小智雅汇 2017-10-13 09:11
We are told that nothing can travel faster than light. This is how we know it is true.
It was September 2011 and physicist Antonio Ereditato had just shocked the world. The announcement he had made promised to overturn our understanding of the Universe. If the data gathered by 160 scientists working on the OPERA project were correct, the unthinkable had been observed.
Particles – in this case, neutrinos – had travelled faster than light.
According to Einstein's theories of relativity, this should not have been possible. And the implications for showing it had happened were vast. Many bits of physics might have to be reconsidered.
Although Ereditato said that he and his team had "high confidence" in their result, they did not claim that they knew it was completely accurate. In fact, they were asking for other scientists to help them understand what had happened.
In the end, it turned out the OPERA result was wrong. A timing problem had been caused by a poorly connected cable that should have been transmitting accurate signals from GPS satellites.
There was an unexpected delay in the signal. As a consequence, the measurements of how long the neutrinos took to travel the given distance were off by about 73 nanoseconds, making it look as though they had whizzed along more quickly than light could have done.
Despite months of careful checks prior to the experiment, and plentiful double-checking of the data afterwards, this time the scientists got it wrong. Ereditato resigned, though many pointed out that mistakes like these happen all the time in the hugely complex machinery of particle accelerators.
Why was it such a big deal to suggest – even as a possibility – that something had travelled faster than light? And are we really sure that nothing can?
We cannot go as fast as light ↑
Let's take the second of those questions first. The speed of light in a vacuum is 299,792.458 km per second – just shy of a nice round 300,000km/s figure. That is pretty nippy. The Sun is 150 million km away from Earth and light takes just eight minutes and 20 seconds to travel that far.
Can any of our own creations compete in a race with light? One of the fastest human-made objects ever built, the New Horizons space probe, passed by Pluto and Charon in July 2015. It has reached a speed relative to the Earth of just over 16km/s, well below 300,000km/s.
However, we have made tiny particles travel much faster than that. In the early 1960s, William Bertozzi at the Massachusetts Institute of Technology experimented with accelerating electrons at greater and greater velocities.
Because electrons have a charge that is negative, it is possible to propel – or rather, repel – them by applying the same negative charge to a material. The more energy applied, the faster the electrons will be accelerated.
You might imagine that you just need to increase the energy applied in order to reach the required speed of 300,000km/s, but it turns out that it just is not possible for electrons to move that fast. Bertozzi's experiments found that using more energy did not simply cause a directly proportional increase in electron speed.
Instead, he needed to use ever-larger amounts of additional energy to make ever-smaller differences to the speed the electrons moved. They got closer and closer to the speed of light but never quite reached it.
Imagine travelling towards a door in a series of moves, in each of which you travel exactly half the distance between your current position and the door. Strictly speaking, you will never reach the door, because after every move you make you still have some distance still to travel. That is the kind of problem Bertozzi encountered with his electrons.
But light is made up of particles called photons. Why can these particles travel at the speed of light when particles like electrons cannot?
New Horizons visited Pluto in 2015
"As objects travel faster and faster, they get heavier and heavier – the heavier they get, the harder it is to achieve acceleration, so you never get to the speed of light," says Roger Rassool, a physicist at the University of Melbourne, Australia.
"A photon actually has no mass," he says. "If it had mass, it couldn't travel at the speed of light."
Photons are pretty special. Not only do they have no mass, which gives them free reign when it comes to zipping about in vacuums like space, they do not have to speed up. The natural energy they possess, travelling as they do in waves, means that the moment they are created, they are already at top speed.
In fact, in some ways it makes more sense to think of light as energy rather than as a flow of particles, though truthfully it is – a little confusingly – both.
Still, light sometimes appears to travel more slowly than we might expect. Although internet technicians like to talk about communications travelling at "the speed of light" through optical fibres, light actually travels around 40% slower through the glass of those fibres than it would through a vacuum.
