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Landmark Experiments

Entry 1017, on 2009-05-25 at 20:26:54 (Rating 1, Science)

It is now 100 years since the British scientist Arthur Eddington performed one of the most important experiments in modern physics which confirmed Einstein's Theory of General Relativity. He observed a total solar eclipse from the island of Principe, off the west African coast. Basically this confirmed that space (and time) really are bent by mass (in this case, the Sun) as Einstein predicted.

The reason we don't usually see this effect is that it is very small. Even a large mass like the Sun only distorts space a small amount so it was necessary to observe stars which were really close to the Sun (or appeared to be close because they were in line). Usually these stars aren't visible because the light the Sun is so much greater than the distant stars (even though the stars could easily be intrinsically brighter) so the eclipse was used to block the Sun's light.

OK, so there's my little history, physics and astronomy lesson but what is the point of all this? Well there are certain experiments, observations and theories which just keep coming up in discussions of science. These experiments are so revolutionary and far reaching that, even years after they were carried out, they still get mentioned in discussions of the relevant fields of science. So I started thinking about what other experiments might be in that category.

The first one I thought of, and another one which is very topical (because of the 150th anniversary of its publication), is the Theory of Evolution. This wasn't really the result of an experiment, it was more about careful and meticulous observation and recording, but I still think it ranks as one of the greatest scientific breakthroughs ever.

Another one which comes up a lot in physics and cosmology is the Michelson-Morley experiment. It was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University. It was designed to detect the ether, which physicists at the time hypothesised the existence of because they needed a medium for wave phenomena, such as light, to travel through. The experiment showed the ether didn't exist, although this was so unexpected to some physicists that there were various attempts to rationalise the result.

A more modern example (and those are difficult because great experiments usually only become obvious after their influence has lasted many years) is the observations of the cosmic microwave background by various satellite observatories, especially the Wilkinson Microwave Anisotropy Probe (WMAP). This mission was launched on 30 June 2001 and helped establish many important characteristics of the Universe, including its age to an accuracy of 1%. WMAP is still working and recent observations of anomalous areas of low temperature could be very significant in the future.

Another older result would be Hubble's (I mean the astronomer, not the telescope which was named after him) observations which showed the Universe was expanding. I do seem to be mainly concentrating on physics and cosmology here but that has always been one of my major interests so I guess that's inevitable. Hubble performed some remarkable precision measurements of galaxies which showed they were all (or almost all) racing away from us. This showed the Universe was expanding which was contrary to what most people thought at the time (including Einstein).

I've got to mention one last experiment which is one of my favourites of all time. And it is related to quantum physics, of course! Its the infamous double slit experiment which demonstrates: how particles are waves and waves are particles, but maybe they're neither or both depending on the conditions; how one particle can be in two places at the same time; and how particles change their behaviour depending on whether they are being "watched" or not.

This experiment is still a mystery: not only can't we explain it but I don't think we even know what it means! Richard Feynman (one of the greatest quantum physicists) often said that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.

Unfortunately, even though its been thought about a lot, quantum mechanics is not only the most successful theory but also the hardest to believe! It seems that reality at its deepest level seems totally unreal to humans who are used to thinking at the macroscopic level.

Finally, what will be the next great experiment? I think we need to know two things (again I'm sticking to the big picture - which is cosmology). First, what is dark matter and dark energy, and second (and most impotrant of all) how do we devise a theory which incorporates both quantum theory and relativity? Yes, its the old theory of everything again. We need an experiment to establish whether string theory or other alternatives can be used for the "theory of everything". There's no sign of that happening yet but one day it will.

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