Faster than light

Albert Einstein once famously remarked of his theory of relativity, “No amount of experimentation can ever prove me right; a single experiment can prove me wrong.” Now, a particle apparently traveling faster than the speed of light may prove him prophetic even as the theory for which he is most remembered faces a new challenge.

The particle in question is called a neutrino, and its velocity was measured over a journey between CERN laboratories near Geneva all the way to a lab in Gran Sasso, Italy, about 730 km away. Investigators working on the project found the particles arriving sixty nanoseconds earlier than they should have been, an amount of time so tiny that it happens seven and a half million times in the time it takes to blink your eyes. That amount of time may seem miniscule, but tiny numbers tend to carry outsized importance in the world of modern physics.

It is not yet clear what the implications are if this measurement is correct; the speed of light is a foundation stone of modern physics, an inviolable speed limit that has withstood half a century of determined scrutiny by the best minds and most advanced instruments in the world. Yet it might be too soon to write an obituary for the theory of relativity. The equipment used for this experiment is among the most advanced ever constructed, but new technology means potentially untested sources of experimental error. And neutrinos themselves are an elusive quarry, having earned the nickname “ghost particles” for their spooky ability to pass through solid matter and, now perhaps, travel through time.

While physicists rush about confirming or falsifying the basis of the experiment, it is interesting to peek ahead at what bizarre new shapes reality might be taking if it all turns out to be true. To get to that point, it behooves us to understand how modern physics sees the world today.

Through Space and Time

Ever since Einstein, physicists have modeled the universe as consisting of four visible dimensions: the three of space and an additional dimension for time. An object has a set amount of “speed” available to move through the dimensions of space and time. One of Einstein's great insights was the realization that the two types of speed are interchangeable, that is by moving quickly through space you must consequently move more slowly through time.

In our everyday lives we have relatively little movement in the three spatial dimensions, and move through time steadily and inexorably. Only at so-called relativistic speeds (near the speed of light, not coincidentally) does the passage of time noticeably slow. At these velocities you actually age slower relative to someone at rest. Put another way, if you travel at a fast enough speed away from a friend who stays motionless, and then return to where you started, the two of you will disagree about how much time has passed.

There is of course a natural limit, a speed at which you are moving so quickly through space that you no longer experience the passage of time, having “borrowed” all movement through the time dimension. We call that upper limit the speed of light.

To illustrate this principle, let's imagine a universe where relativity reigns and the speed of light is 10. We won't worry about units in this example. Thanks to Einstein we know that you move through both space and time simultaneously, so the sum of your speed through space and through time must always equal 10. In this linear universe you can be moving quickly through one, but as a result your speed through the other will be lower such that they always add up to 10.

When standing still you move through time with a speed of 10 (0+10=10). If you are traveling at a modest speed of 2 through space, you experience slower time than when you were standing still, at 8 (2+8=10). If you reach the speed of light, 10, you do not travel through time at all (10+0=10) since all of your motion is devoted to travel through space. Time stops for you when you travel at the speed of light.

This surprising phenomenon is one of the most famous consequences of the theory of relativity, which Einstein derived from a series of thought experiments in 1905. Since then, its conclusions have been corroborated time and time again in physical experiments, including the observation that time slowed (minutely) for astronauts on the Apollo missions. The framework of modern physics has been built on relativity, and central to the theory is the speed of light (in a vacuum) as the universal, inviolable speed limit.

What were to happen if something could exceed the speed of light? Taking our example universe from above, lets look at an object traveling through space at speed 11. We already established that the object's combined speed through time and space must be equal to 10, so its movement through time must be a negative number, -1, to make the math work. In other words, the object must be traveling backwards in time!

There are further problems with accelerating a mass to the speed of light. For one it would require infinite energy. Infinities in physics are warning signs. When you see one of them pop up, then it's either impossible, the question you are asking is nonsense (what color is jazz music), or the theory you are using just can't deal with the answer, and you need a new one to fill in the gaps.

