Quantum Retrocausality for Two-year-olds Huw Price Centre for Time - - PowerPoint PPT Presentation

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Quantum Retrocausality for Two-year-olds

Huw Price

Centre for Time · University of Sydney

Huw Price Quantum Retrocausality for Two-year-olds 1/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The classical case (right)

R = 0 R = 1

τR Beam Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

Malus’ Law: IntensityR=1 = cos2(τR − σR), IntensityR=0 = sin2(τR − σR)

Huw Price Quantum Retrocausality for Two-year-olds 2/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The classical case (right)

R = 0 R = 1

τR Beam Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

Malus’ Law: IntensityR=1 = cos2(τR − σR), IntensityR=0 = sin2(τR − σR)

Huw Price Quantum Retrocausality for Two-year-olds 2/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The classical case (right)

R = 0 R = 1

τR Beam Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

Malus’ Law: IntensityR=1 = cos2(τR − σR), IntensityR=0 = sin2(τR − σR)

Huw Price Quantum Retrocausality for Two-year-olds 2/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The classical case (left)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Beam Polarizing cube set at angle σL

‘T-reversed’: IntensityL=1 = cos2(τL − σL), IntensityL=0 = sin2(τL − σL)

Huw Price Quantum Retrocausality for Two-year-olds 3/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The classical case (left)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Beam Polarizing cube set at angle σL

‘T-reversed’: IntensityL=1 = cos2(τL − σL), IntensityL=0 = sin2(τL − σL)

Huw Price Quantum Retrocausality for Two-year-olds 3/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The classical case (left)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Beam Polarizing cube set at angle σL

‘T-reversed’: IntensityL=1 = cos2(τL − σL), IntensityL=0 = sin2(τL − σL)

Huw Price Quantum Retrocausality for Two-year-olds 3/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Thinking about connection and control

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

1

Polarization as a mechanism

2

What we can wiggle

Huw Price Quantum Retrocausality for Two-year-olds 4/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Thinking about connection and control

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

1

Polarization as a mechanism

2

What we can wiggle

Huw Price Quantum Retrocausality for Two-year-olds 4/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Thinking about connection and control

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

1

Polarization as a mechanism

2

What we can wiggle

Huw Price Quantum Retrocausality for Two-year-olds 4/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the demons of the left?

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Suppose you control the left polarizer, σL, but not the input beams – the Demon of the Left controls those. Can you control the polarization of the

• utput beam, τ?

No! The Demon can make τ whatever She likes, by choosing the appropriate input intensities. (The Demon can “counteract” your wiggles, in

• ther words.)

Huw Price Quantum Retrocausality for Two-year-olds 5/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Nature as the Demon of the Right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τ = τL = τR Beam Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τ? No! Nature absorbs your wiggles in changes in the output intensities, and τ doesn’t change. (So no retrocausality here!)

Huw Price Quantum Retrocausality for Two-year-olds 6/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The quantum case (right)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

QM Malus’ Law: ProbR=1 = cos2(τR − σR), ProbR=0 = sin2(τR − σR)

Huw Price Quantum Retrocausality for Two-year-olds 7/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The quantum case (right)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

QM Malus’ Law: ProbR=1 = cos2(τR − σR), ProbR=0 = sin2(τR − σR)

Huw Price Quantum Retrocausality for Two-year-olds 7/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The quantum case (right)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

QM Malus’ Law: ProbR=1 = cos2(τR − σR), ProbR=0 = sin2(τR − σR)

Huw Price Quantum Retrocausality for Two-year-olds 7/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The quantum case (left)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

‘T-reversed’: ProbL=1 = cos2(τL − σL), ProbL=0 = sin2(τL − σL)

Huw Price Quantum Retrocausality for Two-year-olds 8/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The quantum case (left)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

‘T-reversed’: ProbL=1 = cos2(τL − σL), ProbL=0 = sin2(τL − σL)

Huw Price Quantum Retrocausality for Two-year-olds 8/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The quantum case (left)

R = 0 R = 1

τR Photon Polarizing cube set at angle σR

L = 0 L = 1

τL Photon Polarizing cube set at angle σL

‘T-reversed’: ProbL=1 = cos2(τL − σL), ProbL=0 = sin2(τL − σL)

Huw Price Quantum Retrocausality for Two-year-olds 8/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Connection and control again

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Polarization as a mechanism – intuitively, τL is just whatever ‘piece of machinery’ connects wiggles on the left to changes on the right.

2

What we can wiggle?

Huw Price Quantum Retrocausality for Two-year-olds 9/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Connection and control again

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Polarization as a mechanism – intuitively, τL is just whatever ‘piece of machinery’ connects wiggles on the left to changes on the right.

2

What we can wiggle?

Huw Price Quantum Retrocausality for Two-year-olds 9/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Connection and control again

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Polarization as a mechanism – intuitively, τL is just whatever ‘piece of machinery’ connects wiggles on the left to changes on the right.

2

What we can wiggle?

Huw Price Quantum Retrocausality for Two-year-olds 9/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Connection and control again

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Polarization as a mechanism – intuitively, τL is just whatever ‘piece of machinery’ connects wiggles on the left to changes on the right.

