Science Articles

Where has all the Antimatter Gone?

Thu, 11th Mar 2010

There should be equal amounts of matter and antimatter, so where is it?

Véronique Pagé

Amongst the many gifts physics has presented to science fiction, antimatter is doubtless a highly prized one. It has a catchy name; it is very mysterious and it can 'annihilate' things (and thus be used as a murder weapon, according to Dan Brown). Yet, to physicists, antimatter is somewhat less than exciting and would be almost mundane if it wasn't for one shocking truth about it: it seems to have entirely vanished from our Universe!

It was in the late 1920's that theoretical physicist Paul Dirac, working in Cambridge, Paul Diracpredicted the existence of antimatter. Quantum mechanics was the talk of the town (or at least the university) and Dirac was working on it.

He was trying to incorporate relativity, itself only 20 years old, into a quantum description of the electron. The equations seemed only to say that it was impossible for the electron to obey the laws of both theories. That is, unless it was assumed, and without any experimental evidence, that there existed a new particle, identical to the electron in every respect apart from its electric charge. There simply had to exist an “anti-electron”.

It might have been a great mathematical way to fix the equations, but there was something truly ludicrous about this idea. The electron had been detected more than thirty years earlier. If you wanted to study it, finding it was not exactly difficult: it was more or less everywhere. If a particle existed that was identical to it but with the opposite electric charge, then it should have been equally easy to detect. So where was it? In what is now considered as a stroke of genius, Dirac went ahead despite this issue, and proclaimed the anti-electron existed. Now it was up to an experimental physicists to find it...

Amazingly enough, it only took two years for one of them to do just that. In 1933, Carl AndersonCarl Anderson, of the California Institute of Technology, discovered the elusive particle. Although, as would have been plain to Dirac, anti-electrons are not commonly found on Earth, they do fall on our heads from the sky. In fact, we now know that numerous and various energetic particles are constantly zooming around the Universe. Inevitably, some end up passing through our planet's atmosphere; these, together with the cascade of particles their passage can produce, form what we call cosmic rays. Unknown to Dirac was that cosmic rays do include anti-electrons, which is what Anderson observed. Turning his apparatus heavenwards, he photographed something that would have been inconceivable just a few years earlier - the passage of positively charged “electrons”.

Mirror images

This was certainly great news for Dirac and great news for physics itself: it enabled a quantum relativistic description of matter. Since then a model of all the known elementary particles has been built. In this “Standard Model”, each particle has a corresponding antiparticle. Both are identical in all their properties such as the mass and spin, but they have opposite electric charges.

The Standard Model treats particles and antiparticles very much on an equal footing, and this reflects observed behaviour. Collisions in particle accelerators, for instance, routinely produce antiparticle and particle pairs, and even “anti-atoms” of anti-hydrogen were demonstrated at CERN in 1995.

In fact, there really is nothing exceptional, or indeed exciting, about antimatter – if you were to watch a film set in an anti-world, where anti-people have anti-houses and drink anti-tea, you wouldn't see any obvious difference with the “ordinary” world. But when a particle and its corresponding antiparticle meet, however, something exciting does happen: they both vanish in a flash of light (you might want to keep this in mind if ever the anti-people invite you over for an anti-cuppa).

For this reason, keeping antimatter for any length of time is exceedingly difficult - making sure antimatter doesn't come into contact with matter at all is a real experimental challenge. At CERN, charged antiparticles are maintained in ultra high vacuum jars using magnetic and electric fields. No one has yet found a way of containing electrically neutral antiparticles.

Matter and antimatter are in fact so similar that physicists sometimes describe them as the mirror images of one another. Departures from this matter-antimatter symmetry have been observed, but they are very small. There is a rare particle, called the kaon, which turns into other particles at a slightly different rate than the anti-kaon turns into the corresponding antiparticles. A handful of other exotic particles closely related to the kaon have a similar behaviour. Considering it is so very rare, it does seem a bit odd that this behaviour should be possible at all. Cloud chamber photograph of the first positron ever observed. © Carl Anderson, circa 1937It has, however, been incorporated in the Standard Model quite smoothly, and without altering the general picture of antimatter.

