first conquer the vast distances of space. All About Space investigates the fact and theory behind travelling at the speed of light.
Scientists have recognised the speed of light as the ultimate speed limit of the universe for more than a century now — ever since Albert Einstein put forward his revolutionary special theory of relativity in 1905. Einstein developed his theory in order to account for a series of problems that beset the physics of the time — most importantly the fact that light seemed to travel through a vacuum at the same speed, regardless of the relative motions of source and observer. In other words, light arrives at Earth from a distant star at the same speed regardless of whether the star is moving towards or away from us -think for a moment about how that compares with the behaviour of a tennis ball thrown by someone on a moving train and you’ll see why that’s so weird. The speed of light in vacuum, 299,792.5 kilometres per second (186,282 miles per second), is a universal constant, often written as ‘c’.
In order to explain this strange behaviour, Einstein realised he would have to rewrite the laws of physics from the bottom up. The resulting theory of special relativity mimics the well-established rules of ‘classical’ physics in everyday situations, and only diverges when objects are travelling at speeds comparable to c (so-called ‘relativistic’ speeds). Here, though, things start to get very strange — from the point of view of a distant observer, relativistic objects appear to get shorter in the direction of their travel; to increase in mass (making it harder to accelerate further); and strangest of all, to experience time more slowly.
These weird effects, Einstein explained, were inevitable consequences of the principle of relativity — the long-established idea that the laws of physics behave the same way in all ‘inertial’ frames of reference.
In other words, if a moving train is travelling at a constant speed, then it’s actually impossible for someone on board to conduct an experiment that proves they’re the one in motion, rather than someone doing the same experiment on the platform rushing past the window. (If that sounds weird, then remember how many thousands of years it took people to figure out that Earth is rotating daily on its axis and orbiting the Sun, rather than sitting still among countless spinning celestial spheres.) From the point of view of a person on a relativistic spaceship, everything would seem to be perfectly normal — in fact the strange effects would seem to be happening to the rest of the universe!
Einstein realised that these effects got increasingly extreme towards c, until they spiralled out of control at the speed of light itself: an object travelling at the speed of light would have zero length, infinite mass, and no experience of time. Such a situation was clearly impossible, so travel at light speed was impossible and c set an ultimate speed limit for the universe (photons of light and other radiations can only travel at c because they are completely massless). Special relativity has now been proven by any number of experiments — for example, atomic clocks flown aboard satellites or high-speed jets, have been shown to slow down compared to those in ground-based laboratories, thanks to the effects of time dilation.
The speed limit of c seems to frustrate our dreams of exploring the universe. The scale of interstellar space is daunting — even the closest stars are light years away from Earth, our galaxy is more than 100,000 light years across, and most other galaxies are many millions of light years away, so even if a spacecraft could somehow propel itself to 99.999 per cent of the speed of light, it would still require these kind of timescales to explore the universe. As a result, even the most enthusiastic advocates of interstellar exploration have assumed it would require crews in deep-frozen suspended animation for many years, or enormous ‘generational’ starships that would eventually deliver the descendants of the original crew to a home in a new solar system.
But even though astronomers and physicists accept special relativity as a fact of life, they’ve still discovered a variety of situations — some natural, some highly artificial, in which faster-than-light motion, or at least the illusion of it, can take place.
For instance, one intriguing effect, which at first seems to break Einstein’s rules, is known as a ‘superluminal jet’. These strange phenomena are usually associated with the violent conditions around black holes, and involve streams of genuinelyrelativistic particles shooting out in a jet pointing almost (but not quite) directly towards the Earth. In these special circumstances, light emitted by a particular region of the jet at a later time can ‘catch up’ with that emitted at an earlier stage, giving the impression that the jet itself is moving faster than light (see the ‘Superluminal jet illusion’ boxout on page 33).
So far, all the natural phenomena observed in the universe have turned out to be consistent with relativity and the ultimate speed limit of c. But in March 2011, a team of astronomers at Italy’s Gran Sasso National Laboratory reported measurements that seemed to show that beams of neutrinos fired from the Large Hadron Collider at CERN on the Swiss-French border, some 731 kilometres (454 miles) away, were arriving at their own OPERA detector slightly sooner than expected — so fast, in fact, that they appeared to be travelling slightly faster than the speed of light.
The Gran Sasso team did not really believe that they had found neutrinos breaking the rules of relativity. Instead, their announcement was an appeal to the wider scientific community for help in analysing their experiment and pinpointing possible errors. And indeed, within a year several equipment issues had been identified and the revised measurements brought back into line with expectations.
