What is antimatter?

 

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        In this section you will find answers to the following questions:

  • What is matter? What is it made of?

  • What is antimatter? What is it made of?

  • What happens if matter and antimatter meet?

  • Where do matter and antimatter come from?

  • Is there antimatter in the universe? Where is it?

 

Before we can address the issue of what antimatter is, we should ask ourselves what matter is. The simplest answer would be that matter is what constitutes all that we see around us, the physical world that we inhabit. All substances in that physical world are made of molecules, and molecules are made of atoms. The atom has three constituent parts: electrons (with negative electrical charges), protons (with positive electrical charges) and neutrons (with no charge). Protons and neutrons make up the nucleus, the dense core of the atom. In the atom the electrons fly around the nucleus.

The structure of the atom

The most basic units of matter that we know of are called fundamental particles. Physicists have identified 12 of them that constitute the basis of our whole universe. They can be classified into two big families: quarks and leptons. The protons and neutrons in the nucleus of the atom are made of quarks. By contrast, electrons are fundamental particles themselves. They belong to the family of the leptons.

Antimatter is, to all effects and purposes, the same as matter, except for one feature: its electrical charge. Antimatter is made of particles that have an opposite sign to those of matter. They are called antiparticles. For example, instead of protons, which have positive charge, there are antimatter equivalents called antiprotons, with negative charge. Similarly, the antimatter counterparticle of the electron, is the antielectron, which has a positive charge. Antielectrons are also called positrons, a combination of the terms positive and electron. In sum, antimatter particles are identical to matter particles, except for their opposite electrical charge (and, consequently, opposite magnetic properties).  

Matter and antimatter, a pair of opposites

When a particle of matter meets its corresponding antimatter particle, they both disappear. The process is called annihilation and entails a transformation of all the mass into energy (in the form of photons, in particular gamma rays). 

The opposite process is also possible. Pairs of matter and antimatter particles can be generated from big concentrations of energy. This is what happened during the Big Bang, an explosion so intense that generated matter and antimatter where there only were immense quantities of energy. But, in that case, why is our universe made of matter and not of antimatter? And where is all the antimatter that must have also formed after the Big Bang? Physics can only provide an answer for the second question so far: the original antimatter must have annihilated in contact with the matter that had also been created. Nowadays we can only find minuscule quantities of antimatter as byproducts of high energy physical events, and they are almost immediately annihilate.  The reason why there is a surplus of matter that has given rise to the universe we now inhabit is still unexplained. 

One may wonder whether it not possible that, far from us, there are galaxies or planets made of antimatter that stay around because they are far removed from matter ones. Astronomy answers this question: if there were large amounts of antimatter around us, we should see continuous explosions of gamma rays (high energy photons) resulting from the contact of matter and antimatter at the border between the two regions. There appears to be no evidence that this is the case. Nevertheless, efforts are being made (e.g., the AMS Project) to look for this and other phenomena in areas further and further away in the universe.

Are there "anti-galaxies"?

The discovery of antimatter is one of the most outstanding scientific events of the twentieth century. On many occasions, big discoveries in science are a result of chance. However, the existence of antimatter was predicted before anybody had actually been able to observe it in the real world.

Click here to learn more about the discovery of antimatter