In September 2011, a tentative announcement that neutrinos may have traveled faster than the speed of light sent waves of excitement through the world of physics, and was reflected in popular media. What does it mean?
Updated March 2012: Neutrinos Obey the Speed Limit
In "Neutrinos not faster than light", Geoff Brumfiel reported that the ICARUS experiment confirmed that neutrinos travel no faster than light speed. Earlier, a plausible explanation for the OPERA result was that a loose cable delayed data transmission for the anomalous 60ns.
The remainder of this article is unchanged, as it reflects what was known at the time.
What is the Speed of Light?
The "speed of light" is 299,792,458 m/s (metres per second). This is its speed in the vacuum of outer space; it slows slightly when it passes through clear substances like air or glass. The English letter 'c' is the symbol in Einstein's famous equation, E=mc^2: "energy equals mass times the square of the speed of light".
The usual approximation is 300,000 Km/s (kilometres per second), or roughly 186,000 miles per second.
Whether "light" is treated as an electromagnetic wave or as the subatomic particle called a "photon", Einstein's Theory of Special Relativity states that any experiment will observe light in a vacuum traveling at its constant velocity of c.
What is a Neutrino?
A neutrino is an extremely light-weight sub-atomic particle, where "light-weight" means "having very low mass". There are three types of neutrino; each has a corresponding anti-neutrino. This article will ignore the differences.
Atoms consists of neutrons, protons and electrons. The neutron is the most massive of these three particles, with no electric charge. The proton is almost as massive, with a positive charge. The electron has little mass and a negative charge.
A neutron that is isolated away from an atom's nucleus has a 50% probability of decaying within 13 minutes. "Decaying" means that it breaks apart into a slow-moving proton, a fast-moving electron and an even faster neutrino.
Although this is not the precise type of neutrino that made headlines, the principle is similar.
Neutrinos almost never interact with matter. Usually neutrinos fly through stars or planets more easily than light travels through air.
Why is the Neutrino So Fast?
An imperfect analogy for an isolated neutron is a loaded cannon consisting of the very massive empty cannon, plus a cannonball, plus gunpowder. When the loaded cannon sits motionless on the battlefield, its momentum, or mass times velocity, is zero.
When the gunpowder ignites, the expanding gas forces the cannonball forward at high speed; but some of the burning gunpowder moves even faster as the "muzzle flash". Meanwhile, the empty cannon ricochets backward at a much slower speed.
Sir Isaac Newton's laws of motion require the conservation of momentum, which had been zero. The cannonball and muzzle flash move quickly in one direction, cancelling out the higher mass times lower velocity of the empty cannon going in the opposite direction.
Comparing a neutron to a loaded cannon is not a perfect analogy, since the neutrino is not the propellant although it is lightweight and quick like the muzzle flash.
So the neutrino moves quickly because it has extremely little mass to resist being accelerated by the force that caused the neutron to decay into particles, each with a different mass.
What is Important about the Speed of Light?
Albert Einstein's 1905 Theory of Special Relativity showed that, as an object's velocity approaches the speed of light, its mass approaches infinity. Since mass is the resistance to being accelerated, this means that it becomes more and more difficult to make something go fast enough to reach the speed of light. This is why the speed of light is sometimes called the "cosmic speed limit".
Another important aspect of the speed of light is that it is the speed of every electromagnetic field propagating in a vacuum, based on Maxwell's equations. The speed, based on the value of an ampere and a coulomb, is 300,000Km/s. This ties the speed of light to the speed of everything on the electromagnetic spectrum, from radio waves through visible light to X-rays.
What is the OPERA Experiment?
The OPERA laboratory's objective is to detect neutrinos generated by CERN's LHC (Large Hadron Collider), and determine whether they change from one type to another during their 730 Km straight-line trip through the earth.
Prior experiments at other installations detected fewer neutrinos than expected, for the type that the Sun should be generating. However, there were "too many" of another type of neutrino. The question became, "Do neutrinos change while they travel"?
OPERA needs to know the exact time CERN collides hadrons in order to ignore neutrinos from any other sources. Hadrons are subatomic particles with mass; CERN uses protons or ions of vapourized lead.
What are OPERA and CERN?
OPERA is the acronym for "Oscillation Project with Emulsion-tRacking Apparatus", which is located in Gran Sasso, Italy. CERN is the French acronym for "Conseil Européen pour la Reserche Nucléaire", located in Geneva, Switzerland.
What are the OPERA Experiment's Results?
