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Q & A: Antimatter

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Q:
What is antimatter? How is it used? Does it have effects on people and the environment? Can it travel through matter? I think they should theach this in schools.
- Anonymous (age 15)
A:
The long answer to "What is Antimatter" and its current uses:

Antimatter has many opposite properties to the ordinary matter we are made of. It has a long history, and yes, very small amounts of antimatter are produced routinely in particle physics laboratories every day.

The existence of antimatter was one of the early triumphs of the twentieth century theories of physics. In 1928, Paul Dirac proposed a description of electrons which combined ingredients of quantum mechanics and special relativity and which seemed to work very well. The equations had the odd feature that not only did the solutions describe electrons, but additional solutions described the behavior of particles which behaved a lot like electrons (same mass and spin, for example) but which had opposite charge. If one of these other particles ever interacted with an electron, both would disappear in a burst of energy. This seemed very crazy back in 1928, and people wondered if we should just ignore these other solutions to the equations.

Theories need to be put to the test in nature in order for the scientific method to work, and Anderson did so in 1933. In a cloud chamber, his group discovered that particles just like electrons but with a positive charge (electrons have negative charge) were found to occur naturally, and some of these positively-charged electrons rained down on Earth as a result of cosmic rays interacting with the atmosphere. The new particle was called the positron.

Since then, all particles we know about have been found to have antiparticles, and our theories of particles don't make sense without them. Some particles are their own antiparticles, for example, the photon. Antiparticles can annihilate with particles to make a burst of energy; the opposite reaction also occurs. If you put enough energy together in one place, you can produce a particle and an antiparticle pair together.

In particle accelerators, electrons or protons are accelerated to high speeds and collided with other bits of matter. In the collisions, enough energy is released to make particle-antiparticle pairs. Many accelerators are cleverly designed and can collect the antiparticles and accelerate them in turn, colliding them later with beams of particles in order to study even more interesting collisions. That's what some of us do for a living. Positrons are much easier to make than antiprotons, because they are much lighter. Many other kinds of antimatter are unstable because their matter counterparts are unstable too (i.e., they exist for a very tiny fraction of a second before decaying into more usual stuff).

One note on how little antimatter has been produced (from our answer on making antimatter for space flight:
According to Gerald Gabrielse, a Harvard professor who is doing research on the production and storage of antihydrogen (a form of antimatter), even if you could collect all of the antiprotons ever made (by humans), you couldn't even heat up a cup of coffee with the energy released by annihilation.

Antimatter is opposite to matter in many ways but not all:
1) It has opposite charge
2) It can annihilate with their counterpart matter particles

But antimatter has many similarities
1) A particle and antiparticle have the same mass (as far as we can tell). So they all attract gravitationally just like real matter (no antigravity with antimatter, sorry!)
2) They have the same spin
3) Unstable antimatter particles have the same lifetimes as their matter counterparts.

There are some very slight difference between matter and antimatter which are currently under study. One of the main questions in physics today is: "If you can only produce matter and antimatter in pairs, in equal quantities, then why is there so much matter in the universe today, and so little antimatter?" Experiments today are trying to investigate the differences that can bring this about.

For a good introduction to this and other particle physics topics, and to see what's going on in the field today, check out the particle adventure website at Lawrence Berkeley laboratories in California.

Does antimatter have effects on people and the environment?:

Well, we haven't produced enough of it to have a noticeable effect on anything large. In general, antimatter will not last long after it is produced because each particle will annihilate with a particle as soon as it comes in contact. (Sometimes a particle and an antiparticle will orbit each other for a short while before annihilating -- they are oppositely charged and hence attract electrostatically, but for the purpose of this question, matter and antimatter will annihilate into photons right away). Protons and antiprotons annihilate into other particles called pions which decay themselves into stuff like electrons, photons, muons (which decay into electrons) and neutrinos. So the effect of antimatter is the same as the effect of the radiation produced by its annihilation.

Can antimatter travel through matter?

Well, not really very well. Antiprotons and antielectrons, going slowly, will annihilate with their matter counterparts. If they have lots of energy, they can create sprays of other particles when they strike ordinary matter. In the case of electrons (or positrons), if the energy is high enough, they will create a shower of thousands of electron-positron pairs of lower energy. Eventually these slow down and annihilate. Some particles pass through matter rather easily, like neutrinos. Antineutrinos pass through matter about as easily as neutrinos (not quite the same because of the ability to interact and exchange charge with a nucleus or electron is different). Antimuons pass through matter as easily as muons -- both are found in cosmic rays. There really aren't any muons to annihilate with because they decay so quickly on their own.

Yes they sure should teach this in schools! We've known about antimatter for seventy-six years now!

Tom

(republished on 07/20/06)

Follow-Up #1: antimatter uses

Q:
If antimatter is at least possible, when is the 'antimatter bomb' (currently developing by the U.S. military) coming out? and is antimatter going to be used on medical uses, I mean in nanograms?(not that I want to blow someone up)
- Jacob Kim (age 11)
Vancouver, BC, Canada
A:
Antimatter is routinely produced, but it's very hard to store. Don't expect any antimatter bombs.

Antimatter is now frequently used in medicine in positron emission tomography (PET) scans. Positrons are anti-electrons.

Mike W.

