In ancient Greece there was a controversy about the nature of light. Euclid, Ptolemy and others thought that "light" was some sort of ray that travels from the eye to the observed object. The atomists and Aristotle assumed the reverse. Nearly 800 years after Ptolemy, circa 965 CE, in Basra in what is now Iraq, Abu Ali al-Hasan Ibn al-Haytham (Alhazen) settled the controversy with a clever argument. He said that if you look at the Sun for a long time you will burn your eyes: this is only possible if the light is coming from the Sun to our eyes, not vice versa.
In 1672 another controversy erupted over the nature of light: Newton argued that light was some sort of a particle, so that light from the sun reaches the earth because these particles could travel through the vacuum. Hooke and Huygens argued that light was some sort of wave. In 1801 Thomas Young put the matter to experimental test by doing a double slit experiment for light. The result was an interference pattern. Thus, Newton must have been wrong: light had to be a wave.
The double-slit experiment contains a lot of the best aspects of the weirdness of quantum physics, for example a light shining through a small hole or slit (like in a pinhole camera) creates a spot of light on the screen (or film, or detector). However, light shown through two slits that are close together creates not two spots on the screen, but rather a series of alternating bright and dark lines with the brightest line in the exact middle of this interference pattern. This shows that light is a wave since such a pattern results from the interference of the waves coming from each slit.
However, in the year 1900 physicist Max Planck showed that certain other effects in physics could only be explained by light being a particle. Many experiments followed to also show that light was indeed also a particle (a "photon") and Albert Einstein was awarded the Nobel Prize in physics in 1921 for his work showing that the particle nature of light could explain the "photoelectric effect." This was an experiment whereby low energy (red) light, when shining onto a photoelectric material, caused the material to emit low energy (slow moving) electrons, while high energy (blue) light caused the same material to emit high energy (fast moving) electrons. However, lots of red light only ever produced more low energy electrons, never any high-energy electrons. In other words, the energy could not be "saved up" but rather must be absorbed by the electrons in the photoelectric material individually. The conclusion was that light came in packets, little quantities, and behaved thus as a particle as well as a wave.
So light is both a particle and a wave. OK, kind of unexpected, but perhaps not totally weird. But the double slit experiment had another trick up its sleeve. One could send one photon (or "quantum" of energy) through a single slit at a time, with a sufficiently long interval in between, and eventually a spot builds up that looks just like the one produced when a very intense (many photons) light was sent through the slit. But then a strange thing happened. When one sends a single photon at a time (waiting between each laser pulse, for example) toward the screen when both slits are open, rather than two spots eventually building up opposite the two slit openings, what eventually builds up is the interference pattern of alternating bright and dark lines! Hmm... how can this be, if only one photon was sent through the apparatus at a time?
The answer is that each individual photon must - in order to have produced an interference pattern -- have gone through both slits! This, the simplest of quantum weirdness experiments, has been the basis of many of the unintuitive interpretations of quantum physics. We can see, perhaps, how physicists might conclude, for example, that a particle of light is not a particle until it is measured at the screen. It turns out that the particle of light is rather a wave before it is measured. But it is not a wave in the ocean-wave sense. It is not a wave of matter but rather, it turns out that it is apparently a wave of probability. That is, the elementary particles making up the trees, people, and planets -- what we see around us -- are apparently just distributions of likelihood until they are measured (that is, measured or observed). So much for the Victorian view of solid matter!
The shock of matter being largely empty space may have been extreme enough -- if an atom were the size of a huge cathedral, then the electrons would be dust particles floating around at all distances inside the building, while the nucleus, or center of the atom, would be smaller than a sugar cube. But with quantum physics, even this tenuous result would be superseded by the atom itself not really being anything that exists until it is measured. One might rightly ask, then, what does it mean to measure something? And this brings us to the Uncertainly Principle first discovered by Werner Heisenberg. Dr. Heisenberg wrote, "Some physicist would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist independently of whether we observe them. This however is impossible."
Heres a short Video explaining Uncertainty Principle and the Double Split Experiment
Entanglement is a strange feature of quantum physics, the science of the very small. Its possible to link together two quantum particles photons of light or atoms, for example in a special way that makes them effectively two parts of the same entity. You can then separate them as far as you like, and a change in one is instantly reflected in the other. This odd, faster than light link, is a fundamental aspect of quantum science Erwin Schrödinger, who came up with the name entanglement called it the characteristic trait of quantum mechanics. Entanglement is fascinating in its own right, but what makes it really special are dramatic practical applications that have become apparent in the last few years.
Is it possible that entangled particles are not actually in immediate communication, but are simply programmed to behave in the same way? Much like twins separated at birth who live eerily similar lives - assume the same professions or marry similar spouses.
So what is Entanglement? If you take some property of a particle, the equivalent of color, say the spin of an electron, it doesnt have the value pre-programmed. It has a range of probabilities as to what the answer might be, but until you actually measure it, there is no fixed value. What happens with a pair of entangled electrons is you measure the spin of one. Until that moment, neither of them had a spin with a fixed value. But the instant you take the measurement on one, the other immediately fixes its spin (say to the opposite value).
These quantum bits were every possible color until you looked at one. Only then did it become pink, and the other instantly took on another color.
Einstein among other scientists could not accept quantum entanglement. It seems to throw out the whole notion of cause and effect, so how confident are physicists that quantum entanglement exists and what are the implications for science and the scientific method?
Einstein had problems with the whole of quantum physics which is ironic, as it was based on his Nobel Prize winning paper on the photoelectric effect. What he didnt like was the way quantum particles dont have fixed values for their properties until they are observed he couldnt relate to a universe where probability ruled. Thats why he famously said that God doesnt play dice. I think an even better quote, less well known, was when he wrote: I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will, not only its moment to jump off, but also its direction. In that case, I would rather be a cobbler, or even an employee in a gaming house, than a physicist. Einstein believed that underneath these probabilities were fixed, hidden realities we just couldnt see. That was why he dreamed up the idea of entanglement in 1935. It was to show that either quantum theory was incomplete, because it said there was no hidden information, or it was possible to instantly influence something at a distance. As that seemed incredible, he thought it showed that quantum theory was wrong.
It did take a long time to prove that entanglement truly existed. It wasnt until the 1980s that it was clearly demonstrated. But it has been shown without doubt that this is the case. Entanglement exists, and is being used in very practical ways.
Entanglement doesnt throw away the concept of cause and effect. But it does underline the fact that quantum particles really do only have a range of probabilities on the values of their properties rather than fixed values. And while it seems to contradict Einsteins special relativity, which says nothing can travel faster than light, its more likely that entanglement challenges our ideas of what distance and time really mean. Similarly, entanglement is no challenge to the scientific method. We need to use a different kind of math, but this is still the same science.