The experimental data seemed to show that the neutrinos traveled 0.0025% faster than light. They reached OPERA about 60 ns (nanoseconds, or billionths of a second) faster than light. In other words, the neutrinos would have been about 20 metres ahead of photons, after racing 730 Km.
This result took some three years to compile, based on some 16,000 neutrino detections, out of "gazillions" of neutrinos created at CERN.
How Reliable are the OPERA Results?
The results are both very reliable and somewhat questionable. The result could be stated as "0.0020% to 0.0030% faster than light, with 99.99966% confidence".
The scientists have reliable distance measurements, to within 20 cm of the 730 Km distance: this accuracy is 0.000027% of the total distance. 20 cm is 1% of the excess distance of 20 metres that the neutrinos covered. (Edited Nov. 21, 2011, to clarify the "1%" referene and add that 20 cm is 0.000027% of the total distance).
Using atomic clocks synchronized at both sites, the timing should be accurate to within 10 ns, or one-sixth of the excess time.
The speed of light has been tested by others, and the predicted time should be accurate to within a few nanoseconds.
Despite all the careful measurements, outside observers still wonder if there are any mistakes or overlooked factors. One such is that heavier gravity inside the earth slows the passage of time slightly. Another is whether the time delay for the fast-moving GPS satellites (used for location and synchronization) have been included. These criticisms seem to be unfounded, partly because synchronized atomic clocks were also used.
Other objections to the results cast doubt because of the difficulty in detecting neutrinos. The hadron collision at CERN is not instantaneous; it takes a brief amount of time for all the collisions to occur. They start, rise to a peak, and drop off in a shape like a Bell curve. The detections at OPERA follow a similar curve
They measure the time "from the peak of one curve to the peak of the other". The criticism is that the peak of the OPERA detection curve might represent the neutrinos arriving from the start of the CERN production curve. No reason for this discrepancy has been suggested, but it would preserve the light-speed limit.
For example, the OPERA detectors might stop working after the first neutrinos have been noted. The CERN collisions might twist to send later neutrinos up or down, away from the horizontal plane towards the OPERA detectors. Neither of these have been seriously suggested; but some explanation would be needed.
Another possibility is that the neutrinos travel through an extra spatial dimension to make the distance shorter; this too would keep the real speed below that of light.
Previous observations by other scientists are mixed. FermiLab had reported detecting similar fast neutrinos in 2007, but those scientists were not convinced by their own data due to possible margins of error.
Another objection is that a pulse of neutrinos had been detected from supernova 1987A just a few hours before the light arrived. (Neutrinos escape a stellar core collapse in seconds; light takes a few hours). If those neutrinos had traveled 0.0025% faster than light for the 160,000 light-year journey, they would have been four years ahead of the visible light. The four hours was explained by the neutrinos exited the supernova's centre before the light could escape at the surface.
Could the OPERA Results be Correct?
It is also possible that the OPERA scientists have measured and reported accurately. This could mean that neutrinos are actually tachyons. Outside of Star Trek science fiction, tachyons are purely theoretical particles that always travel faster than light. None had ever been reliably detected before.
As tachyons, these neutrinos might have been created at superluminal speeds; this would avoid the infinite mass and resistance to acceleration at exactly light speed. Perhaps these neutrinos are a waveform that propagates faster than light.
It will take months, at least, for scientists to review the OPERA findings and even longer to re-test. The cosmic speed limit law has not yet been repealed.
Disclaimer: Although the background information in this article may remain correct, the "faster than light neutrino" claims might, in the future, be disproven or be given other explanations.
References:
- Michael Fowler, University of Virginia, " Maxwell's Equations and Electromagnetic Waves ", Sept. 2009, referenced Oct. 14, 2011.
- Tommaso Dorigo, Science 2.0, "A Six-Sigma Signal Of Superluminal Neutrinos From Opera!", Sept. 19, 2011, referenced Oct. 14, 2011.
- R. Nave, Georgia State University, "Fundamental Forces", referenced Oct. 14, 2011.
- Science Daily, "Particles Appear to Travel Faster Than Light: OPERA Experiment Reports Anomaly in Flight Time of Neutrinos", Sept. 23, 2011, referenced Oct. 15, 2011.
- Douglas Perry, Tom's Guide, "Fermilab to Review Super-Light-Speed Neutrinos", Oct. 2, 2011, referenced Oct. 15, 2011.
- Richard McCray, University of Colorado at Boulder, " SUPERNOVA 1987A ", modified Feb. 20, 2004. referenced Oct. 16, 2011.
Mike DeHaan - Copyright (c) Mike DeHaan, B. Math., of DeHaan Services. Well written and well researched freelance articles; ghost writing for clients.
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