Photons (light) and other massless particles cheat because they never have to accelerate to the speed of light. Their lack of mass means that they constantly zip around at the universe's speed limit and can never slow down. Thus the speed of light isn't a speed limit because light is special, with nothing allowed allowed to move faster out of respect. Rather light happens to travel at the fastest speed that is possible, because it has no mass and therefore nothing to slow it down.

This is why the speed of light is not a speed limit like you see on the highway or a barrier like the speed of sound; those limits can be broken with effort or if you are comfortable risking a moving violation. The speed of light is a constant, arrived at through theory and confirmed experimentally, which can't be exceeded without implying spooky things like time travel and the loss of causality. Causality is what makes action lead to reaction, and without it consequences are free to precede the act that caused them. If that turns out to be the case then, in the words of Subir Sakar, head of particle theory at Oxford University, “We are buggered.”

Extraordinary claims, extraordinary evidence

Scientists haven't given up on an understandable universe, yet. The most likely explanation for the faster-than-light neutrinos is simply that there are problems with the experiment that have gone thus far undetected. The investigators were aware of the skepticism likely to meet their announcement, and of astronomer Carl Sagan's famous and relevant aphorism that “extraordinary claims require extraordinary evidence”.

A particle exceeding the speed of light is certainly an extraordinary claim, and the researchers were extremely careful to examine their data errors before presenting it. Their observations are the result of three years of observations and calculations by hundreds of scientists doing their best to prove the strange result wrong. Still, the possibility of error cannot be dismissed until the results are corroborated by another independent experiment. Other labs around the world are likely to begin efforts to replicate the experiment, but may take years to definitively uphold or dispel the results.

In the meantime, there remains the possibility that the results are real, and that neutrinos may travel faster than the speed of light. A number of theories exist that might explain how this is possible, though all are highly speculative. Some of them admit the possibility of time travel, some rely on exotic possibilities like hidden spatial dimensions and wormholes. Before we explore those, however, it might help to understand a bit more about the strange world of neutrinos.

Ghost Particles

The existence of the neutrino was inferred by physicist Wolfgang Pauli in 1930, long before one was ever observed. He noticed some energy going missing during the process of beta decay, a radioactive transformation of one element into another. Since energy can't be destroyed, Pauli theorized that this energy was carried away by a neutral particle which was invisible to the detection techniques of the time. This explanation was eventually accepted by the physics community, but a neutrino wasn't officially detected until 1956.

Twenty six years is a long time to look for something, but neutrinos are elusive quarry. They are very small and carry no electric charge, meaning that they barely interact at all with the type of matter that makes up ourselves and the world we inhabit.

Tap a table with your knuckle or hit your head on a rock and you get the definite impression that they are solid. That is because you are made of the same stuff they are. The atoms in your head, the rock, and all matter consist of two parts. The first is a dense and very small nucleus, which takes up about a millionth of a percent of the volume of an atom, but well over 99% of its mass. The rest of the mass is spread amongst a fuzzy cloud of electrons, which composes the vast majority of an atom's volume.

The illusion of solidity is the result of electron clouds' very strong dislike of each other. Electrons are negatively charged, which means they repel each other vigorously and refuse to share the same space. As the electron clouds which make up a rock meets the electron clouds of another object, say your head, the combined violent repulsion between the two groups rebound against the motion, leaving you with a headache and the very strong conviction that matter is hard.

Neutrinos are very different. Since neutrinos have no charge, electrons don't repulse them the way they do each other, and neutrinos pass right through the diffuse haze of electron cloud. The nucleus is solid and dense enough to stop an incoming neutrino, but the chance of one directly impacting the miniscule nucleus is so small that such events are rare. Thus neutrinos can pass through seemingly solid matter as easily as a bullet passes through fog. In fact about 65 billion neutrinos pass through every square centimeter of your body every second on their way from the sun to deep space, and you never notice a thing. Those neutrinos will continue through you, through the earth, and out the other side unimpeded, a property which has earned them the nickname “ghost particles”.

