We all know that nothing can travel faster than c, the speed of light in a vacuum; or do we? A recent experiment called OPERA at CERN measured some neutrinos travelling fractionally faster than c. To be precise: (v-c)/c is approximately 3x10-5 where v is the observed velocity of the neutrinos. An earlier experiment called MINOS also appeared to have detected superluminal neutrinos; but this result was largely ignored because the superluminality was less than the error margin of the experiment – and because we all know that nothing can travel faster than c.
Jarah Evslin (full disclosure: he's my son) is delivering a paper today at a conference in Beijing on what it may mean for physics if the OPERA and MINOS results are proven to be correct by further experiments. He's a physicist at the Theoretical Physics Center for Science Facilities, Institute of High Energy Physics, Chinese Academy of Sciences.
Cautiously and correctly, Jarah says:
"I will not speculate on whether this claim is correct or incorrect. In my opinion this determination can only be made by future experiments, and it will be made by future experiments perhaps within a few months, probably within a few years. Instead I will ask the following question: If neutrino superluminality is confirmed by future experiments, what does it teach us about physics? In other words, what models are consistent both with OPERA's results, and with other known experimental and theoretical constraints?"
Turns out that a dark energy model he and his colleagues have been working on may explain what might have been observed at CERN.
Any explanation for superluminality must also explain why neutrinos from a supernova observed in 1987 (SN1987A) apparently did not travel any faster than the speed of light and may even have been a little slower. (Actually the SN1987A neutrinos did get here a few hours before the light from the event which created them; but the light gets slowed down a little as it travels through cosmic dust, which most neutrinos sail right through without slowing down. When you allow for that, the neutrinos were NOT moving faster than light in a vacuum). If the SN1987A neutrinos were traveling as fast as OPERA would seem to indicate, they should've arrived 4.8 years before the light since both travelled 160,000 light years from the Tarantula Nebula (my calculation).
Jarah lists four differences between the space neutrinos from SN1987A and those produced for the OPERA and MINOS experiments. At least one of them could account for the observed speed difference.
- Lepton number: the observed space neutrinos were all antineutrinos. Only 2% of those from OPERA were antineutrinos.
- Flavor: the space neutrinos were electron neutrinos; the experiments produced muon neutrinos.
- Energy: OPERA neutrinos are a thousand times more energetic than those from SN1987A.
- Location: "MINOS and OPERA neutrinos traveled almost entirely through solid rock, while SN1987A neutrinos traveled almost entirely through the interstellar medium."
Somewhat counter intuitively, Jarah and his colleagues dark energy theory provides a possible explanation for the neutrinos moving more quickly through a dense medium (the earth) than they do through the near vacuum of space. Vastly oversimplifying (so I can understand what I'm writing), there could be a field associated with mass, but which is not gravity, which accelerates the neutrinos to superluminal speeds where the field is strong. Since the field's strength falls off very rapidly with distance, it doesn't significantly accelerate particles in space or even those at any significant distance from the surface of a massy object. So the MINOS and OPERA neutrinos are accelerated as they travel through the earth while the SN1987A neutrinos loaf along at near c in the near vacuum of space.
It helps in understanding all this to remember that neutrinos are so small that they rarely collide with anything even when they pass through very solid matter. From their very tiny PoV, there are huge gaps between the electrons and nuclei which make up the earth. An experiment called IceCube has determined that less than 20% of the neutrinos which enter the earth around the North Pole collide with anything on the way through the earth to the South Pole; the rest go through the earth and keep on going.
One theoretical objection which has been made to the OPERA results is that neutrino superluminality implies electrons also move faster than the speed of light since neutrinos morph into electrons (not exactly what was said but close enough). Jarah points out: "Of course a theoretical argument cannot disprove an experiment, only a failure to repeat the experiment by another group can do that." In other words, if it happened, it happened. But a model like the one Jarah and his colleagues are suggesting must answer theoretical objections.
Electron superluminality appears to be ruled out both by experiments with very high energy electrons and observations of very high energy cosmic electrons. However, these observations all involve electrons in a near vacuum; electrons, like neutrinos may be superluminal only in a very dense medium. If this is the case, unlike neutrinos, the electrons won't stay superluminal very long because, again unlike neutrinos, they interact strongly though the electromagnetic force; and so will quickly lose energy to their surroundings – especially quickly in a dense medium. That would explain why we haven't yet caught any electrons speeding. In fact it might be that all particles can be superluminal in a dense field, although this is not required by the model Jarah is proposing. If this were the case, it would be, he says "a feature of these models which could potentially be in conflict with [the current theory of] big bang nucleosynthesis." That would be almost as big a deal as exceeding Einstein's speed limits!
Fellow physicists have asked Jarah why he is speculating in advance of confirmation of OPERA's startling measurements. His explanation is simple. Speculation by theoretical physicists like him and his colleagues is orders of magnitudes cheaper than experiments – and may well help to better target future experiments and the precious time and resources of the CERN collider. Moreover (my explanation, not his), something is wrong with our current model of the universe and its genesis. We haven't had any new explanations in physics for a while even though our increasing ability to measure forces us to add "fudge factors" like dark energy and dark matter which we haven't directly observed. Physics typically has a breakout when we discover that some accepted bit of wisdom is not actually correct – like the speed of light in a vacuum being an absolute limit. It helps to ask "what if?"