Evil twins

That said, and as we all know, there is an essential difference between matter and anti-matter, something that was already clear to Dirac in the 20s: there are no anti-electrons on Earth, let alone anti-people, anti-tables or anti-books. In fact, there is no evidence of there being any sizeable “lumps” of antimatter anywhere in the observable Universe. Antiparticles observed in cosmic rays, like Anderson's anti-electrons, are in large part what physicists call secondary; this means that they have been created by the interaction of another particle (here of ordinary matter) with either some interstellar medium or our own atmosphere – both made of matter. Critically, they don’t seem to come from some distant “source” of antimatter.

Moreover, the antimatter CERN uses hasn't been harvested; instead it had to be created through collisions between ordinary protons and nuclei. One possible outcome of such collisions is the simultaneous production of a proton and anti-proton, known as “pair production”. As they have opposite charges, the two species can be prevented from annihilating one another by driving them apart with a strong magnetic field; then the anti-protons can be collected.

Cosy as this sounds, it still leaves a yawning hole in the argument. On the one hand, the current laws of physics seem to preclude any significant difference between matter and antimatter. On the other hand, there clearly is a major difference between matter and antimatter. In fact, the puzzle is even deeper.

The early Universe was a much smaller, denser and hotter place than it is today. In such an environment, collisions (and therefore reactions) between particles happened at an incredibly fast rate. If, in those early moments, matter and antimatter had been, unlike today, present in equal quantities, then something terrible would have happened. Protons and anti-protons would have annihilated, electrons and anti-electrons would have annihilated... there would be virtually nothing left in the Universe! If our Standard Model is to be believed, there can only be something in the Universe today if there never existed any significant amount of antimatter. The Big Bang would need to have generated matter only, and no antimatter whatsoever. The fact that our nearly matter-antimatter-symmetric model should lead to this “annihilation catastrophe” is a major issue in modern physics.

A forensic investigation into outer space

It was unlikely that such a puzzling case would not attract the attention of a large number of (mainly) theoretical physicists. Over the years, it has become clear that there really are only three possible solutions to this issue:

1) We assume that the Big Bang didn't produce any antimatter, so that there has never been any significant amount of antimatter in the Universe.

2) We assume that the Big Bang produced matter and antimatter in equal proportions, and that they still exist in equal proportion; it's just that we live in an immense “matter region” of the Universe, and there are no 'antimatter regions' close enough for us to observe them.

3) We assume that the Big Bang produced matter and antimatter in equal proportions, and that there really isn't any antimatter left; something happened to the antimatter earlier, though our model can't explain it.

Today's physicists are favouring the third option. The first “answer” is considered by many to be unsatisfactory; by merely stating that the Universe started off without antimatter, it is felt it avoids the issue. The second option might be a possibility, though explaining how matter and antimatter ended up segregated on such a scale is no simple task.

But the third set of assumptions forces physicists to look for a mechanism that would effectively create a net amount of matter. Maybe an unknown reaction exists that favours production of an electron than an anti-electron, or a reaction that destroys an anti-proton more often than the opposite reaction destroys a proton? Or perhaps a complicated chain of interactions like this takes place?

For a time there was hope that the peculiar interaction of the kaon might have happened often enough in the early Universe to A cup of anti-tea?explain what we see today. Unfortunately, it seems it was much too uncommon even in the hot, dense first moments of time.

Instead, having taken a leaf from Dirac's book, many physicists now believe that as yet unknown particles may lie at the heart of the “antimatter whodunnit” case. These particles, they suspect, existed only in the early Universe and now cannot be found in nature because they have since decayed into the more common proton, neutron and electron.

