interpretationauthor(s)deterministic? wavefunction real? unique history? hidden variables?...
TRANSCRIPT
Quantum Mechanicsand Scientific Realism
J. Blackmon
Why Quantum Mechanics is Weird
Things you may have heard about:•Schrödinger’s Cat•The Double-Slit Experiment•Heisenberg's Uncertainty Principle•Quantum Entanglement
We will see the Two Paths Experiment, as presented by David Z. Albert.
Quantum Mechanics and Experience
Two Paths Experiment
Initial Observations•Color measurements randomize hardness.•Hardness measurements randomize color.• In two successive color measurements, the second always agrees with the first.• In two successive hardness measurements, the second always agrees with the first.
Given these initial observations,• it appears we cannot discover both color and hardness properties at once. •Once we’ve measured one property, measuring the other “re-sets” the former, effectively erasing our first piece of information.
Two Paths Experiment
New Set-Up (See the board.)•Only green electrons go into a hardness box.•The h and s paths converge and enter the color box where color measurements are made.
Expectations•You might expect the green electrons fed into this new set-up to emerge from the color box 50% red and 50% green.
Observations•However, what happens is that 100% of the electrons emerge green!
Two Paths Experiment
New Set-Up (See the board.)•Only green electrons go into a hardness box.•The h and s paths converge and enter the color box where color measurements are made.
One Interpretation• If all electrons fed in are green, and all emerge green, then all electrons, hard or soft, are green.•But we don’t know which are hard and which are soft.• If only we could check hardness along the way…
Two Paths Experiment
New Set-Up with Moving Wall (See the board.)•Now we have a wall which can block the s path.•When the wall is blocking, only hard electrons get through.•When the wall is not blocking, both hard and soft electrons get through.
Two Paths Experiment
New Set-Up with Moving Wall (See the board.)•Now we have a wall which can block the s path.•When the wall is blocking, only hard electrons get through.•When the wall is not blocking, both hard and soft electrons get through.
Expectations•Because we just saw that 100% of all electrons emerge green, the blocking wall should block the soft half of them and let the remaining ones through.•This should allow us to measurably show that these electrons are both hard and green.
Two Paths Experiment
New Set-Up with Moving Wall (See the board.)•Now we have a wall which can block the s path.•When the wall is blocking, only hard electrons get through.•When the wall is not blocking, both hard and soft electrons get through.
Observations•When the wall is blocking, 50% emerge red and 50% emerge green.
One Interpretation• It is as if the electrons which took path h somehow know whether the wall is in place!
Two Paths Experiment
When the wall is not blocking, which path does the electron take?1. Path h?
No, because electrons which take h have color “re-set”/randomized.2. Path s?
No, because electrons which take s have color “re-set”/randomized.3. Both paths h and s?
No. Whenever we “look”, we find just one electron on one path.4. Neither h nor s?
No. If we block both paths, nothing ever gets through.
Two Paths Experiment
When the wall is not blocking, which path does the electron take?
5. The electron is in a superposition of taking both h and s.
If you’re wondering what this could possibly mean, well…
Two Paths Experiment
When the wall is not blocking, which path does the electron take?
5. The electron is in a superposition of taking both h and s.
If you’re wondering what this could possibly mean, well…
Interpretation Author(s) Deterministic?Wavefunction
real?Uniquehistory?
Hiddenvariables?
Collapsingwavefunctions?
Observerrole?
Local?Counterfactualdefiniteness?
Universalwavefunction
exists?Ensemble
interpretationMax Born,
1926Agnostic No Yes Agnostic No No No No No
Copenhagen interpretation
Niels Bohr,Werner Heisenberg,
1927
No No1 Yes No Yes2 Causal No[citation needed] No No
de Broglie–Bohm theory
Louis de Broglie,
1927, David Bohm, 1952
Yes Yes3 Yes4 Yes No No No17 Yes Yes
von Neumann interpretation
John von Neumann, 1932,John Archibald
Wheeler, Eugene Wigner
No Yes Yes No Yes Causal No No Yes
Quantum logicGarrett
Birkhoff, 1936Agnostic Agnostic Yes5 No No
Interpretational6 Agnostic No No
Many-worlds interpretation
Hugh Everett, 1957
Yes Yes No No No No Yes No Yes
Popper's interpretation
[62]
Karl Popper, 1957[63] No Yes Yes Yes No No (Yes)13 Yes No
Time-symmetric theories
Satosi Watanabe,
1955Yes Yes Yes Yes No No Yes No Yes
Stochastic mechanics
Edward Nelson, 1966
No No Yes Yes16 No No NoOnly for
position16 No
Many-minds interpretation
H. Dieter Zeh, 1970
Yes Yes No No NoInterpretational
7 Yes No Yes
Consistent histories
Robert B. Griffiths, 1984
Agnostic8 Agnostic8 No No NoInterpretational
6 Yes No No
Objective collapse theories
Ghirardi–Rimini–Weber,
1986,Penrose
interpretation, 1989
No Yes Yes No Yes No No No No
Transactional interpretation
John G. Cramer, 1986
No Yes Yes No Yes9 No No14 Yes No
Relational interpretation
Carlo Rovelli, 1994
Agnostic No Agnostic10 No Yes11 Intrinsic12 Yes No No
Hydrodynamic Interpretation
Erwin Madelung,
1927Yes Yes Yes Yes No No No Yes Yes
Quantum Physics, ‘A Snapshot of Foundational Attitudes Toward Quantum Mechanics’•Maximilian Schlosshauer, Johannes Kofler, Anton Zeilinger•2013)•This poll asked 14 questions of 33 QM conference participants.
So what is “superposition”?
Do we take it at face-value, and just accept that ‘superposition’ is a term which merely describes situations in which these kinds of observations are made?
Neils Bohr and the Copenhagen Interpretation take this view. There is nothing more to be said beyond what we observe. QM is complete.
Or should we demand some deeper account according to which something is going on behind the scenes, producing the observations we make?
Einstein, Podalsky, and Rosen (EPR) argue that until QM can explain the observations, perhaps in terms of “hidden variables”, QM is incomplete.
So what is “superposition”?
Logical empiricism appears to favor Bohr.
Verificationism: The meaning of a sentence consists in its method of verification.
“In science there are no ‘depths’; there are surfaces everywhere.” Carnap, Hahn, Neurath, 1929
“What every scientist seeks, and seeks alone, are … the rules which govern the connection of experiences, and by which alone they can be predicted.” Moritz Schlick, 1932
So what is “superposition”?
Similarly, Van Fraassen’s Constructive empiricism appears to favor Bohr.
All we should ask of science is that theories describe the observable parts of the world—that they be empirically adequate.
So what is “superposition”?
And yet, if you feel that something’s been left out, you may have more sympathy with Einstein, Popper, Godfrey-Smith, and many other scientific realists.
“To believe this [the Copenhagen Interpretation] is logically possible without contradiction, but it is so very contrary to my scientific instinct that I cannot forgo the search for a more complete conception.”
“If one abandons the assumption that what exists in different parts of space has its own independent, real existence, then I simply cannot see what it is that physics is meant to describe.”—Einstein
Thank you!