Fifty years after first observing neutrinos, there is a lot we still don't know about them. Originally it was thought that they had no mass and thus traveled at the speed of light, as photons do. Later theories suggested that the neutrino had a small amount of mass and traveled at near but slightly below the speed of light, since particles with mass cannot accelerate to the speed of light. Since we can barely detect neutrinos, it has been quite difficult to pin down their speed, but a fortunate event almost 25 years ago provided a large clue.

On February 23, 1987, at 7:35 a.m. UT, three separate neutrino detectors around the world recorded a 13 second burst of high energy neutrinos far in excess of normal background levels. Three hours later, light from the supernova SN 1987A reached earth after traveling through space for approximately 168,000 years. The supernova was visible to the naked eye and provided astronomers with a rare treat, but proved equally important for physicists looking to measure how fast neutrinos move.

At first glance, it seems like the fact that neutrinos beat the light from the supernova to Earth might support a faster-than-light speed for them. It turns out, however, that this delay was predicted and explained by existing theories. The earliest stage of a supernova includes a massive burst of neutrinos, which may carry away 90% or more of the total energy of the blast. This is shortly followed by a burst of electromagnetic radiation, including light, which is further slowed down (attenuated) as it passes through the gases and debris ejected by the star prior to the supernova. The end result is that neutrinos should form the blast front from a supernova, with light lagging slightly behind, just as predicted. If the measurement of neutrinos' speed made at Gran Sasso was correct for neutrinos in general, the neutrino shockwave would have arrived 18 months ahead of light from SN 1987A instead of 3 minutes. Explaining this discrepancy is one of the greatest challenges facing theorists if the CERN measurement turns out to be correct.

It turns out, though, that there are other glimmers suggesting that neutrinos might not follow posted universal speed limits.

In 2007, a neutrino detector in Michigan measured the speed of incoming neutrinos generated by Fermilab, a high energy physics laboratory in Chicago. The researchers running the experiment measured the neutrinos' speed as slightly above the speed of light, but determined that the inherent inaccuracy of the experiment (the error) was most likely to blame for this seemingly impossible number. In their report they said as much and the scientific community moved on, but the measurements made in Europe cast the possibility of faster than light travel in a more favorable light. Efforts are now under way at Fermilab to update their apparatus to independently verify or dispute the results of the CERN neutrino experiment.

Either way, it seems that neutrinos travel at least at the speed of light, but other experiments suggest that they also have a small but very real amount of mass. Given that an object with mass needs an infinite amount of energy to accelerate to the speed of light, how is this possible?

By never accelerating the neutrino in the first place. There is a loophole in the nature of relativity that can be exploited to allow an object with mass to simultaneously move at the speed of light, provided that object only ever travels that fast. It can never travel slower, never speed up, but like the photon is stuck at one speed from the moment it comes into being to the time it is absorbed or decays.

There are, then, suggestions that our understanding of relativity and the universal speed limit are incomplete, and that neutrinos may be a gateway to investigate further. What if further testing corroborates these faster-than-light neutrinos? If neutrinos can travel faster than the speed of light, is relativity wrong, time travel possible, and every facet of our grasp on reality blown to smithereens? Well . . . probably not.

The shoulders of giants

Scientific progress has not been a smooth road. At infrequent intervals, truly revolutionary discoveries are made which change the context of everything that came before them. Very rarely, however, do they wipe away previous theories entirely. Rather they serve to clarify and extend those theories, offering insight into their underlying reality or expansions of their scope without rendering them entirely invalid.

When Newton formulated his theory of gravitation, it was widely hailed as a breakthrough of historical proportions, for good reason. The movements of planets and stars, comets and fastballs, which had been puzzles since time immemorial, were made explicable by a single, elegant set of mathematical equations. As the theory spread into wide acceptance, many scientists confidently predicted that the universe would be understood in its entirety through mathematical expressions by the end of the nineteenth century.

There was one small hitch in this coup of rationality, namely the erratic orbit of the planet Mercury. The closest planet to the sun, it refused to behave like its brother and sister celestial objects and danced to its own beat, one which unfortunately seemed incompatible with Newton's formulation of gravitation. It wasn't a fatal problem, since Newton's theory held true for every other body in the observable night sky, but it bothered physicists and astronomers greatly.

