Figure 1~ Artist’s conception of the electron electric dipole moment (Adam West).

Everything and Nothing

Ask a physicist “Why is there something rather than nothing?” and you’re likely to provoke consternation. Science, the discipline most concerned with studying nature, may never be able to speak with authority about the reasons behind nature’s very existence, as that question may be forever inaccessible to the scientific method.

But if you ask another, only slightly less audacious question, “Why are there objects in the universe, like planets and stars and people, rather than just light?” then you’ll get us talking! While science may not be able to find out why nature exists, it can certainly try to trace where the stuff within nature came from all the way back to the very beginning: the big bang.

Why indeed is there matter—that is, things that have mass—rather than just pure energy in the form of light? I can’t answer that question here for the simple reason that nobody knows the answer. However, I can tell you about how the electric dipole moment (EDM) of the electron, a quantity so infinitesimal that for all practical, everyday purposes it’s nothing, may hold the key to explaining the existence of all matter, from galaxies to grains of sand.

Two Symmetries

In the next section, we’ll look at what scientific teams like my group are doing to measure the electron EDM and perhaps point the way to a solution to the mystery of matter. In order to understand the connection between the existence of matter and the electron EDM, however, you first need to know about the idea of symmetry in physics and what it can teach us about the universe we live in.

In physics terms, if you can change some property of the universe without changing anything about how the universe behaves, that property is said to be a symmetry of nature. For example, if you magically shifted everything in the universe two feet to the left (without accelerating anything—this is magic, remember!), nothing, physically speaking, would be any different. It would be the perfect crime: there would be no experiment that anyone could do to prove that you had done it.There are two such symmetries of nature involved in the mystery of matter in the universe, and in its solution.

The first is the symmetry between matter and antimatter. For every type of matter particle, there is an antimatter counterpart that is similar in every way—same mass, same size, same shape, same lifetime—except for two peculiarities. First, a particle of antimatter has the opposite electrical charge from its matter partner. Thus, an electron is negatively charged and an antielectron positive, a proton positive and an antiproton negative, and so on. Second, whenever a particle of matter and its antimatter counterpart meet, the two annihilate each other, and their masses are converted into pure energy (typically in the form of light) in accordance with Einstein’s famous relation E=mc2. That is, the amount of energy created by the annihilation is equal to the total mass of the annihilating particles multiplied by the speed of light squared. According to our current best theories, matter and antimatter are fundamentally quite symmetrical: if you swapped all of the matter in the universe for antimatter and vice versa, it would be nearly impossible to tell the difference.

Strangely, however, our universe doesn’t treat matter and antimatter as if they were perfectly symmetrical. Antimatter is incredibly rare. If it weren’t, the universe would be a terrifying place, with bits of it constantly being annihilated in huge flashes of light. Comforting as it may be, the scarcity of antimatter is odd and surprising—if matter and antimatter are so symmetrical, then why does nature seem to show such a strong preference for one over the other? This is the source of the mystery described in the first section: If equal amounts of matter and antimatter were produced in the big bang, they should have quickly annihilated each other, leaving only pure energy in the form of light. Nevertheless, a tiny fraction of matter particles, only about one in a billion of the original cohort, somehow managed to survive, while antimatter all but vanished from the universe. It is these mysterious survivors that make up the entirety of the known universe. [1]

What allowed our matter universe to survive the mass annihilation after the big bang? What mysterious, as-yet unknown process could have produced enough extra matter particles, or destroyed enough antimatter particles to allow us and everything we see to exist? The key to unraveling this mystery may lie in the fact that the symmetry between matter and antimatter is connected to a second near-perfect symmetry of nature called time-reversal symmetry.

