Quantum Mechanics - something just doesn't add up!
I‘m beginning to think we should give up on the idea of reality altogether! Do objects really exist in space and time independent of us, the observer? Classical mechanics answers yes to this. Quantum mechanics answers no! And further still, does the past determine the present? Classical mechanics answers yes to this (remember that physics fundamental called causality – cause always precedes effect). Quantum mechanics answers no!
Quantum mechanics says events in the present can determine what happened in the past. In nature, particles, objects and even reality itself exist only as probability until a measurement is made. It seems reality is a product of consciousness, when we make a measurement and ‘see’ it with our eyes it becomes real, i.e. it only becomes real when we see it. Of course our eyes in this sense are just a conduit to our brains. So it’s only at the moment our brains interpret the measurement that it becomes real since what we see with our eyes is entangled with what we interpret with our brains.
“Anyone not shocked by quantum mechanics has not yet understood it.” Niels Bohr
So here we have the quintessential mystery which has profound theological and philosophical implications and questions the very nature of existence, our own as well as that of our universe. Quantum Mechanics seems to obey only the law of conservation of weirdness! Welcome to the weird and wonderful world of Quantum Mechanics.
Quantum mechanics challenges our view on intuition and common sense and stretches our imagination to the utmost, perhaps beyond what we can ever dare to imagine. Has Mankind reached some sort of a boundary in our ability to understand nature, our ability to discover the truth? If so what’s on the other side of this boundary? Is this the spiritual world?
Quantum mechanics looks at how the world behaves at the atomic level – what goes on inside an atom. Two great scientists, Einstein and Bohr, debated this for years in the first couple of decades in the 20th century. Einstein thought nature was deterministic – one thing determines another. So if we had enough knowledge about all the particles and forces we could predict the future. Quantum mechanics disagreed with this view. It said the universe was indeterminate. So Einstein thought this new quantum mechanical theory was not necessarly wrong per se, but was incomplete, it was saying the wrong thing about the nature of reality (remember Einstein’s words – ‘God does not play dice with the world’).
But wait, what about that law of causality? Where every event has a preceding cause, everything that came into being, you, me, the universe has a preceding cause. Common sense says cause always comes before effect. But not in the world of quantum mechanics! These common sense laws are violated. Quantum mechanics was saying that nature was random, that events can be un-caused and occur for no reason. So for instance radioactive decay events in nature are spontaneous and completely unpredictable due to these quantum mechanical processes. This uncertainty and indeterminacy was central to quantum mechanics. But it goes further. It even went on to declare ‘Instant action at a distance’. Well of all the …….. There is no doubt Einstein had sleepless nights over that declaration as this was inconsistent with his special theory on relativity!
And to think all this torment and suffering unfolded because of that damned two slit experiment! Thanks Mr. Young.
Two slit experiment
When we shine light (photons) through two slits and let it fall on a detection screen in the background we get a statistical interference pattern. Basically what’s happening is that the light travels through the two slits as a wave (think ripples in a pond) and interferes with itself and where the crests of the wave meet up (are in phase) at the screen we get constructive interference (bright lines in the case of light) and where the troughs meet up we get destructive interference (dark lines). But Einstein had previously shown from the photoelectric effect that light was a particle. So how would particles passing through two slits cause interference patterns at the detection screen? And further more when photons were sent through one at a time (it’s possible to fire photons or even electrons like this) this same interference pattern showed up. This made no sense. So scientists decided that they would place detectors at the two slits to establish what was actually happening.
And when the detectors detected the particles at the slits as they go through the slits one at a time it resulted in statistical clumping, rather than interference pattern at the screen as if they were ordinary particles. If we do not detect the particles at the slits it resulted in a statistical interference pattern at the screen as though some kind of wave was involved. Why should there be a difference? When we detect particles at the slits we know which slit the particle went through, when we don’t detect at the slits we don’t know which slit the particles went through. The only difference is we know the path travelled in one scenario and we don’t know in the other. So choosing to know or determine the path (which-path information) of the photons changes the outcome? Weird! How does the photon know we are looking at it?
It seems that the actual conscious act of measuring the data at the slits in the experiment is what decides whether the light was a wave or light was a particle – The Jekyll and Hyde nature of photons (not that there are ‘good’ photons and ‘bad’ photons).
It was the Austrian physicist Erwin Schrodinger that proposed that the photon is not a photon but actually a probability wave, and some of the probability goes through one slit and some through the second slit, and interferes with itself and you get the diffraction pattern at the detector. The probability wave function collapses (instantaneously?) to a physical particle when the measurement is made. So when the measurement is made by a detector (and we observe the results) placed at the double slit the wave function collapsed and a photon passed through the slit. But when the measurement is made at the screen you get a diffraction pattern because the probability wave has passed through the slits and interfered with itself to produce the pattern at the screen.
Delayed choice double slit experiment
In this experiment photons are once again sent through slits, and detected at the slits, then choose between viewing the data (counting them) or erasing the data and check the screen. Note that our choice is made after they hit the screen. And guess what, when we decide to count data we get statistical clumping, when we decide to erase data we get statistical interference! This is nonsensical! Let us say that the experiment was carried out yesterday, and today we decide to count data or erase data – then this effects the results from yesterday! Surely the statistical pattern is determined before we choose to count or not count the data. But it doesn’t work this way in reality. So by extracting ‘which path’ information some time after the photon travels through the slit we can affect its previous behaviour while passing through the slits!
Despite the fact the machines have registered something at the slits and at the screen, there is no information at the screen until we choose to look at it.
