The Problem with Quantum Mechanics

 

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:

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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