Re: Causality
On Saturday, July 13, 2002, at 03:33 PM, Hal Finney wrote: Causality is a fascinating topic and one I hope to learn more about, perhaps via the Pearl book Tim mentioned or another that someone might recommend. Here are some very random and disorganized thoughts on the topic. I may order the Jim Joyce book that Wei Dai is reading...the material overlaps a lot of the Pearl material, based on the Amazon summary and sample pages, but it never hurts to have two views of the same material. BTW, Pearl has a discusson of Newcomb's Paradox and Joyce even devotes an entire chapter to it (based on the TOC Amazon shows). (I haven't commented on Newcomb's Paradox. I read Martin Gardner's column on Robert Nozick's treatment of NP when it first came out, around 1973. Fascinating then, fascinating now. Bu there are a thousand views of Newcomb's Paradox in the naked city, to coin a phrase.) Googling on causality led to a brief web page that summarized 2000 years of philosophical thought in a few paragraphs, http://www.helsinki.fi/~mqsalo/documents/causalit.htm. Generally there seem to be two schools of thought; one is that causality is an artifact of our minds' efforts to understand and interpret the world; the other is that causality has objective reality. One reason I like the recent scientific papers on causality, light cones, universes, toposes, etc. is to move beyond the b.s. college bull sessions about determinacy and suchlike. The modern philosopher's definition of causality has always struck me as somewhat backwards: The cause of any event is a preceding event without which the event in question would not have occurred. This means that A causes B is equivalent to A happening before B, plus the logical proposition, if B then A. You'd think that A causes B would mean if A then B. But philosophers interpret it to mean if not-A then not-B, as stated above (I've read this elsewhere also), which is equivalent to if B then A. That's the contrapositive, always true as stated. (If A then B is the same as if Not B then Not A.) So if we say heavy rains cause flooding we mean that if there were no heavy rains then there would be no flooding; or equivalently, if there is flooding then there must have been heavy rains. This is consistent with there being heavy rains and no flooding. It really doesn't make sense to me. Pearl spends a lot of time discussing these sorts of issues. I regret that I can't summarize in a few paragraphs what it clearly takes Pearl much discussion to explicate. The problem with judging causality is the well known cautionary rule that correlation is not causation. Just because A and B always occur together, and A comes before B, that doesn't mean that A causes B. This is a common danger that scientific researchers have to guard against. It's easy to observe correlation but hard to determine what is the true cause. This is a reason to view causality as just a matter of our perceptions. I don't think so. Causality is a lot more than just a matter of our perceptions. A strange aspect of causality is that to a large degree our laws of physics appear to be time symmetric. This means that you can reverse the time sequence of events and get a physically valid (although possibly unlikely) scenario. If that is so, then if A causes B in the forward direction, can we with equal validity say that B causes A when we look at things backwards? In that case the causal roles are fundamentally arbitrary. Only in fairly simple mechanical situations. A two urn experiment with one urn filled with white balls and the other urn filled with black balls is the classic illustration of increasing entropy expressed in terms of more ways to have things mixed than to have things all of one color. Locally, with just a couple of balls, the process is largely reversible. But not globally. A large literature on why time reversal is meaningful locally, but not globally. Usual example of gas expanding from a cylinder to fill a room versus reversed image of gas moving back into cylinder. (A couple of books: The Physics of Time Asymmetry, P.C.W. Davies, 1974, 1977, and Asymmetries in Time, Paul Horwich, 1987.) Of course in most situations we have a strong arrow of time based on the growth of entropy which allows us to break this apparent symmetry. But this is not always the case. Systems in thermodynamic equilibrium, if considered in isolation, have no arrow of time. Yes, they look like random things bouncing around, which is what they are. Most interesting things involve temperature differences, energy inputs, life surrounded by nonlife, and such. I don't claim to know the nature of time, or what produces the arrow of time. It's not a resolved question. Several of the physics of information conference papers, especially ones by W. Zurek and Charles Bennett, are useful. (But, before I tackle the issue at a deeper level,
Re: Copenhagen interpretation
