

Is quantum mechanics messing with your memory? - jsankey
http://www.guardian.co.uk/science/blog/2009/aug/26/entropy-time-arrow-quantum-mechanics

======
Maro
Here's my understanding of the Maccone paper. Edit: I only skimmed it.

Super-condensed QM intro: In the _formalism_ of quantum mechanics, the
dynamics of physical processes are represented by unitary operators. If Y(0)
is the state of the system at t=0, then the dynamics (change, movement) of the
system is represented by a unitary operator U(t), such that the state of the
system at time t is Y(t)=U(t)Y(0). The technical term unitary roughly means
that it is time-reversible: there exists the inverse of U(t) meaning that the
inverse physical process can also occur (the hidden assumption is that there
is a 1-1 correspondence between operators in the formalism and actual physical
processes). The problem is, if everything is time-reversible, why don't we see
glasses unbreaking or gas that came in to fill the room through a little hole
finding its way out again?

Measurements in QM are _not_ unitary. If you measure something, you can't undo
it. Measurements are in this sense an "add-on" to QM.

The following argument probably pre-dates the Maccone paper: Take a room which
contains a quantum system Q and an observer O. The observer makes measurements
on Q, which are not unitary, so he sees time irreversible processes. Now take
a superobserver O', who is outside the room. To him, the room (Q and O
together) make up a quantum mechanical system Q', which may be described in a
unitary way. This means that the superobserver O' can argue that even though
the observer O only sees time irreversible processes, the inverse process can
in fact occur (glass unbreaking), but they reverse the observer O itself (who
is part of Q'), meaning that he "forgets" what he measured, his ink
"disappears" from his notebook, and so on.

Maccone's contribution seems to be that he identifies O measuring Q with O and
Q becoming quantum entangled and provides some formalism.

Substitute "time-irreversible" for "entropy increasing" for trendiness (eg. to
pick up girls in a bar =)).

~~~
hughprime
I skimmed the paper the other day, but still didn't quite understand what he
was on about.

In particular, I didn't understand why he feels the need to invoke quantum
mechanics at all. Thermodynamic systems don't flow backwards from high-entropy
to low-entropy states simply because there are far more high-entopy states
than there are low-entropy states.

Or to put it another way, what does Maccone think would happen in a purely
classical universe? Would not entropy still increase there?

I find that discussing thermodynamics works terribly for picking up girls in
bars, but maybe I just go to the wrong bars.

~~~
Maro
Susskind answered a similar question during his stat. phys. lectures which you
can watch on iTunes.

Somebody asked the following question: given that entropy is the volume of the
system in phase space and Liouville's Theorem (of classical physics) tells us
that the volume in phase space is conserved, how come entropy increases? His
answer: Liouville's Theorem holds, the volume stays the same, but the blob
representing the system (the system's microstates) spreads out in a fractal
way in phase space, increasing the "coarse-grained" phase volume / entropy.
Then, a "you can't set the initial conditions infinitely precisely" / "you
can't measure inifinitally presicely" type argument is invoked to say that in
terms of real-life reversibility the course-grained volume is what matters, so
entropy has increased despite Liouville.

Explained here:
[http://www.google.com/url?sa=t&source=web&ct=res&...](http://www.google.com/url?sa=t&source=web&ct=res&cd=21&url=http%3A%2F%2Fbooks.google.com%2Fbooks%3Fid%3DR-
jQ4I0VkdoC%26pg%3DPA70%26lpg%3DPA70%26dq%3Dliouville%2Btheorem%2Bentropy%26source%3Dbl%26ots%3DRKI8zXfw-0%26sig%3DtZOa77uJCTs8-XibeaJWf3qhPKk%26hl%3Den%26ei%3DrquWSvulI47Y-QatkeTQCQ%26sa%3DX%26oi%3Dbook_result%26ct%3Dresult%26resnum%3D1&ei=rquWSvulI47Y-QatkeTQCQ&usg=AFQjCNGLutp18ilwZw-
_ZISvOG_oxCbISw&sig2=mk0O2_FIpPZ3etOpTas28g)

------
tilly
I don't buy it for a simple reason. It is easy to look around the macro
environment and see plenty of reasons why entropy has a long way to increase.
For instance for several billion more years the Earth should be receiving
energy from the Sun and radiating it off into outer space, which creates a
sustained low entropy situation enabling something we call "life" to exist.
Therefore the question of the arrow of time seems to me to be explaining why
the macro environment looks that way.

