relevant XKCD
Gravity is not a force – free-fall parabolas are straight lines in spacetime
321–330 of 451 posts
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#322Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
"This ultimately led [Einstein] to the realisation that gravity is best described and understood not as a physical external force like the other forces of nature but rather as a manifestation of the geometry and curvature of space-timeitself." This is in the introduction of the paper you shared...
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#323[deleted]
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#324Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
"This ultimately led [Einstein] to the realisation that gravity is best described and understood not as a physical external force like the other forces of nature but rather as a manifestation of the geometry and curvature of space-timeitself." This is in the introduction of the paper you shared...
Give the commentor some credit -- I doubt they'd link a paper they are not familiar with.
Also in the paper:
"In a certain sense the main effect of curvature (or gravity) is that initially paralleltrajectories of freely falling non-interacting particles(dust, pebbles,. . . ) do not remainparallel, i.e. that gravity is an attractive force that has the tendency to focus matter."
Maybe don't immediately look for ways to discredit someone's contribution to the discourse without examining all of the content they have shared?
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#325my physics teacher would always say that "Gravity is not a force, but we experience a force due to gravity and call it weight"
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#326Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
Isn't gravity somewhat like the forces that one observes in non-inertial frames of reference in classical mechanics?
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#327I've seen this explained elsewhere, and it does look very cool when displayed this way. But perhaps someone with more background can explain to a lay person -- what even is a force? Why does the existence of a transformation that makes movement under a supposed force actually follow a straight line mean it's not really a force? For the other forces (e.g. electromagnetism) can we say that there's _no way_ to exhibit a…
An intuitive way to tell if something is a "real" force (as proposed to a pseudo- or fictitious force) - can someone subject to that force feel it, or equivalently, can an accelerometer measure the acceleration it produces? When you accelerate or decelerate in a vehicle, you feel it. But when you jump out of a plane and accelerate towards the ground, you don't feel a force. Noticing this fact was a big part of what l…
In my high school we were taught the other way - that the gravity on you in free fall is a real force and centrifugal force going round a corner is not.
It depends a bit how you define words like real.
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#328Earlier quoted context omitted.
It's a parabola in a uniform gravity field, an ellipse in a circular gravity field coming from a point mass. So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass. It would be equivalent to a point mass if the Earth were a perfect sphere of uniform density, but it isn't. In reality it's a potato like mass blob that's approximated by what geodesists call the "geoid". So in o…
> So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass. This turns out to be not pedantic but very important if you're guiding an ICBM. And when landing on the Moon, Apollo had to deal with irregularities in the Moon's gravity due to mass concentrations, called mascons. (If you're interested in missile guidance, take a look at the book Inventing Accuracy . Among other thin…
Do we have to use this knowledge to toss nukes at each other? Surely there's something else we could be doing instead?
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#329Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
"This ultimately led [Einstein] to the realisation that gravity is best described and understood not as a physical external force like the other forces of nature but rather as a manifestation of the geometry and curvature of space-timeitself." This is in the introduction of the paper you shared...
> best described and understood not as a physical external force
"best described as not a force" is not the same statement as "is not a force".
In physics and mathematics there can be multiple different, complementarity descriptions of the same thing. In fact, if you can reach the same result by two very different routes that seems to make it more robust.
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#330Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
> Physicist here, gravity is a force, just a different one.
Isn't gravity an acceleration, and weight a force? Otherwise gravity would accelerate heavier objects more slowly.