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Fifth Force

September 12, 2016

In the dim past, when the only manifestation of electricity was the attraction of bits of paper to rubbed amber, and magnetism was known only through attraction of iron to lodestone, electricity and magnetism were considered to be two different things. The development of electrical technology after the time of Alessandro Volta (1745-1827) enabled Hans Christian Ørsted (1777-1851) to discover in 1820 that an electric current will produce a magnetic field.

Figure caption

A Voltaic pile.

In my own experiments, a US dime and US penny, separated by a piece of paper soaked in distilled vinegar, gave 2.01 volts.

(Wikimedia Commons image by Luigi Chiesa, modified.)

In 1831, Michael Faraday (1791-1867) discovered that passage of a magnet through a coil of wire produces electricity, a phenomenon known as electromagnetic induction. In 1865, James Clerk Maxwell (1831-1879) published the paper, A Dynamical Theory of the Electromagnetic Field, in the Philosophical Transactions of the Royal Society. This established the idea that light is an electromagnetic wave, and electricity and magnetism were combined in a single electromagnetic force.

At that point, there were just two known fundamental interactions, gravitation and electromagnetism. The development of nuclear physics resulted in the addition of two other fundamental interactions, the strong nuclear force, and the weak nuclear force, bringing the total number of fundamental forces to four. While one theory suggests that electromagnetism and the weak nuclear force are actually manifestations of a single force, the electroweak interaction, there presently appear to be four forces of nature.

Four forces would have appealed to Aristotle (384-322 BC, see figure), who acknowledged gravity in the sense of objects seeking their desired resting place,[1] and likely knew of the electrostatic properties of amber, since these were described by Thales of Miletus (c.624 - c. 546 BC) and Theophrastus (c. 371 - c. 287 BC). I wrote about Thales and Theophrastus is a recent article (Triboelectric Generators, February 8, 2016).

The four classic elements, and the four qualities of matter.

The four classical elements (Earth ΓΗ, Air ʾΑΗΡ, Fire ΠΥΡ, and Water ʿΥΔΩΡ), along with the four qualities of matter (Dry ξηρον, Wet ʿυγρον, Hot θερμον, Cold ψυχρον).

(Created using Inkscape. Click for larger image.)

Since our knowledge of the fundamental forces has been faulty in the past, there first being two, then three, then two again, and now four, how certain are we that there are just four forces? There might be a fifth force lurking somewhere, just out of reach of our observations. That idea captured the imagination of Austro-Hungarian physicist, Loránd Eötvös (1848-1919), who performed a series of Cavendish balance measurements, now called the Eötvös experiment, in 1885-1919. These experiments found no fifth force to a resolution of 100 parts-per-million (ppm). Subsequent experiments improved this resolution to 0.1 ppm.

In 1986, a group of physicists reexamined Eötvös' data and found a correlation of intermediate range force with a physical property known as hypercharge.[2-3] The presumed fifth force seemed to operate over a range of tens, to hundreds, of meters, and not over cosmological distances where divergence of gravity would have been noted. This observation inspired a series of experiments, the result was not confirmed, but experiments continue to the present.[4]

A fifth force may exist, but not the type that would have been detected in such experiments.[5-10] A recent paper in Physical Review Letters[5] contains an analysis of a 2015 study by Hungarian physicists that detected a possibly new particle.[11] The Hungarian research team supposed that this particle might be a dark photon. The new interpretation is that it might be a "protophobic X boson," a particle that moderates a fifth force between electrons and neutrons at close range. This force is selective and extremely weak, and that's why it's remained undetected until now.[5,9]

X boson mass

Determination of the mass of the discovered particle by the χ2/f method.

This is a small mass in this era of a Higgs boson with a mass of 125 GeV/c2.

(Fig. 5 of Ref. 11.)[11])

The details of the observed decay of excited beryllium-8 nuclei can be explained by the existence of a new particle that's just thirty times heavier than an electron.[6,10-11] If this particle is a fifth-force boson, it has a range of just 12 femtometers (fm). The force operates on up and down quarks and electrons, and it's less for protons than neutrons. What's especially interesting is that it might explain the anomalous magnetic moment of the muon, a problem of long standing.[5]

Beryllium-8 decay

The addition of a proton to lithium converts it to beryllium in an excited state. This excited state apparently releases a fifth-force boson X that decays to an electron-positron pair. (UC Riverside image.)

