__Five Reasons Why Quantum Weirdness is Aptly Named__

Quantum physics is a fascinating field. It is the reigning champion amongst physics theories and is outstandingly successful in terms of its predictive ability and experimental verification. A quantum prediction has never been wrong. This is an incredible feat for any theory but what makes quantum theory extra special is that despite its unprecedented success, it tells us that reality is not just weirder than we suppose but that it is weirder than we *can* suppose. Here are five reasons why this is true:

1. Quantum theory is based on unobservable quantities. One of these is the wave function or wave amplitude of a particle (yes, solid particles act like waves when no one’s watching). The only observable feature of a particle is proportional to the absolute square of this value so the wave function itself can be either negative or positive.

2. The heart of quantum theory is probability. This means that the exact outcome of a single experiment cannot be predicted, but the *probabilities* of the various possible outcomes can be calculated with complete precision. This probability is the above mentioned value proportional to the absolute square of the wave function.

3. The superposition principle. A particle such as an electron has a value for a property called spin. This spin value can be either up or down, but according to quantum theory and probability it isn’t just either up or down, it’s *both* up and down. Of course, when the electron’s spin is actually measured it will be discovered to be in only one direction but the rules state that until such a measurement is made the particle is in both states at once and the state that is actually observed is a direct consequence of those probabilities which can be calculated to such high precision. Superposition can apply to any number of states, not just the two illustrated above involving up and down spin.

4. A particle can interfere with itself in the same way a wave can. Waves are made up of troughs and crests. If a wave passes through a barrier via two closely spaced gaps, the two resulting waves spreading out on the opposite side will interfere with each other yielding a classic interference pattern. Such a pattern results in two crests or two troughs merging to reinforce each other while if a trough merges with a crest the two will cancel each other. The amazing thing is that a series of single point particles also fired through (one of) two closely spaced gaps in a barrier onto a detecting screen, create the same interference pattern of highs (bright spots) and lows (dark spots) as two interfering waves would.

5. Two or more particles separated in space can retain some kind of connection which allows them to operate as a single system. This is called entanglement. Imagine an electron and a positron (anti-electron) annihilating to create two photons. By the rules of spin conservation (total spin before and after an event must be the same) one of the photons must have an upward directed spin and the other a downward directed spin. However, because of the superposition principle, photon 1 is in both an up and down state and so is photon 2. Which state each is in comes down to probability. If the two photons fly apart in opposite directions after their creation and photon 1 is measured three seconds (900,000kms) later and found to have an upward directed spin then at the same moment, the second photon can be known to have a downward directed spin. If photon 1 had been found to have down spin then photon 2 would have had to possess up spin. This may not seem too amazing at first but what it means is that the probabilities (for spin in this case) of the individual particles cannot be considered individually. When the spin of one particle is measured the probability of the other particle’s spin automatically becomes 1 (certain) in the opposite direction. Somehow, across any distance, the particles act in harmony as if belonging to a single system which allows instantaneous communication between its parts. In order for the second photon to know in which direction its spin must be, the first particle’s measured spin direction is somehow communicated to the second particle at a speed that must be greater than the speed of light. Entanglement is essentially the superposition of two or more particles (as opposed to two or more states of a single particle examined in number 3 above).

There are many such contradictory features to quantum physics. The question is do these features penetrate to the heart of reality as it actually is or are they just the outer veneer of a theory that lacks completeness and is missing the whole picture? Albert Einstein argued that quantum physics was just too counter-intuitive to be *the* final theory.[1] He was particularly disturbed by the discovery of the central role given to probability in quantum theory and he refuted this in one of his most famous quotations saying that God is subtle but not malicious.

Despite Einstein’s rejection of quantum theory and the bizarre implications it continues to throw at us, it remains one of the most successful physical theories ever conceived. Perhaps a new theory in 21^{st} century physics will supersede the quantum and revolutionise physics the way quantum theory swept over its classical predecessor at the beginning of the 20^{th} century. Maybe those disturbing probabilities will eventually be revealed as part of a deeper consistency and harmony. Until that day, it seems certain that quantum physics will continue to make its outrageous (but highly accurate) predictions and baffle us all with its lack of respect for the commonsense notions we hold so dear.

[1] This is ironic in a sense because it was Einstein (along with Max Planck, who also rejected quantum theory) who initially expanded on the concept of the quantum and used it to explain some confusing experimental results, giving it a strong push towards the theory it would later become.

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