Why does time only move forward and not backwards?

[Tech2025]

Probably an unanswered question , just wondering what people have to say.

 

[anorlunda]

It is not a trivial question. Start by reading this article, then come back if you still have questions.
https://en.wikipedia.org/wiki/Arrow_of_time

 

[Craftek_Ana]

I'm sure the brilliant minds of this forum can explain what is known this far. I believe it has to do with entropy, where everything goes from order to disorder, at the same time our perception of time is skewed. Past, present, and future all existing at once. Glad you asked also interested in the opinions on this forum.

 

[anorlunda]

I'm sure the brilliant minds of this forum can explain what is known this far. I believe it has to do with entropy, where everything goes from order to disorder, at the same time our perception of time is skewed. Past, present, and future all existing at once. Glad you asked also interested in the opinions on this forum.

You too should read the Wikipedia article.

 

[Craftek_Ana]

You too should read the Wikipedia article.

lol, I am.

After doing a quick Google search and reading what is quoted below I feel more confused. Down the rabbit hole I go.

"There is no connection between classical thermodynamic entropy(which is real) and “time” (which is an illusion), there never was, and there never will be.

No form of "time travel" is possible, for the simple reason that there is no "time," no "time direction," no "flow of time," there is only now — there are no “pasts” and no “futures” to go to! In physics (and in QM), there is only a measure of durations of events, and of the intervals between events: the "fourth dimension of time" in mathematical physics is an abstraction — invented to enable manipulation of "Minkowski space" metrics —- it does not exist in our real Universe.

Recommended reading: J.C.N. Smith's Time — Illusion and Reality.

“Time” has nothing to do with any form of entropy (see What is the Easiest Definition of "Entropy"?), which in turn has nothing to do with “order” or “disorder”: Entropy is measured in units of energy over temperature (i.e, in joule per kelvin (J/K)), as Wikipedia (correctly) states in the article Entropy:

Entropy is an extensive property. It has the dimension of energy divided by temperature, which has a unit of joulesper kelvin in the International System of Units But the entropy of a pure substance is usually given as an intensive property — either entropy per unit mass or entropy per unit amount of substance

I challenge all disinformed fans of the unscientific notion that “entropy is a measure of disorder,” to explain exactly how a “degree of order” — or disorder, as the case may be — can possibly be expressed in units of energy and temperature!"

 

[jbriggs444]

Probably an unanswered question , just wondering what people have to say.

As long as time moves in the same direction for everything, it does not matter whether that direction is labelled "forward" or "backward". Ripples from a rock dropped in a pond will appear to move outward either way.

 

[anorlunda]

Down the rabbit hole I go.

Don't drag us into the hole also. It appears that you hopped all over on the Internet, sometimes to sources unacceptable on PF. Please give a link to every assertion that you make. (If this was not a B level thread, even Wikipedia is not acceptable.)

The OP asked about the direction of time. not "what is time". We have had many threads about "what is time" here on PF. They tend to get philosophical and get shut down quickly, so stay away from that.

 

[Craftek_Ana]

Don't drag us into the hole also. It appears that you hopped all over on the Internet, sometimes to sources unacceptable on PF. Please give a link to every assertion that you make. (If this was not a B level thread, even Wikipedia is not acceptable.)

The OP asked about the direction of time. not "what is time". We have had many threads about "what is time" here on PF. They tend to get philosophical and get shut down quickly, so stay away from that.

Ahh I see, I see. Good to know. Do not want to get banned, relatively new here.

 

[Stavros Kiri]

As long as time moves in the same direction for everything, it does not matter whether that direction is labelled "forward" or "backward". Ripples from a rock dropped in a pond will appear to move outward either way.

What about an exploding rocket? Time reversal there is obviously connected with entropy ... and the two directions are distinguished.

 

[jbriggs444]

What about an exploding rocket? Time reversal there is obviously connected with entropy ... and the two directions are distinguished.

If the direction of low entropy is in the direction that somebody has chosen to label as "future" then time will proceed in "reverse". The two directions are only distinguished by the direction in which low entropy is located.