The solution to this riddle came with Einstein's theory of relativity, which describes gravity as the result mass deforming spacetime in its vicinity, much like a heavy metal weight deforms a rubber sheet it is placed on. Under normal conditions the mathematics of the two theories agree. In regions of high gravity, such as near the sun, predictions made by relativity differ from classical mechanics in small ways, ways that neatly explain Mercury's aberrant orbit. It was an early coup for Einstein's theory and a tectonic shift in our view of the universe, but not one that displaced Newtonian mechanics. Rather it updated old theories, defining their limits without throwing them away.

Faster than light particles notwithstanding, it has been known for decades that relativity is, itself, an incomplete theory. At very small scales and very high energies it gives way, predicting nonsensical infinities and generally becoming an indecipherable mess. Much of late twentieth century physics to the present has been devoted to searching for an expanded theory which explains these discrepancies, updating and smoothing over relativity in the same way that it had modified theories that came before. But laboratory measurements have stubbornly refused to single out such a theory from the many possibilities that have been proposed.

Might the discovery of speeding neutrinos be the break physicists need to fill in our understanding of the universe? What mind-bending new ideas might we be forced to reckon with when that understanding comes? Most importantly, does it mean anything to the rest of the world, or is it only of interest to the ivory towered world of advanced physics?

A shortcut through dimensions

When, 168,000 years ago, the star SN1987A exploded, humans had only recently evolved on Earth. The shock wave from the supernova traveled for the next 168 millenia, spreading out the entire time, until finally reaching Earth. Even in this diminished state, the blast briefly outshone the entire rest of the universe combined (minus the sun, which is so close to the Earth that it's really cheating). The explosion was a titanic release of energy that truly beggars the imagination.

The CERN facility that produces the neutrinos for the Gran Sasso detector sends them off with a hundred times the energy as those launched from that dying star.

The supernova produced far more total energy, of course, by virtue of the massive number of particles involved. But the higher energy per particle produced at CERN may hold the key to explaining why neutrinos in the laboratory seem to behave differently than those observed in the universe, and that brings us to our first possibility for how the seemingly impossible neutrinos might be explained. To picture this we need to borrow a concept from theoretical physics known as M-theory.

Our universe, the only one we know about, has 3 spatial dimensions and a fourth we know as time. In other words, things not only have width, they have height and length and are located in different places at different times. But the fundamental theories of physics don't require 4 dimensions to work. Mathematically they work equally well in 5 or more dimensions, and it turns out that many of the problems facing modern physics can be at least partially explained by assuming the existence of more spatial dimensions beyond the 3 we know, that have somehow remained hidden from observation.

M-theory proposes the existence of no fewer than 11 total dimensions. Within that enormity our own universe may float like a plastic bag adrift at sea, or those dimensions might be rolled up in minute “strings” that stretch throughout the tiniest reaches of reality. The theory also predicts that these dimensions might become observable when sufficiently large energies are involved, such as those present in the particle colliders that produce neutrinos. It's a strange concept to be sure, but par for the course when touring the world of theoretical physics.

Leaping the Gap

Like a car traveling across a valley, a neutrino may travel along the surface of our universe or it may “jump” from one side to the other through the extra dimensions in between. It appears to move faster than the speed of light because it has a shorter path to follow than its low energy companion.

How does this help to explain the faster than light neutrinos? Remember that the neutrinos created at CERN are released with about one hundred times the energy of those from a supernova, a powerful burst not seen since the birth pangs of the universe. At those energies the neutrinos might vibrate with energy so intensely that they temporarily vibrate out of our 3-D universe entirely, cutting briefly through hidden extra dimensions of reality before dipping back into ours.

