311–320 of 451 posts
I want left to be 0 frame acceleration and left to be backwards in time. I kind of also want the graphs to be reverse order too, but I get why it's all presented this way. :)
Here's another 'though experiment' I like which some people disagree with, by not understand reference frames: Light always travels in straight lines. Even when light is experiencing a gravitational lensing and looks to us from earth that it's bending around a star or whatever, from the perspective of the light beam itself, it's moving in a straight line. It's entire reference frame is bent compared to ours (relativi…
> Also if the light wasn't moving straight that would mean it's changing direction, which is the same as an acceleration, and a beam of light traveling thru a gravitational field feels no acceleration, because it's not accelerating. You could have both a deviation (i.e tangential acceleration) and a constant speed.
I heard an interesting question at one point: "how come, when you throw a ball up on Earth, the parabola is so strongly curved? Spacetime is nearly flat, so how can a straight line become such a steep parabola?" I'll answer this question as I understand it, but I only took four lectures of General Relativity before I gave it up in favour of computability and logic, so if there is a more intuitive and/or less wrong an…
The important thing is that gravitation is a distortion in space-_time_, which is way trickier to model as a rubber sheet because you end up with one dimension of space and one of time. If you distort _those_ (also, they don't distort quite like a ball-in-a-rubber-sheet), you can get the results of a ball being thrown up. It's also possible to visualize this for 2 spatial dimensions with a distorted 3d space, but tricky.
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 Earth (weak gravitational field) is mostly curvature of time, namely the spacelike curvature can be ignored, and the g_{00} component of the metric can be seen a a gravitational potential. see p 80 of this:
Earlier quoted context omitted.
> is there any physical way to distinguish these fundamentally different situations? Are you asking if there is a way to distinguish a timelike object from a lightlike object? Of course there is. The fact that, for something that has a very, very small invariant mass, it might be practically difficult does not change the fundamental principle. Also note that the reason it was difficult, for example, to tell whether n…
It still doesn't sound physically distinct any more than distinguishing any continuous quantity as being zero or nonzero. If we measure something that looks like 0, we can't be sure if it's just below the sensitivity of our instruments. For neutrinos, even if we accelerated an rocket and somehow checked if a neutrino was at rest relative to it, we might find that it's not. That means we won't know if we need more spe…
> I'd like to add that even photons have a nonzero upper bound to their possible rest mass. At least they used to. Not sure what you're talking about, their momentum? No object with mass can reach the speed of light and we know they're travelling at that exact speed.
Earlier quoted context omitted.
This is in tune with what happens when you change the timescale in a game engine with proper physics.
I'm not aware of any game engines that simulate general relativity / 4D spacetime. I think what you may be noticing is that, as you reduce the tick frequency of a newtonian physics simulation, parabolas become less accurate as integration error accumulates.
Don't we always consider gravity as "acceleration"? In f = m * a, for objects falling from sky, it will be f = m * g where g is gravity, m is mass & f is force Sorry, I think I am missing the point of the article.
g is not gravity, it’s a constant that quantifies the force that gravity impresses on a mass in Newtonian mechanics
Don't we always consider gravity as "acceleration"? In f = m * a, for objects falling from sky, it will be f = m * g where g is gravity, m is mass & f is force Sorry, I think I am missing the point of the article.
in Newtonian physics only a force can cause acceleration
Earlier quoted context omitted.
> I think the shell theorem says that the gravitational forces cancel if you are on the inside of a hollow sphere and all mass is on the surface. No, it's stronger than that. It says that any spherically symmetric distribution of matter outside a certain radius exerts no "gravitational force" on anything inside that radius, whether there is matter inside that radius or not.
What if my distance to one point on the shell was zero? Would I not feel infinite acceleration towards the massive point on the shell that I was infinitely close to?
But as you get close to stuff, say two atoms, the electrostatic and other forces are far greater than the gravitational ones.