When we choose to look at it the information about where the particles hit the screen will be created, and the result will be determined by our knowledge (or lack of knowledge) of the particles at the slits.
If tree falls in forest and nobody is around to hear it fall did it make a noise….. is the same as asking if a particle goes through double slits, let’s say yesterday, and nobody looks for it at the screen until today then did those particles hit the screen yesterday? Well quantum mechanics says no, they didn’t hit the screen until someone looks for it, and then we can say it hit the screen yesterday.
In 1978 John Wheeler proposed the delayed choice experiment where the method of detecting can be changed after the photons have passed through the double slits. And it seems that the photon doesn’t decide whether it is a particle or a wave at the double slits. It’s the experimenter that decides at a later stage which it is, and this depends on whether the experimenter has or hasn’t knowledge of which slit the photon passed through. If we know which slit it passed through the photon behaves as a particle, if we don’t know which slit the photon passed through it behaves as a wave! The behaviour of the photon at the slits seems to depend on what the photon encounters after the photon has passed through the slits! So the experimenters delayed choice is determining how the photon behaved at an earlier point in time! Can we change the past?
So is it the detectors that are collapsing the wave function? Not according to the delayed choice quantum eraser experiment which rules out the idea that the detectors are collapsing the wave function and seems to imply that it’s our consciousness that collapses the wave function, i.e. that if we are able to determine the ‘which-path’ information then the results are different. Before a measurement is made particles remain in a superposition of two quantum states. i.e. physical reality is not determined until the act of measuring takes place. It’s like the act of observation places it in one state or another. More on the Quantum Eraser later….
In Hamlet, Shakespeare wrote ‘To be or not to be, that is the question’. Well apparently in the world of quantum mechanics there is to be and there is not to be, and there is also something in between!
The Copenhagen Interpretation is a general view on quantum mechanics devised primarily by Niels Bohr and Werner Heisenberg during their research in quantum mechanics in Copenhagen. It deals with the probabilistic and non-determinate description of matter, nature and reality. It is/was the most accepted explanation as to why quantum particles behave the way they do, the wave/particle duality of matter, the two-slit experiment, the superposition of states and the collapse of a mathematical object called the wave function and choosing a state from the many probabilities when we make an observation. So the act of observation changes the universe since an unobserved event has neither happened nor not happened. The outcome is determined when we make observation thereby collapsing the wave function and forcing a probability choice.
Quantum entanglement means particles are linked together in such a way that when a measurement of one particles quantum state (for instance spin or polarization) is taken this instantly establishes the quantum state of the other particle (regardless of distance).
So for instance an elementary particle such as a photon has a property called spin. If a pair of photons are created together one will have a clockwise spin the other will have an anti-clockwise spin, the total spin of a quantum state like this must always be zero.
After separation they do not have a definite state until a measurement is made and the wavefunction collapses. So if we send one of the entangled photons to the other end of the universe and measure its spin, we find that the photon that stays at home responds instantly and has the opposite spin as if they are still connected (entangled) in some mysterious way. Either they remain still connected or information is travelling between them at an infinite speed violating special relativity’s cosmic light-speed barrier and common sense.
It also violates the principle of locality which states that an object is influenced directly only by its immediate surroundings. As distinct from non-locality which is the direct influence of one object on another distant object. But is locality a classical mechanics principle only? It seems to fail in quantum mechanics and this according to Einstein showed serious shortcomings with quantum mechanics rather than it been a locality issue.
The ‘Spooky action at a distance’ comment by Einstein indicated his rejection of non-locality, where regardless of distance apart particles sharing an entangled state are so deeply intertwined that measurement events on one of them seem to instantaneously affect the other. It’s like nature has the ability to transmit information instantly across the universe.
Einstein, one of the creators of quantum mechanics became a ‘quantum’ skeptic and spent much of his life ‘plotting’ it’s downfall. Probably his strongest attack on quantum mechanics was the EPR (Einstein-Podolsky-Rosen) paradox paper from 1935 showed that quantum entanglement violated Einstein’s special theory of relativity re- faster-than-light communication. So they concluded that quantum mechanics was incomplete and proposed an alternative theory called the ‘hidden variables theory’.
However Irish physicist John Bell devised what is known as Bells Theorem in 1964. He was able to prove his theorem through the creation of Bell Inequalities and experiment after experiment showed the violation of these inequalities proving that locality and the ‘hidden variable theory’ was false and quantum entanglement was true.
Aspect, the French physicist tested the EPR paradox in the 1980s and showed that Einstein et al were incorrect. So Einstein, it appears ‘spooky action at a distance’ is a law of quantum mechanics.
Of course today the race is on to understand and make use of this strange entanglement phenomena as it’s thought that if this can happen and it can be exploited and utilized for computers then the power of computers will be almost boundless. And of course entanglement opens up the world of quantum teleportation where the quantum states of photons has already been teleported in experiments. Maybe the ‘beam me up Scotty’ line will be used in real life in the not to distant future.
So for those of us who like to think the moon is there even when you are not looking at it, ‘Houston, we have a problem’! Perhaps it may not be possible to fully comprehend quantum mechanics without fully comprehending consciousness. Remember Einstein’s quote – ‘Reality is an illusion albeit a persistent one’, which ironically could be used to support the theories of quantum mechanics.
The last word for now should be left to Richard Feynman, where he sums up our thoughts on quantum mechanics, “The paradox is only a conflict between reality and your feeling of what reality ought to be.”