CERN experiment will soon confirm
It is surprising that, while antimatter was first observed in 1932, we still don’t know if it falls up or down. That is all about to change, however, as the AEgIS experiment at CERN, the big European home of the Large Hadron Collider, is on track to test a result that could shake the foundations of physics. If we do find it falls up, that could solve a lot of questions we have about the universe like why is there more matter than antimatter and undo some assumptions like whether the universe had a beginning.
At CERN, antihydrogen can be produced from the particle accelerator. This stable atom has a, for all purposes, infinite lifetime. Knowing how it behaves in a gravitational field might reveal whether any violations to our fundamental understanding of nature occur with antimatter. Typically, these antihydrogen atoms, when produced, will be traveling near the speed of light and will not show much tendency to fall. New mechanisms will allow those antihydrogen atoms to be slowed using lasers and magnetic traps, allowing measurements of their behavior in gravity.
For decades, physicists have believed that antimatter must behave the same way in a gravitational field as normal matter from purely theoretical arguments. The nuclei of atoms, they argue, derives a lot of its mass from the energy of the strong force and not intrinsic mass of the protons and neutrons making it up. That energy should be the same whether those are antiprotons or regular protons.
Moreover, general relativity proposes that both matter and antimatter follow geodesics, straight lines in curved space and time. A naive interpretation suggests that antimatter should follow the same path in Earth’s gravity as matter. Indeed, measurements of neutrinos and antineutrinos from distant sources show that they follow about the same path through our galaxy, suggesting that, at least broadly, our galaxy had the same gravitational effect on them.
Another reason is because we know that the collision of matter and antimatter produces photons and photons behave like matter in gravitational fields in terms of experiencing redshift, not blueshift, when escaping a gravitational field. This is despite being their own antiparticle. These have been summed up to argue that any sign that antimatter does not behave the same way as matter in gravity means that there is a violation of the Weak Equivalence Principle (WEP) — the notion that inertial and gravitational mass are the same.
Quantum field theory also argues that matter and antimatter should have the same energy states. This is a consequence of the Charge, Parity, Time (CPT) symmetry. Antimatter is precisely CPT transformed matter and therefore supposedly must have the same gravitational behavior as matter.
For all these reasons, the difference in gravitational acceleration of antimatter versus matter is thought to be about one part in a million. Therefore, it seems as if we already know what to expect from the AEgIS experiment. If some significant deviation were found, like antimatter falling “up”, it would mean a violation of both general relativity’s WEP and CPT.
The successful measurement of antimatter behavior in a gravitational field may shut the door on several cosmologies that assume that matter and antimatter repel one another: the Dirac-Milne Universe and the Lattice Universe models.
Yet, if antimatter is shown to be gravitationally repelled, it need not indicate violations of either WEP or CPT. It may instead indicate an error in how we understand those to work in general relativity.
Given existing quantum theory and measurements of CPT as holding for all matter and antimatter, the Dirac-Milne universe is far-fetched, but the Lattice Universe suggests that WEP and CPT symmetry require matter and antimatter to repel one another under Einstein’s equations of gravity.
This theory challenges the assumption that in gravity mass is equivalent to charge, as in the “C” of CPT symmetry.
If we do make that assumption, then we expect that matter and antimatter would behave the same because quantum mechanics suggests they have the same mass. Gravity, proposed to be a spin-2 graviton field, causes like charges to attract and opposite charges to repel. Case closed.
Yet, general relativity is not so simple, and the masses in it do not behave like charges in CPT symmetry.
Consider that if you want to convert a theory about matter into a theory about antimatter you have to apply not only the “C” operation, changing the “charges” from positive to negative and vice versa, you have to apply the “P” and the “T” operations as well. This has important consequences in general relativity. The “PT” operation results in a change of any vector into its negative in general relativity. That means negative space components and negative time. Richard Feynman interpreted this to mean that matter particles are nothing more than anti particles traveling backwards in time and vice versa.
If you apply these rules to relativistic electrodynamics, you get the obvious results, e.g., electrons repel each other and are attracted to positrons. Antimatter systems obey the same laws as matter systems.
If you CPT transform only a charged particle in an electric field and not both the particle and the field, you get a reversal of the behavior. So, for example, an electron in a field generated by protons is attracted, but a CPT transformation of the electron to a positron is repulsed.
All this is very intuitive for electromagnetic fields.
Now, on to gravitational fields, the CPT transformations behave differently. Instead of four vector fields as in electromagnetics, you have tensor fields like the gravitational metric field. The “charge” is no longer a single number but a momentum vector containing four components. Not the mass at all. This means that while the “C” transformation in electromagnetics flips the sign of the force, in gravity it is not the “C” but the “PT” transformation (i.e., the space and time reversal) that flips the sign. So, the end result is that matter attracts matter and repels antimatter.
This means that if antimatter does behave differently in a gravitational field it is not a violation of the weak equivalence principle at all because that assumes that the reversal of the force is caused by the “C” transformation and not by the “PT” transformation. A reversal of the mass would surely be such a violation, but we haven’t reversed it at all. All we’ve done is applied “PT” to all the vectors and tensors in Einstein’s equations.
This has huge consequences for questions like where is all the antimatter in the universe. It could be that it is lost amid the void between the galaxies, repulsed gravitationally. It may also have implications for the existence of dark energy and whether the universe even had a beginning.
Indeed, a cosmological model of this CPT symmetric universe with about equal amounts matter and antimatter in it does not require dark energy because of antimatter’s repulsive effects. These repulsive effects create the acceleration that the standard model currently attributes to dark energy.
Nor does it require an initial singularity, so no Big Bang, only a period that is radiation dominated, hot and dense billions of years ago but plenty of time before that as well.
Because of the lack of a Big Bang singularity, it has no horizon problem. The horizon problem is the observation that parts of the Cosmic Microwave Background (CMB) in different parts of the sky are too correlated, yet did not have time to become so in the early universe. Since the universe did not have an initial singularity, it had plenty of time to become correlated.
It also does not have a “coincidence” problem. The coincidence problem is the observation that matter and dark energy densities (about 5% and 70%) are similar at the current epoch (to an order of magnitude, meaning they aren’t like 0.1% and 99.9% as they would be in the distant future). Since dark energy is replaced by repulsion between equal amounts of matter and antimatter, there is no coincidence problem.
If AEgIS does measure antimatter falling up, a lot of theoretical ideas about the universe will have to change. Many of the arguments used to justify the assumption that it falls down rest on theoretical premises that are weaker than they appear and perhaps a misunderstanding of how to apply CPT symmetry to general relativity. If, on the other hand, they find that antimatter falls down the same as matter, then that may be a clue that CPT and general relativity don’t work together as one would expect and perhaps another theory of gravity is at work. A third possibility is that they will find that antimatter falls at a different rate than matter and there is some combination of effects going on. With any luck, we will know soon.
Villata, Massimo. “CPT symmetry and antimatter gravity in general relativity.” EPL (Europhysics Letters) 94.2 (2011): 20001.
Villata, Massimo. “The matter‐antimatter interpretation of Kerr spacetime.” Annalen der Physik 527.7–8 (2015): 507–512.
Villata, M. “On the nature of dark energy: the lattice Universe.” Astrophysics and Space Science 345.1 (2013): 1–9.