Transcript The mysterious quantum world and why Einstein was (probably

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
Measured = 1159652180.7+- 0.3
Predicted = 1159652154+-28
• We have never been able to disprove quantum mechanics.
• These wonderful equations give us big problems in understanding what
happens to small things
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:
h
Best uncertainty on position x 
v
v  best uncertainty on speed
h  constant (Planck' s constant) tiny 10
Because h is tiny we never notice it in any
experiment with a big thing.
-34
Implications
h
v
According to quantum mechanics Arvid does not have a
position until the measurement is made. The act of
measurement forced all the atoms in Arvid' s body to
" make a decision" and choose a position. The range
x 
of possible positions is : x  tiny  10-34 m  1millionth
millionth millionth millionth... of a metre.
We never notice the curious laws of quantum mechanics
applying to us because h is small.
Arvid did not exist in a given position until we looked.
There are only different probabilities to find in different
positions. He was in all these different positions and none
of them before the measurement.
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.