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The mysterious quantum world and why Einstein was probably wrong

The mysterious quantum world and why Einstein was probably wrong

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Page 1: The mysterious quantum world and why Einstein was probably wrong

The mysterious quantum world and why

Einstein was probably wrong

Page 2: The mysterious quantum world and why Einstein was probably wrong

Our world and the quantum world

• Our world– Big things– Dust, bullets, cars, people..

• Quantum world– Small things: particles, atoms

Page 3: The mysterious quantum world and why Einstein was probably wrong

Quantum mechanics• A set of equations and postulates which describes how little things (and

ultimately big things) behave.• Take the equations and solve them (by hand or computer)...• This theory works• One typical measurement compared with quantum mechanics prediction

• We have never been able to disprove quantum mechanics.• These wonderful equations give us big problems in understanding what happens

to small things

Measured = 1159652180.7+- 0.3 Predicted = 1159652154+-28

Page 4: The mysterious quantum world and why Einstein was probably wrong

Why is the quantum world mysterious ?• Imagine trying to measure the position of Arvid (a big thing) on

the surface of the earth

• If we had a perfect apparatus, eg perfect rulers, satellites, clocks etc. then we can measure his position to infinite accuracy ? Right ?

Page 5: The mysterious quantum world and why Einstein was probably wrong

Wrong• Quantum mechanics tells us that there is a

fundamental limit to know speed and position of any object (big thing or small thing)

• Even with a perfect experiment:

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anyin it noticenever tiny weis Because

10tiny constant) s(Planck'constant

speedon y uncertaintbest

position on y uncertaintBest

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Page 6: The mysterious quantum world and why Einstein was probably wrong

Implications

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Page 7: The mysterious quantum world and why Einstein was probably wrong

More implications

Its not just position and speed.According to quantum mechanics:

An object doesn’t possess a definite value of quantity, eg energy, momentum etc.

There are only possible values. The act of measurement forces the object to ”make a decision” and choose a value.

Page 8: The mysterious quantum world and why Einstein was probably wrong

You think this is rubbish ?

• You’re in good company• He hated it as well.• He tried to show it was

rubbish by a gedanken (a thought experiment)

Page 9: The mysterious quantum world and why Einstein was probably wrong

Einstein though this would lead to a paradox

A particle decays in space into an electron and antielectron.

An electron can come as two types ”up” and ”down”.50% of the time its ”up”, 50% of the time its ”down”.If one is ”up” the other must be ”down”.

Page 10: The mysterious quantum world and why Einstein was probably wrong

An observer on earth measures the electron

• Before the measurement it was neither ”up” or ”down”• He/she finds it to be ”up”• Instantaneously the antielectron must become ”down”• Physicists don’t like information travelling instantaneously.

It causes big problems.• Information, eg radio signals, *cannot* move faster than

the speed of light otherwise it can be sent backwards in time and create a paradox.

• Eg you can send a message arranging for your grandfather to be killed before he met your grandmother.

• Einstein thought he had won!

Page 11: The mysterious quantum world and why Einstein was probably wrong

Einstein was wrong

• The observer on earth had no control over the measurement

• He/she couldn’t choose the outcome ”up”.• There was a 50% chance of ”up” and 50% chance of

”down”• Information was sent faster than light but we can’t

choose what that information is. • Impossible to write a message and send it back in

time.• Your grandmother is safe.

Page 12: The mysterious quantum world and why Einstein was probably wrong

Summary• Quantum mechanics is the best tested theory in history• Its also strange• An object doesn’t have a position until a measurement is

made – its everywhere and nowhere. The measurement forces it to ”choose a location”.

• We never see this effect because the number h is tiny• If h was big we would see it in our everyday lives• Einstein hated all of this – he thought ”God doesn’t play

dice”• So far, all the measurements indicate that Einstein was

wrong.

Page 13: The mysterious quantum world and why Einstein was probably wrong

But is it useful ?

• Very– Quantum cryptography – Quantum computing.