The Arrow Of Time As Defined By The Forces

Click For Summary

Discussion Overview

The discussion revolves around the concept of the arrow of time and its potential definition through the behavior of forces, particularly gravity. Participants explore whether reversing the arrow of time would necessitate a reversal in the nature of forces, such as gravity's attraction versus repulsion, and the implications of such reversals on particle motion.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that if the arrow of time were reversed, forces like gravity would need to change from attracting to repelling matter.
  • Others argue that time-reversed gravity would still be attractive, as the mathematical properties of forces do not change with time reversal.
  • One participant suggests that if all particles had their velocities reversed, gravity would halt them and potentially reverse their direction, leading to minimal change in the system.
  • Another participant introduces the idea that observing two particles moving away from each other could indicate that time is running backwards.
  • Clarifications are requested regarding the implications of reversing time on the behavior of forces and particle interactions.

Areas of Agreement / Disagreement

Participants express differing views on whether reversing the arrow of time would alter the nature of forces like gravity. There is no consensus on how these concepts interact, and the discussion remains unresolved.

Contextual Notes

Participants acknowledge the complexity of the mathematical relationships involved, with some noting that velocity and acceleration behave differently under time reversal. However, specific mathematical steps and assumptions are not fully explored.

Not A Physicist
Messages
3
Reaction score
0
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?
 
Science news on Phys.org
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.
 

Similar threads

  • · Replies 14 ·
Replies
14
Views
4K
  • · Replies 7 ·
Replies
7
Views
4K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 15 ·
Replies
15
Views
4K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 16 ·
Replies
16
Views
2K
  • · Replies 20 ·
Replies
20
Views
2K
  • · Replies 27 ·
Replies
27
Views
6K