Mercury is the smallest and innermost planet in the Solar System, with an orbital period around the Sun of 88 Earth days. The planet is locked with the Sun in what is known as a spin-orbit resonance and rotates on its axis exactly three times for every two revolutions it makes. This means that, from the viewpoint of someone on Mercury, one day would actually be as long as two Mercurian years.
Dr Cox raised this brilliant point during his new BBC show “The Planets”.
He explained that if you were to drop someone off at a point on Mercury and return after the planet made a full orbit of the Sun, a day from their perspective still would not have passed.
He told viewers on May 28: “Mercury is a world of mystery and apparent contradictions.
“It’s in quite an elliptical orbit, which means it can be as far away from the Sun as 70 million km (43 million miles), but as close as 46 million km (28 million miles).
Brian Cox reveals how to stop time
One day on mercury is twice as long as a year
I could stroll in eternal twilight
“That means that temperatures at midday can rise to 430C on the surface, but at night, because it’s a small planet and it’s got no atmosphere, temperatures fall to -170C.
“It’s also locked into what’s called the spin-orbit resonance.”
Dr Cox went on to detail how if he was to slowly walk across the planet, keeping the Sun at the same point, he would have technically stopped time, as a day would never pass.
He added: “[This] means the planet spins precisely three times on its axis for every two orbits.
“And that, in turn, means that its day is twice as long as its year.
“That means that I could walk over the surface like this at about 2mph and keep the Sun at the same point in the sky.
“I could stroll in eternal twilight.”
This means that if Dr Cox was to travel across the planet faster than it could spin on its axis, he would theoretically be travelling back in time.
It is not the first time the English professor has put forward the idea of time travel.
He previously explained why Albert Einstein’s theory of special relativity is key in understanding how we could travel back in time.
Brian Cox is a physic professor
Mercury orbits the sun once every 88 Earth days
The 20th-century scientist developed the idea that space and time are really aspects of the same thing – space-time.
Special Relativity says that something unique happens when you move through space-time, especially when your speed relative to other objects is close to the speed of light.
Time goes slower for you than for the people you left behind, so you won’t notice this effect until you return to those stationary people.
Dr Cox demonstrated this idea during a BBC Doctor Who special, using a member of the audience to help – Professor Jim Al-Khalili.
He set up the Professor in a moving chair across the stage, with a beam of light moving up and down.
He said in 2015: “Jim is going to be moved along the stage while moving the clock and we will dim the lights so we can see what that looks like from our perspective – stationary relative to Jim.
“We’ve placed a head camera on Jim so you can see the clock in exactly the way we pictured it when it was stationary – relative to us – and the light beam is bouncing up and down between the mirrors.
“But if you look, we’ve got a little video effect on there so you can see the trail of the light we see is tracing out a triangular pattern across the stage.”
The experiment then showed how the audience had a different perspective to the light than the Professor.
Dr Cox added: “But what we saw there from the audience perspective was: as Jim moved the light took a triangular path as it bounced across the stage.
“Here is what Einstein’s suggestion that the speed of light is constant for all observers implies.
“See this path is obviously longer that this path, but if we all agree on the speed of light, then it is obvious that it must take the light longer for the moving clock than it does for the stationary.
“Moving clocks run slower – this is true – time really did pass at a different rate for Jim than it did for you.”