Einstein’s theory of special relativity tells us that anything with mass cannot travel faster than the speed of light in a vacuum.
And that’s a big problem when it comes to travelling between star systems.
If we ever want to travel to, ahem, galaxies far, far away, we’ll need to find a way of getting there within our lifetime. For example, travelling to Alpha Centauri, one of our closest galactic neighbours 4.35 light years away, would take approximately 70,000 years if we made the journey at the same speed as Nasa’s Voyager 1 probe. Even Yoda would struggle with that time scale.
One way to push the boundaries of space exploration is to travel faster than light, which is a mindboggling, 670,616,629mph, or 1.07bn km/hr. By comparison, the fastest manmade spacecraft – Nasa’s Juno Probe – briefly reached 165,000 mph (266,000 km/h).
But according to our understanding of the laws of physics, it’s impossible to break ‘c’, the cosmic speed limit set by Albert Einstein.
The big problem
The main barrier that we – and most particles – have is mass. Any object with mass accelerates, gaining energy, but it always needs more to accelerate further.
So, propelling us to the speed of light would take an infinite amount of energy. ‘There is simply no fuel source big enough to accelerate you or I to light speed,’ Peter William Millington, a research fellow at the University of Nottingham explained.
But that hasn’t stopped scientists trying to find a workaround to this mammoth problem. Teams at the CERN laboratory in Switzerland tried to get neutrinos – the lightest known particles in the universe – to exceed the speed of light but failed.
However, their efforts were not without drama.
In 2011, the OPERA (Oscillation Project with Emulsion tRacking Apparatus) team made an announcement that promised to rewrite our understanding of the universe, by saying they had ‘high confidence’ that neutrinos had travelled faster than light, giving science fiction fans immediate hope that spacecraft might be possible. Theoretically, at least.
However, it turned out that the results were wrong due to a faulty cable connection in the GPS system used to time the particles. This made their journey look around 73 nanoseconds speedier than it was.
So for now, at least, we won’t be able to travel beyond the speed of light based on our current understanding of the laws of physics.
Mind and space-bending work arounds
The race is on find new ways of exploring space without exceeding the speed of light, but they all take advantage of Einstein’s general theory of relativity, which is our best theory so far to explain gravity.
This theory tells us that space and time can be warped by the presence of energy, and they are warped in different ways depending on the form of that energy.
‘By warping and bending spacetime in the right way, and by manipulating the right kinds of matter or energy, a super-advanced civilisation might be able to bend spacetime enough to fold it back on itself, building shortcuts between points in spacetime,’ Dr Millington explained.
This gives rise to hypothetical phenomena like wormholes, which, in theory, could allow a spacecraft to zip one part of the universe to another thanks to such holes in the fabric of spacetime, or cosmic shortcut.
For example, imagine if two star systems are 10 light years apart, it would take 10 years to travel from one to another, but if you sent a light signal through a wormhole cutting the distance to five light years, this would mean the distance between the star systems through the wormhole was half the distance of the normal spacetime path and the light signal would take five years to arrive.
‘Thus, the light travelling through the wormhole would appear to be travelling at twice the speed of light compared to the speed of the light taking the normal spacetime path,’ explained Gerald Cleaver, professor of physics at Baylor University, Texas.
‘Both beams of light were travelling at exactly the speed of light c (in a vacuum). But one had a shorter path to traverse. That’s how wormholes would work and provide apparent faster-than-light travel, when in reality, it would be shorter path travel),’ he added.
But while wormholes are possible in theory, none have ever been found.
Another possibility borrows from Star Trek, showing that fact could be as strange as fiction. Proposed by theoretical physicist Miguel Alcubierre in 1994, the ‘Alcubierre drive’ (which is a little like a sci-fi warp drive) describes a situation in which a spacecraft can be placed inside a ‘warp bubble’. Theoretically, spacetime is squashed in front of it to pull it forward, while space-time behind the craft is expanded, effectively pushing it along, enabling it to appear to travel faster than light, despite spacetime acting normally outside the bubble.
Back to reality
Of course, these scenarios are not without their problems, not least because any space craft would require incredibly advanced technologies and an infinite amount of energy to exploit anomalies in spacetime.
But even if we found a miraculous infinite fuel source, could we survive such a fast journey?
Some scientists have suggested we may have to be put in a state of suspended animation or live in communities on ‘generation ships’ so that the descendants of original astronauts would reach a destination light years away (with the first intrepid explorers dying en-route) while others believe we would simply be killed by radiation, G-force or be ripped apart if we traveled through a wormhole.
Yet, despite these incredible problems, scientists continue to ponder just how we might explore the stars.
‘With the universe, the numbers are not in our favour,’ Dr Millington told Metro.co.uk, explaining that due to the vast distances involved between star systems, ‘even if the first Homo sapiens had set off somewhere over 300,000 years ago, they still wouldn’t have gotten very far.’
So while the idea of sci-fi-inspired space travel may seem silly, studying far-fetched solutions to our best theories, like wormholes, pushes physicists to the limits of their understanding.
‘It is by asking how our current laws of physics behave in extreme settings that we push them to breaking point and can hope to improve on them and correct them,’ he said.
Yet, without new laws or unimaginable technological progress, the Millennium Falcon and U.S.S. Enterprise will remain part of science fiction.
‘Only faster than light travel would enable exploration of by any intelligent species beyond its local stars. Nothing like Star Trek’s ‘United Federation of Planets’ could ever exist without Faster than light ships,’ Professor Cleavers said.
Sarah Griffiths is a freelance science and technology journalist who has written for the BBC, MailOnline and New Scientist
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