“Let me allow myself at this point the luxury of expressing a strong personal opinion, which would certainly be contested vigorously by some of my colleagues. It seems to me that the many-worlds interpretation is nothing more than a verbal placebo, which gives the superficial impression of solving the problem at the cost of totally devaluing the concepts central to it, in particular, the concept of ‘reality’. When it is said that the ‘other worlds’ are ‘equally real’, it seems to me that the words have become uprooted from the context which defines their meaning and have been allowed to float freely in interstellar space, so to speak, quite literally meaningless. I believe that our descendants two hundred years from now will have difficulty understanding how a distinguished group of scientists of the late twentieth century, albeit still a minority, could ever for a moment have embraced a solution which is such manifest philosophical nonsense.”
September 13, 2010
A. J. Leggett disagrees with Eliezer Yudkowsky
Posted by teageegeepea under Uncategorized[15] Comments
Entitled to an Opinion 
September 14, 2010 at 12:22 am
Link please…
September 14, 2010 at 12:24 am
I also reject MWI but Leggett’s reason is a bad one. It seems to be founded on a confusion of epistemology with ontology. Arbitrary suppositions of unobserved entities are a bad thing, but if positing many worlds really were the best way to make sense of QM, then almost by definition, that would be the ontology to favor.
The fundamental problems of MWI are mildly technical, and involve relativity, probability, and need for a “preferred basis”. Taking these in reverse:
Preferred basis problem. Just as you can decompose an n-dimensional vector with respect to any of an infinite number of orthonormal bases, a quantum state vector in Hilbert space can be reexpressed as a superposition with respect to any of an infinity of possible sets of basis vectors. From a position of state-vector realism, Hilbert space should be the primary reality and wavefunctions which privilege positional observables are just one possible basis. Yet you need to pick a basis in order to extract a set of worlds from a state vector.
Probability problem. If you naively count the worlds postulated by MWI as all existing equally, the probabilities come out wrong. You need to ‘weight’ the worlds by a factor which in the Copenhagen interpretation is supplied by Born’s probability rule. But there is no endogenous derivation of this rule from MWI, although there have been several attempts to obtain it.
Relativity problem. I think this is the sharpest criticism from a physical perspective: the ‘worlds’ of MWI just aren’t relativistic objects. The lesson of special relativity is that there is no objective universal time, but the worlds of MWI only exist in a particular time-slice. They are only defined relative to a particular reference frame, which means that if those worlds are the fundamental reality, then that reference frame’s notion of time is ontologically preferred.
Purely theoretical progress in physics usually comes from a close attention to the technical frontier of the subject. The fact that MWI has hardly even engaged with QFT (the relativistic extension of QM) indicates that it’s a lagging and artificial construct. I would say that really arcane stuff like Gukov and Witten’s “Branes and quantization”, and Witten’s “Analytic continuation of Chern-Simons theory”, is where you will find the seeds of a new perspective on QM that will eventually flower into a new ontological interpretation. Those papers are motivated by an attempt to make modest incremental progress on pressing ‘technical’ questions of mathematical physics, like how to do the Chern-Simons path integral. I expect a new understanding of QM to come from there, and not from formulaic ideas like Bohm or Everett, and not from the musings of experimental physicists like Leggett (or Zeilinger, to name one other among many).
September 14, 2010 at 9:19 pm
Kevembuanga, it’s from a book. “The Problems of Physics”. No page to link to.
Mitchell, have you read Barbour’s “The End of Time”? That seemed to me like a way to resolve MWI with issues of time. But I don’t know much about relativity and only took a half-semester of quantum.
The familiar forces of gravity & electromagnetism have attractive or repulsive forces, and do not involve the swapping of states. The color interaction causes quarks to exchange colors through the transmission of colorful bosons. The actual strong force between nucleons is said to be just a manifestation of that color force, like the static force between objects resulting from the electromagnetic force. The weak force can involve both the swapping of flavor/charge through charged W/X bosons or simple scattering of neutrinos through neutral Z bosons. Is there a simple explanation of why some of the forces act like classical forces and others cause the swapping of states?
September 15, 2010 at 12:47 am
Electromagnetism is the same sort of interaction as weak and strong forces, except there’s only one “state”. You may have read that their symmetry groups are respectively U(1), SU(2), SU(3). SU(n) is the group of nxn complex matrices that are unitary and have unit determinant. Each such matrix describes a “rotation” in n-dimensional complex vector space – it preserves “length” (vector norm) and “angles” (inner product of two vectors).
