Saturday, June 05, 2010

CP-violation and our origins

I was honored to be invited to an event with some exceptionally distinguished and intellectually independent participants whose names and whereabouts probably shouldn't occur here before I ask anyone for permission. So unless I miss my flight or something like that, you won't "see" me at least until Friday.

But before I leave, let me mostly agree with Sean Carroll:
Marketing CP-violation (Cosmic Variance)
To summarize, I do agree with him that it is unfortunate to use titles such as A new clue to explain existence for articles about experiments that study technical issues of the CP-violation. Dennis Overbye of The New York Times is arguably among the best U.S. newspaper journalists who write about science but yes, I do think that the meme shown especially in the titles - and, to a lesser extent, in the bodies of such articles - is misleading.

However, Overbye is very far from being the first one who has use the theme.

His article is about the recent statistical claims by D0 about a new source of CP-violation. Closely related, older claims by D0 were recently rejected by CDF, the competitors of D0 at the Tevatron.

You can see that I was always careful to use the term "CP-violation" in the very title - which is the first hint that I tend to agree with Sean Carroll. In fact, it took me some time to understand what the articles about "our origins" were actually all about.

The case that the journalism is fine

Some people defend the meme in Dennis Overbye's article. Let me borrow the words of one of the seriously sounding advocates of this position, a Cosmic Variance commenter nicknamed Baal.

He thinks that a fairer representation of the logic being used by Overbye is:

1. Baryogenesis requires a new, previously unknown source of CP violation.
2. We may be seeing (or looking for) a new, previously unknown source of CP violation.
3. Therefore, what we are seeing (or looking for) may be related to baryogenesis.

Consequently, the description is honest and also good physics, Baal thinks. Overbye did a very good job - and it's also right for the journalists to make science more interesting. Baal also criticizes Sean for de facto replacing "may be" by "is" in the third statement above. Let's start with the last point.

Well, almost everything "may be" - but in my opinion, an article hyping a certain idea can't be retroactively justified by saying that it only said that those things "might be true". Clearly, the articles were written because they wanted the readers to believe that the statements "were" true.

The global warming propaganda is the most notorious example where the ambiguous meaning of "may be" is being abused all the time. Ridiculous scenarios are being described as something that "may be" true, with the predetermined (and intentional) effect that the readers will think that the statements "are" really true or they "ar probably true" rather than extremely unlikely speculations. And whenever it's pointed out that these reports are dishonest, their authors say that they just wrote that it "might be" true.

If Sean calls it a case of journalistic dishonesty - and I guess he would hide that global warming is the best example of this bad practice - I completely agree with him.

Problems with the positive attitude

A few paragraphs ago, I copied Baal's logic trying to justify that the title was just perfect. I find it problematic for many reasons - reasons that go from technical details about the early cosmology to fundamental issues such as the motivation for science, its very definition, and the selection of people who should do it.

Because I expect to be losing readers along the way, let me start with the more comprehensible, philosophical issues.

Religion is no longer the motivation for science

The first point is concerned with the seemingly universal meme that "high-energy physics tries to answer the deepest questions about our origin." Well, theoretical high-energy physics is about the deepest phenomena in Nature but I don't share the idea that it is only or primarily about the questions "how things arose". It is equally or more importantly about "how things actually work today - and all the time" which have nothing to do with the creation.

It's often said that the LHC will restore the Big Bang and that's why it should be viewed as interesting by the readers. The LHC will create similar energies per particle but that's it: so I don't share this logic. If we lived in an eternal universe without any beginning, and if there would have been no very high temperature in our history, we would still study short-distance i.e. high-energy physics for its own sake. It's just important because other phenomena would still reduce to high-energy physics.

Physics wants to understand how the things work today and how they work at any moment. It's not just about our origins. In fact, the most distant events leading to our origins represent just a small portion of physics - and for a good reason. The world was pretty boring and chaotic and there were not "excessively" many interesting things going on at the very beginning.

The origins are exciting for people who think about these issues somewhat religiously. But I am actually not certain whether the notion that "physics is all about the origins" attracts some people. I am not even sure whether it makes religious people more excited about physics. My guess is that the answer is No. After all, they may prefer a completely different theory of our origins - but they may still be interested in the scientific mechanisms that make the world work today.

Also, I disagree that it's correct to bring people to physics by telling them these comments that "this science is all about our origins". I have never been biased towards these "historical sciences", if you allow me to use the term within natural sciences, and I think that this "historical bias" is a manifestation of a non-physics approach to the reality. Physics must always include some phenomena that are observed today and, ideally, also predictions for the future. It can't look into the past all the time (even though the past is a part of science, too - and it is, strictly speaking, the only source of all the empirical facts that we can already use).