The experimental research team didn't claim a fifth-force particle, since it wasn't clear whether the particle was a matter particle or a force-carrying particle.[6] Follow-up experiments should happen quite quickly, since this is a low energy nuclear reaction, and many laboratories have the required equipment.[9]

While the Standard Model has persisted for quite some time, resilient against a barrage of experimental tests, it can't explain gravity, and it doesn't explain dark matter particles.[10] If a fifth-force boson has been discovered, this new force might be unified with the weak and strong nuclear forces to obtain a more fundamental force.[6]

Says Philip 'Flip' Tanedo, an assistant professors of physics at the University of California, Irvine ,
"We think that the Hungarian anomaly is interesting and our model is proof that consistent theories can be constructed... We're not saying that a fifth force has been discovered – only that we can pass the first consistency check... The next big check is for other experiments to confirm the anomaly. Our paper lays down the framework for how other types of experiments can definitely check or refute the original Hungarian result. If it ends up being real, that would be a huge deal in our field."[8]

As one physicist was quoted in the Daily Mail, "We explore ideas... Probably most of ideas are wrong—but we learn from them, and we propose better ideas."[10]


  1. Aristotle, "Physics," Book IV, Part 1, R. P. Hardie and R. K. Gaye, Trans., MIT Internet Classics Archive.
  2. Ephraim Fischbach, Daniel Sudarsky, Aaron Szafer, and Carrick Talmadge, and S.H. Aronson, "Reanalysis of the Eötvös experiment," Phys. Rev. Lett. vol. 56, no. 1 (January 6, 1986), pp. 3-6, DOI:http://dx.doi.org/10.1103/PhysRevLett.56.3.
  3. L. Bod, E. Fischbach, G. Marx And Maria Náray-Ziegler, "One Hundred Years Of The Eötvös Experiment," Hungarian Academy of Sciences Web Site, August 31, 1990.
  4. T. A. Wagner, S. Schlamminger, J. H. Gundlach, and E. G. Adelberger, "Torsion-balance tests of the weak equivalence principle," arXiv, July 10, 2016.
  5. Jonathan L. Feng, Bartosz Fornal, Iftah Galon, Susan Gardner, Jordan Smolinsky, Tim M. P. Tait, and Philip Tanedo, "Protophobic Fifth-Force Interpretation of the Observed Anomaly in 8Be Nuclear Transitions," Phys. Rev. Lett., vol. 117, no. 7 (August 12. 2016), Article No. 071803. A version of this paper can be found at arXiv.
  6. UCI physicists confirm possible discovery of fifth force of nature, University of California, Irvine, Press Release, August 15, 2016.
  7. Jonathan L. Feng, Bartosz Fornal, Iftah Galon, Susan Gardner, Jordan Smolinsky, Tim M. P. Tait, and Philip Tanedo, "Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays," arXiv, August 11, 2016.
  8. Sean Nealon, "Nuclear Puzzle May Be Clue to Fifth Force," University of California, Riverside, Today, August 17, 2016.
  9. Weston Williams, "Have scientists discovered a fifth fundamental force of nature?" Christian Science Monitor, August 16, 2016.
  10. Mark Prigg, "New hints of a fifth fundamental force: 'Revolutionary' X boson could rewrite laws of physics - if it really exists," Daily Mail, August 15, 2016.
  11. A.J. Krasznahorkay, M. Csatlós, L. Csige, Z. Gácsi, J. Guly´s, M. Hunyadi, T.J. Ketel, A. Krasznahorkay, I. Kuti, B.M. Nyakó, L. Stuhl, J. Timár, T.G. Tornyi, and Zs. Vajta, "Observation of Anomalous Internal Pair Creation in 8Be: A Possible Signature of a Light, Neutral Boson," arXiv, April 7, 2015.

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