This sounds like an insurmountable conundrum, but even if these particles no longer exist in the real world, maybe they can be re-created, and CERN's Large Hadron Collider is attempting to do just this.

Though none of these particles might end up being the main suspect, it can be hoped that they will help shed a little more light on the strange case of the annihilation catastrophy. Whatever comes out of the collisions there should prove inspiring to physicists... and to science fiction writers too!

Subscribe Free

Related Content


Make a comment

Hmm... I thought that one of the possibilities was that matter and anti-matter were created in nearly, but not quite perfectly equal proportions...

The kaon asymmetry must have had some cause, as well as a corresponding effect, even if we haven't yet found what caused the asymmetry or the subsequent consequences of it.  That sort of thing cannot be irrelevant. LeeE, Tue, 16th Mar 2010

There's a real out-of-the-box answer to this one.

I don't know if you know, but the only stable static particles with mass are the electron and the proton along with their antiparticles. The neutrino doesn't count because it isn't static. The neutron doesn't count because it isn't stable. Nor are pions or the rest of the "zoo". Even quarks don't count, because we've never seen a free quark. So you've got four particles to consider - the electron and positron, and the proton and antiproton.

Draw a little 2 x 2 table and put the electron into column 1,1 then put the positron in column 1,2. Think about their properties, and then think about the properties of a proton. Now, ask yourself this: between the electron and the positron, which does the proton most resemble? They all have the same spin 1/2. The proton mass is 1836 times the electron and positron mass. However the proton charge is +1, which matches the positron rather than the electron. So put the proton in column 2,2 under the positron, and put the antiproton in column 2,1 under the electron. 

Now add the headings MATTER and ANTIMATTER and ask the question again. Where has all the antimatter gone? Farsight, Tue, 16th Mar 2010

Thank goodness all the answers in science are not "out-of-the-box". How boring would that be? Geezer, Wed, 17th Mar 2010

Hi thedoc, Farsight, LeeE, Geezer and all,

This answer is not "out of the box" either... Interesting article but gives few real clues. Of course the answers might be "already well understood"... just not acceptable in the framework of current theories. I would like to propose an "old" idea that has no disproof but appears to cause quite a few frowns on the brows of those who believe in "tachyons" (which I think are purely fictional). These theories appear to be superficially incompatible. It beggars the mind to think that a theory such a tachyons can get "legs and begin to run" without any physical proof. The theory I suggest you all "reconsider" is the Feynman-Stueckelberg Interpretation for Antimatter.
Antimatter - Feynman-Stueckelberg Interpretation
Put simply... antimatter is simply a state of matter in which it is traveling into the past. The creation of particles and antiparticles still "works" and there is vast amounts of energy involved but rather than thinking that matter and antimatter is all about "the explosive release of energy" we consider it as the creation of "mirror matter" as a "matter wave reflection" in a mirror that projects it backwards through time. Matter wave reflections associated with elliptical quantum corrals producing matter wave phantoms... which have all the physical properties including the chemical properties of the original "imaged" atoms... have been recently demonstrated at the IBM Almaden Laboratories.

