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

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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

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

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 ?

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:

Implications

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.

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)

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”.

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!

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.

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.

But is it useful ? Very – Quantum cryptography – Quantum computing.

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