In 1915 in Berlin, at the height of WWI, Albert Einstein presented a revolutionary new theory of gravity – the General Theory of Relativity. It has since become one of the most successful theories ever, passing every observational test thrown at it and predicating cosmological phenomena such as the Big Bang, black holes and gravitational waves. But the theory has also given scientists sleepless nights because it makes one thing pretty much unavoidable: time machines…
It comes down to the fact that, in Einstein’s theory, time is not absolute, ticked off by a universal clock with which everyone agrees, but instead is relative. “I can’t talk to you in terms of time – your time and my time are different,” wrote the English novelist Graham Greene.
According to Einstein, the rate at which time flows for someone depends on how fast they’re moving relative to you and the strength of the gravity they’re experiencing. If you can find a way to jump from a region where time flows at one rate to a region where it flows at slower rate, you can go back in time – you’ll have made a time machine.
The recognition that time is not what you think it is goes back to the Special Theory of Relativity that Einstein published in 1905, and it all hinges on the unique properties of the speed of light. Einstein realised that nothing can travel faster than light – it is the cosmic speed limit of our Universe. This makes light uncatchable by anything. He also discovered that intervals of space and time stretch like elastic as massive objects move through them. By a cosmic conspiracy this means that everything measures exactly the same speed for a light beam, no matter how fast that thing is travelling or in which direction.
To be a little more precise, moving clocks run slow. So, if someone flies past you – and it has to be at a speed approaching 300,000 kilometres per second – then their clock will run slow compared to yours. If they could ever reach the speed of light – which is impossible for a material body, though possible for a massless entity such as a particle of light (a photon) – time would come to a complete standstill.
OPTION 1: Travel faster than light
Denne historien er fra November 2018-utgaven av BBC Earth.
Start din 7-dagers gratis prøveperiode på Magzter GOLD for å få tilgang til tusenvis av utvalgte premiumhistorier og 9000+ magasiner og aviser.
Allerede abonnent ? Logg på
Denne historien er fra November 2018-utgaven av BBC Earth.
Start din 7-dagers gratis prøveperiode på Magzter GOLD for å få tilgang til tusenvis av utvalgte premiumhistorier og 9000+ magasiner og aviser.
Allerede abonnent? Logg på
World's First Malaria Vaccine
The World Health Organization’s director-general hails ‘historic moment’ as mass immunisation of African children begins
Is River Pollution Putting The Species In Jeopardy Again?
Ten years ago, it was jubilantly announced that o ers had returned to every county in England. But is river pollution putting the species in jeopardy again?
The Big Burnout
Long hours, low pay and a lack of appreciation — among other things — can make for a stressful workplace and lead to burnout. It’s something we should all be concerned about, because over half of the workforce reports feeling it
Putting Nature To Rights
More countries are enshrining the right to a clean environment into law. So if a company or government is impinging upon that right, you could take them to court
Mega Spaceship: Is It Possible For China To Build A Kilometre-Long Spacecraft?
Buoyed on by its successful Moon missions, China has launched a five-year study to investigate the possibility of building the biggest-ever spacecraft
Are We Getting Happier?
Enjoying more good days than bad? Feel like that bounce in your step’s getting bigger? HELEN RUSSELL looks into whether we’re all feeling more cheery…
“Unless the Japanese got the US off their backs in the Pacific, they believed they would face complete destruction”
Eighty years ago Japan’s surprise raid on Pearl Harbor forced the US offthe fence and into the Second World War. Ellie Cawthorne is making a new HistoryExtra podcast series about the attack, and she spoke to Christopher Harding about the long roots of Japan’s disastrous decision
Your Mysterious Brain
Science has mapped the surface of Mars and translated the code for life. By comparison, we know next to nothing about what’s between our ears. Over the next few pages, we ask leading scientists to answer some of the most important questions about our brains…
Why Do We Fall In Love?
Is it companionship, procreation or something more? DR ANNA MACHIN reveals what makes us so willing to become targets for Cupid’s arrow
Detecting the dead
Following personal tragedy, the creator of that most rational of literary figures, Sherlock Holmes, developed an obsession with spiritualism. Fiona Snailham and Anna Maria Barry explore the supernatural interests of Sir Arthur Conan Doyle
