# Question Regarding Perception of Time

1. May 25, 2013

### Billiards1974

First off, forgive me if I don't word this properly. I'm a furniture restorer by trade and have recently begun thinking about how everything works. Specifically, I have been thinking about the possibility of our perception of time and the actual flow of time.

From what I understand, Einstein theorized that time could flow in either direction. With that being said, is it theoretically possible that time is CURRENTLY flowing in a "backwards" direction but, our perception of it is actually "forward"?

Thanks,
Billiards1974

2. May 25, 2013

### Staff: Mentor

Hi Billiards 1974, welcome to PF!

I don't think that Einstein ever theorized that. Do you have a specific quote or source you are refering to?

The laws of physics appear to be time-reverse symmetric for the most part. Perhaps that is what you are discussing, but I don't recall any comments by Einstein along those lines.

3. May 25, 2013

### Billiards1974

Sorry, about bringing Einstein into this.... It is theoretically possible for time to flow in either direction, isn't it?

My question is, if it is possible for time to flow in either direction, could it be that time actually is flowing in the direction opposite from what we perceive? And, if time, in fact, could be flowing in that direction, can it be mathematically expressed?

4. May 25, 2013

### nitsuj

The symmetry Dalespam mentions is literal, no different than the symmetry of your reflection in a mirror.

A classic "visual" for this is to imagine a video recording playing in forward or reverse. In both cases the mechanical physics is the exact same.

What is apparently directional and closely correlated with time is entropy. However these are two distinct concepts. In other words from a purely physical mechanics perspective there is no "direction" to time.

What that means with respect to your question "possible for time to flow in either direction, isn't it?" There is no direction or "flow" of time, therefore it doesn't makes sense to ask if it can flow in either direction. Saying it's symmetric is descriptive and accurate, either "side" of that symmetry is not forward or backward, it's just equal but opposite.

It does make sense to ask your question about entropy, however I know little to nothing about entropy, plus it's not the right sub-forum for such a topic.

Last edited: May 25, 2013
5. May 25, 2013

### Staff: Mentor

So, scientifically the concept you are refering to is called T-symmetry. The Wikipedia page on T-symmetry used to be pretty good, but apparently now it has some sort of certificate errors:
https://en.wikipedia.org/wiki/T-symmetry

Mathematically, you would express it as $t \mapsto -t$. You can then apply that mapping to different laws of physics and see what would happen. For example, Newton's 2nd law:

$f=m a = m \frac{d^2x}{dt^2}$
$t \mapsto -t$
$f = m \frac{d^2x}{d(-t)^2} = m \frac{d^2x}{dt^2} = ma$

So Newton's 2nd law is unchanged under time reversal. The same thing is true for all of the fundamental laws except for the weak nuclear force. Also, even with symmetric laws, if you have certain initial conditions you can get asymmetry as well, such as the big bang and the second law of thermodynamics.

6. May 25, 2013

### Stricklandjr

Moving in time

7. May 25, 2013

### Billiards1974

I'm REALLY trying to stay away from the philosophical. But, this would explain our perpetual state of "now".

8. May 25, 2013

### Stricklandjr

It is not philosophy if it is correct physics. Mr. TheBC believes it is true physics and he gave sayings of Einstein where he said that the universe is 4 dimensional and Einstein is the greatest physicist to ever live and I don't think he would make a saying of such big importance if it was not good physics since he gave his whole life to physics.

9. May 25, 2013

### Billiards1974

How is the law different for weak nuclear force? (Again, I am a COMPLETE novice here.)

10. May 25, 2013

### Staff: Mentor

This is a non sequiter. Even the greatest physicist to ever live is allowed to make philosophical statements, as well as order lunch, complain about their neighbors, whisper romantic nothings, and any other thing he might want say. Simply because a physicist says it doesn't make it physics.

11. May 25, 2013

### Staff: Mentor

Basically, the time asymmetry of the weak interaction (also called CP violation) allows quarks to change their flavors in ways that wouldn't happen the same if time were reversed.

12. May 31, 2013

### dayalanand roy

Dear Billiards
Though I am not a physicist, I have tried to understand time. Time does not flow at all. So no question of flowing in any direction. Things flow, ie, change/move as per thermodynamics or the forces acting on them. In order to measure their rate of change/motion, we apply a parameter of time. But this 'time' too, as Einstein suggested, is not absolute.
Since we see mostly irreversible changes in nature, like our kids growing, our aging, breaking of a cup etc, we beleive in a flowing- and in one direction only, time. But what about melting of an ice cube into water and then again freezing back to ice. Does time flow here in a reverse direction here? No.
regards
Dayalanand