Strange as it sounds, according to time-reversal symmetry, nearly all the known laws of nature are exactly the same when you play them forwards as when you play them on rewind. Watch a movie of two billiard balls colliding, a planet orbiting a star, or a gyroscope spinning, and you won’t be able to tell whether the projectionist is rolling the film forwards or backwards. (An aside: I’m sure you can think of plenty of video clips where you could tell the difference between forwards and rewind—for example, a wine glass spilling, a cow munching grass, or a bomb exploding. The reason you perceive an arrow of time in these instances has to do with the statistical unlikelihood of wine flowing back into the glass, grass being resurrected out of the cow’s mouth, or a huge volume of smoke, shrapnel, and sound concerting itself back into a small inert shell. All of the fundamental physical processes in these backwards-seeming events are perfectly allowed by physics.)

Sound pretty weird? It gets even weirder. According to a deeply mathematical interpretation developed by physicists Richard Feynman and Ernst Stueckelberg in the mid-1900s, particles and anti-particles can be regarded as time-reversed versions of each other. This means that in preferring matter over anti-matter, the universe is breaking time-reversal symmetry on a much larger scale than our current understanding of physics says should be possible. Understanding the connection between the matter-antimatter asymmetry and the breaking of time-reversal symmetry is a subtle yet huge conceptual step: It means that in addition to looking for direct evidence for what happened during the big bang, scientists can also do experiments in the laboratory to look for time-asymmetric effects that might have played a role in the early universe.

Searches for New Physics and the ACME Experiment

The predominance of matter in the universe tells us that there must be some new physics, meaning undiscovered particles and interactions, that breaks time-reversal symmetry and could therefore assist in the creation of excess matter or the destruction of some of the antimatter in the early universe. Whatever they are, these new particles must be hard to see under ordinary conditions, or we would have seen them by now. In order to find them, either we have to create some extraordinary conditions for them to show up in, or we have to look very closely at ordinary matter under ordinary conditions to see the tiny effects these new particles cause. Some experiments, like the Large Hadron Collider at CERN, are taking the former approach by smashing particles together at huge energies and seeing what pops out. Other groups are taking the latter approach by searching for small, time-asymmetric effects in simple, well-understood systems (see, for example, [2,3,4]). My group, the Advanced Cold Molecule Electron EDM (ACME) Collaboration, belongs to this second category. We are looking for the breaking of time-reversal symmetry in an ordinary, everyday particle: the humble electron.

My group is searching for the electron’s electric dipole moment (EDM). If you imagine the electron as a round ball of negative charge, the EDM would look like a tiny bump of extra charge on one of its poles. The existence of this bump, which has never been seen before in this or any particle, would break time-reversal symmetry, as illustrated in figure 1. The time-reversed version of an electron with an EDM would actually be a different particle, which does not exist in nature! We know, however, that the electron doesn’t break time-reversal symmetry on its own. Therefore, if we find an EDM, it must be caused by the same undiscovered,time-asymmetric particles pulling it out of shape. By studying their effect on the electron, we can learn more about these particles and their possible role in generating matter in the early universe.

Figure 2~ An electron can be visualized as a spinning ball of charge like the blue ball on the left in this cartoon. The EDM appears as a small bump on the electron’s bottom, and the white arrow indicates the direction the EDM is said to be pointing. The black arrow indicates the direction in which the electron is spinning. In the time-reversed version, the electron spins the other way, but the bump still remains on the bottom. There is only one type of electron in the universe, so one of these two versions of the electron must not exist (though nobody knows which one yet!). Therefore, nature has broken time-reversal symmetry by preferring one version over the other. This is why the search for the electron EDM is a search for time-asymmetry.

Because the effect we are looking for is so small, we use some tricks to make it as easy to see as possible before we measure it. We use a beam of thorium monoxide (ThO) molecules, which can be produced in large numbers and which have a huge internal electric field—the same sort of thing that makes your hair stand up after rubbing it with a balloon, except that the fields in our molecule are a few million times larger than those around your hair. We put these molecules in an electric field in the lab and use laser light to get all of the electrons pointing in the same direction. As the molecules fly through our apparatus, the electrons’ EDM (assuming they have one) causes them to precess around the molecules’ huge internal field, in the same way a spinning top slowly precesses—that is, its axis swings in circles—around the pull of gravity. (The video in reference 5 gives a good explanation of precession.) After a little while, we use another laser to determine how much the electrons have precessed, and from that we can determine the size of the EDM—if it’s big enough to see at all.

