I have been prompted to write this blog, instead of chilling with a glass of wine after a busy week and watching a movie on TV, because of the flurry of comments via email and Twitter that I have received today regarding the latest news from the Opera neutrino experiment.
It’s entirely my own fault. After the first announcement back in September I volunteered on Twitter, then on BBC television to eat my boxer shorts on live TV if this result is proven to be right. Now, many people mistakenly believe that this second repeated experiment is the confirmation needed for me to fetch the ketchup.
Let me begin by making two statements that I hope are very clear and that I can refer back to if necessary:
- The result from Opera is still only a measurement, not a discovery
- I would absolutely love it if it were true and particles could indeed travel faster than light. It’s heaven for physicists because it means the whole of modern physics is back up for grabs again. We would need something to replace Einstein’s theories of relativity or at least a way of fixing them.
OK, so, briefly, what is all the fuss about? Well, neutrinos are tiny elementary particles that are almost weightless and which pretty much ignore the presence of all other matter. We all have millions of neutrinos streaming through our bodies that arrive from space, mainly from the Sun. And they do this even at night because those neutrinos can pass right through the whole of the earth (when the sun is on the other side) before coming up through the ground, up our feet and leaving to continue through space. Now, neutrinos are so light that they are able to travel almost at the speed of light. We know there are three types of neutrinos (electron neutrinos, muon neutrinos and tau neutrinos). I won’t go into the technical details. Basically, the most common, the electron neutrinos are produced in what is known as beta decay inside the nuclei of atoms.
The Opera experiment involved timing a beam of mostly muon neutrinos between their point of origin at CERN in Geneva and the arrival point at a the Grand Sasso Lab in Italy (which has special detectors than can capture these elusive particles. The travel distance is 730 kilometers and the neutrinos appear to be able to cover this at a speed faster than light. Basically, they arrive 20 billionths of second sooner than light would were it travelling in a vacuum. Of course, even though these neutrinos are travelling underground, it’s as though they are moving through empty space since they don’t interact or bump into anything.
Maximum speed limit
There is nothing that annoys people more about Einstein’s theory of relativity than its claim that nothing can travel faster than light. Why can we not conceive of anything moving at a speed of over a billion kilometres per hour? Granted, this is a stupendously high speed to which nothing that we know of (apart from subatomic particles) can get close, but special relativity seems to be saying that the laws of nature forbid anything from going faster.
This is hard to stomach if you haven’t followed the logical steps and the careful experimental tests of Einstein’s relativity. I do not plan to go through the details but will instead try and give a flavour of why physicists are so confident that there is a universal speed limit. You see in a sense it is not light that is so special that it holds the speed record, but rather that way space and time themselves are intertwined in our universe implies that there is a maximum speed limit beyond which those laws of physics break down. In our universe this speed happens to be 299,792,458 metres per second, or 186,282 miles per second. Light, because it has no mass, is able to travel at this speed. In fact, in the vacuum of empty space, light is unable to speed up or slow down but is constrained to always move at this speed.
There are a number of ways to explain why the speed of light is the upper speed possible in our universe. One method is by using algebra. (Oh great, you’re thinking, that will really convince me; a load of equations full of Greek symbols is just what is needed to put my mind at rest.) I will not go into all the gory details. Suffice it to say that, in special relativity, speeds get added up in a very strange way.
It also turns out that the faster an object moves the heavier it becomes, and the harder it gets to make it go even faster. The closer it gets to the speed of light, the larger its momentum becomes, but this is by virtue of its increasing mass, not its velocity. Consider what happens to an object’s mass when it moves very fast. The single most important consequence of the equations of special relativity is how mass and energy are related. Einstein showed that mass can be converted into energy and vice versa. The two are related through the equation E=mc2, which tells us how much energy is locked up in any given mass. The c stands for the speed of light, and thus the quantity c2 (the speed of light times itself) is a very large number indeed and explains how we can get so much energy out of a small amount of mass. This equation suggests that that we can think of mass as frozen energy.
Since a moving object also has energy due its motion (called its kinetic energy), its total energy will be the sum of the energy frozen as mass when it is not moving plus its kinetic energy. The faster it moves the more energy it has. This means that the real mass of an object will be due to its frozen energy plus the energy due to its motion. Most of the time the frozen energy of an object (its mass) is so much more than the energy of its motion that we can ignore the latter and take the mass to be constant. But as the speed approaches that of light the kinetic energy becomes so great it can exceed the frozen energy. Thus the mass of a fast moving object is much greater than its mass when stationary.
