B The Arrow Of Time As Defined By The Forces

AI Thread Summary
The discussion centers on the concept of the arrow of time and its relation to forces like gravity. One participant questions whether reversing the arrow of time would necessitate gravity repelling matter instead of attracting it. However, others clarify that even in a time-reversed scenario, gravity remains attractive, and the mathematical implications show that while velocity may reverse, acceleration does not. The conversation also touches on how particles would behave if their velocities were reversed, suggesting that forces would still act to bring them back to their original states. Overall, the idea that reversing time fundamentally alters the nature of forces is challenged by the principles of physics.
Not A Physicist
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I was wondering why the arrow of time can't be defined by the direction in which the forces hold their observed values. It seems to me that if the arrow were to be reversed this would necessitate a reversal of the forces' ability to attract or repel. For example: wouldn't gravity have to start repelling matter? If this is so, shouldn't this be a candidate in the debate over how to define the arrow of time?
 
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Not A Physicist said:
For example: wouldn't gravity have to start repelling matter?
No. A time reversed parabola pointing down is still a parabola pointing down. Time reversed gravity is still attractive.

Not A Physicist said:
It seems to me that if the arrow were to be reversed this would necessitate a reversal of the forces' ability to attract or repel.
It turns out, if you work through the math, that velocity reverses but acceleration remains the same.
 
Dale said:
No. A time reversed parabola pointing down is still a parabola pointing down. Time reversed gravity is still attractive.

It turns out, if you work through the math, that velocity reverses but acceleration remains the same.
I understand that, but if time started running backwards wouldn't gravity slow it down and pull it back around to its usual direction?
 
I don't understand what you are asking. It seems like I already answered that question. Can you please clarify?
 
Dale said:
I don't understand what you are asking. It seems like I already answered that question. Can you please clarify?
Sorry. It seems to me that if all particles had their velocity reversed, gravity (and the other forces) would just bring many of them to a halt and turn them around again, with the result that nothing much changed.
 
I think the OP is thinking of a system of for example two particles that in forward time attract each other. By opposite argument, you could tell time is running backwards by noticing that those two particles are moving away from each other.

I am not knowledgeable enough to make the counterargument solidly (I only have a vague idea how I would argue it), so I'll leave it up to others here.
 
Not A Physicist said:
Sorry. It seems to me that if all particles had their velocity reversed, gravity (and the other forces) would just bring many of them to a halt and turn them around again, with the result that nothing much changed.
Yes, that is correct.
 

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