Ask Ethan #31: Why are we made of matter? (Synopsis)

“You may not feel outstandingly robust, but if you are an average-sized adult you will contain within your modest frame no less than 7 × 10^18 joules of potential energy—enough to explode with the force of thirty very large hydrogen bombs, assuming you knew how to liberate it and really wished to make a point.” -Bill Bryson

At the end of every week, I entertain a reader-submitted question or suggestion for our Ask Ethan series, and every once in a while I get a question that science simply doesn't have the complete answer to. For this week, it's the question of the matter-antimatter asymmetry.

Image credit: Andrew Harrison of Image credit: Andrew Harrison of

But just because we don't know everything about why our Universe -- everywhere we look -- appears to be made up of matter and not antimatter doesn't mean we don't know some amazing and powerful things about how it happened!

Image credit: RHIC collaboration, Brookhaven, via Image credit: RHIC collaboration, Brookhaven, via

Go and read the full story, and learn about the frontiers of our scientific knowledge!


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Very nice, Ethan! Clear explanation of the Sakharov conditions, and thanks for the shout-out to both B-Bbar CP violation (my old experiment, BaBar!) and to the fact that we _didn't_ actually explain what we were going after :-)

The only place left within the (enlarged) Standard Model to look is the neutrino mixing matrix, and I don't have a whole lot of hope for that. Affleck-Dine would be nice, if SUSY looked more promising than it seems to today.

The upside to all of this is that we _know_ there's something beyond the SM. The downside is that we haven't a clue where to look for it yet.

By Michael Kelsey (not verified) on 05 Apr 2014 #permalink

Great explanation, thank you. Until now my suspicion was that the antimatter got stable a bit earlier than the matter and collapsed into black holes or decayed into dark energy. How would we know if a black hole consisted of antimatter anyway?

By Peter Krahulik (not verified) on 06 Apr 2014 #permalink

Given that we can only observe a relatively small region of the universe, how do we know that the entire universe is made of matter? Could it not be that the observable universe happens to be in region of matter, and there are other vast regions of antimatter beyond what we can observe?

@Vershner #3: Our limits on "made of antimatter" regions are much smaller than the observable Universe, let alone the "whole" Universe (i.e., including the part at least 5 times larger in radius outside our current horizon).

[ My description below is based on a 1996 CERN review paper on the subject, which I have not been able to track down again :-( ]

The limits are built up rather similarly to the way the cosmological distance ladder works.

1) Earth is made of matter. (Duh :-).
2) The solar system is made only of matter, because otherwise we would observe gamma rays from meteorites, from interactions with the solar wind, etc. This case is strengthened by our probes, but is not dependent upon them.
3) The local galactic neighborhood (50-100 parsecs or so) is made of matter. Otherwise, we would observe a gamma-ray "bow shock" from the Solar System plowing through an antimatter interstellar medium.
4) The galaxy as a whole is made of matter. If there were large regions (molecular clouds, etc.) of antimatter, we would observe strong regional gamma emission from the boundary surfaces. We also have limits from antimatter cosmic rays.
5) The same "gammas from boundary regions" applies out as far as the Virgo cluster.

Beyond that, we can't set strong limits just because the detection rate is low. There "could" be antimatter superclusters, and the boundary annihilation rate would be low enough that we wouldn't (yet?) have enough data to see it.

However, at those scales we have a different argument. If you want to create big regions made of antimatter, and other big regions made of matter, then you need to have a mechanism to separate them, and to keep them separate.

Recall that the early Universe was quite thoroughly mixed, simply because photons and matter/antimatter pairs were in chemical equilibrium. Once the Universe cools below the pair-production temperature, you're left with _everywhere_ being a mixture of matter and antimatter. There's no known mechanism to separate them.

On the other hand, CP violation, which can (as Ethan described) lead to an intrinsic excess of matter (by definition) over antimatter, _is_ a known mechanism which we have observed and measured with precision. Our measured values (for epsilon-prime in the kaon system, and the unitarity triangle in the B0 system) are not large enough to explain the cosmological excess (which means there is more CP violation we haven't yet observed).

However, the existence of CP violation as a known part of physics is sufficient to prefer it as a minimal hypothesis, compared to invoking some new and unknown method of separation.

By Michael Kelsey (not verified) on 07 Apr 2014 #permalink

No human is born with Thought. This is because Thought-Thinking is the direct result of Language with Language being the direct result of Speech. Afterall, without a language, you cannot think, A Simplicity which Eludes. As such, the fuller the Speech, the better developed the Thought process. Merely squealing, sqwarking, bellowing, barking, etc, will not do. The Righting This, That & The Other of “Cruelty to Animals” be aware and beware.

It may seemed that Thought is the great gift of Speech but what if the reality is completely the opposite? Afterall, “Sun Tzu” alluded to this reality via his, “Keep your Enemy closest”. Should you disagree with The Military Academies and their Might is Right, that is. For Thinkers 101, be they saving or worshipping, once Thought had been acquired, It can never be removed. Not by humans anyway – should Thoughtlessness be not only possible but is the reality sustaining The Material Cosmos, oka The Periodic Table.

The reason The Double-edged Sword of Thought was gifted is for humans to realise Conscience, Conscience being the consciousness and thence awareness that self is not only a reflection of Self, but self is neither Animal/Gross-Masculnity nor Vegetation/Gross-Femininity. Afterall, without Thought, there can be no Conscience. Disagree so soon? Well, just ask your “pet” what is that image it is seeing in the mirror. And why the seemingly combative tone? Because you had already disagreed before you had even begin to realise other than merely “knowing all about eet”.