In reality, the photons are still travelling at 300,000km/s, but they are encountering a kind of interference caused by other photons being released from the glass atoms as the main light wave travels past. It is a tricky concept to get your head around, but it is worth noting.
Similarly, special experiments with individual photons have managed to slow them down by altering their shape.
Optical fibers carry information ↑
Still, for the most part it is fair to say that light travels at 300,000km/s. We really have not observed or created anything that can go quite that quickly, or indeed more quickly. There are a few special cases, mentioned below, but before those, let's tackle that other question. Why is it so important that this speed of light rule be so strict?
The answer lies, as so often in physics, with a man named Albert Einstein. His theory of special relativity explores many of the consequences of these universal speed limits.
One of the important elements in the theory is the idea that the speed of light is a constant. No matter where you are or how fast you are travelling, light always travels at the same speed.
But that creates some conceptual problems.
Imagine shining light from a torch up to a mirror on the ceiling of a stationary spacecraft. The light will shine upwards, reflect off the mirror, and come down to hit the floor of the spacecraft. Let's say the distance travelled is 10m.
Now let's imagine that the spacecraft begins travelling at a hair-raising speed, many thousands of kilometres per second.
When you shine the torch again, the light will still seem to behave as before: it will shine upwards, hit the mirror, and bounce back to hit the floor. But in order to do so the light will have to travel diagonally rather than just vertically. After all, the mirror is now moving quickly along with the spacecraft.
The distance the light travels therefore increases. Let's imagine it has increased overall by 5m. That is 15m in total, instead of 10m.
And yet, even though the distance has increased, Einstein's theories insist that the light is still travelling at the same speed. Since speed is distance divided by time, for the speed to be the same but the distance to have increased, time must also have increased.
Yes, time itself must have got stretched. That sounds wacky, but it has been proved experimentally.
Time can slow down or speed up ↑
It is a phenomenon known as time dilation. It means time travels slower for people travelling in fast-moving vehicles, relative to those who are stationary.
For example, time runs 0.007 seconds slower for astronauts on the International Space Station, which is moving at 7.66 km/s relative to Earth, compared to people on the planet.
Things get interesting for particles, like the electrons mentioned above, that can travel close to the speed of light. For these particles, the degree of time dilation can be great.
Steven Kolthammer, an experimental physicist at the University of Oxford in the UK, points to an example involving particles called muons.
Muons are unstable: they quickly fall apart into simpler particles. So quickly, in fact, that most muons leaving the Sun should have decayed away by the time they reach the Earth. But in reality muons arrive at Earth from the Sun in great numbers. This was something scientists long found difficult to understand.
"The answer to this puzzle is that the muons are generated with so much energy that they're moving at velocities very near the speed of light," says Kolthammer. "So their sense of time, if you will, their internal clock, actually runs slow."
The muons were "kept alive" longer than expected, relative to us, thanks to a real, natural bending of time.
Light travels from the Sun to Earth ↑
When objects move quickly relative to other objects, their length contracts as well. These consequences, time dilation and length contraction, are both examples of how space-time changes based on the motion of things – like you, me or a spacecraft – that have mass.
Crucially, as Einstein said, light does not get affected in the same way – because it has no mass. That is why it is so important that all of these principles go hand-in-hand. If things could travel faster than light, they would disobey these fundamental laws that describe how the Universe works.
That sums up the key principles. At this point, we can consider a few exceptions and caveats.
For one thing, while nothing has ever been observed travelling faster than light, that does not mean it is not theoretically possible to break this speed limit in very special circumstances.
Take, for instance, the expansion of the Universe itself. There are galaxies in the Universe moving away from one another at a velocity greater than the speed of light.
Another interesting situation concerns particles that seem to be expressing the same properties at the same time, no matter how far apart they are.
This is called "quantum entanglement". In essence, a photon will flip back and forth between two possible states at random – but the flips will exactly mirror the flipping of another photon somewhere else, if the two are entangled.
