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The art of guestimation

By Matin Durrani

Enrico Fermi was a real lover of back-of-the-envelope “guestimation” calculations and was fond of posing them to his would-be PhD students.

The Quantum to Cosmos festival is on now in Waterloo, Canada

He famously asked how many piano tuners there are in Chicago and in July 1945 calculated the strength of the first atomic-bomb test blast by dropping pieces of paper before, during and after the explosion.

It is that ability of physicists to make rough “ball-park” estimates, off the cuff, of various quantities that inspired today’s “Art of Guestimation” event at the Quantum to Cosmos festival in Waterloo, Canada.

Holed up in the Princess Twin cinema were three young physicists – Sarah Croke and Robin Blume-Kohout from the Perimeter Institute for Theoretical Physics and Robert McNees from Loyola University in Chicago — who were given 10 minutes to answer various “Fermi questions” put to them by the audience before the gong went.

We had time for five questions, which are listed below, along with the panel’s answers. As with all these things, there are no right or wrong solutions. The point of the session was to show the logical way that physicists think when they want approximate solutions.

How much memory would an individual person need to store everything they could see in a lifetime? About 1 exobyte – on the assumption that the eye works like a movie film, storing visual information at about 30 frames a second, with each frame being stored in high definition (1920 x 1080 pixels) and with each pixel needing 32 bits to store colour. (The panel ignored what happens when you sleep, which would only open another can of worms.)

How many humans have ever lived since Homo Sapiens first walked on the planet? This question has been asked before – it’s about a hundred billion. Very roughly speaking, there as many people alive now as have ever lived.

How many “eh”’s would a typical Canadian say in a lifetime? (Bit of a silly one this – the “joke” is that Canadians say “eh” a lot.) The panel’s answer was seven million, assuming Canadians talk for three hours a day, that each sentence lasts five seconds and one in 10 sentences include the word “eh”. Eh?

How many Loonies are there in circulation? (No, we’re not talking mad people, but Canadian one-dollar coins.) This got the panel really stuck – their final answer was between two and four hundred million before the gong went.

How much salt is there in the Atlantic Ocean? The critical point was knowing how much salt there is in a litre of sea water. Just multiply that number by the volume of the ocean to give, ooh, about 10 to the power 19 kg.

The session was a lot of fun. Although I am not sure if this kind of event has ever been done before, I reckon it could be a winner at other science festivals too. It certainly got the audience involved, which has to be a good thing.

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  1. Matin,
    Thanks for coming to see us! It’s pretty neat to get a writeup in Physics World.
    I did a little research afterward on “How many loonies are in circulation.” According to the Royal Mint’s statistics, about 930 million loonies have been minted since 1987. If all of them are still in circulation, then our answer of 300 million is correct to within a factor of 3!
    Actually, over half of the loonies were minted in the first 3 years. So, although the coin has proved exceptionally durable, some of the coins have been taken out of circulation due to wear — meaning that we were probably correct to within a factor of 2. Not bad, I think!

  2. James Hultman

    “How many humans have ever lived since Homo Sapiens first walked on the planet? This question has been asked before – it’s about a hundred billion. Very roughly speaking, there as many people alive now as have ever lived.”
    Since when does a hundred billion equal six and a half billion? What am I missing?


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