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!