Quantum arrow of time

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Except few ARTIFICIAL examples, ALL realistic devices (inculding our brains) are significantly IRREVERSIBLE.
On the contrary. According to the article all the devices are essentially PASSIVE and REVERSIBLE. Including observers. Including the real ones, like you. It is just that any observers are unable to observe the erasure of their own memory, and thus are unable to observe the decrease of entropy (and reverse time flow). So you don't need to invent reversible quantum computers and simulate consciousness to get you answer, you are as reversible as these computers, and the time flows for you in both directions, but you (and any other observer) fundamentally unable to remember the reverse flow.
 
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  • #27
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Erasing information increases entropy. This is well known and it's the celebrated Landauer's principle.

If I am not wrong the article is "assuming" or somehow justifying the converse argument:
"Decreasing entropy must necessarily erase information and therefore such a phenomenon cannot be tracked."

If he gave a concrete, PHYSICAL example -- as to why and how the information is erased from the "observer's" memory - that would be more striking.

I don't want to be ignorant since this is published and all but I just don't buy it at the moment. It doesn't make sense.
 
  • #28
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Erasing information increases entropy. This is well known and it's the celebrated Landauer's principle.
If I am not wrong the article is "assuming" or somehow justifying the converse argument:
"Decreasing entropy must necessarily erase information and therefore such a phenomenon cannot be tracked."
Landauer's principle is: "erasing information increases entropy of the environment". And it is in no way is converse with the argument in the article: "a reverse time flow requires the memory erasure of the observer" or "decreasing entropy must necessarily erase observer memory".
 
  • #29
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Landauer's principle is: "erasing information increases entropy of the environment". And it is in no way is converse with the argument in the article: "a reverse time flow requires the memory erasure of the observer" or "decreasing entropy must necessarily erase observer memory".
First, environment is not very well defined. What is environment in Landauer's principle? Without CONCRETE, physical examples - this will not get clear at all.. I have clear examples on that, which would require another discussion. Physics of information is still a widely researched subject and there are many subtle points.

Back to the point:
Is there a reasonable system that you (or the author) can imagine where decreasing entropy results in the erasure of information from the observer's head?
 
  • #30
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Is there a reasonable system that you (or the author) can imagine where decreasing entropy results in the erasure of information from the observer's head?
No, of course. For me it is some kind of logical excercise.

If Nature in fact was 'secretly' violating 2nd law then we would feel some strange gaps in our memory.
 
  • #31
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I can even provide a very simple example of such machine: mirror labyrinth. You pass light ray there... and it reflects many times.

With the reversible inputs such device simply translates the inputs, like glass translates light. If becomes completely decohered with the environment. Like glass or a mirror does not change when reflects light.
Note, that potentially, you can make an observer out of such a machine. That observer would be a completely passive, reversible device. And note that it is not really different from a real human observer, which in a sense is too, completely passive, reversible device. Fundamentally, there is no such thing as 'irreversible', 'active' device.
 
  • #32
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No, of course. For me it is some kind of logical excercise.

If Nature in fact was 'secretly' violating 2nd law then we would feel some strange gaps in our memory.
Yes, of course. :smile:
The author was talking about the real human observers. Enjoying the symmetric backward/forward time flow. Completely unable to remember the former.
 
  • #33
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Note, that potentially, you can make an observer out of such a machine. That observer would be a completely passive, reversible device. And note that it is not really different from a real human observer, which in a sense is too, completely passive, reversible device. Fundamentally, there is no such thing as 'irreversible', 'active' device.
Hm... no...

Say, you invented a 'mirror camera'. Mirror camera has a matrix with ideally reflecting cells. At some moment also reflecting covers are quickly closed, trapping light inside. So light is contained within the cells without any decoherence and collapse, to be released later and analyzed.

However, it only appears that that 'mirror camera' is able to store information. Without decoherence, light (or darkness) inside cells is still in superposition. Bob's actions far away from the device can change thru entanglement the properties of the light inside. Can we say that it is actually 'storing' something?
 
  • #34
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Hm... no...

Say, you invented a 'mirror camera'. Mirror camera has a matrix with ideally reflecting cells. At some moment also reflecting covers are quickly closed, trapping light inside. So light is contained within the cells without any decoherence and collapse, to be released later and analyzed.

