If we reverse time and this implies running everything backwards in physics, do we include gravity in the set of things that are reversed? Then everything would fly off the face of the earth in reverse-time.
Reversing time on an attracting force still gives you an attractive force. Velocity is reversed, but acceleration isn't.
Imagine a ball being thrown up and then falling down, in a parabola. Reversing a video of that still gives you a video of a ball in a normal parabola trajectory.
The history of the universe channel on YouTube has an episode called 'what is time' that goes over symmetries like this. Lots of atomic/quantum scale interactions are time reversible like this, but not quote all, and that may be where time arrives from.
Well worth watching that episode as it does far better explaining than I do.
If you stop there it looks like you're right, but you also must change the definition of velocity to account for the new time.
v = dx/dt = -dx/du
+F = m dx/du
So the direction of gravity (the force F) stays the same when you flip time. I can explain that without the math by pointing out that if you took a video of a ball being thrown up and caught and played it in reverse, it would still depict a ball being thrown up and caught.
> I can explain that without the math by pointing out that if you took a video of a ball being thrown up and caught and played it in reverse, it would still depict a ball being thrown up and caught.
That's amazing, thanks. The portion where you caught the ball in forward time is equivalent to throwing the ball in reverse time.
If we change the analogy of throwing a ball to firing a gun into the air - does the analogy still work? Since when we fire the gun up, the bullet will travel faster up than it will travel down due to terminal velocity in forward time. How is that phenomenon explained in reverse time?
Instead of predominately striking the bullet in a way that causes it to slow down, the molecules in the air will predominately strike it in a way that causes it to move faster, in what looks like an unbelievable (but still physically possible) run of good luck.
So it seems like if we reverse time, we reverse entropy and that as time approaches 0, we would effectively be reversing the big bang and instead have the big collapse.
Another thought experiment that comes to mind is compressed gas in a cylinder. When we open the valve, the gas in the cylinder comes out. In reverse time, the gas would go back into the cannister and the valve would close after the gas went back in. Very low probability of that happening in forward time, though not not 0.
Though it seems weird, because why does the gas go into the cylinder? Because further into reverse-time, something sucked it all out (in forward-time, this machine is the compressor that put the gas in the cylider.) This hurts my brain!
In the way down -sky to gun - the molecules in the air will give it energy to accelerate more than gravity alone would. Before that - in the way up - air molecules will cause it to move upwards at constant speed until conveniently they stop doing so.
> unbelievable (but still physically possible)
Physically possible - but in the same sense that the second law is not a physical law.
" it's literally like playing a movie in reverse" seems overly authoritative for something we haven't observed. I have seen Tenet, but it's a work of fiction.
Have we observed reflected gravitational waves? In reverse-time, where would they originate from if they presumably rippled out into space and didn't collide with anything in forward-time?
.... they would "originate" from all the locations the gravity waves spread to and converge on the source.
Tenet has nothing to do with this- I'm just explaining it as it was covered in my many physics classes that covered the nature of the arrow of time (https://en.wikipedia.org/wiki/Arrow_of_time) and how I interpret it in terms of what seems most likely/least unlikely.
In these "it's like taking a video of throwing a ball in the air and allowing it to land on the ground, then playing it in reverse" examples, I can't help but think of Newton's first law. If an object at rest stays at rest, how does the ball leap from the ground? Where does the impulse come from? Reverse time seems too far fetched for me, or at least the simplified naïve version of it does.
It also seems a bit misleading, since in that scenario a ball is intentionally thrown so that it comes down the same way.
Let's consider something else - imagine an accretion disk of space dust slowly pulling itself together to form a planet. Play that in reverse, and you have the a planet slowly coming apart piece by piece. Imagine reversing the impact that created the moon. The moon comes apart piece by piece, creating an accretion disk around the earth, which then all moves and hits one area of the earth, and there several parts of it (and part of the earth itself) move together to form a separate planet, which then launches itself from the earths surface into space, flies around the sun a few times, and then slowly breaks apart piece by piece into another accretion disk.
