The Problem with Quantum Mechanics
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| Credit - geralt: https://pixabay.com/illustrations/physics-quantum-physics-particles-3871216/ |
Quantum mechanics is currently seen
as ‘the best description we have of the nature of the particles that make up
matter and the forces with which they interact.’ [1] The key ideas
of quantum physics include wave-particle duality and quantised properties. Wave-particle
duality is the concept that all physical entities behave as both waves and
particles simultaneously. For instance, electrons undergo diffraction which is
a wave effect. Meanwhile, radiation is emitted in small discrete packets or
quanta called photons. These photons can be considered as particles. Each
photon contains an amount of energy. Therefore, light energy is quantised.
Quantum mechanics has been
incredibly successful since its development in the early 20th
century. Most notably, they underpin the science behind transistors, fundamental
to modern electronics, which are composed of semiconductors: the energy bands
in semiconductors are related to the discrete energy levels (quantised energy
states) which electrons can occupy. *So, with all its successes, what could
possibly be wrong with quantum mechanics?
The Quantum Measurement Problem
‘I’m not so sure that quantum
mechanics will survive as anything but an extraordinarily good approximation in
the centuries to come’ – Steven Weinberg, 2020. [2]
This quote by the physics Nobel
laureate was made in reference to one of the underlying issues with quantum
mechanics: the involvement of observers – they are included in the postulates
of quantum mechanics. Although all theories require knowledge about what an
observer sees or experiences so that they can be tested experimentally, the
fundamental principles should not rely on the observers. What the observer
finds should be a consequence of the postulates rather than being part of them.
[2] For instance, in classical mechanics, when you observe a system,
‘you see what is really there.’ [3] When you are not observing the
system, it behaves the same as if you were: the motion of stars and planets is
not affected by you looking at them. It is not so plain sailing when it comes
to observations in quantum mechanics.
Probably the most famous experiment
which highlights the impact of observers is the single particle double slit
experiment:
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| Double-slit experiment; Credit: https://commons.wikimedia.org/wiki/File:Double-slit.svg |
Electrons are fired one at a time
towards a screen with two slits. Over time, an interference pattern builds up
on a receiving screen. Interference is a wave effect. Therefore, the result
suggests that each electron passes through both slits as a wave. This wave then
interacts with itself to produce the interference pattern. [4] To test
this hypothesis, you can place a detector at the top slit which beeps when an
electron travel through it. Surprisingly, when the experiment is set up in this
way, the detector beeps half the time. This indicates that the electron passes
through each slit 50% of the time. Furthermore, an interference pattern is no
longer seen on the receiving screen. Instead, two bands are seen – this is the
expected pattern if you fire particles at the slits. Clearly then, the electrons
behave differently when there is a detector. Placing the detector at the top
slit is an act of measurement. Therefore, this measurement or observation
changes the system fundamentally. What really happens when a measurement is
taken is still open to debate. This major issue is known as the quantum measurement problem. [3]
*Read more about band theory here:
https://phys-talk.blogspot.com/2020/07/semiconductors-part-1-introduction-band.html;
for more on semiconductor types and doping, check out this post: https://phys-talk.blogspot.com/2020/08/semiconductors-part-2-doping-types-of.html
Sources:
1. New Scientist: https://www.newscientist.com/term/quantum-physics/
2. Steven Weinberg via St Cross
College, University of Oxford: https://www.youtube.com/watch?v=J9Tuhh4vzjg&t=1491s
3. Sean Carroll via Science and
Cocktails: https://www.youtube.com/watch?v=-kxmR82QMN8
4. Matt O’Dowd, PBS Spacetime: https://www.youtube.com/watch?v=p-MNSLsjjdo&t
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