One way in which all the matter and antimatter ratios can be effectively taken care of is if they never mix... if at the instant of the big bang which created matter from pure energy (lets say from a burst of intense photonic light) matter is confined to travel forward in time while all the antimatter was confined to travel backwards in time. The CPT Symmetry insists that if we consider a positron (anti-electron) to have all reversed properties as a "special electron" then the property of time is also reversed too.  The fact that the matter-antimatter creation event can occur without the intervention of matter (sparking the vacuum!) and has been performed at SLAC in 1997 adds credibility to this interpretation. It does take a little wind out of the sails for quark-gluon soups being needed in the Baryogenesis event but who cares if it is "easier" to do so it should happen in preference to the more energetic event possible. In fact matter antimatter creation naturally separates out into two "particle bubbles" due to the time phases being apparently "incompatible" it would seem.... due to this time separation process... and we see this today in pair production from the objective "third-time" we record in a Laboratory. The idea that matter and antimatter existing in the one space even for an instant do not naturally annihilate seems absurd... but the fact has been experimentally proven recently adding some loose support for this "phase separation process" as a fundamental property of matter and antimatter. "Classical Grand Unification" may still occur at high temperatures but may not be the only "unification" possible. Naturally since the point of creation is the natural "zero" of time any backward traveling in time antimatter simply went into a spacetime foliation of relatively negative time... A separate mirror universe. There would be no annihilation of either matter or antimatter since these states were separated from the start of our universe and aside from that single instant of creation and very brief subsequent period where this event carried "creation" forward there was no further mixing of matter or antimatter in our universe or in the mirror universe since they now were existing in two different non-overlapping time streams... in the identical way in which Feynman Diagrams have always indicated. Antimatter created in our time is attempting to travel back to the big bang and enter the parallel foliation but "mostly" (and not surprisingly) it mostly undergoes an annihilation event long before it travels those billions of years backward in time. Please note that the matter particle that was created in the creation event does not need to be the particle annihilated later on by the antiparticle... any identical particle will do!

What appears to happen in the annihilation event is the transfer through time of a normal particle in the present to an identical and entangled particle in the past using the Feynman-Stueckelberg Time Travel Paradigm which I have illustrated here for an electron but the same routine applies for any antiparticle.

This illustration shows a particle such as an electron (particle A) progressing normally forward in time along the vector A from the origin to the coordinates S,T = 1.5,1 then encountering it's own antiparticle (particle B) moving along the vector B and annihilating. The two particles vanish liberating a pair of photons (... the two blue lines). This can now be reinterpreted as a particle creation event in which two mirror particles (electron positron) are created at the coordinates coordinates S,T = 2,0 then progressing forward in time as an electron (particle C) along vector C and positron (particle B) along vector B.  What the Feynman-Stueckelberg Time Travel Paradigm suggests is that particle A and particle C are the same particle... the entire event causes the particle A to be transported back through time as particle B along the vector B in the negative time direction to the new coordinates. The two electrons A and C are copies of itself existing  on different time lines and it is not impossible for them to potentially "harmlessly" encounter "each other" before particle A performs this time travel "trick". The time travel is the annihilation event itself which is actually not an ending but a new beginning as particle C in it's own past.

There is an interesting further point to be made here is the energy emitted and absorbed in the form of photon pairs are "truly time symmetric since" since nature does not care where the photon source was or where the sink is ... the source of the energy and the sink of the energy could be simply a quantum catalytic event in which these two are connected together as this nett reaction require no nett input if "correctly configured" as an emitter-absorber event pair we see in waveguides. This is in general agreement with the Wheeler-Feynman Absorber Theory Interpretation of Quantum Mechanics which has been given additional support recently by the enigmatic results from a test of Hardy's Paradox where the event phenomena become part of a more general eventspace which can involve the uncollapse of events that have been recently demonstrated. It would just mean that both emission and absorption would need to occur in the "near field" to allow this to occur. Experimentally all these features can be shown individually to exist while the existence of tachyons cannot be shown to exist.

No new particles needed.... So much experimental support and not a scrap of real investigative analysis being able to dispute this process. So I believe "everything old is new again"... if you can time travel.    Considering the universal use of Feynman Diagrams of which this is "sort of" one of them it is strange to me that these ideas are given no credence in todays papers and instead so much is allowed to an demonstrably unproven idea such as tachyons to rule the roost. Why the prejudice? I think we are forced to wait out the events unfolding at CERN to see if this is still on the table and certain other theories that are still currently unproven are proven to be "unprofitable".  Are we so needy of finding a way to make more weapons that we have forgotten these still sound proposals waiting in the wings that have the potential to "correct" our theories of cosmology?

Cheers Good Elf, Fri, 19th Mar 2010

While I think there's some mileage in the idea of reversed time phenomena, they do require a superimposed parallel universe, as I think you go on to mention.