Two scientists each studying their own photon will therefore get the same results at the same time, faster than the speed of light.
However, in both these examples it is crucial to note that no information is travelling faster than the speed of light between two entities. We can calculate the Universe's expansion, but we cannot observe any faster-than-light objects in it: they have disappeared from view.
As for the two scientists with their photons, while they might achieve the same result simultaneously, they could not confirm the fact with each other any more quickly than light could travel between them.
Galaxies are flying away from us ↑
"This gets us out of any problems, because if you are able to send signals faster than light you can construct bizarre paradoxes, under which information can somehow go backwards in time," says Kolthammer.
There is yet another possible way in which faster-than-light travel is technically possible: rifts in space-time itself that allow a voyager to escape the rules of normal travel.
Gerald Cleaver at Baylor University in Texas has considered the possibility that we might one day build a faster-than-light spacecraft. One of the ways to do this might be to travel through a wormhole. These are loops in space-time, perfectly consistent with Einstein's theories, which could allow an astronaut to hop from one bit of the Universe to another via an anomaly in space-time, a sort of cosmic shortcut.
The object travelling through the wormhole would not exceed the speed of light, but it could theoretically reach a certain destination faster than light could if it took a "normal" route.
But wormholes might not be available for space travel. What if instead you actively distorted space-time in a controlled way, to travel faster than 300,000km/s relative to someone else?
Wormholes would be handy, if they exist ↑
Cleaver has investigated an idea known as an "Alcubierre drive", proposed by theoretical physicist Miguel Alcubierre in 1994. Essentially, it describes a situation in which space-time is squashed in front of a spacecraft, pulling it forward, while space-time behind the craft is expanded, creating a pushing effect.
"But then," says Cleaver, "there's the issues of how to do that, and how much energy it's going to take."
In 2008, he and graduate student Richard Obousy calculated some of the energies involved.
"We worked out that, if you assume a ship that's about 10m x 10m x 10m – you're talking 1,000 cubic metres – that the amount of energy it would take to start the process would need to be on the order of the entire mass of Jupiter."
After that, the energy would have to continue being provided constantly in order to ensure the process did not fail. No-one knows how that would ever be possible, or what the technology to do it would look like.
"I don't want to be misquoted centuries from now for predicting it would never come about," says Cleaver, "but right now I don't see solutions."
Faster-than-light travel, then, remains a fantasy at the moment.
But while that may sound disappointing, light is anything but. In fact, for most of this article we have been thinking in terms of visible light. But really light is much, much more than that.
Visible light is only part of the electromagnetic
Everything from radio waves to microwaves to visible light, ultraviolet radiation, X-rays and the gamma rays emitted by decaying atoms – all of these fantastic rays are made of the same stuff: photons.
The difference is the energy, and therefore their wavelength. Collectively these rays make up the electromagnetic spectrum. The fact that radio waves, for instance, travel at the speed of light is enormously useful for communications.
In his research, Kolthammer builds circuitry that uses photons to send signals from one part of the circuit to another, so he is well placed to comment on the usefulness of light's awesome speed.
"The idea that we've built the infrastructure of the internet for example and even before that, radio, based on light, certainly has to do with the ease with which we can transmit it," he points out.
He adds that light acts as a communicating force for the Universe. When electrons in a mobile phone mast jiggle, photons fly out and make other electrons in your mobile phone jiggle too. It is this process that lets you make a phone call.
The jiggling of electrons in the Sun also emits photons – at fantastic rates – which, of course, produces the light that nourishes life on Earth.
Light is the Universe's broadcast. That speed – 299,792.458 km/s – remains reassuringly constant. Meanwhile, space-time is malleable and that allows for everyone to experience the same laws of physics no matter their position or motion.
Who would want to travel faster than light, anyway? The show it puts on is just too good to miss.
By Chris Baraniuk