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Antimatter is a term in particle physics. Antimatter is a material composed of antiparticles. These have the same mass as particles of ordinary matter but have opposite charge and properties, such as lepton and baryon number.
What is it made of?
In physics, all the elementary particles, or the basic building blocks of the things we can touch, come in pairs. Each particle has what is called an antiparticle. This may look and act just like the regular particle, except for one major difference. An example is the electron and the positron. They both weigh the same, and act the same, but the electron has a negative electrical charge, while the positron has a positive electrical charge, which is where the positron (positive + electron) gets its name.
Other antimatter particles are the same way, where they have the same weight, and look and act the same as regular particles, but their electrical charge is the opposite of regular particles. Antihydrogen, for example, has the positron, which is positively charged, orbiting around an antiproton, which is negatively charged, which is the opposite way regular hydrogen looks, which has the electron (negative charge), orbiting around a proton (positive charge).
Albert Einstein found a formula that can show how much energy a certain amount of something has, whether it is matter or antimatter. This formula is [math]E=mc^2[/math], and is one of the most well known equations. In simple terms, if you take the mass of something and then multiply it by the speed of light, and then multiply it by the speed of light again, you will get how much pure energy a given piece of something has. Since the speed of light is such a big number, this means that even a small amount of matter can have a lot of energy (it has been projected to be 4 times more effective per mass than nuclear fission).
Antimatter was predicted from the expanded form of [math]E=mc^2[/math], which is [math]E^2=m^2c^4[/math]. You can take the square root of each side of the equation, since it is equal. However, any real square root has two answers. You can think of the negative root as being antimatter.
The reason this is important to understand antimatter is because scientists found that when matter and antimatter touch each other, the amount of energy that is released comes very close to the amount of energy [math]E=mc^2[/math] says should be all together in those two pieces. The reason is that each particle of matter, when it touches its antiparticle in the antimatter world, both change over into pure energy, or annihilate each other. This release of such a high amount of energy is why a lot of science fiction writers use antimatter for fuel in their stories. For example, author Dan Brown uses antimatter in "Angels and Demons" as a very powerful weapon. It is also being looked at as a fuel source for real-life missions to outer space in the future.
Theory of the Big Bang
Many scientists think that in the first few moments after the Big Bang, which created the universe a very long time ago, both matter and antimatter were mixing together. Since there was just a little bit more matter than antimatter (scientists still have not figured out why this happened yet), whatever was left over after most of the matter and antimatter annihilated into energy became the Universe we see today. Physicists do not yet know for sure that equal amounts of matter and antimatter were created, and because of this, they are also wondering where the antimatter went, and if any was left over from the beginning of the universe. Another theory is that there is lots of antimatter on the other side of the universe, hidden far beyond our vision. They could have formed their own galaxies and solar systems too.
Because antimatter can make so much energy, it can be used for a lot of things, such as fuel for going into outer space, or in our cars. The problem is that antimatter is very very expensive to make, and is almost as expensive to store, since it cannot touch regular matter. It takes several hundred million dollars to make less than one-millionth of a gram of antimatter. In fact, it is the most expensive and rare substance on Earth. Since it is so expensive, this means that antimatter is not practical to use as a weapon or as an energy source, because so little of it is obtainable.
Recently, however, scientists have trapped antimatter for over 16 minutes (1000 sec. total).
It does have a use in medicine, because a special kind of scanner called the PET, which stands for positron emission tomography, uses positrons to go into the human body. Doctors can look at the way the positron changes into energy inside a person's body, and be able to tell if something is wrong inside a person. This kind of machine works differently than an X-ray machine or a magnetic resonance imaging (MRI) machine, and can help doctors see things that those other machines cannot see.