2

What we can wiggle?

Huw Price Quantum Retrocausality for Two-year-olds 9/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating the Demon of the Left

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Again, suppose you control the left polarizer, σL, but not the input photons – the Demon of the Left controls those. Can you control τL? Yes! . . . so long as the Demon has to put the photon on one channel or

• ther. In this case, either τL = σL or τL = σL + π/2 – so you can’t fix τL

completely, but you can restrict it to just two possibilities.

Huw Price Quantum Retrocausality for Two-year-olds 10/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Defeating Nature on the right

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

Similarly, suppose you control the right polarizer, σR, but not the output beams – the Nature controls those. Can you control the polarization of the input beam, τR? Yes! . . . so long as Nature has to put the photon on one channel or

• ther. In this case, either τR = σR or τR = σR + π/2 – so you can’t fix τR

completely, but you can restrict it to just two possibilities. (Retrocausality!!)

Huw Price Quantum Retrocausality for Two-year-olds 11/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Making some assumptions explicit

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Time-symmetry: This is what gets us from the existence of τL to the existence of τR

2

Discreteness: If Nature can ‘spread’ the photon between both output channels, the retrocausality goes away – we’re back to the classical case.

Huw Price Quantum Retrocausality for Two-year-olds 12/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Making some assumptions explicit

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Time-symmetry: This is what gets us from the existence of τL to the existence of τR

2

Discreteness: If Nature can ‘spread’ the photon between both output channels, the retrocausality goes away – we’re back to the classical case.

Huw Price Quantum Retrocausality for Two-year-olds 12/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Making some assumptions explicit

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Time-symmetry: This is what gets us from the existence of τL to the existence of τR

2

Discreteness: If Nature can ‘spread’ the photon between both output channels, the retrocausality goes away – we’re back to the classical case.

Huw Price Quantum Retrocausality for Two-year-olds 12/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Making some assumptions explicit

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Time-symmetry: This is what gets us from the existence of τL to the existence of τR

2

Discreteness: If Nature can ‘spread’ the photon between both output channels, the retrocausality goes away – we’re back to the classical case.

Huw Price Quantum Retrocausality for Two-year-olds 12/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Making some assumptions explicit

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Time-symmetry: This is what gets us from the existence of τL to the existence of τR

2

Discreteness: If Nature can ‘spread’ the photon between both output channels, the retrocausality goes away – we’re back to the classical case.

Huw Price Quantum Retrocausality for Two-year-olds 12/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

Making some assumptions explicit

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

Time-symmetry: This is what gets us from the existence of τL to the existence of τR

2

Discreteness: If Nature can ‘spread’ the photon between both output channels, the retrocausality goes away – we’re back to the classical case.

Huw Price Quantum Retrocausality for Two-year-olds 12/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

What we get from this case

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

For sure: A demonstration of how the simple, intuitive ideas at the basis of

• ur causal reasoning – connections and wiggles – can lead us to retrocausality.

2

Maybe: A simple, intuitive answer to some of the deepest quantum puzzles.

Huw Price Quantum Retrocausality for Two-year-olds 13/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

What we get from this case

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

For sure: A demonstration of how the simple, intuitive ideas at the basis of

• ur causal reasoning – connections and wiggles – can lead us to retrocausality.

2

Maybe: A simple, intuitive answer to some of the deepest quantum puzzles.

Huw Price Quantum Retrocausality for Two-year-olds 13/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

What we get from this case

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

For sure: A demonstration of how the simple, intuitive ideas at the basis of

• ur causal reasoning – connections and wiggles – can lead us to retrocausality.

2

Maybe: A simple, intuitive answer to some of the deepest quantum puzzles.

Huw Price Quantum Retrocausality for Two-year-olds 13/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

What we get from this case

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

For sure: A demonstration of how the simple, intuitive ideas at the basis of

• ur causal reasoning – connections and wiggles – can lead us to retrocausality.

2

Maybe: A simple, intuitive answer to some of the deepest quantum puzzles.

Huw Price Quantum Retrocausality for Two-year-olds 13/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

What we get from this case

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

For sure: A demonstration of how the simple, intuitive ideas at the basis of

• ur causal reasoning – connections and wiggles – can lead us to retrocausality.

2

Maybe: A simple, intuitive answer to some of the deepest quantum puzzles.

Huw Price Quantum Retrocausality for Two-year-olds 13/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

What we get from this case

R = 0 R = 1

Polarizing cube set at angle σR

L = 0 L = 1

τL, τR Photon Polarizing cube set at angle σL

1

For sure: A demonstration of how the simple, intuitive ideas at the basis of

• ur causal reasoning – connections and wiggles – can lead us to retrocausality.

2

Maybe: A simple, intuitive answer to some of the deepest quantum puzzles.

Huw Price Quantum Retrocausality for Two-year-olds 13/14

Classical Polarization 101 Playing with blocks Quantum Polarization 101 Playing with quantum blocks Caveats Conclusion

The End

Huw Price Quantum Retrocausality for Two-year-olds 14/14