If some people enter physics just because of our "origins", because of motivations that are largely religious in character, they're likely to be disappointed. You may increase the number of people who begin to study physics because of this reason but I am not sure whether you will increase the total quality of the science or the total number of the people who will be happy in it.

So I completely agree with Sean that it would be much better to bring people to science by telling them the truth. And the statement that these experiments at the Tevatron are primarily performed to learn about our origins is, in my opinion, untrue. This research is especially focusing on the physical laws that hold now and that will always hold. And the violations of C-, P-, CP- (yes, yes, subtle but certain yes), and (extremely unlikely) CPT-symmetry are interesting for their own sake.

(If you have ever met ugly posters called Antiluboš, you may have begun to believe that C-, CP-, but also CPT- symmetry is broken in Nature because they had really nothing to do with me. But CPT is actually not violated. It's an error in terminology for those dirty bastards to call themselves Antiluboš: they're not even made out of antimatter.)

Sean did a good job in explaining what they're about. The violation of these symmetries is kind of shocking - even for the laymen, I guess. Wouldn't you think that the phenomena must work in the same way behind the mirror? They don't. If people are not interested in this stunning fact without some special misleading justifications, there has to be a problem somewhere. But you can't fix the problem by linking these things to topics that are not really related.

The previous paragraphs were about the difference between "historical sciences" and "sciences about the present and the phenomena that exist eternally". And I argued - and Sean Carroll would probably agree - that this research of CP-violation at the Tevatron is more about the latter than the former, so the links to the early cosmology are misleading.

But now we must realize that Overbye's title doesn't even talk about "baryogenesis" - something that will be discussed later. It talks about "our existence". There's another huge gap between "baryogenesis" and "our existence", and I dedicate the following paragraphs to it.

Baryogenesis: an introduction

In our world, protons and neutrons are far more numerous than antiprotons and antineutrons. The same holds for electrons vs positrons. To summarize, our Universe is dominated by matter rather than antimatter. It's pretty clear that there can't be big regions occupied by antimatter in our present Universe - or in its recent past. Such "regions of antimatter" would lead to violent and easily visible annihilation, among many other problems.

This fact - that protects us against a speedy death caused by the annihilation with the omnipresent antimatter - is surprising from a physics viewpoint. It's because the number of protons and neutrons - the "baryon number" - is positive for matter. But it's conserved in the Standard Model. So it seems that the "baryon number" has always been positive.

But if the Universe were created from a tiny seed, its initial baryon number should be zero. So something had to happen that changed "B=0" to a nonzero value of "B". In fact, to create the matter without antimatter as we see it today, you need to violate several other assumptions.

These violations are called "Sakharov's necessary conditions" for the observed baryon asymmetry to be created by the cosmological evolution. They include the violation of the "B" conservation law, C-violation (asymmetry between matter and antimatter), CP-violation (asymmetry between matter here and antimatter in the mirror), and non-equilibrium dynamics during the relevant part of the Universe's life.

Similarly, you also need to create a lepton-antilepton asymmetry - and similar conditions for the dominance of electrons over positrons include the violation of "L", the lepton number. (This violation may also easily arise in neutrino oscillations if they are Majorana particles.) These two asymmetries might be linked to each other. One "asymmetric" process is usually able to ultimately create both asymmetries. If the asymmetry creation "starts" with the violation of "B", we call the process "baryogenesis", while for "L", it is called "leptogenesis".

A part of the life of our Universe had to include one of these processes - that may arguably be hard to separate because both "B" and "L" may be violated at the same moment. In that ambiguous case, the process is usually called "baryogenesis" because the light leptons are being disciminated by the fat hadrons that we call "baryons". ;-)

Fine, so I didn't complain in the previous paragraphs. What's the problem?

Baryogenesis is far from being the only key to our existence

The point is that I disagree with the identification between "baryogenesis" (or "leptogenesis") and "our creation". Baryogenesis is an interesting process - and it implies that when most of the matter and antimatter annihilated, there was something left - but there are many other, equally fundamental processes that were necessary for our existence.

Pretty much everything that has happened since the Big Bang (and, more speculatively, even before it, if that's relevant) and that is studied in high-energy physics and cosmology is necessary for our existence, at least if you really mean "ours". These key processes include
  • the creation of anything: there could also be nothing; clearly, this philosophical question due to Leibniz can't be resolved by technical considerations about muons although Leibniz's proclamation - why is there something rather than nothing - could be chosen as a better title than Overbye's title
  • the selection of the right shape of the hidden dimensions - assuming that you realize that they exist - and the selection of 3+1 dimensions (whose existence should be accepted by everyone) as those that are destined to grow
  • the inflationary period: it (or something more speculative that replaces it) is needed for the space to become large, nearly flat, and to dilute many exotic particles that could be killing us today, among other things
  • reheating - the end of inflation: it is needed to create lots of "pretty ordinary matter" out of the kinetic energy of the inflation field
  • baryogenesis or leptogenesis: that's needed to create the asymmetry so that antimatter is not going to completely destroy the matter in a minute
  • nucleosynthesis: when the Universe cools down to the QCD temperatures, hydrogen which is crucial for our Sun to exist - and other light elements - are born out of the quarks
  • decoupling: the atoms are born
  • structure formation: needed for the seeds of galaxies to arise, which is needed for galaxies; inflation was needed for some pre-requisites here
and so on, and so on. You may add the whole history of geology, biology (evolution), and even the history of the human race if you wish - because they're parts of physics, too. Our live depends on pretty much all these things. It's just not true that "baryogenesis" plays a privileged role for our existence.

