Kinkajou : I think now we arrive in the modern era.
Erasmus : yes now we arrive at the “EPR” paradox. This theory was developed by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935. The theory was based on the concepts of locality (local relativistic causality) and realism, often also described as indicative of the presence of “local hidden variables”.
By showing how the predictions of quantum mechanics flew in the face of reality as seen by the common man, they hoped to debunk the concept of quantum mechanics.
Einstein Bosen Podolski Paradox
In a 1935 paper, Einstein and his colleagues noted that one way to get around spooky action at a distance would be to assume that each particle always travelled with some hidden knowledge of the other's state before the particles were measured.
Spooky Physics Einstein
Erasmus : Einstein was not a big fan of the crazy new physics and opted to use much more mundane explanations for this phenomenon. Einstein held that when two widely separated particles are measured to have the same property; it is not because they two particles randomly chose to have this property, but because they were identically programmed at the start to have the same property.
The correlation between the behaviour of widely separated photons is evidence that the photons were imbued with identical properties when emitted, not that they are subject to some form of bizarre long distance quantum entanglement.
The next battleground on quantum mechanics followed the physicist John Stewart Bell publishing a paper in 1964 showing mathematically that it is possible to discriminate between the predictions of quantum mechanics theory and of “local reality” theory.
Bell's theorem is often described as a “no-go theorem”.
This is a theorem states the particular situation is not physically possible. In its simplest form, Bell's theorem states: No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.
Bell showed that if a local hidden variable theory holds, then these correlations would have to satisfy certain constraints, called Bell inequalities. However, for the quantum correlations arising in the specific example considered, those constraints are not satisfied, hence the phenomenon being studied cannot be explained by a local hidden variables theory.
Erasmus : The basic tenet of quantum mechanics is the Heisenberg Uncertainty Principle. A particle according to quantum mechanics cannot have a definite position and a definite velocity. A particle cannot have a definite spin about more than one axis (clockwise or anticlockwise).
So it is possible to define a situation whereby two particles such as photons can be demonstrated to show entanglement with the effect able to be demonstrated at speeds exceeding the speed of light. In effect, a quantum mechanical particle is bypassing or obviating the predictions of the “theory of relativity”.
So for our two photons:
Newtonian (Mechanical) physics would predict that the particles randomly acquired the same properties at the time they were emitted. They would be measured to have the same properties, because they both always had the same properties. Einstein held this view.
Quantum physics would predict that the properties of the two particles were acquired randomly at the time they were measured, long after they were emitted. Up to that point each particle existed in a probability state where all possible states are possible. The Quantum physicists such as Heisenberg, De Broglie, Schrodinger, and Bohr held this view.
Since two particles can have their properties “determined” at exactly the same time in widely different places, there can be no faster than light communication between the two particles. Yet these same particles would give the same property measurement. They act as if they are entangled and that the space between them does not exist.
Kinkajou : So now we have two explanations of this phenomenon. Which could be true?
Erasmus : The problem then becomes how can you differentiate between these two explanations? An Irish physicist John Bell had proposed that this could be proved experimentally. By the 1980s, the jury was in. Einstein was wrong.
Experiments have been performed which demonstrate violations of” Bell’s inequality”. The realist view of the nature of matter is wrong. The quantum mechanical explanation of the nature of matter is correct.
The famous experiments include those of John Clauser and Stuart Freedman (1972) and Alain Aspect et al. (1981).
Clouds and Freedman jury rigged a detector that measured photon polarisation. They fired thousands of pairs of photons from a scented photon source towards detectors facing opposite to each other. The statistical results were consistent with quantum theory only.
The French physicist Alain aspect tested the instantaneousness of entanglement. The results were consistent with quantum mechanical theory demonstrating faster than light effects.
Bell himself struggled with the concept of determinism in the quantum mechanical universe. He proposed that there is no need for particle A to generate faster than light signal to tell particle B about its physical state, because the universe including particle A or E nose with the measurement and its outcome will be.
