goes: "What is gravity?", "It is a force!", "Is gravity really a force?", "Perhaps not!")
How is this not helping us attaining a deep understanding? How is this not advancing science? How is this any different from any science?
> The gravitational force doesn’t change based on your kinetic energy.
This is not correct.
Gravity is a function of everything that has energy (see the stress energy tensor). It’s a very unusual “force” in this regard. This leads to interesting situations like a perfectly mirrored box of light (if you could have such a thing) weighing more on a scale than a box without light in it. Also, a spinning cue ball or a compressed spring weigh more than a stationary ball and an uncompressed spring. Granted, these differences are so small as to be undetectable by any equipment we could ever build, but it’s still interesting to think about.
> If it turns out gravity is not really a force ...
It definitely isn't. The word "force" implies a Newtonian model for physics. Under general relativity, gravity isn't seen as a force but a consequence of curved space-time.
The title was probably chosen by an editor without deep knowledge of the subject. It doesn't accurately describe the article's content either.
> I think this article is missing the biggest thing that makes gravity different: it arises from and affects everything with energy, which includes everything that we know exists.
To be fair, one of the physicists (Daniel Harlow) does say
> gravity is the only force that is felt by all kinds of matter
> The explanation of why gravity is only positive is a complex one, but physicists believe it's almost axiomatic
Isn't that a consequence of relativity, the speed of light being constant by definition? I'm not sure that makes any sense, though, what is the physical unit of gravity N? Jf you mean classical mechanics, than there has to be a negative counterforce by the three axioms of newtonian mechanics and it's just a question of frame of reference.
Yeah, this fact made me cringe at explanation #2: the guy keeps saying things like "gravity is the only force that is felt by all kinds of matter". Stuff doesn't "feel" gravity; it's not a force, it's just the shape of spacetime. Also, gravity doesn't just affect matter; photons are not matter, but are affected by the curvature of spacetime (i.e. gravity).
/me not a physicist; but I'm not a baby either, and I don't appreciate babytalk oversimplifications.
> GR says that the Newtonian law of gravity is an approximation that makes reasonably accurate predictions when the spacetime curvature is small and all relative motions are slow compared to the speed of light.
These are merely the aforementioned caveats.
> It does not say that the Newtonian interpretation of that equation is correct.
If the interpretation were wrong, and that's not a force, then the amount of force in that equation would be 0.
With no force, Gm1m2/r² = 0.
However, that's not the modification that GR applies to this, though.
> Beauty represents the forces of nature: electromagnetism, the weak force, the strong force and gravity.
The point of General Relativity is that gravity is not a force. I'm familar with standard argument that Newtonian force is a suitable approximation. But he is being careless with a very fundamental concept. Gravity is either a force or it is not. In physics it looks like it is a force when the author needs a force and it is not a force when the author feels like it.
Not a physicist so excuse the ignorance, but do we understand gravity at all?
I mean afaik we can observe and predict it's behaviors but do we understand what underlying force causes it, and if it potentially has a counter force.
Not really.
[1] https://en.wikipedia.org/wiki/Gravityreply