The problem with reversing time in our time dimension is that there appears to be no origin from which the reversed time can start from i.e. our 'normal' time has an origin in the Big Bang, but a reversed time flow would seem to need to start from the end of the universe, and as far as we can tell, there's nothing to suggest that time will ever stop, even if entropy leads to the eventual flattening of all energy.  Hence, setting the origin for reversed time at the opposite end of 'our' time dimension seems to be impossible as 'our' time dimension doesn't appear to have an end.

It should be noted that if our time dimension does have an end then it suggests that the future is predestined.  This isn't really 'against the rules' though either. LeeE, Fri, 19th Mar 2010

Hi thedoc, Farsight, LeeE, Geezer and all,

Thank you for your questions LeeE...
The are only "superimposed" at the instant of creation after which they separate out from each other due to the reversed time in each "phase" and their "temporal motion" into separate foliations.
Just look at the conventional number line which applies to time as well... The natural origin for time in our Universe is at the Big Bang... that is where zero time exists "for us". Reverse time for our Universe starts there and runs backwards on our time line towards larger negative numbers in the opposite direction. At T=0 when all the matter in our Universe began to form half of it instantly moved "backwards" into the negative time foliation region where it was "out of reach" of contact with normal matter. It did not have to time travel far to get there at the time it was created... no "temporal distance" at all really!. It has nothing to do with entropy since entropy only deals with increasing scalar time which increases in both our Universe and in the Mirror Universe in the same way today. The reversal along the time line is accompanied in the mirror universe by the appropriate reversal of sign of the charge and party making the "antimatter" appear exactly the same type of matter in that universe that we currently experience in our universe with all the same properties... or are we the "antimatter" Universe?? This is an application of CPT Symmetry we always see in our universe in which there is conservation. Feynman made specific note that this is one way it could be interpreted. There is no way to tell which "matter" our entire universe is collectively composed of since the physics is now identical to any other mirror matter universe when observed from "within" and the two separate universe now evolve together though along different directionally pointing time-lines and perhaps trajectories in the hyperspace our universe evolved from originally .... where all this energy originally came from.

We can "observe" reversed time in our Universe in "small events" with the production of antimatter of any kind. So the lab frame's "absolute time" is 14 billion years or so along the positive direction of our collective time line (ignoring differences due to relativistic time dilation due to motion) being the one observer time which has one particular direction along the time axis.

For particle creation events lab time "perceives" one state of matter progressing (the "normal" particle)along the positive direction of time while the Lab is at the same time "forced" to view  "antiparticles" progressing along the negative time direction along our time-line as reversed time starting at a "local zero" of the two particle system but time before the common zero of the two particle system. The antiparticle is being now forced to "find" it's way back to the "big bang" in our universe continually "winding it's tiny personal clock backwards" which is before it's local zero creation date. Unfortunately it usually encounters another particle in our Universe and annihilates it "well before then" as a pair canceling out the mutual time-lines into nulls (photon trajectories). All the while the normal particle that was created in the event has it's clock "roughly" in step with our clocks in the Lab but only differing with a different local zero.

All particle experiments ignore the "local clocks" deemed to be embedded within individual elementary particles. We currently have no way of actually "reading" these internal clocks which should all normally have a creation date "zero" around 14 billion years ago. We know time for them does exist since this aspect of relativistic time dilation is one of the most experimentally confirmed facts known for collective observed particles.