To an outside observer it might appear that the neutrino moved faster than light if the particle happened upon a shortcut during its brief hop out of our universe (see sidebar). In visualizing this , an analogy might help. Imagine our universe as a flat surface, like the surface of the Earth. Everything that happens in the universe takes place on this surface, while the air that surrounds it represents the extra dimensions proposed by M-theory. Those extra dimensions are normally inaccessible to the residents of our world, who have yet to invent air travel, and they must get from place to place by following the natural contours of the surface.

In some places, like a valley, the surface of this universe is naturally curved. Residents of the universe will travel from one side of the valley the long way, from top to bottom to top, without ever realizing that there is a shortcut through the air above them. If one of their vehicles could be imbued with enough extra power, say by a ramp and a turbocharged engine, they could temporarily leap out of their universe entirely and into the air, crashing down on the opposite site of the gorge. To an observer on the far side of the valley it would seem that the airborne pioneer was able to move much faster than his earthbound companion, even if he never actually reached a higher speed, because he took a shorter path to the other side.

In essence, by producing neutrinos imbued with such large amounts of energy, the researchers at CERN may have supercharged them enough to bounce in and out of our 3-D world. They appear to us to be moving faster than the speed of light, though from their perspective they have merely taken a shortcut.

Tiny wormholes

There is more than one way to take a shortcut through the world of the subatomic. Consider the wormhole, a staple of speculative fiction with a pedigree reaching back to Einstein himself. In its basic form a wormhole refers to two regions of spacetime which are pinched together, forming a bridge that offers a shorter path between two points than is normally available. Like with the previous scenario, an object passing across that bridge wouldn't technically exceed the speed of light, but it could traverse two distant points almost instantaneously.

Small Tunnel Ahead

Wormholes have been theoretically described since the 1930s, but never observed. It may be that small, energetic particles can traverse microscopic wormholes while larger, slower particles will not fit.

While the theoretical basis for the existence of wormholes is sound, one has never been observed. This could be because they form on scales too small or too brief for current instruments to detect. A neutrino, thanks to its small size, might be able to slip through one of these tiny, short-lived wormholes for just long enough to spurt ahead, again fooling an observer into believing that it has moved faster than the speed of light. Larger and less energetic particles, like those that make up matter, can't fit through these wormholes and are doomed to taking the slow path.

Unfortunately neutrinos aren't the only small particles, and there is no obvious reason why faster than light behavior of this kind shouldn't be possible for particles of similar size, like electrons. Given the vast amount of attention that has been devoted to such particles, it is suspicious that none of this behavior has been heretofore observed. If there is something special about neutrinos that predisposes them to skipping across the surface of dimensions, or tunneling through microscopic wormholes, theory has yet to describe it.

Both theories, involving hidden dimensions and tiny wormholes, exploit loopholes in the theory of relativity to move between two points more rapidly than light can, without locally exceeding the speed of light. Thus they avoid the thorny issues of time travel and how to accelerate a particle to faster than the speed of light by abusing a technicality. The third hypothesis tackles these problems head on, positing a particle that actually experiences time flow in the opposite direction as the rest of the universe.

Tachyons, born to speed

Objects moving faster than the speed of light have drawn fascinated speculation since Einstein first suggested it as a speed limit. It shouldn't be any surprise; scientists are as drawn by the prospect of surpassing limits and breaking rules as the rest of us. It was clear from the equations that such a particle would have some weird properties, for instance experiencing moving through time backwards and traveling slower the more energy it had. This seemingly impossible particle was christened the tachyon.

If a neutrino can really travel faster than the speed of light, instead of stepping through some kind of shortcut, it is behaving like a tachyon and must be experiencing time in the reverse order that we do. This seems like a straightforward admission of the possibility of time travel, and in a way it is. But you shouldn't be surprised to hear that there is a way to cheat that may strip time travel of its most vexing consequences.

Let's revisit those two companions, one of whom travels while his friend remains behind. When they reconvene to compare clocks, they will find that the traveler has aged less than the person who stayed put. Now let's allow him to travel faster than light, and see where that takes us.

