Time asymmetry. was: This Week's Finds (Week 26)
john baez
In article <CH2vu...@cantua.canterbury.ac.nz>
w...@math.canterbury.ac.nz (Bill Taylor) writes:
>>Why is the future different from the past?
>John gave a nice account of Boltzmann's attempted explanation, noting it was..
>>a rather desperate solution to the problem of temporal asymmetry
>...and suggesting that it is a..
>>great achievement of modern cosmology has been to offer us an alternative
Both these remarks were actually made by Price, in the paper I was
quoting. But anyway, I agree.
>Maybe so. But it seems a long way to go when there may be a quicker route.
>Is it *really* believable that the early (or final) conditions of the
>universe explain why rocks never hop up out of mud pools? (Apart of course
>from the fact that strict initial conditions may be needed to ensure that
>the rocks and pools are there at all.) But given rocks and pools, it's hard
>to credit that early cosmological conditions have anything further to do with
>them.
Given rocks, pools, and the like... yes. But if you ponder it a moment
you'll see that the key thing is the sunlight and the dark of night that
keep the earth in its far-from-equilibrium state, which is a
prerequisite for interesting things to happen like rocks falling into
pools. Now ask why the sun emits light rather than absorbing it and you
are smack dab into cosmology! I really urge everyone to read Zeh's
book,
The Physical Basis of the Direction of Time, by H. D. Zeh, Second
Edition, Springer-Verlag, 1992. ISBN 3-540-54884-X or 0-387-54884-X
for an expanded and much clearer version of what I just said.
>To the naive observer like me it seems "a folly without warrant", almost.
>So why do rocks not fly up ? The 2nd law of thermodynamics is often invoked,
>and this is obviously connected, but it strikes me as being unsatisfactory
>as an *explanation*. Rather more, it is just another way of describing
>irreversibility in a certain context. To use it as an explanation is (IMHO)
>what John has dubbed "downright circular".
>
>So leaving aside the macro-statistical, and the cosmological, where do we
>look to deal with the nasty fact that..
>
>>temporal asymmetry is not explicable ... by a time-symmetric physics.
>
>This is the key, that gets the knickers in a twist. Physics is said to be
>"time-symmetric". The equations of physics, whether Newton's, or Maxwell's,
>or Schrodinger's, or whatever, are always noted to be time-symmetric; and
>this is no doubt true. But I would humbly submit that the equations of
>physics are NOT ALL of physics. There IS more to physics than just the
>equations, and the extra is clearly *not* time-symmetric. We don't need
>any fancy new mystical ideas here, just standard boring old college physics
>and its QM underpinnings.
>
>Irreversibility may not be apparent in the various standard equations of
>physics, but it is surely due to the universality of "friction", or less
>colloquially, *damping* or hysteresis. Friction or damping is known to us all,
>but seemingly does not arise from the time-symmetric laws mentioned above.
>And yet friction/damping is clearly an irreversible process, or rather
>ensures that many other processes *will* be irreversible.
>So one is forced to conclude that the equations mentioned above are not
>"all" of physics, in some sense. And surely isn't this already
>well-known?
For better or worse, friction is well-known to be a consequence of the
time-symmetric laws of physics, *given* appropriate initial conditions.
Again try Zeh for details.
>"Friction" of a sort is already present in the microscopic events of QM.
Hmm, I don't like the sound of that.
>The standard interpretation of QM provides for this fact, surely ? Rocks
>don't pop up, because if we stopped the universe(!!) shortly after one had
>flopped down, and reversed every particle/photon's motion, (oh well it's
>only a thought experiment), we would *not* see the exact reversal. Sure, it
>would start out exactly reversed, but then little errors would creep into
>the scenario, as damping and friction took their inevitable toll.
Hmm. Are you arguing that the laws of physics are only approximately
time-reversible?? Now in fact this is true due to the time-symmetry
violation of the weak interaction but I have a feeling that is NOT what
you are referring to. You almost seem to say that there are some fundamental
"damping and friction" terms in the laws of physics. That is certainly
not common knowledge. But below you suggest that what's really to blame is
the probabilistic nature of quantum theory....
>If we
>stopped things while the rock was falling, then reversed, we might just
>see it go back up on its ledge; but if we let it hit the swamp and let the
>waves die down, then reversed things, no way could it get back up.
>
>Why ? Because of this basic quantum irreversibility. It happens virtually
>any time there is degradation due to heat radiation, and elsewhere too.