The analogy in our universe's case is it started by an injection of "light" from a higher dimensional source (in this case from outside the particle - antiparticle system defining the local zero of energy and  time)... I mean absolutely everything in our universe started from an object so tiny that it would fit easily inside a fundamental particle. The matter and antimatter of our "big bang" separated into two "bubbles" in the same way primary particles separate... their different directionally pointing and oppositely signed time-lines temporally protecting and isolating the different forms of matter from each other. Both particles (particle - antiparticle) each in their own right defining their own developing and evolving space and time. As previously mentioned this process has been performed in the Lab using a pair of photons for the low energy case of electron - positron pairs (it is "hard to do"... the "raw event" requires more energy than CERN and only the accelerator at SLAC plus one of the then most powerful laser sources in existence was able to perform this event for the simple case of electron-positron production. It is still way beyond our technology to perform this "sparking the vacuum" for protons - anti-protons using pure light but in principle it should be a simple extension of the same phenomenon. I would also point out that photons which are bosons and obey Bose-Einstein Exclusion Rules exist in a state of superposition and do not normally "crowd each other out" so everything "a universe" might contain might be packed within that "first event".

Out Of Pure Light, Physicists Create Particles Of Matter

.... Fiat Lux!


PS: The difference in energy causing this separation between matter and antimatter "could be" interpreted as being related to the "Weak Force" and shows this "asymmetry" in the physics we are able to see today. Good Elf, Sat, 20th Mar 2010

More brevity is required. LeeE, Sun, 21st Mar 2010

Good Elf, I really must talk to you about time! Farsight, Sun, 21st Mar 2010

The Sun and the cosmos are currently powered by dynamic competition between neutron repulsion and gravitational attraction.

Matter/antimatter asymmetry is a problem if the universe is finite and all matter was came into being at some hypothetical time like the Big Bang.

Matter/antimatter asymmetry is a not problem if the universe is infinite and oscillates between:

1. Expansion as compact nuclear matter dissociates, and

Neutron stars => Neutrons => Hydrogen

2. Contraction after the neutron stars are gone and neutron repulsion no longer counters gravitational attraction:

Hydrogen => He => C => . . . . Fe => Neutron stars

This was discussed earlier here:

With kind regards,
Oliver K. Manuel
om, Tue, 1st Jun 2010

You've been listening to Sean Carroll a bit too much Gazeteler. Farsight, Fri, 1st Oct 2010

Ironically, pehaps the answer is in the film "la double vie de Veronique". In this Veronique catches sight of her doppleganger, but the doppleganger has a mysterious illness and dies! John, Sun, 12th Dec 2010

As a doctorate in English. The article is a good literary piece.

Scientifically, I was utterly bamboozled by it. ChristianHuffines, Sat, 19th Feb 2011

Good Elf theory implies a negative mass for antiparticles, which i doubt... CPT ArkAngel, Sat, 19th Feb 2011

Good Elf is questioning 'time' CPT, not defining anti particles. And if time indeed can act as it seems on a Feynman diagram and are reversible as physics assume? Time is indeed a very strange thing in relativity. Even though my arrow, as far as I can measure, is a unchanging one it will differ when I start to compare it with other frames. The twin experiment do state that we can 'time travel' although not as measured inside our own frame of reference. Maybe something similar is imaginable for the reversed temporal direction, not that I see how for the moment? yor_on, Sat, 19th Feb 2011

As a competitor theory the Iron Sun deserves more discussion particularly in light of NASA's most recent measurement indicating that E (mass) bends space. Posting from above: The Sun and the cosmos are currently powered by dynamic competition between neutron repulsion and gravitational attraction. Matter/antimatter asymmetry is a problem if the universe is finite and all matter was came into being at some hypothetical time like the Big Bang. Matter/antimatter asymmetry is a not problem if the universe is infinite and oscillates between: 1. Expansion as compact nuclear matter dissociates, and Neutron stars => Neutrons => Hydrogen 2. Contraction after the neutron stars are gone and neutron repulsion no longer counters gravitational attraction: Hydrogen => He => C => . . . . Fe => Neutron stars This was discussed earlier here: With kind regards, Oliver K. Manuel katesisco, Sat, 20th Aug 2011

lol, you are so confused Ian, Sun, 24th Feb 2013

See the whole discussion | Make a comment

Not working please enable javascript
Powered by UKfast
Genetics Society