Encyclopaedia corner: black holes and kilograms

28 July 2020

Interview with 

Adam Murphy, The Naked Scientists


5kg shown on the dial of a scale.


Phil Sansom checked in with Adam Murphy in encyclopaedia corner after the physics & space round...

Phil - Adam - scores from the contestants after that?

Adam - Right so scores after the physics round: we have Chris with five; Megan coming up after that with seven; but thanks to some pretty serious joker play, Bhavesh is on fourteen!

Phil - Oh my goodness. That's a huge performance. Any interesting answers in there that you picked out over in encyclopaedia corner?

Adam - The first one I have to point out is Bhavesh bringing up that ergosphere in black holes, because that's a really weird part of physics. That is a bit where spacetime itself gets pulled around by the black hole. You can't stay still in the ergosphere. It's really weird.

Phil - You have to move towards it? What happens?

Adam - No, you have to move around. You just can't stop.

Phil - There’s no way to do it?

Adam - Because the fabric of space is pulling, so you have to pull with it.

Phil - Wow. Wow. I'm so glad I found that out.

Adam - Everything about black holes is really cool. Like if you get sucked into a black hole, the word scientists use for what will happen to you is called spaghettification.

Phil - Oh goodness! Goodness gracious me. I can imagine what they mean by that.

Adam - Yeah. Slowly you turn into a noodle as you go in.

Phil - Now am I right that that happens because the force of gravity on your feet is so much bigger than that on your head that it actually makes a physical difference, and stretches you out?

Adam - Yeah. Even on Earth there's more gravity at your feet than there is at your head, but at a black hole that gets so, so much worse than you end up being noodled.

Phil - Wow. What a horrible thing to happen.

Adam - Exactly. But a wonderful fact.

Phil - Apart from black holes, any other interesting tidbits?

Adam - So one of the questions there was about kilograms, and we are in a new era of the kilogram, because until 2019, the kilogram was just a block of metal that sat in a vacuum chamber in Paris. And that was the kilogram.

Phil - What do you mean, ‘the kilogram’?

Adam - As in, that was the definition of the kilogram. If you changed that block of metal in Paris, you changed what a kilogram was.

Phil - How strange! I'm thinking that every time I want to know what a kilogram is, I've got to go to the block and compare them.

Adam - Yeah. So the block was called’ Le Grand K’ that was sitting in Paris. And what would happen was other countries would go to Le Grand K - or the international prototype kilogram, for its proper name - and they make copies. And then that would become the German kilogram or the Eastern seaboard of the United States kilogram. And then from those measuring companies would make their own standards. And that's how they'd calibrate weighing scales and things like that.

Phil - What was it made of?

Adam - Platinum and iridium, because that doesn't rust and won't change on its own.

Phil - Now scientists, I know, love to be precise.

Adam - Oh, very.

Phil - Especially physicists. How on earth could they get away with having a block of metal? Because you say it's not going to rust, but surely it might lose a bunch of atoms here or there. And physicists love being that precise. Would that actually make a difference? How do they avoid that?

Adam - It did make a difference. So what they would do is every couple of years they'd measure like Germany's kilogram and the UK's kilogram and all the ones around the world. And what they found was they were diverging; some were getting heavier, some were getting lighter, some were staying the same, and everyone was thinking, “well, if all these ones are changing than the one we've got is definitely changing”. And the race was to find something that could replace the kilogram, because the metre used to be the same until we replaced it with how far light travels in a certain amount of time, which is based on a constant. So we were looking for a constant.

Phil - And what's come out?

Adam - It's based on a mathematical constant called the Planck constant, which is a really, really small number. But the Planck constant also has units of kilogram metre squared per second, which means that if you've got a good definition of a metre and a good definition of a second, then you can pull out a definition of the kilogram that's based on those two.

Phil - What's the Planck constant?

Adam - The Planck constant is this tiny, tiny, tiny amount, zero point 34 zeros, and then like a six. And what it does is it relates the energy of a photon, a tiny single particle of light, to what colour that photon is.

Phil - And because quantum mechanics, of course, it's called quantum because the stuff comes in quantised, discrete amounts.

Adam - Yeah.

Phil - The Planck constant is a constant because of the way this quantum world works. Is that what you're saying?

Adam - Yeah. So you've got a single quantum, a photon of light; if it's this particular colour, it will always have the same amount of energy.

Phil - So is the new kilogram some weird bit of maths involving the Planck constant, a bunch of other stuff like the speed of light, in order to get to a kilogram.

Adam - That's exactly what it is, yeah. So now we don't need to go to blocks in Paris, and that block can have a nice retirement in a museum somewhere.

Phil - I'd love to buy the kilogram on eBay. Wouldn't that be fun?

Adam - Yeah. I own the kilogram. Deal with it.

Phil - That's fantastic.


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