Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Gravity is not a force – free-fall parabolas are straight lines in spacetime

timhutton.github.io

Post reply on HN
Print view View original

41–50 of 451 posts

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#41

Post by dreamcompiler »

Smaug123 wrote: post #14

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…

I think of it as being because the ball has mass and photons don't. So Newton's Gm1m2/r^2 = 0 for photons, and you have to use Einstein to measure the "force" on a photon.

But because massive objects also cannot move as fast as photons, we're probably both saying the same thing from two different perspectives.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#43

Post by Smaug123 »

sohkamyung wrote: post #36
Smaug123 wrote: post #14

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…

Off topic, but in reality, the trajectory of a ball thrown on Earth is not a parabola, but an ellipse [1]: > under the laws of gravity, a parabola is an impossible shape for an object that's gravitationally bound to the Earth. The math simply doesn't work out. If we could design a precise enough experiment, we'd measure that projectiles on Earth make tiny deviations from the predicted parabolic path we all derived in…

Good point; this becomes more obvious if you imagine throwing the ball up and then immediately collapsing all the mass of the Earth into a single point at the centre. What path does the ball follow now? It's probably following a path we would more usually call an "orbit", and it sure looks a lot like an ellipse. Now just put the mass of the Earth back where it was, and notice that the ball hits the ground before it can trace out very much of its orbit.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#44

Post by nobody9999 »

chrisseaton wrote: post #9
shireboy wrote: post #8

I saw this and the related Veritasium video, but am still scratching my head about something. Does this mean that away from all gravity a body does not experience any time? ie a person on a spacecraft stopped in intergalactic space would not age relative to persons on planets?

How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times?

>How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times?

Yes. But the effect of the distortion of space-time that we call "gravity" is subject to the inverse square law[0].

This means that, as Newton described: "The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance. The force is always attractive and acts along the line joining them"

As such, while objects are affected by the distortion of space-time, the effect is diminished (but not eliminated) by increased distance from the mass that's distorting space-time.

Or at least that's how I understand it.

[0] https://en.wikipedia.org/wiki/Inverse-square_law

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#45

Post by dmurray »

sohkamyung wrote: post #36
Smaug123 wrote: post #14

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…

Off topic, but in reality, the trajectory of a ball thrown on Earth is not a parabola, but an ellipse [1]: > under the laws of gravity, a parabola is an impossible shape for an object that's gravitationally bound to the Earth. The math simply doesn't work out. If we could design a precise enough experiment, we'd measure that projectiles on Earth make tiny deviations from the predicted parabolic path we all derived in…

A good point to make, but if one is going to be that pedantic one shouldn't call it "impossible". A parabola is possible with the right wind, air resistance, gravity from nearby mountains, etc, and in practice those have a larger impact on almost any suborbital projectile than the difference that turns the trajectory from a parabola to an elliptical arc.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#46

Post by aspenmayer »

Retric wrote: post #37
mrfusion wrote: post #20

Earlier quoted context omitted.

Are we flat in the time dimension? Or what is our time size?

It’s something of a philosophical question. We tend to think of distant galaxy’s as if we are viewing them via some FTL means with a single consistent now. NASA for example tracks distant Mars probes like that rather than marking timing based on when the signal was received. Alternatively, you can think of everything that could impact you as something of a now light cone. The second view has the universe existing as…

I’m reminded of the film Donnie Darko, but I won’t say more as to not spoil anything for anyone.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#47

Post by chrisseaton »

nobody9999 wrote: post #44
chrisseaton wrote: post #9

Earlier quoted context omitted.

How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times?

>How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times? Yes. But the effect of the distortion of space-time that we call "gravity" is subject to the inverse square law[0]. This means that, as Newton described: "The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional t…

> but not eliminated

So... you can't get away from it. That's what I said isn't it?

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#48

Post by ithkuil »

Smaug123 wrote: post #30
vbezhenar wrote: post #24

Earlier quoted context omitted.

Are we moving through time with constant speed? Or we're constantly accelerating through time?

Under special relativity, everyone and everything moves at a constant speed `c` through spacetime. If you feel like you're not moving, it's because all your speed is being put towards travelling faster through time. Conversely, if you manage to move very fast through space, the world around you will appear to speed up, because you've had to trade off some of your forward travel through time so as to travel in space;…

And at the same time, in your new frame of reference, you're still moving at exactly c through the time and 0 through space. But your time axis is no longer parallel with the time axis of the rest of the world.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#49

Post by grenoire »

atoav wrote: post #15
Smaug123 wrote: post #14

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…

This is in tune with what happens when you change the timescale in a game engine with proper physics.

I think this is more of an integration method error than an analogous case.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#50

Post by saagarjha »

sohkamyung wrote: post #36
Smaug123 wrote: post #14

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…

Off topic, but in reality, the trajectory of a ball thrown on Earth is not a parabola, but an ellipse [1]: > under the laws of gravity, a parabola is an impossible shape for an object that's gravitationally bound to the Earth. The math simply doesn't work out. If we could design a precise enough experiment, we'd measure that projectiles on Earth make tiny deviations from the predicted parabolic path we all derived in…

Speak for yourself. My balls reach escape velocity and then make corrections to assume e=1.
Post reply on HN