>The rock falls; the energy is converted to sound and mud waves, these die
>away into heat, which is radiated away. Heat ? The thermal motion of the
>molecules is reduced by knocking together; these knocks occasionally send
>an electron up to a higher orbit (reducing the thermal recoil); the electron
>soon drops down again; releasing a photon. Now we reverse all this. The
>photon comes in... knocks the electron back up.... NO IT DOESN'T ! This is
>already a critical point. There is *no* guarantee that the photon coming
>back exactly to where it was created, would be absorbed by the electron
>(now with the opposite momentum). It might be, but most likely not. These
>interactions have an irreducible probabilistic component, as I understand it,
>which is not intrinsically part of Schrodinger's equation, but just derived
>from it via the amplitude. This real amplitude of the complex wave function
>in some way produces time asymmetry, even though the function itself
>satifies the time-symmetric equation.
>
>So anyway; most of these "reversed" photons will miss their "proper" electrons;
>though soon hit something else of course - the mud would re-heat a little,
>but would not regather a converging wave to hurl out the rock.
>
>So there is my naive view, at overly boring length. It is these micro-events
>everywhere, all the time, that produce irreversibility, that produces the
>damping and the friction. The equations governing the events are symmetric,
>but the quantum uncertainties involved ensure that the events themselves
>will not (usually) reverse exactly. And so we get damping, cooling,
>thermodynamics, permanent records, memories, and all the other macroscopic
>paraphernalia of an irreversible universe.
>
>So my puzzlement at why physicists so often get their knickers in a twist
>comes down to this - why is it not clear that the equations of physics
>are not all the physics "there is" ? That something else, essentially
>irreversible, (unlike Newton/Maxwell/Schrodinger), is also needed, and
>that we already know what it is - essential quantum uncertainty.
>Simply put; physics is *not* time-symmetric.
Basically, this is what you should ponder. You present an argument why
it is very unlikely for a rock to jump out of the water: even given the
*exactly* correct initial conditions, quantum effects might be expected
to make it very improbable for the vibrating muck's energy to concentrate in
such a way that the rock pops up. But: Given that the equations of QM
are time-symmetric (apart from the weak force), the exact same argument
can be used to argue that it is extremely unlikely for a rock to fall
into the water!!! (Just make the substitution t -> -t.)
The only ways out of this are to assume that something about quantum
indeterminism picks out a preferred direction of time, or that something
about the *state* of the universe is time-asymmetric.
Try Chapter 4 of Zeh, "The quantum mechanical arrow of time." Quoting a
wee bit, "The quantum mechanical probability interpretation contains an
indeterminism of controversial origin. Most physicists seem to consider
it as representing a "real" or "fundamental" (dynamical) indeterminism,
and some of them even as the ultimate reason for the thermodynamical
arrow of time." *Some* of them, indeed, think that the "collapse of the
wavepacket" is the root of the arrow of time. But as Price's remarks
suggest, these folks are probably not in the majority. Zeh's detailed
analysis should make this clear why.
Archimedes Plutonium
The difference for me in writing a textbook on physics rather than mathematics or chemistry or biology, is that I have to invent new concepts, not there before. For example, I replace the entire 4 Maxwell Equations with 6 Equations of AP - EM theory, and making 6 laws. And reinventing the Electromagnetic Spectrum. Of course in AP physics, a atom has no nucleus for the inside of an atom consists of true electrons as muons thrusting through a proton torus tube producing Dirac magnetic monopoles by converting Space into electricity. All of the Maxwell equations are wrong in some aspect, worst of all is the Gauss's law of no magnetic monopole, that is the worst of them. And of course the particle that JJ Thomson found in 1897 was not the atom's electron, but was what Dirac was searching for by the 1930s-- the magnetic monopole as 0.5MeV particle.
So, well, in writing my TEACHING TRUE PHYSICS, 1st year College, I have to produce an entirely different book of teaching physics, one that no one who took College Physics in the past can recognize.
I have made it student friendly by making mathematics ultra ultra easy. And it is the shameful mathematics of Old Physics that has turned many students off of and away from physics.
As promised, the world needs the successor textbook of Physics since the Feynman Lectures of the 1960s, and that is what AP's textbook promises. The New Age physics textbook that will carry physics forward for the next 30 years, perhaps 1,000 years if fate is kind to me.
AP
King of Science, especially Physics