But what if particles could travel faster than light? In 1967 Columbia University physicist Gerald Feinberg put forward the suggestion that an entire universe of new hypothetical particles with speeds greater than that of light might exist. These ‘tachyons’ (from the Greek word for swift) would avoid the problem of travel at с itself by never travelling that slowly However, other theoretical physicists soon showed that the tachyon properties Feinberg proposed would have different consequences from those he predicted. What’s more, the widely accepted models of particle physics seem to work pretty well without faster-than-light particles, so tachyons remain little more than a fascinating thought experiment.
Another particle phenomenon, however, really does seem to break Einstein’s golden rule. This is quantum entanglement — an effect in which two atomic or subatomic particles interact with one another in such a way that certain properties or ‘quantum states’ become intrinsically linked together. According to quantum theory, on the smallest scales particles have an uncertain, wave-like nature, with properties that only become fixed when they are measured. In theory, it should be possible to prepare a pair of entangled particles, then separate them by a great distance (perhaps taking one away in a spaceship while keeping the other one on Earth). When the state of the one particle is finally measured, the other particle’s complementary state also ‘collapses’ instantaneously, wherever it is in the universe, and can then be measured. Einstein himself memorably referred to entanglement as «spooky action at a distance», but since his time it has moved beyond the realms of theory into reality. In September 2012, scientists at laboratories on the islands of La Palma and Tenerife demonstrated the phenomenon of simultaneous collapse across a record distance of 143 kilometres (89 miles).
In the future, quantum entanglement could form the basis of a new generation of superfast computers, but does the phenomenon really break the speed of light? Not strictly, say the experts, explaining that the phenomenon only enables observers at two locations to instantaneously discover the same information — nothing is communicated directly from one observer to the other.
This, then, is the fundamental rule of relativity — that it’s impossible to transmit information faster than the speed of light. If it was then causality itself, the universal law of cause and effect, could be undermined — for instance, you could send a message that would arrive before you had even sent it, and the recipient might then send one back in the same way, arriving in time to stop you sending them the original message and creating a paradox.
However hard we might try, it seems that the cosmic speed limit is unbreakable — so are we doomed to stay trapped forever in our small corner of the universe, with dreams of star travel limited to science fiction or the most ambitious, one-way-ticket projects? Not necessarily — we can’t break the speed of light, but Einstein’s own work gives rise to ways in which we could bend it.
Ten years after the special theory, Einstein published his theory of general relativity, which remodelled space and time as a four-dimensional ‘manifold’ that can be warped by the effects of gravity. He suggested that the bending of space and time around massive objects such as stars could create similar effects to those seen at relativistic speeds in special relativity. For instance, the three dimensions of space might become compressed or curved, while the time dimension could become extended. It’s another idea that sounds crazy, but was developed from the simplest principles and was soon proved to be correct thanks to the discovery of ‘gravitational lensing’ — the way in which the straight paths of light rays travelling through space are deflected as they pass through the warped space close to massive objects.
One common way of thinking about general relativity is to think of space-time as a rubber sheet in which heavy objects create dents or ‘gravitational wells’. The heavier or more concentrated the object, the deeper the well and the greater the effect on its surroundings. In 1935, Einstein and his colleague Nathan Rosen published a paper in which they outlined the theoretical possibility that two such wells in different regions of the universe might connect together, creating a shortcut known as an Einstein-Rosen bridge. Today, mostscience fiction fans know the concept better by the popular nickname coined by physicist John Wheeler in 1957.
Wormholes offer a potential shortcut that gets around the cosmic speed limit — in theory the distance connecting two points through a wormhole might be much shorter than the distance across normal space — so you could fly down a wormhole at conventional or ‘subluminal’ speed and still reach your destination more quickly than a beam of light taking the long route between the same two points.
Wormholes are a fascinating area of study for theoretical physicists, but they still present huge problems. Although nothing in general relativity forbids their existence, their existence could undermine causality and even (as American cosmologist Kip Thorne proved) be used to build a time machine. For this reason, some experts have speculated that a real-life wormhole would have to connect with a parallel universe rather than another part of our own.
What’s more, in 1962 John Wheeler showed that the forces around any naturally forming wormhole would cause it to close up within an instant of its creation — sustaining an artificial wormhole would require the use of ‘exotic matter’ with negative energy density (something which sounds entirely fantastical but may not be as far-fetched as it sounds — see our interview with Dr Richard Obousy, the president of Icarus Interstellar, on the following page).
But even if wormholes do turn out to be impossible, things aren’t quite as bad as they seem. If a starship could achieve sufficient speed for the effects of special relativity to kick in, it could take advantage of the time dilation effect. While everything would seem normal for the crew, their experience of time would be slowed down considerably compared to the surrounding universe, so that a journey of several dozen light years at near-light speed might only take a few months for those on board the starship. Such a mission would be a one-way ticket into the future, leading to some rather strange phenomena — for example, an explorer might witness their twin sibling left behind on Earth age much more rapidly. Sceptics about Einstein’s theories have tried to suggest there is a paradox here — surely both twins should see the same phenomenon? But this is ignoring the fact that special relativity only applies to ‘inertial’ (non-accelerating) situations — and the explorer twin will likely spend a great deal of their voyage either accelerating or decelerating.
