5 ms·

It is because in both cases, you can work out that you'll never make an observation that really proves it one way or the other, but you might come up with some

It is because in both cases, you can work out that you'll never make an observation that really proves it one way or the other, but you might come up with some laws of physics that are more conveniently expressed in one way than another. They're both ultimately questions about your coordinate system, and in fact it is easier to invert time than it is to work out how everything's supposed to function in a rotating+sun-orbiting+precessing+... coordinate system.

nanidin 4y ago
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.
- entropicdrifter 4y ago
  Gravity is a universal constant. If you reverse time, you just reverse the order of cause and effect, not what the effect is. Does that make more sense to you?
  - nanidin 4y ago
    If we reverse time, would gravitational waves flow backwards?
    - dekhn 4y ago
      yes. it's literally like playing a movie in reverse.
      - nanidin 4y ago
        " 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?
        dekhn 4y ago
        .... 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 https://en.wikipedia.org/wiki/Arrow_of_time) and how I interpret it in terms of what seems most likely/least unlikely.
  - bigbacaloa 4y ago
    "gravity is a universal constant" contradicts Newtonian mechanics, special and general relativity.
    - nanidin 4y ago
      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.
      - bnralt 4y ago
        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.
- whatshisface 4y ago
  Let's check. F = ma F = m dv/dt u = -t dv/dt = -dv/du - F = m dv/du 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.
  - nextaccountic 4y ago
    > 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. I need to rewatch Tenet some day
    - novaRom 4y ago
      I watched it 4 times. Only then I started to understand what's happening. How great and unique this movie really is.
      - mkl 4y ago
        I felt like I understood it the first time, but didn't think it was very good. Was that your initial reaction?
  - nanidin 4y ago
    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?
    - whatshisface 4y ago
- theemathas 4y ago
  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.
  - deleted 4y ago
    [deleted]
  - pixl97 4y ago
    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.
sidlls 4y ago
It isn't because we have the observations we have. It might be mathematically easy to invert time, but empirically it's not so simple. Mathematical symmetries and expressions are nice, but then the actual physical consequences and requirements (e.g., material, time, energy inputs) sort of force one's hand. Also, geocentrism is actually disproven by numerous observations, which in fact guided the mathematical formulation of our current physics. One thing is (almost) certain: the laws of physics as we know them (i.e. the Standard Model and General Relativity) are incomplete ("wrong"). That doesn't mean any old model is equivalent or as useful as either, though.
- whatshisface 4y ago
  >Also, geocentrism is actually disproven by numerous observations, which in fact guided the mathematical formulation of our current physics. The validity of a coordinate system with the earth stationary at the center is guaranteed by the general principle of relativity. To get the stars to circle around it you would add a radially increasing potential in classical mechanics, or some coordinate shenanigans in GR. These coordinate systems are used in aerospace engineering to get convenient expressions of L1 points, etc. I know it sounds hard to believe, but if you're willing to bite the bullet that "the center of the universe" has no physical meaning, then earth can't be not the center of the universe any more than it can be.
  - prerok 4y ago
    To be honest, I have no idea what you are talking about. Certainly, there is merit in practical calculations when we are all this close to the ground where it all seems either flat or at least geo-centric. But... the movements of the other planets in our solar system were really strange to model in geo-centric models. In short, each of the other planets should rotate around an imaginary axis to compensate for their changing positions in the sky. Of course, all this is talk about in the context of our solar system. As for us being the center of the universe, well, the same argument holds for any other point in the universe. So, I think it's less likely than me winning the lottery in the next five minutes :)

nanidin 4y ago

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.

entropicdrifter 4y ago
Gravity is a universal constant. If you reverse time, you just reverse the order of cause and effect, not what the effect is. Does that make more sense to you?
- nanidin 4y ago
  If we reverse time, would gravitational waves flow backwards?
  - dekhn 4y ago
    yes. it's literally like playing a movie in reverse.
    - nanidin 4y ago
      " 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?
      - dekhn 4y ago
        .... 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 https://en.wikipedia.org/wiki/Arrow_of_time) and how I interpret it in terms of what seems most likely/least unlikely.
- bigbacaloa 4y ago
  "gravity is a universal constant" contradicts Newtonian mechanics, special and general relativity.
  - nanidin 4y ago
    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.
    - bnralt 4y ago
      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.
whatshisface 4y ago
Let's check. F = ma F = m dv/dt u = -t dv/dt = -dv/du - F = m dv/du 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.
- nextaccountic 4y ago
  > 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. I need to rewatch Tenet some day
  - novaRom 4y ago
    I watched it 4 times. Only then I started to understand what's happening. How great and unique this movie really is.
    - mkl 4y ago
      I felt like I understood it the first time, but didn't think it was very good. Was that your initial reaction?
- nanidin 4y ago
  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?
  - whatshisface 4y ago
theemathas 4y ago
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.
- deleted 4y ago
  [deleted]
- pixl97 4y ago
  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.

sidlls 4y ago

It isn't because we have the observations we have. It might be mathematically easy to invert time, but empirically it's not so simple. Mathematical symmetries and expressions are nice, but then the actual physical consequences and requirements (e.g., material, time, energy inputs) sort of force one's hand. Also, geocentrism is actually disproven by numerous observations, which in fact guided the mathematical formulation of our current physics. One thing is (almost) certain: the laws of physics as we know them (i.e. the Standard Model and General Relativity) are incomplete ("wrong"). That doesn't mean any old model is equivalent or as useful as either, though.

whatshisface 4y ago
>Also, geocentrism is actually disproven by numerous observations, which in fact guided the mathematical formulation of our current physics. The validity of a coordinate system with the earth stationary at the center is guaranteed by the general principle of relativity. To get the stars to circle around it you would add a radially increasing potential in classical mechanics, or some coordinate shenanigans in GR. These coordinate systems are used in aerospace engineering to get convenient expressions of L1 points, etc. I know it sounds hard to believe, but if you're willing to bite the bullet that "the center of the universe" has no physical meaning, then earth can't be not the center of the universe any more than it can be.
- prerok 4y ago
  To be honest, I have no idea what you are talking about. Certainly, there is merit in practical calculations when we are all this close to the ground where it all seems either flat or at least geo-centric. But... the movements of the other planets in our solar system were really strange to model in geo-centric models. In short, each of the other planets should rotate around an imaginary axis to compensate for their changing positions in the sky. Of course, all this is talk about in the context of our solar system. As for us being the center of the universe, well, the same argument holds for any other point in the universe. So, I think it's less likely than me winning the lottery in the next five minutes :)