3 ms·

I really don't understand the question "is gravity a force?" It reminds me of the Feynman story: > "Say, Pop, I noticed something. When I pull the wagon,

I really don't understand the question "is gravity a force?" It reminds me of the Feynman story: > "Say, Pop, I noticed something. When I pull the wagon, > the ball rolls to the back of the wagon. And when I'm > pulling it along and I suddenly stop, the ball rolls > to the front of the wagon. Why is that? > > "That, nobody knows," he said. "The general principle is > that things which are moving tend to keep on moving, and > things which are standing still tend to stand still, unless > you push them hard. This tendency is called 'inertia,' but > nobody knows why it's true." Now, that's a deep > understanding. He didn't just give me the name. I don't get why this is "a deep understanding." Why should someone have to have some deep reason for believing that something at rest won't spontaneously begin moving? Should you also need a profound explanation for why things don't spontaneously change their mass or sprout kangaroo legs and start singing "La Bamba?" And what does it mean to ask if "gravity" is not actually a "force?" The idea of a "force" is just an abstraction we use to describe our observations. How is it a deep question to ask whether we give gravity the label of "force" or not?

retube 16y ago
Inertia is a very, very subtle concept. Few people (incl myself) really have a good understanding of what is meant by inertia and why it is so perplexing. There are some great examples I've come across in the past that demonstrate, but I'll have to look them up. In the mean time take a look at the "Inertial frames" section on the wikipedia page: http://en.wikipedia.org/wiki/Inertia http://en.wikipedia.org/wiki/Inertia
ars 16y ago
The deep understanding part is that he explained what is going on, rather than, as many science books and science teachers do, just naming it. Knowing the name of something does not help you understand it. The idea of a force is that it's a prime mover - it's not caused by something else. The practical implications in the case of gravity are zero, but maybe we'll figure out something cool due to the implications someday.
maggit 16y ago
> 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?
- mryall 16y ago
  > 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.
  - maggit 16y ago
    I humbly bow to anybody's superior knowledge of physics. To clarify, I wasn't actually trying to say anything about physics, just about science. So it would have been better phrased as a hypothetical; If we assume that gravity is currently best modeled as a force, why is it valuable (nay, essential) to question whether or not it actually is a force? I hope I answered that hypothetical properly :)
  - ScottBurson 16y ago
    > Under general relativity, gravity isn't seen as a force but a consequence of curved space-time. But why does the presence of matter cause spacetime to curve? If you want to argue that gravity isn't a force, I don't think it's enough to say "it's a consequence of curved spacetime". I think you have to answer the question of why it curves. I'm not aware of an answer to that question. If there isn't one, as I suspect, then I would say, there's your force of gravity: you're just describing it as acting on spacetime itself rather than on the objects in that spacetime.

retube 16y ago

Inertia is a very, very subtle concept. Few people (incl myself) really have a good understanding of what is meant by inertia and why it is so perplexing. There are some great examples I've come across in the past that demonstrate, but I'll have to look them up. In the mean time take a look at the "Inertial frames" section on the wikipedia page: http://en.wikipedia.org/wiki/Inertia http://en.wikipedia.org/wiki/Inertia

ars 16y ago

The deep understanding part is that he explained what is going on, rather than, as many science books and science teachers do, just naming it. Knowing the name of something does not help you understand it. The idea of a force is that it's a prime mover - it's not caused by something else. The practical implications in the case of gravity are zero, but maybe we'll figure out something cool due to the implications someday.

maggit 16y ago

> 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?

mryall 16y ago
> 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.
- maggit 16y ago
  I humbly bow to anybody's superior knowledge of physics. To clarify, I wasn't actually trying to say anything about physics, just about science. So it would have been better phrased as a hypothetical; If we assume that gravity is currently best modeled as a force, why is it valuable (nay, essential) to question whether or not it actually is a force? I hope I answered that hypothetical properly :)
- ScottBurson 16y ago
  > Under general relativity, gravity isn't seen as a force but a consequence of curved space-time. But why does the presence of matter cause spacetime to curve? If you want to argue that gravity isn't a force, I don't think it's enough to say "it's a consequence of curved spacetime". I think you have to answer the question of why it curves. I'm not aware of an answer to that question. If there isn't one, as I suspect, then I would say, there's your force of gravity: you're just describing it as acting on spacetime itself rather than on the objects in that spacetime.