The question then becomes is everything we do predetermined and known by the universe? Is indeed any concept of free will or choice possible in a quantum universe?
Quantum mechanics gives answers that are a set of probabilities all existing at the same time. This is totally unreal.
As Schrödinger pointed out, quantum mechanics seems to say that you could create a situation where a cat was both alive and dead at the same time, and we never see this. But this is in fact a very curious piece of ammunition to use against quantum mechanics.
We already have a very good nontechnical word for a mixture of possibilities coexisting at the same time-we call it the future. Unless we believe that all events are predetermined, which would be a very dismal view of the world; this is what the future must be like.
Of course, we never experience it until it becomes the present, when only one of the possibilities takes place, but the actual future-as opposed to our prediction of one version of it-must be something much like what quantum mechanics describes.
The final point is a little vague but more fundamental. If we accept that the future is not fixed, we expect it to contain surprises. Crudely speaking, this is not very plausible in a world where particles have continuous trajectories and an infinite amount of information is freely available.
It is much more plausible in a world that is in some way discontinuous, where the available information is limited.
Kinkajou : Tell so the experiment works.
A classical simple experiment shows some aspects of this proof. Two spin detectors were place at opposite ends of a room: about 13 metres apart. A container of high energy calcium atoms was placed midway between them. It is known that each calcium atom when returning to its normal low energy state, will emit two photons travelling back to back, whose spins are perfectly correlated.
When the settings of the detectors were varied randomly, i.e. whether they detect clockwise or anticlockwise spins), the detected properties of spin correlated with statistical distributions of results that fitted the quantum theory model, not the classical Newtonian model.
The Classical Newtonian view of this experiment is:
Since the two objects were spatially separate a measurement on one object could not possibly have an effect on the second object.
More precisely, since nothing travels faster than the speed of light, if a measurement on object were to change the second object ( i.e. to make it take on an identical spin0, there would have to be a delay in this happening, a delay at least as long as the time taken for the light to travel between the two detectors..
But in the experiment, the two particles are examined by the detectors at the same time. Therefore whatever is learned about the first particle must be a feature that the second particle already possessed, completely independent of whether we take the measurement at all.
The Quantum view is that the photons acquired their measured properties at the time they were measured. Though Quantum mechanics shows that particles randomly acquire the measured property, randomness can be linked across space.
Pairs of identical particles don’t acquire their random properties independently. Entangled particles though spatially separated, operate in a linked or dependent fashion.
Quantum Correlation Experiment
So the difference between the two views is that the probability results of a large number of experiments on paired photons would follow either a result distribution predicted by Quantum mechanics or a result distribution predicted by Newtonian mechanical physics. This is the result
The local realist prediction (solid lines) for Quantum correlation for spin (assuming 100% detector efficiency). The Quantum mechanical prediction is the dotted (cosine) curve.
The result was that the distribution curve followed the Quantum distribution not the classical Newtonian distribution. So Quantum mechanics was born
The twin photon experiment has recently been undertaken by Dr. Nicolas Gisin of the University of Geneva and his colleagues. Dr Gisin sent pairs of photons in opposite directions to villages north and south of Geneva along phone optical cables. When the photons reach the end of these fibres they were forced to make random choices between alternative equally possible pathways.
Classical physics predicts that the independent choices of the photons would bear no relationship to each other. But the results were correlated, the independent decisions of the paired photons always matched, even though there was no physical way for them to communicate with each other.
Gisen’s major contribution to quantum mechanical theory lay in his enabling a link across an optical fibre pathway several kilometres long.
Electron Spin Image
Illustration of Bell test for spin-half particles such as electrons. A source produces a singlet pair, one particle is sent to one location, and the other is sent to another location.
A measurement of the entangled property is performed at various angles at each location. The scheme for measurements on photons looks very similar: the quantum state is different but has very similar properties.
