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The Big Picture

Page 19

by Carroll, Sean M.

as the “crowning edifice” of this hierarchy. Subsequently, the daz-

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  zling success of physics at describing the microscopic world has flipped

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  things around in some people’s minds; they prefer to focus on the deepest,

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  most fundamental way of talking about reality. Ernest Rutherford, a New

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  Zealand– born experimental physicist who was as responsible as anyone for

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  discovering the structure of the atom, once remarked that “all of science is

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  either physics or stamp collecting.” It should come as no surprise that scien-

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  tists who are not physicists— the very large majority of scientists, in other

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  words— would beg to differ.

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  From the point of view of emergence, the question becomes: how new

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  and different are emergent phenomena? Is an emergent theory just a way of

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  repackaging the microscopic theory, or is it something truly novel? For that

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  matter, is the behavior of the emergent theory derivable, even in principle,

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  from the microscopic description, or does the underlying stuff literally act

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  differently in the macroscopic context? A more provocative way of putting

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  the same questions would be: are emergent phenomena real, or merely il-

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  lusory?

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  As you might imagine, these questions lie front and center when we

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  start talking about knotty issues such as the emergence of consciousness or

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  free will. Sure, you think you’re making a choice about whether to have that

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  last slice of pizza or virtuously resist the temptation, but are you sure you

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  really are? If the underlying laws of nature are deterministic, then isn’t your

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  volition simply an illusion?

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  But the independent reality of emergent phenomena is an important

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  issue even when we stick to physics. Philip Anderson won the Nobel Prize

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  in Physics in 1977 for his work on the electronic properties of materials. He

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  is a “condensed matter” physicist— someone who thinks about materials,

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  fluids, or other macroscopically tangible forms of matter here on Earth, as

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  opposed to an astrophysicist, atomic physicist, or particle physicist. In the

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  1990s, when the US Congress was contemplating the fate of the Supercon-

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  ducting Super Collider particle accelerator, Anderson was called to testify

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  as an expert in physics who was not directly involved in particle physics. He

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  told the committee that the new machine would doubtless do good work,

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  but any discoveries it would make would be utterly irrelevant to his own

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  research. That was honest, and accurate, if a bit frustrating to the particle

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  physicists who hoped the whole field would present a unified front. (Con-

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  gress canceled the SSC in 1993; a competing machine, the Large Hadron

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  Collider, was built in Europe, and went on to discover the Higgs boson

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  in 2012.)

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  Anderson’s comments were based on the fact that an emergent theory

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  can be completely independent of more fine- grained comprehensive de-

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  scriptions of the same system. The emergent theory is autonomous (it works

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  by itself, without reference to other theories) and multiply realizable (many

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  microscopic theories can lead to the same emergent behavior).

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  Anderson would be interested in questions about, for example, how cur-

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  rent flows through a particular kind of ceramic. We know that the material

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  is made of atoms, and we know the rules by which electricity and magne-

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  tism interact with those atoms. For the questions Anderson cares about,

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  that’s all we need to know. We can think of the theory of atoms, electrons,

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  and their interactions as the emergent theory, and anything more fine-

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  grained than that as a microscopic theory. The emergent theory has its own

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  rules, independent of any purported lower levels. And it may very well be

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  multiply realizable. Anderson doesn’t need to worry about the quarks zip-

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  ping about inside an atomic nucleus, or about the Higgs boson itself, and

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  certainly not about superstring theory or anything that tries to give a more

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  comprehensive microscopic description of matter. (For much of his work,

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  W h At E x I S t S , A n d W h At I S I l l u S IO n ?

  he doesn’t even need to know about atoms, as he is working at an even

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  higher level of coarse- graining.)

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  Given this situation, condensed- matter physicists have long argued that

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  we should think of emergent phenomena as truly new, not “merely”

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  smeared- out versions of some deeper description. In 1972 Anderson pub-

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  lished an influential article entitled “More Is Different,” arguing that every

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  one of the multiple overlapping stories we can tell about nature deserves to

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  be studied and appreciated for its own sake, rather than focusing primarily

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  on the most fundamental level. He has a point. A famous problem in

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  condensed- matter physics is to find a successful theory of high- temperature

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  superconductors, materials through which electrical current can flow with-

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  out resistance. Everyone working on the problem believes that such materi-

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  als are made out of ordinary atoms, obeying the ordinary microscopic rules;

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  knowing that has been of essentially zero help in guiding us toward an

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  understanding of why high- temperature superconductivity happens at all.

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  There are several different questions here, which are related to one another

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  but logically distinct.

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  1. Are the most fine- grained (microscopic, comprehensive)

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  stories the most interesting or important ones?

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  2. As a research program, is the best way to understand mac-

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  roscopic phenomena to first understand microscopic phe-

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  nomena, and then derive the emergent
description?

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  3. Is there something we learn by studying the emergent level

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  that we could not understand by studying the microscopic

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  level, even if we were as smart as Laplace’s Demon?

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  4. Is behavior at the macroscopic level incompatible— literally

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  inconsistent with— how we would expect the system to be-

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  have if we knew only the microscopic rules?

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  Regarding question 1, it’s obviously a subjective matter. If you’re inter-

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  ested in particle physics, and your friend is interested in biology, neither is

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  right or wrong; you’re just different. Question 2 is a bit more practical, and

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  the answer is fairly obvious: no. In almost all cases of interest, we might

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  learn a little bit about higher levels by studying lower ones, but we’ll learn

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  more (and more quickly) by studying those higher levels themselves.

