In the first six episodes of our new series, Everything You Wanted to Know About…, we talked to physicist and BBC presenter Prof Jim Al-Khalili.


We asked Jim the most-searched-for physics questions on Google and answered burning questions from our readers and listeners, and Jim gave us a guided tour of the key ideas and unsolved mysteries in physics.

In this wide-ranging podcast, we talk about space and time, multiverses and Hot Tub Time Machine physics. Here are six things that stuck with us:


In effect, just thee particles make up the Universe, and all the complexity we see within it

“Everything that we see around us is only made of three types of particles: two types of quarks and electrons. So you have the up quark and the down Quark. Not very imaginative names but they make up the particles, the protons and neutrons, that make up the nucleus of an atom.

"Then buzzing around that nucleus are the electrons. Those three particles alone make up all atoms, and its combinations - how many of them there are per atom, how they are arranged and how they fit together ultimately gives us the variety of different atoms and chemical properties of the everything we see around us.

"So it is quite incredible that from such a small number of ingredients we can get such infinite variety”


The Earth’s core is younger than its surface

“One of the predictions of Einstein's General Theory is that gravity changes space and time. In fact, gravity slows time down. So the stronger the gravitational field, the slower clocks will run.

"Now the Earth has its own gravitational well. So the deeper you get into the Earth, the deeper into the well you go and the slower time will run. So, at the centre of the Earth, a clock would tick by, counting the seconds, the minutes, the years, the billions of years, at a slightly slower rate than a clock on the surface of the Earth.

“We haven’t actually sent a clock to the core of the Earth, but we so trust Einstein’s theory – because we’ve tested it so carefully, that we know this is correct. And it works out that over the course of the Earth’s life, five or so billion years, the Earth’s core, will have aged cumulatively around two and half years less than its surface.

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"This is more than a theory, because, after all, it’s the basis on which our GPS systems work. The mobile phone that you use to locate your position works only because scientists and engineers have deliberately slowed clocks down on satellites since they tick slightly more quickly than those on the surface of the Earth.”


Quantum effects may play out inside our bodies

“There are certain phenomena and mechanisms that have been discovered over the last 10 to 20 years inside living cells that look like they need non-trivial quantum mechanics to explain them.

"So by non-trivial, I don’t just mean quantum mechanics that describes the rules of how atoms bond together, that's standard chemistry. I mean things like quantum entanglement, particles being interconnected across space; quantum tunnelling, particles being able to do the equivalent of a ghost moving through a brick wall; quantum superposition, particles being in two places, two states at once.

"Certain phenomena like photosynthesis or the way enzymes catalyse reactions inside living cells, examples like that, look like they need quantum mechanics to help them out. So quantum biology is a coming together of quantum physics, computational chemistry and molecular biology to see whether these phenomena really do play a role in life.

"Can life exist without quantum mechanics? And maybe quantum mechanics is just going along for the ride, in which case it will be a bit boring. But is there some functional advantage? Has life evolved the ability to utilise the trickery of the quantum world to give it a leg up?

"That's what makes it fascinating and it's still an open question.”


You can never reach true Absolute Zero

“It's similar in a way to the fact that we can never travel at the speed of light. Matter particles can't quite attain the speed of light. They can get closer and closer to it, but never quite reach that speed.

"We can get closer and closer to absolute zero, but ultimately, we get down to the quantum scale where you can have the lowest possible quantum of energy, but you can't get below that.

"There will always be some sort of quantum fluctuation, quantum vibrations at the lowest possible energy that you can have, what's called the ground state. You can never have absolutely no motion, absolute zero.

"There is always a little bit of fuzziness thanks to quantum mechanics.”


Jim rates Looper as a good time travel film, Hot Tub Time Machine 2 not so much

“I'm sort of rather disappointed that it's [is time travel possible?] is still one of the highest search terms for a question on Google in terms of physics, because I wrote a book on this 21 years ago. Clearly people aren't reading my book. Otherwise, they wouldn't have to ask this question.

"But that's okay. That's fine. It's amazing just how many films about time travel there have been over the years. And they've they vary widely in their quality. Some are intelligent and really smart: I remember Looper, which is really great, great movie. It really gets you thinking going back and forth its sort of almost, all sort of makes sense.

And you get others like Hot Tub Time Machine and believe it or not, there was Hot Tub Time Machine 2, which is two hours of my life I will never get back. I don't know why I watched that to the end. It was awful.


You can fall into a black hole gently

“When you fall into a black hole you undergo spaghettification. You get stretched as you fall through an event horizon of a collapsed star, because the gravitational potential is so different across just the length of a human body. So if you fall in feet first, your feet are pulled much more than your head. And I suspect that’s not a pleasant experience. Not that I’ve tried it.

"But supermassive black holes, which reside at the centre of galaxies, are so big that you probably won’t even know the moment that you’d fallen through the event horizon until you wanted to turn around and come back again and realise it’s impossible. So that would be much gentler than falling into a stellar black hole.

The World According to Physics by Jim Al-Khalili is out now (£12.99, Princeton University Press).
The World According to Physics by Jim Al-Khalili is out now (£12.99, Princeton University Press)

Listen to more episodes of the Science Focus Podcast about physics:



Daniel BennettEditor, BBC Science Focus

Daniel Bennett is the Editor of BBC Science Focus. He is an award-winning journalist who’s been reporting on science and technology for over a decade, writing about the science of serials killers, sandwiches, supernovae and almost everything in between.