The traveler steps inside his craft and goes on his way. As he moves faster and faster he observes the universe around him slowing down. At the moment he reaches the speed of light time around his ship stops, and he goes even faster. Like a video on rewind the universe around him begins to retrace its steps, sending the traveler back in time. Finally he slows his ship to a halt and gets out to compare clocks with his friend, who is surprised to see him. The traveler went back to a moment before he ever left, and his friend here has no memory of him leaving in the first place! At this point the time traveler is free to kill his grandfather to prevent his own birth, bet on the winning team for the Superbowl, and generally do the kind of paradoxical things that make thinking about time travel such a headache. Because of these logical inconsistencies, many insist that time travel must be impossible.

Of course we've run into problems accelerating objects to the speed of light before. Neutrinos with mass vexed an earlier crop of physicists because they traveled at the speed of light, but needed an infinite amount of energy to accelerate to that speed. The solution was that they never accelerated at all; they came into existence at that speed, spent their entire existence that way, and were incapable of slowing down. What if our tachyon-like neutrinos did the same thing, moving faster than light at all times without ever slowing down?

That is exactly how tachyons are theorized to work, and that slight alteration to the example has profound consequences.

We again have two observers, but they don't know each other. They can't, because one was born charging along faster than the speed of light, and the other is living a sedate life in the slow lane along with the rest of us. The best they can hope for is a moment while they speed past, where they can look briefly into the others world.

For the longest time the stationary observer doesn't see anything. The fact that a particle is speeding faster than light towards his exact location is invisible to him, because the tachyon outruns its own light! By the time the light from the tachyon reaches our observer, the tachyon itself will have arrived and will be speeding past and away.

As it passes, our observer suddenly sees both the light from the particle's approach and the light lagging behind it as it speeds away. Effectively he now sees two images of the tachyon, one traveling along the particle's path and the other retracing the particle's steps in the opposite direction.

It's much easier to see this with pictures than with words, so take a look at the figure. The key point is that, because a tachyon outruns the very light that lets you see it, watching one pass by is really to watch two tachyons spring into existence, one moving in the direction the tachyon is actually traveling and the other appearing to retrace its steps in the opposite direction.

To make things even more surreal, since the tachyon is obeying the rules of relativity by never moving slower than the speed of light, it also experiences time subjectively moving in reverse when compared to the rest of the universe. While watching the tachyon move along its faster than light course, the observer sees anything traveling with it aging backwards! This isn't an optical illusion or a trick, anything moving faster than light actually experiences events moving in the opposite order that the rest of the universe experiences them.

Looking at it from the point of view of the tachyon makes that clear. When it begins life, it is already moving faster than light. It zips through the universe, outrunning its own light, and at some point passes our stationary observer. In that instant of interaction, the tachyon would see the observer's world passing in reverse. Anything he “dropped” would be seen to leap off the ground back into his hand. If he were writing something, the tachyon would see ink being soaked into his pen as he traced his letters backwards across a page, erasing them. A sneeze would be a bizarre act where the observer sucked up a cloud of spittle from the air in one mighty breath.

All right, so does this mean we can send signals back in time? If tachyons exist (or if neutrinos can act like tachyons), then in a sense time travel is possible. Put more precisely, a tachyon will travel through time by experiencing everything in our universe backwards. But the key point is that this is how it experiences time, but we still see it moving forward, through time, from A to B as any other particle does (albeit at faster than the speed of light). The fact that it disagrees, and sees itself as moving from B to A, is interesting but not paradox-causing because it can never slow down and join our frame of reference, thus it can never travel back from B to A to relay a message back in time.

Thus the universe remains neatly segregated into the slow lane, where time passes as we are accustomed, and the fast lane, populated by tachyons and their relatives, where time travels in reverse. And never the two shall meet. Paradoxes are avoided, time travel technically allowed but stripped of its logical inconsistencies, and faster than light neutrinos, maybe, explained.

So what is true?

The simple fact is that we don't really know what is happening in the experiment yet. We don't even know if the results are true, and it will take a long time and many experiments to bear that out. In the meantime we can look to the horizon to imagine what frontiers await exploration, and how seeing the world in a new light, supported by what we know about physics and what we might learn, could transform our universe forever.