MWI is a fully deterministic theory, but it is not the only deterministic theory consistent with QM. I believe that 't Hooft's theory is more natural from the point of view that universes are programs. It is hard for me to understand how you get interference between ``nearby´´ universes or programs. According to 't Hooft QM would arise in a ``single universe´´ References: [1] Quantum Gravity as a Dissipative Deterministic System http://arxiv.org/abs/gr-qc/9903084 [2] Determinism in Free Bosons http://arxiv.org/abs/hep-th/0104080 [3] How Does God Play Dice? (Pre-)Determinism at the Planck Scale http://arxiv.org/abs/hep-th/0104219 [4] Quantum Mechanics and Determinism http://arxiv.org/abs/hep-th/0105105 - Oorspronkelijk bericht - Van: Bruno Marchal [EMAIL PROTECTED] Aan: [EMAIL PROTECTED] Verzonden: vrijdag 12 juli 2002 17:30 Onderwerp: Copenhagen interpretation Saibal wrote: This all assumes that photons, electrons, etc. are real. We don't know that. If you were Einstein, and you were faced with Bell's result, you could have concluded that the nonexistence of local hidden variables implies that elementary paricles don't exist. They are mere mathematical tools to compute the outcome of experiments. The real underlying theory of Nature could be still be deterministic. Recently 't Hooft has shown how QM can emerge out of a deterministic theory. In this case QM has to be interpreted according to the Copenhagen interpretation. But QM-without-collapse *is* a fully deterministic theory. In QM-without-collapse the indeterminism is a first person indeterminism quite comparable to what appears in classical self-duplication process (if I may repeat myself). It seems to me that 't Hooft theory is very speculative, but then I am not sure I fully understand it, for sure. Bruno Yahoo! Groups Sponsor -~-- Save on REALTOR Fees http://us.click.yahoo.com/Xw80LD/h1ZEAA/Ey.GAA/pyIolB/TM -~- Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
Re: Copenhagen interpretation
Gordon wrote: Saibal Mitra wrote: This all assumes that photons, electrons, etc. are real. We don't know that. If you were Einstein, and you were faced with Bell's result, you could have concluded that the nonexistence of local hidden variables implies that elementary paricles don't exist. [Gordon] I dont know how you come to that? Very simply, if Nature is described by a single universe local classical deterministic theory and if local hidden variables are excluded experimentally, the only way out is that the objects to which the hidden variables are supposed to be associated with, don't exist. [Saibal] They are mere mathematical tools to compute the outcome of experiments. The real underlying theory of Nature could be still be deterministic. Recently 't Hooft has shown how QM can emerge out of a deterministic theory. In this case QM has to be interpreted according to the Copenhagen interpretation. [Gordon] Why, if anything it would be closer to Bohm(1952 Mech version) or MWI(1957 version) than saying than CI,In fact HV say that they is something beneath lower sub levels where CI dont,or aleast can explain them ! Funny thing is that 't Hooft and some other who first laught about Ontological theory are now look at them for answers, however the one they found are too simple and may have more problem when takening it onto a broader view? I would prefer to look further into Bohm/Hiley or Deutsch/Dewitt area myself! I never studied Bohm theory in any detail. From what I know it makes the same predictions for experimental outcomes as ordinary quantum mechanics. According to 't Hooft's theory QM would have to break down at the planck scale. Also, there would be a limit on the performance of quantum computers. Saibal
Re: Is Reality as function of Reference Frame?
Dear Saibal, The idea that photons, electrons, etc. are real or not might make sense if we consider the role of virtual particle/wave fluctuations involved in the Hawking Black Hole evaporation process and the Unruh effect. From what I have read (cf. Kip S. Thorne, et al), it seems that the reality (or unreality or virtuosity) of particles/waves depends on the reference frame of the observer - inertial (free- falling) or non-inertial (accelerated), etc. Perhaps consideration of the context of observation might help us get past this difficulty. Would you happen to know the reference to 't Hooft's paper? Kindest regards, Stephen - Original Message - From: Saibal Mitra [EMAIL PROTECTED] To: Bruno Marchal [EMAIL PROTECTED]; scerir [EMAIL PROTECTED] Cc: FoR [EMAIL PROTECTED]; [EMAIL PROTECTED] Sent: Friday, July 12, 2002 8:11 AM Subject: Copenhagen interpretation This all assumes that photons, electrons, etc. are real. We don't know that. If you were Einstein, and you were faced with Bell's result, you could have concluded that the nonexistence of local hidden variables implies that elementary paricles don't exist. They are mere mathematical tools to compute the outcome of experiments. The real underlying theory of Nature could be still be deterministic. Recently 't Hooft has shown how QM can emerge out of a deterministic theory. In this case QM has to be interpreted according to the Copenhagen interpretation.
Re: Causality
Tim May writes: One reason I like the recent scientific papers on causality, light cones, universes, toposes, etc. is to move beyond the b.s. college bull sessions about determinacy and suchlike. Yes, there is a lot to be said for this perspective. In some cases, science is able to find concrete and testable answers to questions which have long been in the realm of philosophy. After all, science was originally called natural philosophy. A large literature on why time reversal is meaningful locally, but not globally. Usual example of gas expanding from a cylinder to fill a room versus reversed image of gas moving back into cylinder. (A couple of books: The Physics of Time Asymmetry, P.C.W. Davies, 1974, 1977, and Asymmetries in Time, Paul Horwich, 1987.) Another good book on this topic, more philosophically oriented, is Huw Price's 'Time's Arrow and Archimedes's Point'. He has a web page at http://www.usyd.edu.au/philosophy/price/TAAP.html which has a chapter from the book and some reviews. This book actually made a good case for a notion which I had always thought to be absurd, namely that if the universe's expansion were to reverse and become a contraction leading up to a big crunch, time might reverse in the contracting phase. Hal Finney