There, something for your Thought but do realise, because merely knowing is woefully inadequate for the purpose, that the duality of Thought [because Thought’s duty is to seed Doubt/The-Addiction-To-Repetition/Duality/Vacillation] is symbolised by that ancient depiction of “The 2 Horns of The Devil”. Afterall, another Simplicity which Eludes is that when it comes to the materially intangible, should such a beast exists, of course, without first being Materially Intangible, how would anyone be able to realise what is Absolute/Real and what is Unreal/Relative? By merely thinking? Thoughts like “Anti-Matter”? Perish the Thought, thougtless Awareness being not only The Secret of The Ages but Thoughtless Awareness leads onto Doubtless Awareness – the state of being able to visualise The Spiritually Benevolent. The moment when Gambling becomes Choice. It is benevolent to realise that Gambling under perfect conditions will realise neither gain nor loss and that Gambling under human[ised] conditions will always realise a loss – of reality at the very least.

@Peter #2,

I am not an expert so I will defer to anyone who is who wishes to correct me, but I would think that Hawking radiation would be of inverted parity for an antimatter black hole. We should therefore see gamma rays from annihilation events corresponding to such an antimatter black hole.

Another question for those with more expertise than I have: what explanations are there for gamma ray bursts? Is it possible that these could be produced by matter-antimatter annihilations? I'm guessing not, or Ethan would have mentioned this in his article. What is it about them that rules out annihilation events?

@Sean T #6: Hawking radiation is thermal, and by construction which particle of the pair escapes is random, so I don't think your analysis is quite right. In particular, it violates the no-hair theorem, so I "know" it's wrong, even if I can't point to specifics.

GRBs appear to be mostly quite normal supernovae, but with a jet aligned with our line of sight (so we get much more intensity than you'd expect from isotropic emission). The gamma-ray spectrum from GRBs is continuous, not 511 keV lines. There is a kind of supernova ("pair instability") which involves antimatter, but in the form of internal pair production, not pre-existing "chunks".

By Michael Kelsey (not verified) on 08 Apr 2014 #permalink


Thanks for the clarifications. It makes sense, then, that GRB's are not evidence for annihilation; annihilations must produce discrete spectrum gammas. Even if the total mass of the annihilating particle/anti-particle pair is different from 511 keV, it would still be a discrete energy. Therefore, postulating some heretofore unknown particle would not explain GRB's.

As for Peter's question, I guess then it's theoretically possible for a black hole to be made of antimatter, at least in the sense that were such a hypothetical antimatter black hole to exist, there would be no observation that could distinguish it from a matter black hole. However, that begs the question of how such a black hole would form in the first place. If we have a mix of matter and antimatter, both would behave identically with regard to gravitational collapse. There would be no reason why antimatter should collapse into black holes while matter remained more widely distributed. I assume that would be the best answer.

I am a mathematician with limited knowledge, but if there is no observation that could distinguish a matter from a antimatter black hole, I still believe this gives a plausible explaination. The travelled paths of the spawned matter and antimatter particle are different, and the event of being caught in a black hole is probabilistic. In effect, the number of uncaught matter and antimatter particles would go up and down as a random walk, and begin to stabilize at some value with the expansion and cooling of the universe. Our universe simply stabilized by chance to a state where matter particles were in majority. Magnifying this effect, is that the uncaught matter and antimatter particles are annihilating. Say the number of matter particles goes from M to M-K and antimatter from A to A-K. Then clearly, the share of uncaugt particles that are matter, M-K / (M+A-2K), increases with the number of annihilations K.

By Henrik A. Friberg (not verified) on 08 Apr 2014 #permalink

Curiosity question (probably answered in Physics 101)- the examples that I see of matter-antimatter annihilations involve matched particles, i.e. proton-antiproton pairs. What happens between an unmatched pair, i.e. proton-antineutron, interaction?
Also, since they don't seem to interact with much, could a neutrino-antineutrino pair escape annihiliation after initial formation?

By Bill Wood (not verified) on 09 Apr 2014 #permalink

@Bill Wood #10: Those are great questions! For your first case, p - nbar (we conventionally label antiparticles with a bar over the symbol), both particles are actually composites made of quarks. The proton is (uud), while the neutron is (ubar dbar dbar). At low energies, you'll probably have the (u-ubar) and the (d-dbar) annihilate, producing some gammas or maybe pi-zeros, while the u-dbar can interact weakly to produce a positron and neutrino.

At high energies, you have much more complicated interactions, not just annihilation, and the results will be similar to plain proton-proton collisions (e.g., at the LHC).

Neutrinos are a special case. They don't couple to phonons, so nu-nubar annihilation only goes through the Z0 boson. If the total energy is below two electrons, then the Z0 will decay into a pair of neutrinos. At higher energies (above 1.02 MeV), you can get electron pairs, or muon pairs (above 414 MeV).

In fact, we expect that there exists a cosmological background of relic neutrinos, left over from when the temperature dropped below the electroweak scale (about 1 TeV). These neutrinos and antineutrinos today would have a temperature around 2.18 K (they decoupled earlier than photons, so have had more time to cool).

By Michael Kelsey (not verified) on 09 Apr 2014 #permalink