 

[Stavros Kiri]

If the direction of low entropy is in the direction that somebody has chosen to label as "future" then time will proceed in "reverse". The two directions are only distinguished by the direction in which low entropy is located.

Then

Ripples from a rock dropped in a pond will appear to move outward either way.

is not true

 

[jbriggs444]

Then

is not true

If a rock spontaneously rises from a pond in the middle of a circle of converging ripples, the ripples will appear to move outward because the direction of low entropy is in the direction that you have chosen to call "future".

 

[Stavros Kiri]

If a rock spontaneously rises from a pond in the middle of a circle of converging ripples, the ripples will appear to move outward because the direction of low entropy is in the direction that you have chosen to call "future".

The smaller the circles the lower the entropy ... thus more into the "future", thus inwards. q.e.d

 

[jbriggs444]

The smaller the circles the lower the entropy ... thus more into the future, thus inwards. q.e.d

So your claim is that ripples in ponds always converge inwards over time? That certainly does not match my experience.

Edit: I see now the scare quotes around "future" and agree with what you wrote.

 

[Stavros Kiri]

So your claim is that ripples in ponds always converge inwards over time? That certainly does not match my experience.

No, only if you choose the false direction, as you suggested.
"Time reversal" is a tricky thing! ...
[In Particle Physics it may be a symmetry, but not in everyday life ...]

 

[richrf]

Time as a scalar quantity measurement (clock time) can be negative (or reversed) as often is the case in in mathematical equations.

The OP implicitly (or explicitly) is teetering to time as it is experienced in life, which is a manifestation of memory moving from the past towards the future and never is experienced in reverse. This time is probably more of a philosophical discussion than a scientific discussion.

 

[AlexCaledin]

It's fun to derive the necessary philosophy from Feynman! His key words are NATURE KNOWS, in the QM part of the Lectures. So, Nature keeps incrementing Her knowledge, using the Spacetime as the organizer to keep it in order. Naturally, the organizer is being filled (with actual events) from one side forward, in a certain direction

 

[Vanadium 50]

So, Nature keeps incrementing Her knowledge, using the Spacetime as the organizer to keep it in order.

I don't think this makes sense. It's certainly not science.

I also think this question is ill-defined. What does it mean for time to "run backward"? What experiment could you conduct that would show this? You would quickly find yourself asking the question "backward with respect to what?" Presumably some universal clock that measures absolute time...

 

[Stavros Kiri]

I also think this question is ill-defined. What does it mean for time to "run backward"? What experiment could you conduct that would show this? You would quickly find yourself asking the question "backward with respect to what?" Presumably some universal clock that measures absolute time...

I guess you can restate it by asking if there is one preferred well-defined direction [of time]

 

[Vanadium 50]

You can ask all sorts of ill-defined things. What experiment could you conduct that would show this?

 

[Stavros Kiri]

You can ask all sorts of ill-defined things. What experiment could you conduct that would show this?

In the micro-world there may be time-reversal symmetry (equations invariant under the time-reversal symmetry operator/transformation), but not in Thermodynamics or Statistical Physics. You can use the direction of increasing Entropy (2nd Law of Thermodynamics) to figure out the direction of time ...

 

[Vanadium 50]

Stavros, you are ignoring what I am writing.

 

[Stavros Kiri]

Stavros, you are ignoring what I am writing.

What experiment? You seem to be the experimentalist ... you figure it out

 

[Stavros Kiri]

Stavros, you are ignoring what I am writing.

What experiment? You seem to be the experimentalist ... you figure it out

What I mean is that you haven't proven that such an experiment is not possible or it doesn't exist, nor it's impossible that it doesn't, per se or a priori, or even purely scientifically, unless/until that is proven. I offered you a starting point:

In the micro-world there may be time-reversal symmetry (equations invariant under the time-reversal symmetry operator/transformation), but not in Thermodynamics or Statistical Physics. You can use the direction of increasing Entropy (2nd Law of Thermodynamics) to figure out the direction of time ...

