The Radio Times logo

5 big questions about life, the universe and everything answered by Professor Brian Cox

Why is Earth round? Why are plants green? Why is the sky blue? Is time travel possible? How did life begin?

Published: Monday, 4th July 2016 at 11:12 am

Why is Earth round?

“In the solar system you find that small objects, less than 500 kilometers in diameter, are not spherical. You’ve got to be big enough that the gravitational force overcomes the strength of rock and ice. Why does gravity make spherical things? It doesn’t care about angles.”


Why are plants green?

“Plants absorb red and blue light for photosynthesis – why not green as well? Why aren’t they black? It’s probably one of the frozen accidents in biology – there are two kinds of chlorophyll. They’re both good at red and blue but not green. Green light is wasted. Why? Because chlorophyll was good enough. That’s the thing. Life’s not optimal.”

Why is the sky blue?

“Sunlight is scattered in all directions by particles in Earth’s atmosphere. Blue light waves are shorter and smaller, so they’re scattered more than the other colours and we see more blue in the sky.”

Is time travel possible?

“That’s not really the question. It’s the distance between events that matters. Like, what’s the distance in space and time between getting up in the morning and going to bed at night? That’s how Einstein’s theory sees distance – as moments or events in space in time. Events in space and time aren’t gone; they’re still there. So your memories of an idyllic summer day in 1973 with your mum and dad – that’s still there. Really there’s no definition of the present – you don’t say the past is gone and the future’s yet to come. It’s just a series of points that all exist. But you can’t loop back.”


How did life begin?

“You need liquid water, carbon, stuff like hydrogen sulphide and an energy source that flows through the system. You’re led to volcanic activity of some description in an ocean that has all of the above. That’s probably it.”


Sponsored content