Will experimental anomalies lead to new physics?

A proton: two up quarks and one down quark

Historical anomalies in physics

It is often said that experimental anomalies lead to new physics. This is actually a bit overstated. Actually, the vast majority of experimental anomalies turn out to have more prosaic explanations — errors in the experimental setup or analysis, or errors stemming from invalid applications of the theory.

Nonetheless, a few experimental anomalies in years past have led to important new advances in the field. A few examples are:

  • In 1887, Michelson and Morley compared the speed light in two perpendicular directions, hoping to measure the speed of Earth with respect to the “luminiferous ether” that was presumed to pervade all space. Surprisingly, no difference was found. The anomaly was finally explained in 1905 by Einstein’s theory of special relativity, which eliminated any need for the existence of an ether.
  • In 1859, Urbain Le Verrier found that the precession of Mercury’s perihelion disagreed from Newtonian theory by 38 arc-seconds per century. This was ultimately resolved in 1917, again by Einstein, with his theory of general relativity.
  • In the late 1880s, applications of classical Newtonian physics and Maxwellian electromagnetics yielded three puzzling results: (a) theory suggested that blackbody radiation should increase without bound at shorter wavelengths, but experiments found that it does not; (b) theory suggested that the photoelectric effect (light shining on a metallic surface ejects electrons) should decrease gradually with very dim light; instead the effect cuts off completely below a certain level; and (c) most puzzling of all, a hydrogen atom should be unstable, because its electron should emit radiation, lose energy and quickly collapse to the nucleus. These anomalies led to the development of quantum theory.

Today physicists are paying increased attention to several anomalies that have arisen in recent experiments, with the thought that perhaps one or more of these anomalies will indeed be the spark that leads to new physics beyond the “standard model” that has prevailed in the field since the 1970s.

The proton radius anomaly

The proton radius anomaly stems from the fact that careful measurements of the radius of a proton find it to be slightly larger when orbited by an electron than when it is orbited by a muon (a charged particle similar to an electron but over 200 times as massive).

In particular, measurements of the proton’s radius when orbited by an electron (such as in a hydrogen atom) find a radius of approximately 0.877 femtometers (i.e., 0.877 x 10-15 meters). But separate measurements of the proton’s radius when it is coupled with a muon (known as “muonic hydrogen”) find a radius of 0.84 femtometers. The two measurements differ by significantly more than the error bars of the two sets of experiments.

Both groups plan additional experiments, including some experiments of a completely different design. The new experiments should better clarify whether or not the effects are being driven by a “new force.”

For additional details, see this Quanta report by Natalie Wolchover.

The neutron lifetime anomaly

Researchers have noted for some time that two different experimental procedures (“bottle” and “beam” measurements) give different values for the lifetime of a neutron. Until recently the discrepancy between the two types of experiments was small enough that no one paid much attention, but that situation is changing.

In “bottle” experiments, neutrons are stripped from atomic nuclei and then observed decaying while confined in a “bottle.” Measurements of the neutron lifetime in this type of experiment over the past 20 years have found an average lifetime of 879.3 seconds. In “beam” experiments, physicists generate beams of neutrons and tally the number of protons that emerge from the beam due to neutron decay. These experiments have produced an average lifetime of 888 seconds.

Again, no one paid much attention until recently, when both experiments further refined their methods. The latest bottle method measurements have an error bar of plus or minus 0.75 seconds; the latest beam method measurements have an error bar of plus or minus 2.1 seconds. Yet the two values differ by 8.7 seconds.

What is going on? Some have suggested that neutrons may be decaying into one or perhaps two dark matter particles, but others have dismissed this possibility. In the meantime, both groups are pressing forward to further reduce errors bars and uncertainty. Within a year or two improved experimental results should be available.

For additional details, see this Quanta report also by Natalie Wolchover.

The Hubble constant anomaly

Perhaps the most troubling anomaly is a disagreement over the Hubble constant, namely the rate of expansion of the universe. This subject has a long history, with many papers and even several books written on the topic.

One experimental method to find the Hubble constant stems from analysis of measurements of the cosmic microwave background (CMB), based on data from the Planck satellite. Recent data (2015) yield H0 = 67.8 plus or minus 0.9 (the units are “kilometers per second per megaparsec”).

The other approach is to employ more traditional techniques, based on observations of Cepheid variable stars. In 2016, a team of astronomers using the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope obtained the value H0 = 73.24 plus or minus 1.74.

Needless to say, these two values do not agree — they differ by roughly 8%, and which is significantly greater than the combined error bars of the two measurements. Both groups have planned new experiments to further clarify the difference and, of course, to try to explain it.

Some new data has already arrived. In April 2018 the research team operating the “Gaia” spacecraft released an initial database of over 1.7 billion stars in the Milky Way. Buried in this data were 50 new Cepheid variable stars. Combining these data with earlier data has only sharpened the discrepancy. As project leader Adam Riess (co-recipient of the 2011 Nobel Prize in physics for the discovery of the accelerating universe) explains it, “Not only is [the discrepancy] confirmed, but it’s actually reinforced.”

If these differences hold up, they could spell big trouble for our understanding of physics and cosmology. Perhaps the prevailing big bang paradigm is seriously wrong; perhaps our conjectures about the natures of dark matter or dark energy are incorrect. Things could really get interesting if no solution is found.


In spite of the temptation to jump to conclusions, throwing out the standard model and big bang cosmology, caution is in order. After all, as mentioned above, in most cases anomalies are eventually resolved, usually as some defect of the experimental process or the application of the theory.

A good example of a resolved experimental anomaly was the 2011 announcement by scientists working at Italy’s Gran Sasso National Lab that neutrinos emitted at CERN, near Geneva, Switzerland, had arrived at the Gran Sasso Lab (in a underground lab in the Alps) 60 nanoseconds faster than they would had they traveled at the speed of light, which is, of course, the unbreakable speed limit for any particle. As it turns out, the experimental team later discovered that the discrepancy was due to a loose fiber optic cable, which had introduced an error in the timing system.

Another example came to light in April 2018: improved astronomical data is slowly closing the door on claimed anomalies and alternatives to Einstein’s theory of general relativity.

Readers may recall the August 2017 announcement that the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) had detected the collision of two neutron stars. Subsequent analysis by Miguel Zumalacarregui, a physicist at the Institute for Theoretical Physics in Saclay, France, found that these results severely constrain several alternative gravitational theories, such as the Modified Newtonian dynamics (MOND) theory and the tensor-vector-scalar (TeVeS) variation of MOND, which had been advanced to explain certain anomalies in the rotational speed of stars in some galaxies. In particular, the near-simultaneous observation of both a gamma ray burst and a LIGO wave, just 1.7 seconds apart, “brutally and pitilessly murdered” the TeVeS theories, according to Paulo Freire of the Max Planck Institute for Radio Astronomy.

Additional constraints to MOND-type theories come from observations of a newly discovered triple system, consisting of a pulsar and two white dwarfs. Careful analysis finds that these motions are “totally in agreement” with general relativity, according to Anne Archibald of the University of Amsterdam. For additional details, see this Quanta article by Katia Moskvitch.

Even if some theories are overturned, there is no need to despair. As Enrico Barausse of the Astrophysics Institute of Paris, who has researched MOND-type theories for years, explained,

This is what we do all the time, put forward a working hypothesis and test it. … 99.9 percent of the time you rule out the hypothesis; the remaining 0.1 percent of the time you win the Nobel Prize.

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