Also, any effect above is linked to some observations that some scientists - or physicists - are doing today. So it means that all physicists - or at least all cosmologists and all high-energy physicists - have the same right to claim that they study the origins of our existence as the Tevatron muon-counters.

I would find the scientific journalism that always boils down to this single cliché pretty stupid because physics is simply not about this single cliché: it's much more accurate to say that religions (and metaphysics) try to answer these deepest questions. So if someone is not interested in any of the "more detailed" questions about our origins, he's simply not interested.

It makes no sense to tell him that physicists are working on something else, or that they are working on the right thing for different reasons. Such a statement is not really true, so if you lead someone to believe it, you will create at least as many problems as the good stuff that you may have produced.

The truth simply has to be important for science - and if the laymen decide not to find you after you tell them what you actually study and why, they simply have the right to decide in this way.

Leptogenesis, not baryogenesis, is closer to these particular observations

This point is a very small technical observation but I decided to dedicate three special paragraphs to it. The articles claim that the CP-violation experiments are related to "our existence" because "baryogenesis" is related to "our existence" - I have already discussed this weak link - and because these experiments are linked to "baryogenesis".

But a funny observation is that the particular experiment actually found an asymmetry between muons and antimuons - and these particles are leptons rather than baryons. Their baryon number is zero. So if this very effect were directly relevant for cosmology, it would lead to no baryon violation (at least not in this first step) but rather to a lepton-antilepton asymmetry which could help to sustain a violation of the lepton conservation law. (The Tevatron experiments themselves don't violate the conservation of "L", of course.)

So it would be more reasonable to say that this effect may tell us about our existence because of its links to "leptogenesis". The very fact that this reasoning hasn't taken place is evidence that our origin - or "baryogenesis" - is not really an important consideration that the physicists have in mind when they study these possible new CP-violating effects. It's purely a trained cliché for journalists.

Baryogenesis probably occurs at a much higher energy scale, making this experiment irrelevant

Finally, there's one more reason why these experiments are unlikely to directly teach us about baryogenesis or leptogenesis: baryogenesis and/or leptogenesis were probably taking place when the temperature of the Universe (multiplied by Boltzmann's constant) was much higher than 2 TeV (which is the energy probed by the Fermilab collider).

So completely new effects were probably dominant for the creation of the matter-antimatter asymmetry. It's very clear that at much higher energies, the relevant collisions numerically differ from the 2 TeV collisions at the Tevatron. And it's also clear from theoretical arguments that the impact of the CP-violating effects may be much higher than it is at low energies.

It's useful to know that there's no problem to violate "B" or "L" at very high energies. If a black hole decays, it may convert atoms to photons, despite their nonzero "B" and "L". Only truly conserved charges linked to gauge symmetries - and "B-L" is the only charge that may arise from a gauge symmetry but doesn't have to - have a reason to be conserved during black hole evaporation.

It's also conceivable that various CP-violating terms such as the QCD theta-angle are significantly stronger at very high energies.

Summary and gays of the Universe

To summarize, I wasn't as irritated by the coverage as Sean Carroll - because I would have otherwise written about it already. But I do think that Sean Carroll is more right than wrong.

There exists one related meme that I find "dumber than necessary": the notion that "we don't understand 96% of the Universe"; we only understand the 4% minority, so to say. Brian Cox is a great retailer of science but I do think that this statement is somewhat dumb. Why?

Because knowledge and understanding is not measured in kilograms. If something or someone is heavier, it doesn't mean that he or she or it will inevitably represent a bigger part of our knowledge and memory. It doesn't mean that many more books or articles will be written about him just because of the fat.

Dark matter and dark energy are analogous to the fat. Their mass is greater than the mass of the visible baryonic matter - but that doesn't mean that they're smarter, more organized, more complex, more interesting to study. Sometimes we can say: quite on the contrary. It's very likely that when people know "everything that is worth knowing", dark matter as well as dark energy will only be described in a very small minority of their books (or their pages).

Moreover, both dark matter and dark energy have already been partially understood. In the case of the dark matter, we expect that we will also learn about one additional species of elementary particles that dominates dark matter - which is not a real paradigm shift (although the related discovery of supersymmetry could be counted as one).

