Category Archives: Physics

No Faster-Than-Light Neutrinos Yet

At the end of September, a paper was published with the provocative conclusion that neutrinos had been measured to travel faster than the speed of light. It was big news, and was widely reported in the media as a reasonably established finding (when in reality it had been released on arXiv, an open-access physics portal, but was not yet peer reviewed, and was subject to much eyebrow raising from physicists at large). In the past week, news has come out that there were some systematic sources of error, and the results are not necessarily accurate.

Here’s a quick recap of what the original experiment entailed:

  • a beam of neutrinos was generated at the Large Hadron Collider (LHC), and the beam was aimed through the Alps (ie, underground) to a detector in Italy called OPERA about 730 km away
  • the signal leaving the LHC is timestamped, using a highly accurate (and very carefully calibrated) GPS system
  • as the neutrinos leave the LHC, a light signal is sent to the same location in Italy via a fibre optic cable. This provides a time-of-flight for a light signal to compare to the time-of-flight for the neutrino signal.
  • when the light and neutrino signals are detected in Italy, they are timestamped using the GPS system, and the times of flight cane be compared
  • this repeated over and over again to reduce statistical errors in the measurements
Schematic diagram for the experiment.

The scientists reported that there was a difference in the times-of-flight of about 60 ns, with the neutrino beam reaching the detector before the light signal. Now, the scientists involved have found two sources of error, which, indicentally, mirror my hunch from when this first hit the news. First, the GPS equipment is operating far outside of its normal operating range, and so my not behave exactly as expected; this error is thought to produce a faster neutrino speed. But the second source of error is a loose connection for the fibre optic cable that carries the light signal, introducing a delay that may account for the difference in time elapsed for the two signals (light and neutrino).

I’m not at all surprised at this — when the paper was first put out, I thought it was only a matter of time before a systematic source of error was found. General relativity has held up spectacularly in every experimental test undertaken, and it would take a lot to upend all of that.

I was surprised that the group released their paper as early as they did, and without initial peer review, and I think that speaks to the authority that the scientific community confers on the CERN collaboration. If this exact paper had been written be a group at a small, less renowned institution (assuming they had all the equipment to do the experiment), would it have ever seen the light of day? Would a smaller group release very controversial results which naturally invite a huge amount of attention from popular media, without even subjecting them to peer review first? Would they ask for scrutiny from all and sundry, not just their peers?

I doubt it, because the stakes are too high. For a smaller, less authoritative group, the hit that their reputation could sustain could be devastating. No-one’s going to give CERN side-eye at everything they do from here on, because it’s CERN! They’ve done incredible work and have a very solid reputation. But if a group without that name backing did the same science and presented it that same way, there’s a good chance it’d be either ignored, chalked up to poor science or ineptitude, and probably wouldn’t be given much of a second look.

I bring this up not to cast aspersions at CERN — they do excellent work — but rather to highlight the weight that an authoritative name can carry in the scientific word. We like to think that science is objective and speaks for itself, and that good science will get the recognition it deserves regardless of who does it. This is not true — there’s plenty of excellent scientists who face all sorts of barriers (monetary, linguistic, social, etc) in their attempts to get their science visible. This isn’t to say that crackpots don’t exist — there’s plenty of people with a tenuous grasp of general relativity who insist that “Einstein was WRONG!”, though they are generally easily refuted — but this is a prime example of how having a big name collaboration helps people take a second look at your work, rather than just writing it off as an errant result.

I don’t think that this paper would’ve gotten as much attention from the scientific community (or the popular media, but they’re not the best arbiter of what’s new and important in science) had it not come from such an authoritative group. I think that, like me, most scientists would’ve said “there’s probably an error in the GPS or the cabling” and left it at that. An awful lot of scientists said that this time, but they typically said it after they’d read the paper; I doubt many of them would’ve bothered to read the paper had it not been from an authoritative collboration. It’s a disappointing realization, but it’s always good to have a reminder to check for our unconscious (or conscious) biases.


Electric Fields

In the last post, I glossed over the bit about how lightning has a hard time passing through air, so I thought I’d clarify (and hopefully this’ll be clear enough that I don’t need to keep up with this string of addenda and clarifications and can write about something new).

From the last post:

The net difference in electrical potential builds up, until the neutral air and water vapour in between the positive and negative regions can no longer sustain the difference, and a lightning bolt discharges the electrical energy. Air is a very good electrical insulator (ie, it is difficult for an electrical current to pass through the air), so a very large electric field can be sustained in the cloud before a lightning bolt discharges the stored energy, and returns at least part of the cloud to a neutral electrical state.

So what exactly is an electric field? It’s a region where, if a charged particle is placed, it will experience an electric force. It’s just like a magentic field: when a magnet (for example, a compass) is placed in a magnetic field (like the Earth’s planetary magnetic field), it experiences a force that aligns it (ie, the compass needle) in a particular way. Similarly, a charged dropped into an electric field will experience a force that pushes it in the field. Electric fields are created by a distribution of charges, either discrete or continuous:

A point charge and a lump of continuous charge, both with electric field lines.

The green lines represent the electrical field.

Of course, the force experienced by a charge dropped into a field depends on the sign (positive or negative) of the charge. A negative charge will experience the opposite force that a positive force experiences, ie, the arrow heads all point in the other direction.

With lightning, it’s not a point test charge dropped into the cloud that creates the bolt, but rather that the charge distribution itself cannot be sustained any longer, and a bolt transfers charge from one region of the cloud to another and neutralizes the field.

Heavily charged cloud with two lightning bolts.

The bolt travels through air, and air is not a vacuum, so the physical properties of the air (or any material that charge is attempting to move through) will affect how easily the charge can move through the material. Materials (and by materials I mean any state of matter, so it can include say glass, water, and air) can generally be classified as either insulators or conductors, depending on a property called conductivity. Electrical energy has a hard time travelling through insulators (like glass) which have a low conductivity, while it passes easily through conductors (like metals), which have high conductivity.

The electrical conductance (or resistance, depending on how you look at it) depends on the atomic or molecular structure of the material. Metals are excellent conductors of electricity, due to their atomic structure. Metals, by definition, have a lattice structure, with loose electrons that can propagate throughout the metal and are not confined to a single atom.

Schematic of metal lattice of positive ions, with an electron that is free to propagate through the lattice.

A very small piece of a metal lattice. The positive ions (nuclei) form the lattice, and the negative ions (valence electrons) can propagate throughout the lattice.

These loose electrons, which are negatively charged, carry electrical charge throughout the material. When a charge is applied at one end of a piece of metal, that charge quickly and efficiently propagates throughout the metal, as the loose electrons move throughout the metal in response to the introduced charge.

But not all materials are metals, and many do not have such loose electrons to acts as charge carriers. Glass, for example, is an excellent insulator, because it has an amorphous, non-crystalline molecular structure. There are no loose electrons, and so few loose charge carriers, and an applied electrical charge has difficulty propagating through the material.

A schematic of glass's crystal structure.

Glass does not have a regular lattice structure, so electrons do not propagate easily through it.

Air is like glass, in that even though there is plenty of room for particles to move freely, there are very few charge carriers to transfer electrical energy from one location to another. This means that a very large electric field can be sustained across a cloud, as small amounts of charge have a hard time flowing freely between the two regions. There is no such thing as a perfect insulator, and eventually the air cannot sustain the electric field, but since it is such a good insulator, the bolt that discharges the energy is very energetic, which we see as a bright flash and hear as a clap of thunder.