(published on 10/16/09)

Follow-Up #2: Producing particle-antiparticle pairs.

Q:
Quote:"If you put enough energy together in one place, you can produce a particle and an antiparticle pair together." Well, but how do you know which particle-pair will come out, after putting a huge amount of energy in one place? Or are there any necessary cicumstances to 'produce' an electron-positron-pair or an proton-antiproton-pair? What do you think, where's all that "antimatter" in our world? If both was produced in the same amount in the BigBang - so 50% matter and 50% anti-matter. Probably most of them annihilated within the 'first second', but the rest of matter formed our "today's Universe". So is it possible that there is somewhere else our anti-universe, that looks absolutely the same as ours? (Because both have the same amount and the same "starting-point" the bigbang) I hope it's clear what I want to say^^ Greetings, Dani
- Dani (age 19)
Austria
A:
Good questions Dani.
 
The pair production process occurs during collisions between two other particles.  The first requirement in pair production is that all of the basic conservation laws must be obeyed:  energy, momentum, angular momentum, charge,  lepton number, strangeness, etc.    Other than that, anything goes.   The different rates of pair production can depend on available kinetic energy of the final particles, (phase space considerations) and the relative coupling strengths of the possible interactions. It's a crap shoot, to use the vernacular.  However the relative probabilities can be calculated.

As you point out there seems to be more matter around than antimatter: why? This is a profound question and is one of the puzzling questions of today.   The necessary conditions for matter-antimatter asymmetry was first pointed out by the Nobel Prize winner Andrei Sakharov:  see  http://en.wikipedia.org/wiki/Baryogenesis
It could be there is a neighboring universe that has the asymmetry going the other way.   We don't know.

LeeH


(published on 12/23/09)

Follow-Up #3: Energy from antimatter?

Q:
Great answers. But there's just one thing I'm still confused with. Apparently we can use the energy produced from antimatter and convert it for use. What could we use this energy for? And how could we get energy from something that will be destroyed if it comes into contact with any form of matter?
- Lane (age 16)
Raleigh NC
A:
When antimatter annihilates with ordinary matter it mainly produces photons and other particles like mesons which in turn decay into more photons, electrons, etc.  These secondary particles will be absorbed by surrounding matter and the net result is a heating of the absorbing material.   In principle one could use a beam of antimatter to heat up water to make steam and run a turbine to generate electricity.  However from an economic point of view it won't pay because the cost of generating the antimatter far exceeds the payback by many orders of magnitude.

LeeH

(published on 11/18/10)

Follow-Up #4: the wrath of Newton

Q:
Dear Sir/Madam: Newton NEVER used the word Attraction but only Gravitating and very angry with whom could believe that Gravitation is Inherent to Matter. Gravitation is Pushing. See please the New Paradigm by Dr. Carezani dubbed by short Autodynamics at: http://autodynamicslborg.blogspot.com/2011/05/quantun-universal-gravitation_06.html Regards. Lucy Haye PH. D.= SAA's representative
- Lucy Haye (age 69)
Long Beach, CA-USA
A:
OK, but what  is gravitation pushing? The New Paradigm? Or something not yet legalized?

Mike W.

(published on 09/27/11)

Follow-Up #5: Matter, anti-matter, and electric charge

Q:
In the universe we have matter and we also imagine about the anti matter but do we have something which is between matter and anti matter something like neutral matter?
- Razin Shaikh (age 13)
Navsari, Gujarat, India
A:
Any charged fundamental particle can have an associated anti-particle with opposite sign charge.  Neutral matter does exist such as the neutron, and has an anti-matter counterpart that is also neutral.  An anti-neutron has the same electrical charge as the neutron, zero, but it is different in other ways.   It can annihilate with a regular neutron releasing the combined rest mass energy into a burst of secondary particles such as pions or other small mass particles.   The total energy is conserved, as usual.
We have answered many similar questions on anti-matter on our website.   Just type in antimatter into the search box and you will find lots of them.

LeeH

p.s. Photons are indeed their own antiparticles, and thus would count as "neutral" in your  broad sense of that word. /mw




(published on 08/15/12)

Follow-Up #6: Is antiparticle spin opposite?

Q:
Hi. In your description of the properties of antimatter you say that the quantum spin is the same as a normal particle. But on the wikipedia page (http://en.wikipedia.org/wiki/Antimatter) it says that both charge and spin are opposite for antiparticles. Who is correct?
- Steve (age 34)
UK
A:
Usually when we describe the spin of a particle, we just mean s= 0, 1/2, 1, ....Basically what we're doing there is giving the eigenvalue, s(s+1), of the S2 operator. That's the same for a particle and its antiparticle. It wouldn't even mean anything to talk about opposites for that.

I guess when WP says that the spin is opposite, they're referring to any participation in processes that violate time-reversal symmetry. It's only such processes that would really distinguish between a spin and one pointing the opposite way. If you've got one of these rare weak processes for some particle, and you look at that process for its antiparticle (also looking in a mirror) it will look the same only for the time-reversed version, which means the one with opposite-pointing spin, not the opposite absolute s. The formal way to say that is that there's an exact symmetry, CPT (charge, parity, time) and that in some cases there are weak violations of CP compensated by violations of T.

Mike W.

(published on 03/04/13)

Follow-up on this answer.