Figure 3~ This is the apparatus used by the ACME collaboration to do the experiment described in the text. The blue box on the left is the source of the ThO beam, the silver-colored chamber in the middle is where the experiment is performed, and the table in front of it is full of mirrors, lenses, and optical fibers for shooting laser beams into the apparatus. The five metallic disks on either side of the experiment chamber help to protect the experiment from being affected by the Earth’s magnetic field.

Looking Closer Still

I wish I could end by announcing a climactic discovery, but what makes science so frustrating and fascinating is that nature sometimes keeps her mysteries well veiled.

We succeeded in our goal of performing an experiment ten times more sensitive to the electron EDM than any previous measurement. However, even with our record-breaking sensitivity, we did not detect an EDM and couldn’t prove that it exists [6.7]. On the bright side, we still learned something new: we now know that the electron EDM is even smaller than anyone ever knew before. Our result can be used to set a new upper limit on the possible size of the electron EDM of about 10-28 electron charge centimeters.” What this tiny number with a funny unit means is that if you were to magnify the electron to the size of our solar system, the size of the bulge on its charge distribution could not be any larger than the width of a grain of rice.

This is both disappointing and tantalizing. There are many theories that attempt toexplain the predominance of matter in the universe, but many of the most promising ones are gradually being ruled out as our EDM measurements get better and better. All this gives us hope that the EDM—and the key to the mystery of our matter universe—may be just around the corner. For my group and others in our field, there’s nothing for it but to keep looking.

Figure 4 ~ This chart shows some theoretical predictions for the electron EDM (in electron charge centimeters) in colored bands: purple, blue, and red bands indicate different types of theories. The grayed-out areas show which of those theories have been and are being disproved by the experimental upper limits on the size of the electron EDM set since 1990. For example, Naive SUSY, the theory band in blue on the bottom left, predicted that the electron EDM would be about 10-26 electron charge centimeters. Since the ACME experiment (together with the Imperial College London and Berkeley experiments before it) showed that the electron EDM is more than 100 times smaller than that prediction, Naive SUSY has been fairly conclusively ruled out.

Elizabeth Petrik is a 7th year PhD student in the Doyle group at the Harvard Department of Physics.

References

[1] Eric Sather. (1996) The mystery of the matter asymmetry. Beamline Spring/Summer:31-37. Full text available: http://www.slac.stanford.edu/pubs/beamline/26/1/26-1-sather.pdf

[2] LiveScience. Shape of the electron is surprisingly round. http://www.livescience.com/14322-electron-shape-standard-model-particle-physics.html

[3]  CERN press office. ATRAP experiment makes world’s most precise measurement of antiproton magnetic moment. Released March 25, 2013. http://press.web.cern.ch/press-releases/2013/03/atrap-experiment-makes-worlds-most-precise-measurement-antiproton-magnetic

[4] Fortson N, Sandars P, Barr S. (2003) The search for a permanent electric dipole moment. Physics Today June:33-39. Full text available: http://scitation.aip.org/content/aip/magazine/physicstoday/article/56/6/10.1063/1.1595052

[5] Veritasium. Gyroscopic Precession. YouTube video. https://www.youtube.com/watch?v=ty9QSiVC2g0

[6] ACME Collaboration: Baron J, Campbell W C, DeMille D, Doyle J M, Gabrielse G, Gurevich Y V, Hess P W, Hutzler N R, Kirilov E, Kozyryev I, O’Leary B R, Panda C D, Parsons M F, Petrik E S, Spaun B, Vutha A C, West A D. (2014) Order of magnitude smaller limit on the electric dipole moment of the electron. Science 343(6168):269-272. Abstract available: http://www.sciencemag.org/content/343/6168/269.short. Preprint: http://arxiv.org/abs/1310.7534.

[7] Scientific American. Electron appears round, squashing hopes for new physics theories. http://www.scientificamerican.com/article/electron-spherical-electric-dipole-moment/

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