# B The Arrow Of Time As Defined By The Forces

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1. May 13, 2017

### Not A Physicist

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?

2. May 13, 2017

### Staff: Mentor

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.

3. May 14, 2017

### Not A Physicist

I understand that, but if time started running backwards wouldn't gravity slow it down and pull it back around to its usual direction?

4. May 14, 2017

### Staff: Mentor

5. May 14, 2017

### Not A Physicist

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.

6. May 14, 2017

### rumborak

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.

7. May 14, 2017

### Staff: Mentor

Yes, that is correct.