In the case of dark energy, we may be ignorant about the mechanisms that determine its very small size but we may actually be already knowing everything we need to know to be able to predict its impact on everything else in the Universe. So it's pretty unlikely that our knowledge will have to expand 25 times for us to understand just dark matter and dark energy.

And it seems that only dark energy, but not dark matter, may be hiding a truly conceptual mystery about Nature. But we're not sure about it. It may also be hiding nothing. For example, the value of the cosmological constant may also be determined anthropically in which case the future physicists will only confirm or prove what some physicists already "know" today.

I think that the journalists should try to describe what science is actually all about - and many of us may be surprised to learn that the readers actually care about it more than about some discredited, constantly repeated quasi-religious clichés.

And that's the memo.


  1. We are taught that matter is composed of atoms and that atoms have the same number of electrons and protons. Does this mean that there are the same number of each in the universe? If there were more protons how would we detect them? Could they be dark matter?

  2. Dear Harlow,
    good questions.

    Atoms are neutral composites of electrons, protons, and neutrons.

    It means that the number of electrons is equal to the number of protons and the total electric charge is zero.

    There also exists matter types that don't respect this criterion - they're not strictly made out of "atoms". They may be made of ions.

    Ion is what you get from an atom if you add or remove an electron or many of them. You can do it with the fox's tail and ebonite/hard-rubber sticks, and other combinations.

    Ions are even more important in chemistry - in solutions, all chemicals come in ions. For example, salt is Na Cl but Na+ and Cl- are not atoms but ions and they move freely in seawater.

    The higher temperature you consider, the more natural it is for matter to move separately - ions, electrons, protons etc. Plasma is what you get at high temperatures. Plasma is not made out of atoms but free electrons and protons etc.

    At any rate, it's true that you can locally create an arbitrary (in principle) excess or deficit of the negative or positive charges. That's what capacitors in electric circuits are doing all the time. One side contains much more negative charges and the other one has the positive charges.

    It's easy to detect the excess: there's electric field so the people who have hair (and dry hair) will see their hair going in the direction of the electric field, among hair-free methods to detect the electric field. ;-)

    However, it seems that they charges are always cancelled by the opposite ones somewhere in the Cosmos. The electric charge conservation is an absolutely valid law of Nature that can never be violated - because the electric U(1) symmetry associated with this conservation law is a gauge symmetry and gauge symmetries can never be broken.

    Because the Universe arguably began with a vanishing - or at least negligible (a few elemenentary charges? Even this is impossible in compact space) - electric charge, it still has to have a vanishing electric charge.

    However, that doesn't mean that the number of electrons equals the number of protons exactly. There are other charged particles, too.


  3. leptogenesis and bariogenesis is like:
    To have left shoe makers based on Mars and right shoe makers based on Earth. When we finally got to Mars we found that they guessed how much we needed. It is not possible without communication.

    DM is not needed at all:
    'A relativistic time variation of matter/space fits both local and cosmic data'

  4. The journalist "origins" meme that you see often enough has its roots as much in theoretical physicists as it does in journalists.

    Lots of theoretical physicists use "time elapsed from the big bang" as their primary way of explaining the notion of "high energy scales." The implicit reason that they do this, I think, is that cosmological implications make clear why anyone would care about the behavior of forces that aren't observed in any natural conditions, and the existence of particles that are grossly unstable and have no observed large scale impact on the universe.

    An alternative to the "origins" meme, and I agree with you that it is problematic and probably inaccurate in explaining the subjective reasons that experimenters have for doing fundamental physics research, is that physicists are looking for the architecture of the universe (or the rule book, if you will) and that this is proving phenomenally ellusive.

    The frame I like is one used by Feynman, that explaining the last 1% of what happens in the universe takes 99% of the model. This, rather than the 96% of stuff is unknown frame, is often more useful.

    One can get very far, in terms of practical applications, with quantum electrodynamics, the periodic table in lieu of QCD, a very black box empirical model of beta decay in lieu of a well developed weak force model, and general relativity with a cosmological constant. But, the riddle is why there is so much detail out there that can be observed and so many differences in possible solutions to the remaining detail, which is necessary to explain the tiny phenomena that these models don't explain?

    Dark matter is probably the not fully solved problem with the most applications in terms of applying theory to observations (and you are right to note that dark matter and dark energy are not complete mysteries, even though we don't fully understand them either).

  5. why cp is not violated in stronger interactions? the conservation of cp,in the case,would be linked to the violation of pt-that implies the differences between the particles and antiparticles.

  6. if appear the asymmetry between electrons and positrons could appear the polrarizations of photons,then the spacetime is not isotropics and homogenous.the quantic vaccum is polarizated..,