I'm not sure where I heard it, but one idea that I like is that it ties to the
Big Bang and the expansion of the universe. Most people don't realize how
complicated conservation laws get in general relativity. For instance it is a
matter of opinion whether or not energy is conserved! (See
[http://www.math.ucr.edu/home/baez/physics/Relativity/GR/ener...](http://www.math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)
for details.) In particular suppose a photon sets out on a long journey, gets
red shifted by the expansion of the universe, and consequently loses energy.
What happened to the energy of the photon?

This question is highly relevant to the issue of entropy. Current theory says
that we had a very hot and almost uniform universe, which then expanded
rapidly. As it expanded any clumps that formed stayed hot, while the space
between them cooled off, thereby leading to an increased thermodynamic non-
equilibrium. Worse yet, local gravity pulled the clumps together resulting in
interesting things like stars, which further increased the amount of non-
equilibrium stuff going on.

Of course this pushes the question of the arrow of the time down to cosmology
and general relativity. But given that physicists debate whether conservation
of energy makes sense in general relativity, I'm fairly comfortable with
accepting that the laws of thermodynamics are on even shakier ground there.
Which makes the physical asymmetry seem not so bad to me.

------
mechanical_fish
I'm not convinced that this is a really _great_ article. But it may be as good
as it's reasonable to expect a one-page popular article on this subject to be.

It is incredibly difficult to write an article like this. You can't assume
that your audience knows what entanglement is. You can't assume that they know
what entropy is. And even if you thoroughly understand those two things it
isn't exactly clear what this research is trying to say. It reads like
something that is being badly translated out of the original mathematics.

------
Tichy
Isn't memory just a couple of changed atoms? So erasing memory would be
equivalent to making atom configurations unchanged. But if atom configurations
don't change, entropy doesn't change. So the whole theory seems absurd.

(Disclaimer: I don't really know QM very well)

------
newsdog
You mean ARE quantum mechanics messing with your memory, and those quantum
mechanics should go back to fixing quantum cars!

------
Confusion
_When you observe any system, according to Maccone, you enter into a "quantum
entanglement" with it._

That's a nice theoretical claim, but in practice it is just plain false. Two
macroscopic objects, like a human and a cup of coffee, at room temperature,
entangled? Give me a break. We have to go trough a hell of a lot of trouble to
create entangled pairs of photons to do experiments with. Interactions with
the environment break the entanglement, which is exactly what we are doing all
the time.

 _Maccone's solution is to suggest that in fact entropy-decreasing events
occur all the time_

Of course they do. The point is those fluctuation in the 'wrong' direction
last an extremely short period of time. We have no trouble observing temporary
decreases in entropy: the 2nd law of thermodynamics holds over macroscopic
periods of time and that has been verified experimentally. Laplace's daemon
doesn't exist, but he can deceive you for a nanosecond.

The problem with these damned brilliant mathematician-physicists is that they
really loose touch with the boundary conditions imposed by physical reality.

~~~
hughprime
_That's a nice theoretical claim, but in practice it is just plain false. Two
macroscopic objects, like a human and a cup of coffee, at room temperature,
entangled? Give me a break._

That idea is pretty common in (so-called) many-worlds interpretations. In such
interpretations, the wavefunction never collapses: the "collapse" of the
wavefunction is just the observer becoming entangled with the system. This, of
course, has all sorts of disturbing implications, but arguably still makes a
lot more sense than any other interpretation.

------
rfreytag
(I realize this article is very light but its is an interesting point of
departure...)

So are all the 'positive thinkers' correct? Is life but a giant breadth-first
search where we strive for some optimum to only remember the last path we
took?

I'd be willing to give up some memories to bring Dad back.

[Pathos apparently gets downmodded - interesting].

~~~
sp332
The answer is no.

The downmodding is, I think, because people are sick of metaphysical
misinterpretations of QM.

