The mysterious quantum world and why Einstein was probably wrong

1. The mysterious quantum world and why Einstein was probably wrong

2. Our world and the quantum world • Our world • Big things • Dust, bullets, cars, people.. • Quantum world • Small things: particles, atoms

3. 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

4. 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 ?

5. 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:

6. Implications

7. 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.

8. 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)

9. 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”.

10. 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!

11. 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.

12. 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.

13. But is it useful ? • Very • Quantum cryptography • Quantum computing.