In theory, if a spacecraft could reach sufficient speed, time dilation would allow humans to make not just interstellar, but even intergalactic trips in manageable time spans. But there are huge practical challenges in generating the power needed to reach such speeds — suggested mechanisms include controlled nuclear explosions, and the transfer of energy from powerful laser beams.
Perhaps the most intriguing possibility of all, though, is the ‘warp drive’ whose principles were outlined by Mexican physicist Miguel Alcubierre in 1994. Alcubierre’s research described how a bubble of space-time, driven forward by the expansion of space-time behind it and contraction in front of it, could theoretically propagate across the universe at hyperfast speeds, without requiring the spacecraft within this bubble to break the light-speed barrier in its own frame of reference. Alcubierre’s idea was purely theoretical, but it has so far stood up to scientific criticism, and scientists such as Dr Richard Obousy of Icarus Interstellar have now proposed ways in which such a drive might be created in practice.
Researching warp drives and wormholes.
Dr Richard Obousy, president of Icarus Interstellar, tells All About Space how science is beginning to encroach on the stuff of science fiction.
What first got you interested in the subject of interstellar travel?
My interest in space and space exploration was galvanised by my interest in astronomy. I remember in my early teens happening upon a book on astronomy which opened my eyes to the vastness of the universe and helped me realise that there’s so much more than our small planet. I also developed an interest in science fiction around the same time and read everything I could by sci-fi greats like Clarke and Asimov, which compelled me to explore the technical aspects of interstellar flight.
Conventional proposals for travel to the stars still present huge technical challenges, even if they don’t require groundbreaking physics…
Yes, because the stars are so far away one typically has two options. Either you travel very fast, or you take a very long time to get there. For the first option chemical rocket fuel is inadequate to make trip times on the scale of human lifetimes, so one has to look into more exotic means. One proposal made initially by the British Interplanetary
Society in the Seventies called Daedalus involved detonating thermonuclear pulse units inside a reaction chamber at a rate of 250 detonations per second over a period of several years. This would have allowed the spacecraft to reach a top speed of 12% the speed of light.
But if you get close to the speed of light, strange things start to happen.
Yes, Einstein showed us that, as an object’s velocity increases, relativistic effects manifest themselves. One example is time dilation, famously explained through the ‘twin paradox’.
In this example twin A stays on Earth, and twin B crews a starship travelling close to the speed of light. When twin B returns to Earth she’s aged but a few months, while twin A is now in old age.
But even with time dilation, you’re still looking at very long journey times — are there any other options?
You’d be looking at long trip times from the context of an Earth-based observer. Loopholes in the restrictions of special relativity have been appearing in physics for some time. The two most famous examples are the wormhole and the ‘warp drive’. Wormholes are essentially tunnels through space-time that connect two distant points and allow for near-instantaneous travel between them. A warp drive is a specific distortion of space-time that contracts the space-time in front of a ‘craft, allowing one to travel by manipulating space instead of moving through it. Space-time itself isn’t restricted by special relativity, which is why these constructs are so fascinating.
However, all faster-than-light (FTL) schemes are speculative at this stage and require exotic matter to create and prodigious amounts of energy. None are practical with today’s technology.
You’ve recently been looking at how a warp drive might operate in practice?
That’s not strictly correct. I was looking at a model of dark energy, which is an energy field that has the effect of causing space-time to inflate. I examined the energy requirements necessary to artificially amplify the magnitude of dark energy in the vicinity of a spacecraft to create the type of spacetime modification one would need.
But the nature of dark energy is a big mystery in itself.
It’s called theoretical physics for a reason. Under the models I looked at, dark energy can be explained through an enigmatic form of energy known as Casimir energy, which has a non-zero value in the higher dimensions of space that have become popular in recent ‘superstring’ theories of the universe’s fundamental structure. Dark energy itself is an artefact of higher-dimensional Casimir energy, showing up in the four dimensions that we can perceive.
My analysis showed that the energy requirements to create a warp bubble could be reduced dramatically — instead of requiring the mass energy in a typical galaxy to create the warping effect, ‘all’ you’d need is the mass energy contained within the planet Jupiter.
And by manipulating space in this way, you could actually reach or exceed the speed of light?
Yes, one could stimulate space-time to behave in such a way that it mimics the classical warp drive.
Is there any hope of testing it?
Some work I did in 2009 indicated that it could be tested with energy levels accessible by the Large Hadron Collider. I’d like to encourage theoreticians not to shy away from using theoretical physics to explore novel propulsion paradigms.
It was barely 70 years from the discovery of the subatomic constituents of the atom to the first working nuclear thermal rocket. First comes the physics, next comes the technology.