Consider an up quark. You can think of it as having a color, red, green, or blue. But you can also use the superposition principle and think of it as being in a quantum state c_red |red> + c_green |green> + c_blue |blue>. The c’s are the complex coefficients and together they are a three-component vector. Now consider a “field” which is not vector-valued, but matrix-valued at every point, and the matrix is 3×3 and the matrix elements are complex numbers. This is the gluon field and the matrix elements are (reading left to right, top to bottom), red-antired, red-antigreen, red-antiblue, green-antired, green-antigreen, green-antiblue, blue-antired, blue-antigreen, blue-antiblue. The logic of this is that if the quark was in a pure red state (1,0,0), and it was acted on by the pure red-antiblue matrix (I won’t write out the matrix elements), the quark would acquire the pure blue state (0,1,0), just by standard linear algebra.
So if we write down field equations, we’re going to have a kinetic term which describes how the quark color vector evolves, a kinetic term which describes how the gluon color matrix evolves, and an interaction term which describes how quark and gluon interact. Actually it’s more complicated, because the gluon matrix field self-interacts – there are only eight independent degrees of freedom, because the matrix has to be unitary (in order to preserve “lengths” in complex color space, analogous to the orthogonal matrices which implement rotations in real space); the gluon components aren’t completely free to vary independently of each other.
To sum up, what you think of as a change of state in the quark – red turning blue – can also be thought of as a change in its ‘color vector’. And the ‘swapping of states’ just comes from the conservation laws of gluon evolution – that gluon can only be absorbed by a blue quark turning red.
You can think the same way about the weak force, only now there are two states. There are many particles which interact weakly, but they all come in pairs, e.g. up quark and down quark, or electron and neutrino. So instead of thinking of up and down as different particles, you could think of them as different states of a single entity, the “first-generation quark”, with a state c_up |up> + c_down |down>. The weak field is again a matrix – this time from SU(2), the group of matrices which “rotate” two-dimensional complex vectors – and unitarity means there are only three independent degrees of freedom, which correspond to W+, W-, and Z. (Sort of. It’s more complicated because those are massive particles, and they’re actually combinations of the “primordial” SU(2)-valued field and the Higgs field. But this is why you have three W/Z particles, rather than some other number – because they are implementing rotations in a two-dimensional space.)
When we get to electromagnetism, we have a one-component vector and a 1×1 matrix. That is, the electron is not part of a larger “multiplet” which is transformed by its interaction with the electromagnetic potential. That potential – which in its quantum form is described by a single complex number, the 1×1 matrix – simply rotates a single complex coefficient, the complex phase of the electron wavefunction.
I’ve been imprecise in a few places, but hopefully you’ll get the gist of what I’m saying.
Gravitation, as I said before, is a different sort of force, because it acts on a space-time vector (energy-momentum) rather than on a vector of abstract states, and that gives the resulting field theory a different structure. Though I should note that in higher-dimensional theories like string theory, the “abstract vector” (like the color vector) does correspond to geometric properties of the extra dimensions, so there is a sense in which, ultimately, the gauge theories are unified with gravity.
September 15, 2010 at 1:04 am
As for Barbour, if you look in his book, at the end of Chapter 9, he says that the old view in relativity (the view of Einstein and Minkowski) was that space-time is made of events at points, but that instead we should think of it as made of “extended Nows” – what we would call, in the usual terminology, a spacelike hypersurface. This is where he goes wrong – he wants to think these hypersurfaces are fundamental, rather than the events which compose them. Classically, while a space-time has a unique decomposition into a set of point-events, ordered by their causal or metrical relationships, there are an infinite number of ways to slice it into equal-time hypersurfaces corresponding to a choice of inertial reference frame. Barbour wants to treat these hypersurfaces as all equally real and as independently existing, even though it’s the same constituent point-events being recycled in the different slicings.
This approach to quantum gravity isn’t at all Barbour’s invention, as his book makes clear, but he takes the explicit step of reifying the hypersurfaces and saying “they’re all real”. I don’t rule out that some version of this formalism is physically valid, but this interpretation of it can’t be right. Intuitively, as I said earlier, anything which says that frame-dependent hypersurfaces are the ultimate reality is at odds with the spirit of relativity, and the fact that the same point-events show up in different hypersurfaces is a tip-off that it’s wrong to treat them as independently existing entities.
