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Asked by flozzah to Joel, Kristian, Tim, Venus, Zachary on 14 Nov 2013. This question was also asked by thelonepotatoe.
Question: what is your favourite scientific experiment?
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Kristian Harder answered on 14 Nov 2013:
Ha, you are probably expecting me to vote for the CMS experiment at the LHC, or any massive particle physics experiment that I’ve been involved in, right?
Wrong!
For me, the winner is….THE DOUBLE SLIT EXPERIMENT!
Specifically, the version of it that sends single electrons through the double slit at a time.
The reason is that this experiment is the single most impressive demonstration of how wrong our intuitive ideas really are about what matter is. You might do this experiment with a laser in school. Shining a laser through two slits, you get a stripey pattern on a screen behind the slits, because light behaves like a wave, and waves can add up or cancel out, depending on whether peak of one wave hits peak of the other wave, or peak hits trough. We call this “interference”, and only waves can do that. So far so good.
Now, do this experiment with electrons. Electrons are said to be particles. So they can’t really cancel out, can they? Well, they do! Even with electron beams instead of laser light, you get interference patterns. So that’s lesson one in quantum physics: particles sometimes also behave like waves. Ok. Kind of acceptable for most people.
Finally, do this experiment with electrons again, but slow it down so much that only one electron gets sent at a time. Now it has nothing to interfere with, it must go either through one or through the other slit, and there can’t be an interference pattern, right?
WRONG! If you do this over and over again and mark where the electrons ended up hitting the screen, you*still* get an interference pattern! The only explanation for that is that each electron basically goes through both slits at the same time, and does some wave-like interference *with itselfMATOMO_URL
If that isn’t odd, then I don’t know what. 🙂 But *that* is what matter is like.
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Tim Hollowood answered on 15 Nov 2013:
My favourite experiment is known as the Greenberg-Horne-Zellinger or GHZ experiment done in 1998 I think. The results is very profound because it proves that quantum theory with all its strange features is the correct theory of nature.
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Zachary Williamson answered on 15 Nov 2013:
Probably the Homestake experiment.
It was a neutrino experiment in the 1960s. Neutrinos are very elusive particles with almost no mass, and they interact with almost nothing. We don’t know all that much about them today: in the 1960s we knew almost nothing.
So this experiment was built to detect neutrinos being emitted by the Sun. It was a hugely ambitious experiment that was technically ahead of its time. It used a giant tank of, well, bleach. The important thing is that the bleach contained a lot of chlorine atoms. Every so often, one of the Sun’s neutrinos would hit a chlorine atom, knock out a proton and turn the atom into an argon atom.
Every week or so, the amount of argon atoms produced would be counted. And this amount was *tiny*, we’re talking about a few atoms per week.
Long story short, the fellow running the experiment, Raymond Davis, found only 1/3 of the argon atoms he expected.
The theoretical conclusion was that most of the Sun’s neutrinos had turned into something else before they reached his experiment, which was correct! His experiment could only measure one type of neutrino (the type the Sun was emitting), but the Sun’s neutrinos were changing into other neutrino types as they traveled.
But…nobody believed him! The poor man had made one of the greatest discoveries in modern experimental physics, but everybody thought he’d just made a mistake. After all, he was trying to collect about 6 atoms a week out of a giant tank of chlorine, and the results contradicted established theories about neutrinos. It took *fifty years* until later experiments proved him correct. It was a marvelous experiment that was far ahead of its time, it’s just such a shame that everybody ignored its results.
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Joel Goldstein answered on 18 Nov 2013:
My favourite experiment used to be adding bicarbonate of soda to vinegar (or vice versa). I did it a lot as a kid, until my mum got fed up with malt vinegar stains all over the furniture.
If I want to be a bit more serious, I would have to say the 1956 experiment carried out by C.S. Wu on the radioactive decay of cobalt 60. First of all, this was a very cleverly designed experiment. Secondly there are too few female physicists, so it’s great to see that one was leading the field 60 years ago. But most importantly, this experiment showed that there was a fundamental difference between left and right, which had never been understood before.
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