Rather you responded with a totally ambigious statement! I had to guess what you meant! [Then you went ahead and reported me!? ...]

Do you have a solution or proof for any claims? I am more than willing to hear! ...

 

[Vanadium 50]

1. It is not my job to come up with a counter-example to my own argument.
2. All you have succeeded in convincing me is that further discussion is a waste of time.

 

[Stavros Kiri]

1. It is not my job to come up with a counter-example to my own argument.
2. All you have succeeded in convincing me is that further discussion is a waste of time.

1. Nor you have proven or can prove your argument either! ...
2. It would also help if you were not kind of cantankerous, sometimes.

Have you read the whole thread? There was a dispute, not exactly what you're asking though, and was resolved back then (about whether, at least in everyday macroscopic life, we can tell apart the two possible directions of time):

As long as time moves in the same direction for everything, it does not matter whether that direction is labelled "forward" or "backward". Ripples from a rock dropped in a pond will appear to move outward either way.

(Proven wrong:)

The smaller the circles the lower the entropy ... thus more into the "future", thus inwards. q.e.d

No, only if you choose the false direction, as you suggested.
"Time reversal" is a tricky thing! ...
[In Particle Physics it may be a symmetry, but not in everyday life ...]

 

[jbriggs444]

As long as time moves in the same direction for everything, it does not matter whether that direction is labelled "forward" or "backward". Ripples from a rock dropped in a pond will appear to move outward either way.

(Proven wrong:)

Nope. The statement is correct. Though it seems that you had not actually agreed with it.

No matter which direction time "flows", a person inside the universe where it does so will experience a progression from low entropy past to high entropy future.

If someone outside the universe chooses to invoke time reversal symmetry and claim that the flow of time is actually reversed, that is of no consequence to the inhabitants of the universe in question.

 

[Stavros Kiri]

Nope. The statement is correct. Though it seems that you had not actually agreed with it.

You have to review carefully the thread too. We had agreed on it:

The smaller the circles the lower the entropy ... thus more into the "future", thus inwards. q.e.d

(Thus it violates the entropy law in that case [of reversed time]. Thus we can use that argument to pin-point the increasing entropy direction as the correct direction of time ...)

You had agreed on it:

Edit: I see now the scare quotes around "future" and agree with what you wrote.

 

[jbriggs444]

You have to review carefully the thread too. We had agreed on it:

(Thus it violates the entropy law in that case [of reversed time]. Thus we can use that argument to pin-point the increasing entropy direction as the correct direction of time ...)

You had agreed on it:

Right. We had an agreement. But then you claimed disagreement. Please do not do that.

 

[Stavros Kiri]

Right. We had an agreement. But then you claimed disagreement. Please do not do that.

You did that not me

 

[jbriggs444]

You did that not me

I am not the one who wrote "proven wrong".

 

[Stavros Kiri]

I am not the one who wrote "proven wrong".

Yes, for your original statement, which later on I assume you realized its falsity: in reversed time, circles go inward (to a decreasing and not increasing entropy), while you said always outwards ... . Simple.

 

[jbriggs444]

Yes, for your original statement, which later on I assume you realized its falsity: in reversed time, circles go inward (to a decreasing and not increasing entropy), while you said always outwards ... . Simple.

The original statement was and is correct.

Edit to add... Your objection appears to have been that there is only one correct labeling for past and future. My suggestion is that if we are to give meaning to the notion of "time flowing into the future", one approach is to note that the labeling is arbitrary.

However, as we both agree, operationally the result is the same. The observable progression for inhabitants of a time-reversed universe is indistinguishable from that for a time-normal universe. Time still "flows" from the direction of low entropy to high entropy.

We both agree that one might as well label the direction of low entropy as "past". It would appear that our disagreement is purely about words and not about physics. However, I dislike to see "proven false" applied to a disagreement about words.

 

[Stavros Kiri]

It's not just about words.

Time still "flows" from the direction of low entropy to high entropy.

Is incorrect, if you switch "past" and "future". Just make yourself a simple diagram and think carefully.