Mirror symmetry
The image you see in a mirror is not real – quite obviously so, since it will have a watch on its right arm if you have a watch on your left arm! It is what is called a virtual image. Your brain interprets the light reaching your eyes as having travelled in a straight line from its source, when in fact it has travelled along a 'crooked' line – it has been reflected from the surface of the mirror. So your brain 'sees' an image the same distance 'inside' (or behind) the mirror as you are standing from the mirror.
The fact that the reflected light is interpreted as having travelled in a straight line from 'inside' the mirror is what changes the handedness of the image your brain 'sees'.
Before the mid-1950s, scientists believed that nature was 'mirror symmetric' – that the laws of physics should not have an intrinsic handedness. In other words, they believed that if you were viewing an elementary particle interaction in a darkened room, you could not tell whether you were viewing the real process, or the image of the real process in a mirror. However, evidence emerged to suggest that one of the four fundamental interactions between elementary particles, namely the weak interaction, could in fact distinguish left from right.
An experiment confirming this suspicion was carried out in 1958 by a female physicist, Chien-Shiung Wu. Prior to the experiment, the Nobel Prize-winning Italian physicist Wolfgang Pauli had stated, "I do not believe that the Lord is a weak left-hander, and I am ready to bet a very high sum that the experiments will give symmetric results." He lost his money!
Antimatter
All matter in the Universe is built up from a relatively small number of elementary particles, which include quarks (the constituents of protons and neutrons) and electrons (which, together with protons and neutrons, make up atoms). Associated with each elementary particle is an 'antiparticle' which also occurs in nature. The antiparticle has the same mass as the particle, but has the opposite charge.
A particle and an antiparticle can combine (or "annihilate") to produce a photon, or a particle of light. Conversely, a particle-antiparticle pair can be produced from a photon. So there is a type of symmetry between particles and antiparticles in these processes. However, for some reason not yet entirely explainable, processes that occurred shortly after the creation of the Universe in the Big Bang resulted in a greater number of particles than antiparticles. This is why we see most matter in the Universe made up of protons, neutrons and electrons rather than antiprotons, antineutrons and positrons (the name given to antielectrons).
There is no evidence to suggest that some parts of the Universe contain a preponderance of particles while other parts contain a preponderance of antiparticles – which is probably lucky, as if our Earth happened to venture into a region of the Universe with a preponderance of antiparticles, we may all end up as photons (light)!
Superfluid
One of the most puzzling phenomena observed in the first part of the 20th century was the behaviour of helium at very low temperatures – with 'very low' meaning just two degrees above ABSOLUTE zero (-271ºC). At these temperatures, the density of helium remains constant independent of temperature, but does not solidify. Normal liquids do not do this.
But the most significant and bizarre observation is that at these really cold temperatures, helium will conduct heat better than ANY metal, and shows NO viscosity. The complete absence of viscosity means that the helium can flow without friction – a hallmark of superfluidity.
For liquid helium this means that some truly odd things happen: when cooled in a hanging dish, a thin film will form and creep up and over the sides of the dish as if it were soup in a ladle determined to get back to the pot! These strange properties were later understood to be very natural examples of purely quantum phenomena.
Helium is just one example of matter that is able to exist in its lowest possible energy state as a collective whole, where all particles have exactly the same energy and even the same identities. Being indistinguishable while simultaneously occupying the same lowest possible energy state is a characteristic of what are called "bosons".
Sometimes bosons display this curious kind of condensation into a 'super' phase. Examples are superfluid helium, electrons in superconducting metals and high temperature superconducting ceramics, and most recently, Bose-Einstein condensates of heavy atoms caught in optical laser traps.
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