''You start with an ultraviolet photon and split it into two photons. One goes one way and the other goes another way, both to identical interferometers. And interferometer is a device for separating and then recombining beams of light.
Entering its own interferometer, each photon must make a random decision as to whether it will travel a long pathway through the device or a short one. Then you look for a correlation between the pathways taken by the photons in their respective interferometers.''
In Gisen’s experiment as in earlier experiments, no signal of any kind was transmitted between the photons. However each photon new what had occurred to its distant twin, and its response dovetailed with its twinned response.
By having a long pathway, an accurate measurement of the speed of the effect could be made. The conclusion was that the response took less than 1/10000 of the time a light beam would have needed to carry the news from one photon to the other. In effect a speed 10,000 times the speed of light at a minimum.
Physicists call this a ''collapse of the wave function.'' The amazing thing is that if just one particle in an entangled pair is measured, the wave function of both particles collapses into a definite state that is the same for both partners, even separated by great distances.
Even more amazing is the knowledge that, due to the phenomenon of superposition, the measured particle has no single spin direction before being measured, but is simultaneously in both a spin-up and spin-down state. The spin state of the particle being measured is decided at the time of measurement and communicated to the correlated particle, which simultaneously assumes the opposite spin direction to that of the measured particle.
Quantum entanglement allows qubits that are separated by incredible distances to interact with each other immediately, in a communication that is not limited to the speed of light. No matter how great the distance between the correlated particles, they will remain entangled as long as they are isolated.
Quantum Teleporting Communication
The key word is “instantaneously.” The entangled particles could be separated across the galaxy, and somehow, according to quantum theory, measurements on one particle should affect the behaviour of the far-off twin faster than light could have travelled between them.
Special Theory of Relativity
Another theory explains quantum entanglement using special relativity. According to this theory, faster-than-light communication between entangled systems can be achieved because the time dilation of special relativity allows time to stand still in light's point of view. For example, in the case of two entangled photons, a measurement made on one photon at present time would determine the state of the photon for both the present and past at the same moment.
This leads to the instantaneous determination of the state of the other photon. Corresponding logic is applied to explain entangled systems, i.e. electron and positron, that travel below the speed of light.
Kinkajou : So what are the key concepts that we learn from quantum mechanics?
Many of the concepts are things we’re not used to dealing with in the real world. Most events in quantum mechanics are defined as events that are a set of probabilities all existing at the same time. This seems to mean instant awareness or linkage between elements of matter even separated by large distances.
Quantum mechanics tells us that we cannot know where everything is every instant in time. Particles can be many places at one time until they are observed. When there are observed are found one location. Quantum mechanics call this the “collapse of the wave function”.
The “classic” illustration of this is the experiment of passing a steady stream of electrons through two slits (figure 5).
Instead of the simple shadows we would expect if the particles were just particles, we see an interference pattern, as if the electrons have dematerialized into a wave and passed through both slits at the same time.
Figure 5: A schematic diagram of the two-slit experiment
Two Slit Experiment
If we deliberately try to observe where the electrons go, we see them as particles somewhere else, but the interference pattern disappears. In effect, the problem is that we cannot say what the particles look like only when they cannot be seen.
Erasmus : However in spite of our frustration with the quantum mechanical world, there are a number of constants.
Energy is conserved.
Energy and matter are made of quanta which have discrete values and not all possible values in between.
Dr AXxxxx : all this fuss about nothing. You people are idiots. It should be obvious looking at the real world that it cannot work in the classical sense.
Imagine one proton and one electron facing each other, stationary in a vast empty space. They will attract each other with electromagnetic force. This force is carried by particle called the photon. Matter and energy are into equivalent. This means the photon has a mass.
When the electron and photon attract each other, the momentum of the system pair changes. The mechanism of change of momentum occurs through the interchange of photons, (Which have a mass equivalent).
Remember our system has no kinetic energy initially as the particles are stationary relative to each other. We can remove thermal energy from system by redefining the system as existing at absolute zero.
