Why does the past hypothesis let us believe our memories?

[olamosey]

In Sean Carroll’s excellent series of lectures about time (Mysteries of Modern Physics: Time), he talks about the past hypothesis: the hypothesis that entropy was lower in the past. He says "it is ultimately that we know more about the past because of the past hypothesis that what we call causes precede what we call events." And he says “It is the past hypothesis that let's us believe our memories.” But is he not just saying that we know more about the past because we know more about the past? And how does this determine what I personally believe about the past? Until a year ago I didn’t know what the past hypothesis was, and I believed my memories.

 

[Andrew Mason]

Thanks for mentioning thus lecture series. I listened to this YouTube clip:

While his explanation of time relating to entropy is not wrong, it seems to me that it is an unnecessarily complex explanation.

Time is necessarily unidirectional because in order to reverse time and return to a previous time, t, every observable object and particle in the universe would have to return to the exact state as it existed at time t. Now the second law of thermodynamics tells us that this cannot happen. But so does common sense.

AM

 

[Andrew Mason]

Just a short follow-up to give a better answer to your question.

The second law of thermodynamics or the past hypothesis does not explain why we can rely on our memories. The process of memory formation involves a local decrease in entropy. (All that is required to achieve a local decrease in entropy is a larger increase in entropy elsewhere).

An event recorded in our memory results in parts of our brain undergoing a thermodynamic process that results in a reduction in entropy in our brain cells. Our brain gets progressively more organized as memories grow.

So memory formation and it reliability just requires a local reduction in entropy. This local reduction results in an increase in the entropy overall, which is perhaps Sean Carroll's point.

Carroll seems to be saying that the concept of cause and effect relies on the second law - that somehow we rely on the fact that entropy, overall, increases in order for there to be causal relationships. I don't think this is correct because entropy is a statistical concept that applies to large numbers of particles. We can observe cause and effect at the atomic level.

AM

 

[anorlunda]

Carroll seems to be saying that the concept of cause and effect relies on the second law - that somehow we rely on the fact that entropy, overall, increases in order for there to be causal relationships.

I agree. The second law is true statistically, as is all of thermodynamics. Causality can be applied to a single particle or a single pair of particles; a realm where you can not use statistics.

 

[olamosey]

Thank you for your answers.

Andrew, in that youtube clip, Carroll says (around 16 minutes)

“As you go through your life, as you are remembering things and predicting things about the future you are constantly relying on the fact that the entropy of the universe was very low near the big bang. You might not have known that you were relying on that fact, but it is crucially important to how we live our lives.”

This is basically what he said in his lecture series that prompted my question. I knew nothing about entropy until a year ago, so how did it affect how I remembered things and predicted the future?

Andrew, you say that the past hypothesis does not explain why we can rely on our memories, and that makes sense to me. I think I am misunderstanding what Carroll is saying.

 

[sysprog]

I knew nothing about entropy until a year ago, so how did it affect how I remembered things and predicted the future?

Shouldn't it be taken as obvious that you don't have to know about something for it to affect you?

 

[olamosey]

Shouldn't it be taken as obvious that you don't have to know about something for it to affect you?

That's true... so how exactly does the fact that the entropy of the universe was very low near the big bang affect how I remember things?

 

[sysprog]

That's true... so how exactly does the fact that the entropy of the universe was very low near the big bang affect how I remember things?

Except in the sense that entropy being lesser or greater affects everything, I don't agree with the notion that it discernably affects how you or I remember things (although I wouldn't disagree with the idea that the fact of entropy affects memory) -- I think that neural action potentials are more proximately causal regarding that . . .

 

[olamosey]

Time is necessarily unidirectional because in order to reverse time and return to a previous time, t, every observable object and particle in the universe would have to return to the exact state as it existed at time t. Now the second law of thermodynamics tells us that this cannot happen. But so does common sense.

Andrew,
1 I don't understand what your answer has to do with my question.

2 If time is unidirectional, then why do physicist talk about reversing time?

 

[anorlunda]

2 If time is unidirectional, then why do physicist talk about reversing time?

https://en.wikipedia.org/wiki/Time_reversibility

Physics​

In physics, the laws of motion of classical mechanics exhibit time reversibility, as long as the operator π reverses the conjugate momenta of all the particles of the system, i.e.

(T-symmetry).

In quantum mechanical systems, however, the weak nuclear force is not invariant under T-symmetry alone; if weak interactions are present, reversible dynamics are still possible, but only if the operator π also reverses the signs of all the charges and the parity of the spatial co-ordinates (C-symmetry and P-symmetry). This reversibility of several linked properties is known as CPT symmetry.

Thermodynamic processes can be reversible or irreversible, depending on the change in entropy during the process.

 

[olamosey]

anorlunda, I was responding to Andrew Mason's comment "Time is necessarily unidirectional because in order to reverse time and return to a previous time, t, every observable object and particle in the universe would have to return to the exact state as it existed at time t. Now the second law of thermodynamics tells us that this cannot happen. But so does common sense."

If time is unidirectional, then it doesn't make sense to talk about reversing time.

 

[anorlunda]

If time is unidirectional, then it doesn't make sense to talk about reversing time.

Yes, it sounds like a riddle. In terms of mechanics, it is. In terms of statistics, it's not reversible.

Consider the first 10 seconds of this video. You can't tell if the movie is playing forwards or backwards. The mechanics of those particles, including collisions with each other and collisions with the walls, work the same forwards or backwards.


Now consider these two photos. According to probability and statistics, the one on the left becomes the one on the right given enough time. If you stir the water, it takes less time. It never goes in the other direction. It is irreversible. But it is not the physics of mechanics that forbid reversal, but rather probability, statistics and the 2nd law of thermodynamics.

So again, it sounds like a riddle. Time can be reversible or irreversible depending on the context and on which laws you're talking about. In other words, "it depends...".

If you study physics, you'll likely learn first of "mechanics", and later of "statistical mechanics."

 

[Andrew Mason]

Andrew,
1 I don't understand what your answer has to do with my question.

2 If time is unidirectional, then why do physicist talk about reversing time?

I don't know if physicists do talk about reversing time. All sorts of absurdities result from just the concept of reversing time.

There may be some academic reason to imagine time reversal, however. Richard Feynman has pointed out that the time reversal of the state of a particle provides a description of the anti-particle state. For example, reversing the flow of time for an electron moving toward a positive charge under coulomb attraction would describe the state of a particle of the same mass as an electron being repelled by the positive charge - ie. a description of the state of the electron anti-particle, the positron.

AM

 

[olamosey]

I don't know if physicists do talk about reversing time. All sorts of absurdities result from just the concept of reversing time.

But the T in CPT symmetry is time! I thought that modeling what particles do if time reversed was a big part of this.

 

[anorlunda]

"I don't know if physicists do talk about reversing time. All sorts of absurdities result from just the concept of reversing time."

But the T in CPT symmetry is time! I thought that modeling what particles do if time reversed was a big part of this.

You're still over reaching. The 2nd law applies to the universe. So there is no way that time reverses. However, certain parts of the physics of mechanics are reversible (as shown in post #12). That is not the same thing as saying that time in the whole universe reverses.