September 15, 2010 at 11:50 am
Time doesn’t really exist, it’s a mental interpretation of things moving around. Gravity is an illusion as well. What appears to be an independent force is really a return to form, as in a stretched spring recoiling.
September 15, 2010 at 9:21 pm
Very informative, as always, Mitchell. And you’re not even getting paid or attractive readers to your own blog for this.
Carter: The non-existence of time is Julian Barbour’s argument in “The End of Time”. According to general relativity the gravitational force is supposed to be the result of the curvature of space-time and indistinguishable from acceleration. A recoiling spring is subject to tension forces, but both it and gravity are useful examples when discussing potential energy.
September 16, 2010 at 11:56 am
I’ve never heard of this Barbour fellow, the next time you see him tell him to quit stealing my ideas.
Regarding gravity, I was referring to the views of Dr. Erik Verlinde.
September 16, 2010 at 6:31 pm
Barbour is not just denying the existence of “time”, he’s denying the existence of change. It’s a type of Parmenidean occasionalism in which all that exists are “time capsules”, an infinity of platonically disconnected, static arrangements of matter in space, some of which contain conscious beings also transfixed in an illusion of change, memory, and continuity of flow. You may think you are reading and have been reading this paragraph and this sentence, but according to Barbour, that’s an illusion. The you reading this sentence is not the you reading the first sentence… It’s difficult to convey just how false to phenomena this view is. Yet apparently it holds an appeal for people who are good at thinking in terms of mathematics and other abstractly objectifying frameworks.
Verlinde’s idea descends from various forms of emergence in string theory, such as the AdS/CFT duality and the matrix model of M theory. But his idea suffers from an elementary difficulty. More recently (see second half of the talk) he’s trying to base his idea on something other than entropy.
September 30, 2010 at 4:02 am
I’m as impressed as TGGP. What’s your day job again Mitchell Porter? I aspire to be as fluent in the narrow expertise of maximizing my persistence odds as you appear to be in theoretical physics (cosmology?) among other topics.
September 30, 2010 at 10:50 pm
He had a top-level post at LessWrong with a bit about himself:
http://lesswrong.com/lw/2qo/lets_make_a_deal/
He sometimes blogs at climateundercontrol.wordpress.com
October 5, 2010 at 3:47 pm
Interesting. Mitchel Porter reminds me of a less successfully precious Eliezer Yudkowsky, even though he may be a more good faith thinker on the FAI problem.
I hope the bleg works out for him -he reminds me of the general problem of wasted/unexploited talent.
Relatedly, he reminds me of the problem of high IQ people with impulse control disorders or conscientiousness deficiencies like the notorious example of Elliot Spitzer.
Personally, I’m concerned with my own impulse control but the evidence on paper (credit score, work product evaluations, BMI) is that my impulse control and conscientiousness are deviantly high. Nevertheless I struggle daily with mundane things like diet, spending, and the more mundane aspects of work and life management (not to mention longer term persistence optimization).
October 5, 2010 at 7:44 pm
I’m also skinny and careful with my money (the downside is I have no credit history). But I’m prone to procrastination & laziness. I suppose there are more productive things I could be doing, but I don’t feel much obligation to be more productive. I’m very easily contented.
The fact that you “struggle” may not say much, as I presume its normal to do so. The unusually conscientious may be more aware and concerned with shortcomings of willpower. The religious tend to score higher in conscientiousness and also always ask God to strengthen that aspect of themselves.
I’ve never heard the phrase “Successfully precious” before. Googling results in mostly false positives. I’m reminded of an old Something Awful bit from when they seemed to be tiring of their own joke.
October 6, 2010 at 2:37 am
I think the term “precious” entered my microsocial analysis lexicon with the movie “Precious”, which I think was brilliantly titled. Previously I thought of it in terms of telescopic philanthropy, pets, and “babies”, but I think precious may be a superior descriptor for those whose value is derived from the emotional enjoyment they offer more powerful parties or audiences.
BTW I haven’t seen the movie yet, but it does seem to me to be a telescopic philanthropy positioning (for fat black women).
October 7, 2010 at 10:44 pm
Chip Smith declares “Precious” to be camp here. His review of Inglourious Basterds put it in the grindhouse/exploitation horror genre.