However, it only appears that that 'mirror camera' is able to store information. Without decoherence, light (or darkness) inside cells is still in superposition. Bob's actions far away from the device can change thru entanglement the properties of the light inside. Can we say that it is actually 'storing' something?
It doesn't matter that from the outside point of view the photons inside are still in superposition. There would be the inside observer point of view, observer, which 'state of mind' corresponds to the intricate interference patterns in this mirror labyrinth. Note, that these interference patterns correspond to the classical information - bits, not qbits.
 
  • #35
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Umm. Two things:
1) If time flows symmetrically, then why would an observer only remember the "forward" flow? Shouldn't it be possible for an observer to live in the symmetric flow and remember only the "backward" flow?
Exactly! This is also the basis of this comment:
http://xxx.lanl.gov/abs/0912.1947
 
  • #36
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I just skimmed the first page and I think the core issues here is the same as the various interpretations of probability.

As we know, entropy is a constructed (unique or not) measure of missing information, or alternatively a kind of measure of the probability of a macrostate given incomplete knowledge about the microstates, or a measure of the complexion number. If we take the subjective probability view here, then clearly also this entorpy measure is subjective (or observer relative), and thus there is a contradiction in imagining.

A frequent observation of entropy decreasing processes would be as to say that we frequently observer improbable events. Well, if we do repeatadly observer so called "improbable" events, then clearly the prior is revised and at some point it is not improbable anymore.

Anyway this all fits well, within a evolutionary view. If the observers are the ones that ENCODE the entropy measures, there should be an evolutionary selection in favour of "constructive codes". A encoding system that consistentely chooses to encode and retain only the most improbable events, would be extremely unfit/non-constructive and would not be very ubiquitous.

I think what he says that "physics cannot study those processes where entropy has decreased" is sensible if we interpret is as

"it's improbable that physical observers has evolved and persisted, that encodes it's own environment as improbable"

If anyone thinks this looks circular, then the circling, is the progress of evolution - there isn't necessarily a beginning and an end to everything. The physical basis of the measure "improbably past" as seen by any observer, simply doesn't exists in the actual past, since the observer itself is a result of evolution itself. So there is no need to pick infinitely unlikely initial conditions, since the complexion number itself, is reduced to triviality in the past, along with the observers.

/Fredrik
 
  • #38
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About the human brain, there are some brain research indicating that the brain is evolved in order to "predict the future", and NOT in order to "remember the past". Clearly there is a difference in survival values between these to traits. An biological organism that in absurdum try to say rememeber only hte most recent past like a "recording device" would be easily put out of competition.

The brain "forgets" or even "modifies" part of the "actual time history", for the benefit of the "expected future". Remembering certain things, has a survival value, and since the brain is limited, remembering everything is not an option. This is why expectations of the future, can distort human memories of the atcual past.

But if entire community or physical environment works like this, there is no objection notion of the "actual past", and at some point it does not matter.

/Fredrik
 
  • #39
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No, if 'forward flow' is dominating then 'backward observers' are not stable or not formed at all.
You are missing the point. The paper by Maccone attempts to explain WHY "forward flow is dominating", or more generally, WHY one direction has a different role than the other one. Not to say what are the consequences IF it does, but to explain WHY it does so in the first place.

The Maccone paper fails to do what it attempts to do because it uses a circular argument; it "explains" the time arrow from an assumption that a time arrow already exists.
 
  • #40
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The paper seems to me to be an unnecessarily circuitous way of answering the question of why we observe an arrow of time. I agree with those who think that a new, fundamental law of motion is needed. That is, insofar as the current laws of physics are time-symmetric, then they're not describing the fundamental motion.

So, what might be a plausible candidate for such a fundamental law?

It seems to me that an expanding spherical wave shell is worth considering -- and preferable to the current ambiguities of 2nd LoT interpretations.

So, in this view, the radiative arrow of time is the fundamental arrow of time. Disturbances propagate away from their sources. In an ideal, absolutely contiguous, homogeneous medium, the propagation from a point source is isotropic, describing a perfect sphere and the rate and extent expansion is proportional to the energy imparted at the origin of the disturbance. Energy disperses, and dissipates.

Such a fundamental law of motion (the basic wave mechanic) is easy to illustrate in everyday circumstances. Drop a pebble into a smooth pool of water. As the surface wave expands more or less unidirectionally, the energy imparted by the pebble disperses along the wave front and dissipates (the amplitude of the surface wave steadily decreases).

Of course, if such a fundamental law of motion is postulated, then the question of why all waves in all media conform, more or less, to this general dynamic is obviated -- and the arrow of time (and I think maybe some other puzzling foundational issues as well), on any and all scales, is simply and physically accounted for in a readily understandable way.
 

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