Top Eleven: Ernest Rutherford

Posted by Chad Orzel on February 8, 2006
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The eighth of the Top Eleven is an experiment by the man who set the gold standard for arrogance in physics.

Who: Ernest Rutherford (1871-1937), a New Zealand-born physicist who famously declared “In science, there is only physics. All the rest is stamp collecting.” He was awarded the Nobel Prize in Chemistry in 1908.

When: He’s nominated for the alpha-particle scattering experiments that showed the existence of the nucleus, an 1909.

What: Rutherford is famous for carrying out early experiments with radioactive substances. Among other achievements, he coined the terms “alpha, beta, and gamma” for the different types of particles emitted in radioactive decays, showed that alpha particles are helium-4 nuclei, and demonstrated that radioactive atoms decay into different atomic species.

His most famous experiments involved the scattering of alpha particles off a thin gold foil. At the time, it was believed that atoms consisted of negatively charge electrons embedded in a sort of positively-charged mush (“Like raisins in a pudding” in one famous description). Rutherford hoped to learn something about their structure by bombarding a sample of gold with alpha particles– the positively charged alphas passing through the foil should be deflected by the positive charges in the atom, and the angle of deflection should convey some information about the structure.

In the “plum pudding” model of the atom, you would expect that alpha particles would only be deflected by small angles. Rutherford therefore set a couple of new students, Marsden and Geiger, to measuring background levels by placing their detector (a fluorescent screen that would light up when hit by an alpha particle) at a very large scattering angle– to get to the detector, particles would have to hit the foil and be deflected almost straight backwards, which was thought impossible.

To everyone’s surprise, Marsden and Geiger saw lots of particles deflected at large angles. The always quotable Rutherford later described it thus:


It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.

Why It’s Important: Such large-angle scattering is absolutely impossible in the “plum pudding” model of the atom. In order for the alpha particles to be deflected backwards, the atoms would need to contain a very compact and very massive positively charged nucleus, accounting for most of the mass of the atom.

Rutherford worked this out, and was able to both derive a formula that exactly matched Marsden and Geiger’s data, and to deduce the size of the nucleus. This led to a radical re-thinking of the atomic model, into the “solar system” type picture everybody learns in grade school. Problems in that model led to Bohr’s semi-classical model of hydrogen, and then to the development of the full quantum theory.

Rutherford’s scattering experiments also pretty much established the standard techniques of nuclear and particle physics. Everything we know about the structure of matter at the subatomic scale, we’ve learned by flinging tiny energetic particles at one another.

You can find more details about the experiments at the hyperphysics pages on Rutherford scattering, and you can also play with a nifty little simulation of the scattering process.

Reasons to Vote for Him:: Completely revolutionized our understanding of the atom, invented nuclear physics, produced a number of fabulous quotes, paving the way for future curious characters in physics.

Reasons to Vote Against Him: Physics envy.

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Comments

  1. #1 Jeff
    February 8, 2006

    You’re really getting hard up on reasons not to vote for these experiments, aren’t you?

  2. #2 Ross Smith
    February 8, 2006

    After his comment about physics and stamp collecting, he must have been a trifle embarrassed to receive the Nobel Prize in Stamp Collecting. I seem to recall chemist Isaac Asimov getting some mileage out of this in his popular science essays.

    Meanwhile, in a more recent recognition of his achievements, his face adorns New Zealand’s $100 bill.

  3. #3 Chad Orzel
    February 8, 2006

    Jeff: You’re really getting hard up on reasons not to vote for these experiments, aren’t you?

    Yeah, it’s kind of hard to find serious counter-arguments.

    Ross Smith: After his comment about physics and stamp collecting, he must have been a trifle embarrassed to receive the Nobel Prize in Stamp Collecting. I seem to recall chemist Isaac Asimov getting some mileage out of this in his popular science essays.

    One of his contemporaries noted that Rutherford was awarded the prize for discovering that radioactive decays turned one element to another, and noted that the most remarkable tranformation was that of Rutherford himself from physicist to chemist.

    I also enjoy the story (reported in one of the textbooks I have) that he heard he had won a scholarship to Cambridge while digging potatoes on his family’s farm in New Zealand, and threw down his shovel, declaring “This is the last potato I will ever dig!”

    (In the “At your age, Mozart was dead” file, the same book reports that it was only thirteen years between digging his last potato and winning the Nobel Prize. Of course, it was easier back then. Or something.)

  4. #4 CCPhysicist
    April 30, 2008

    Viewed due to side comment in a 2008 blog:

    Several nit picks:

    It is incorrect to claim that Rutherford was trying to test Thomson’s plum pudding model that was published in 1910. I trust you can figure out why. Thomson’s model was a failed attempt to explain the same data, one that failed due to lack of imagination. He assumed that positive charge was still a fluid (maybe because it was the positive current that flowed in wires) and that the only particles were the electrons that had been seen a decade earlier.

    You correctly point out that Geiger and Marsden did the “Rutherford experiment”, but forget to mention that Rutherford does not even appear as a co-author on the 1909 paper! Rutherford suggested it and undoubtedly participated in the data acquisition, but he does not get credit for the Geiger and Marsden experiment. Rutherford’s name is associated with it because he was the sole author of the 1911 paper explaining the data by what we still call “Rutherford scattering”.

    It is quite wrong to refer to them both as “new students”. Geiger was a 3rd year post doc (PhD at 24). Marsden was, indeed, a student – a 20 year old undergrad supervised by Rutherford.

    You incorrectly state that Rutherford measured the size of the nucleus with that experiment. He did not. He only established an upper bound on its size.