You can now see the problem of trying to attain light speed. Imagine an accelerating train engine pulling a single carriage. What if, for every ten kilometres per hour faster that it goes, another carriage is added. It would therefore have to work harder just to maintain its speed. The faster it goes the more carriages it has to pull, and the more power it needs. In the same way, the faster a body moves the heavier it will seem, and the harder it will be to make it go any faster. To accelerate it up to the speed of light would require an infinite amount of energy, which is impossible.
Finally, the real real real clincher is this: If anything can travel faster than light in our frame of reference, then we will always be able to find another frame of reference (i.e. another perspective from someone moving relative to us) in which it will appear to be moving backwards in time. Remember of course that if Einstein is right then all frames of reference are equally valid (all motion is relative). In this new frame, causality is violated – that is, causes have to come before their effects, otherwise we are left with a paradox. For instance, if A were to shoot B with a faster than light bullet, then it will appear to some observers as though the bullet is moving backwards from B to A’s gun. That is B is shot before A pulls the trigger, so he could decide not to after B is shot!!
See how crazy violation of causality is, and just how much this neutrino experiment needs to explain away???
Could Einstein have been wrong?
Ultimately, the speed of light being the maximum speed limit is written into the fabric of reality itself. But what if we’re wrong? Is there a way of understanding this result? The simple answer is that we cannot with our current theories and understanding. We would need to overhaul the whole of modern physics, and we would need to find a way of explaining away the thousands of other experiments that over the past century have all confirmed that nothing can go faster than light. We may have to bring back the aether, or modify Einstein’s equations. We would have to explain why no other neutrino experiment showed such a result, and why none of the trillions of neutrinos coming from supernovae manage to exceed light speed.
So, yes of course Einstein could be wrong. The whole point of a scientific theory is that it is there to be shot down – to be shown to be false by new experimental evidence or to be replaced with a better, more accurate or more profound theory that explains more about the universe. But… extraordinary claims require extraordinary evidence, and Einstein’s ideas have been checked too carefully for too long for one experiment to come along and destroy all that. But of course that is all it would take if this experiment is proved correct.
Nobel prize winner, Sheldon Glashow, together with Andrew Cohen have predicted that such faster-than-light neutrinos would have to be radiating electrons and their antiparticles, positrons, all along their route from CERN to Grand Sasso via a process called vacuum Cerenkov radiation and hence lose energy. This is not seen. It’s a bit like an aircraft that manages to break the sound barrier silently and without a sonic boom. It just isn’t possible folks.
So, what would it take for it to be possible. I reckon there are two possibilities (there are other more exotic ones that are rather too speculative):
a) Einstein was wrong and there is an aether: technically, what is known as Lorentz invariance is violated here and there is a preferred frame of reference.
b) Einstein was wrong and Lorentz invariance has to be modified: technically, there may be nonlinear correction terms in the mass-energy relation.
I am not prepared yet to buy into these, or notions of tachyons (hypothetical faster than light particles), or wormholes as shortcuts through space-time or replacing the electroweak theory, etc. All this technical hot air basically means I prefer to appeal for now to Occam’s razor and go for the simplest explanation: there is still an error in the experiment.
So what could be wrong with the experiment?
I should say that this experiment is a highly complex one and has been carried out with the utmost care and attention to detail. I am a theoretical physicist not an experimentalist so I certainly refuse to insult my colleagues at CERN and Grand Sasso by trying to point out where they may have gone wrong. They know where uncertainties still lie. So far, they have ruled out one potential source of systematic error.
Not all the scientists involved in the experiment wanted to sign the paper because they were themselves yet to be convinced. After this second check, four of the physicists who had not signed the paper in September now agreed to sign it, but four more who had signed the first one now asked for their names to be removed from the new one.
Having said this, here are a few potential problems:
1. The neutrinos are produced via a complex process: protons from the SPS at CERN are fired in pulses at a carbon target, producing new particles: pions and kaons, which decay to produce muons and neutrinos. The muons are stopped in detectors while the neutrinos continue on to Italy. The start of their journey time is itself not recorded directly but is started from the timing of the proton beam and so the long process has to be subtracted away from total time to leave just neutrino’s travel time.
2. At both ends there are complicated electronics that may contain tiny systematic timing errors.
3. The timing has to be done via GPS satellite. We know that GPS systems only work if we carefully take into account Einstein’s theory of relativity. It seems strange to me that Einstein’s equations (both special and general relativity) need to be taken into account to measure something that is proving them wrong. It just doesn’t make sense. In any case, the experimenters haven’t ruled out an error in the GPS relativistic timing.
The experiment needs to be re-run independently by other particle physics laboratories, and plans are currently underway for this to take place in Japan and the US, but it will take some months at least.
I am happy to eat my boxers on live TV. It would be a small price to pay for the thrill of so much new physics. But let’s not be too hasty just yet, eh?