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  It’s at question 3 where things become contentious. One point of view

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  would say: if we completely understand the microscopic level, which has a

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  domain of applicability that strictly contains that of the emergent theory,

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  we know everything there is to know. Whatever question you have could,

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  in principle, be translated into the microscopic language and answered

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  there.

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  But “in principle” covers a multitude of sins here, or at least one very big

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  sin. This perspective amounts to saying “You want to know if it will rain

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  tomorrow? Just tell me the position and velocity of all the molecules in the

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  Earth’s atmosphere, and I’ll get to calculating.” Not only is that wildly un-

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  realistic; it’s also ignoring the fact that the emergent theory describes true

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  features of the system that might be completely hidden from the micro-

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  scopic point of view. You might have a self- contained and comprehensive

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  theory of how things behave, but that doesn’t mean you know everything;

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  in particular, you don’t know all of the useful ways of talking about the

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  system. (Even if you know how every atom in a box of gas behaves, you

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  might be blind to the important fact that the system can also be described

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  as a fluid.) From that perspective— the correct one— we really do learn

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  something new by studying emergent theories for their own sakes, even if

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  all the theories are utterly compatible.

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  Then we have question 4, where all hell breaks loose.

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  •

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  We’re now entering into the realm known as strong emergence. So far we’ve

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  been discussing “weak emergence”: even if the emergent theory gives you

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  new understanding and an enormous increase in practicality in terms of

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  calculations, in principle you could put the microscopic theory on a com-

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  puter and simulate it, thereby finding out exactly how the system would

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  behave. In strong emergence— if such a thing actually exists— that wouldn’t

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  be possible. When many parts come together to make a whole, in this view,

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  not only should we be on the lookout for new knowledge in the form of bet-

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  ter ways to describe the system, but we should contemplate new behavior. In

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  strong emergence, the behavior of a system with many parts is not reducible

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  to the aggregate behavior of all those parts, even in principle.

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  The notion of strong emergence is a bit puzzling, on the face of it. It

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  starts by admitting that there is a sense in which a big macroscopic object,

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  such as a person, is made up of smaller constituents, such as atoms. (In

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  quantum mechanics, remember, this division into constituents isn’t always

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  possible, but that’s not the subtlety that strong emergentists usually have in

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  mind.) It further admits that there is a microscopic theory, one that will tell

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  you how an atom will behave in any particular circumstance. But then it

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  claims that there is an effect on that atom by the larger system of which it

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  is a part— an effect that cannot be thought of as arising from all of the other

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  atoms individually. The only way to think of it is as an effect of the whole

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  on the individual parts.

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  I can imagine focusing on one particular atom that currently resides as

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  part of the skin on the tip of my finger. Ordinarily, using the rules of atomic

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  physics, I would think that I could predict the behavior of that atom using

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  the laws of nature and some specification of the conditions in its

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  surroundings— the other atoms, the electric and magnetic fields, the force

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  due to gravity, and so on. A strong emergentist will say: No, you can’t do

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  that. That atom is part of you, a person, and you can’t predict the behavior

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  of that atom without understanding something about the bigger person-

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  system. Knowing about the atom and its surroundings is not enough.

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  That is certainly a way the world could work. If it’s how the world actually

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  does work, then our purported microscopic theory of the atom is simply

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  wrong. The nice thing about theories in physics is that they are very clear

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  about what information is needed to predict the behavior of an object, and

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  also clear about what the predicted behavior actually is. There’s no ambigu-

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  ity in what that atom is supposed to do, according to our best theory of

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  physics. If there are situations in which the atom behaves otherwise, such

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  as when it’s part of the tip of my finger, then our theory is wrong and we

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  have to do better.

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  Which is completely possible, of course. (Many things are possible.) In

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  chapters 22 to 24 we’ll dive more deeply into how our best theories of phys-

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  ics work, including the remarkably successful and unforgiving frame
work

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  of quantum field theory. Within quantum field theory, there is no way for

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  new forces or influences to play an important role in what atoms do in my

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  body— or, more precisely, all of the possible ways this could happen have

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  been ruled out by experiments. But it’s always conceivable that quantum

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  field theory itself is just wrong. There’s no evidence that it’s wrong, however,

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  and very powerful experimental and theoretical reasons to think it’s right,

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  within a very wide domain of applicability. So we’re allowed to contemplate

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  alterations in this basic paradigm of physics— but we should be aware of

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  how dramatically we are changing our best theories of the world, just in

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  order to account for a phenomenon (human behavior) that is manifestly

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  extremely complex and hard to understand.

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  •

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  We may or may not need to bite the bullet of strong emergence in order to

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  understand the relationship between the atoms of which we are made and

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  the consciousness we all experience. But it’s our duty to figure out how they

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  are related, given that both atoms and consciousness exist in the real world.

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  Or do they?

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  There is a continuum of possible stances toward the way that the differ-

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  ent stories of reality fit together, with “strong emergence” (all stories are

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  autonomous, even incompatible) on one end and “strong reductionism” (all

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  stories reduce to one fundamental one) on the other. A strong reductionist

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  would be someone who not only wants to relate macroscopic features of the

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  world to some underlying fundamental description but also wants to go

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  further by denying that elements of the emergent ontology even exist, under

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  some appropriate definition of “exist.” The real problem with consciousness,

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  according to this school of thought, would be that there’s no such thing.

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  Consciousness is merely an illusion; it doesn’t really exist. In the context of

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  philosophy of mind, this hard-core flavor of reductionism is known as elim-

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  inativism, since its proponents want to eliminate talk of mental states en-

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  tirely. (Naturally, there is a rich zoo of different types of eliminativism, each

 

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