Now what happens when these particles are some distance apart, for example in a big empty universe? There will be a stream of photons inter-changing between the two particles. But because it takes a finite time to travel between the particles, if the particles are far enough apart there will be considerable energy lost in transit at least for a time.
You may answer will particles a small segment of energy is very very small especially in view of Planck’s constant.
Rewrite the above example. There is now a mole of protons orbiting in space equidistant to the single electron. A Mole represents 6.02×10 to 23 particles (protons). Perhaps we might have 10,000 moles of these particles in orbit stationery positions distributed over a hemispherical shell with locations equidistant to the electron.
Now we have effectively 6.02×10E27 particles attracting our electron, each requiring a photon of energy to be transferred to allow energy and momentum to be conserved during the “attraction” reaction.
Ridiculous you say. That’s a lot of particles. Actually 10,000 moles of hydrogen protons weigh about 10 kg. Not that hard to imagine at all is it?
So now our proton cloud is sucking 10E27 photons out of our electron. And this occurs recurrently for each second of our electromagnetic “attraction” reaction. We are now losing serious mass from our electron.
(Remember Planck’s constant is of the order of 10E37). However our electron is really small, and cannot sustain the loss of mass. Also as an elementary particle with discrete mass and energy and with initially no kinetic energy, and no thermal energy, it is not able to lose mass.
So where does this scenario take us in Newtonian physics? Our electron, locked in an electromagnetic embrace with its cloud of equidistant protons essentially evaporates, as its mass is converted to electromagnetic photonic energy, travelling to transfer “attraction” to the oppositely charged particle.
We do not believe this occurs.
So we’re forced to a realistic explanation of events, which just happens to concur with a quantum mechanical theories.
Photons are exchanged between the electron and the proton and to travel at speed of light between the two particles. They can be intercepted in transit. However, it is also obvious that they still maintain a presence at the points of origin.
This means the mass in the electron and the proton is conserved, until photon exchange occurs, at which point the mass is still conserved. Our electron does not evaporate.
This exchange would need to occur essentially instantaneously, at the end of the transit, as our electron must conserve mass to remain elementary particle. So in our explanation, we are forced to accept that the photon energy exists both at its source and in transit at the same time, until exchange occurs.
You cannot explain conservation of energy and momentum without accepting this principle.
So the photons in transit exist both in transit and at the point of origin until exchange occurs. Our explanation of events in Newtonian physics concurs with a quantum mechanical explanation of events.
It’s just perhaps not quite what we expected or how we understood it. In quantum mechanical terms, the wave function of the photon overlaps both electron and proton, and is dependent on time and distance.
What is interesting is that the energy is both at its point of origin and in transit at the same time. Experiments or observations can detect the energy at different points, but again it can’t really exist at those points, (unless observed,) as this would force our electron to evaporate as a horde of electromagnetic photons try to transfer energy and momentum to the cloud of protons.
I think it is becoming obvious that there is another problem with our conceptualising this interaction. Is becoming obvious that perhaps time and distance artefacts of our conceptual process of the universe.
While we have these unusual explanations for what is occurring, perhaps you need to consider that in “some” dimension the proton and electron exist adjacent and contiguously. Also while time exists in our perceived universe, it does not appear to exist in this alternate dimension.
If you can accept, that the concepts of time and space are artefacts of our perception of the universe, perhaps there is nothing really “spooky” at all about how these particles interact.
The shadows of these particles that are so evident within our universe, belie a very different reality of the nature of these particles existing in a different dimension to the one which we perceive daily.
Since time and space to not exist in this alternate dimension, events can occur at any distance and can ignore the limitations of the speed of light. “Spooky” physics is the norm. It is our universe and our perception of it that forms a “special case” of reality.
Erasmus : Wow!
Kinkajou : Could be!
Let’s get back to something that we can sink our teeth into. Tell us about the quantum mechanical properties that we can use to advance human technology and cause changes in our world, albeit what we believe to be the real world.