Matter and Antimatter Help (page 3)
Matter And Antimatter
The Cosmos can do strange things to matter. Strange, that is, according to what is considered “normal” here on Earth. Some scientists think that these extreme manifestations—antiparticles, neutron stars, geometric points with infinite density, and such—are as common in the Universe as any other form of matter.
The notion of antimatter is as old as science fiction. You know how this kind of story can go. A space ship lands on a planet that turns out to be made of antimatter, and the journey comes to an unexpected, abrupt, and catastrophic end. A rogue band of space aliens comes to Earth with an antimatter bomb and sells it to the highest bidder. What is the reality of antimatter? Let’s look at the nature of matter first. In simplistic terms, matter consists of three types of particles: the proton , the neutron , and the electron .
Protons are too small to be observed directly, even with the most powerful microscopes. All protons carry a positive electrical charge. The charge on every proton is the same as the charge on every other. Every proton at rest has the same mass as every other proton at rest. Most scientists accept the proposition that all protons are identical, at least in our part of the Universe, although they, like all other particles, gain mass if accelerated to extreme speeds. This increase in mass takes place because of relativistic effects; you’ll learn about this later.
While an individual proton is invisible and not massive enough to make much of an impact all by itself, a high-speed barrage of them can have considerable effects on matter. Protons are incredibly dense. If you could scoop up a level teaspoonful of protons the way you scoop up a teaspoonful of sugar—with the protons packed tightly together like the sugar crystals—the resulting sample would weigh tons in Earth’s gravitational field. A little ball made of solid protons would fall into the Earth and cut through the crustal rocks like a lead shot falls through the air.
A neutron has a mass slightly greater than that of a proton. Neutrons have no electrical charge, and they are roughly as dense as protons. However, while protons last for a long time all by themselves in free space, neutrons do not. The mean life of a neutron is only about 15 minutes. This means that if you gathered up a batch of, say, 1 million neutrons and let them float around in space, you would have about 500,000 neutrons left after 15 minutes. After 30 minutes, you would have approximately 250,000 neutrons remaining; after 45 minutes, there would be about 125,000 neutrons left.
Neutrons can last a long time when they are in the nuclei of atoms. This is a fortunate thing because if it weren’t true, matter as we know it could not exist. Neutrons also can survive for a long time when a huge number of them are tightly squeezed together. This happens when large stars explode and then the remaining matter collapses under its own gravitation.
An electron has exactly the same electrical charge quantity as a proton but with opposite polarity. Electrons are far less massive than protons, however. It would take about 2,000 electrons to have the same mass as a single proton.
One of the earliest theories concerning the structure of the atom pictured the electrons embedded in the nucleus, like raisins in a cake. Later, the electrons were imagined as orbiting the nucleus, making every atom like a miniature star system with the electrons as the planets (Fig. 14-1).
Still later, this view was modified further. In today’s model of the atom, the electrons are fast-moving, and they describe patterns so complex that it is impossible to pinpoint any individual particle at any given instant of time. All that can be done is to say that an electron will just as likely be inside a certain sphere as outside. These spheres are known as electron shells . Unless there is an external force or force field acting on the atom, all the electron shells are concentric, and the nucleus is at the center of the whole bunch. The greater a shell’s radius, the more energy the electron has. Figure 14-2 is a simplistic drawing of what happens when an electron gains enough energy to “jump” from one shell to another shell representing more energy.
Generally, the number of electrons in an atom is the same as the number of protons. The negative charges therefore exactly cancel out the positive ones, and the atom is electrically neutral. Under some conditions, however, there can be an excess or shortage of electrons. High levels of radiant energy, extreme heat, or the presence of an electrical field can “knock” electrons loose from atoms, upsetting the balance.
Equal And Opposite
The proton, the neutron, and the electron each have their own “nemesis” particle that occurs in the form of antimatter . These particles are called antiparticles . The antiparticle for the proton is the antiproton , for the neutron it is the antineutron , and for the electron it is the positron . The antiproton has the same mass as the proton, but it has a negative electrical charge equal and opposite to the positive electrical charge of the proton. The antineutron has the same mass as the neutron. Neither the neutron nor the antineutron have any electrical charge. The positron has same mass as the electron, and it is positively charged to an extent equal to the negative charge on an electron.
You might have read or seen in science fiction novels and movies that when a particle of matter collides with its nemesis, they annihilate each other. This is indeed true. The combined mass of the particle and the antiparticle is completely converted into energy, according to the same Einstein formula that applies in nuclear reactions:
E = ( m + + m – ) c 2
where E is the energy in joules, m + is the mass of the particle in kilograms, m – is the mass of the antiparticle in kilograms, and c is the speed of light squared, which is approximately equal to 8.9875 × 10 16 m 2 /s 2 .
If equal masses of matter and antimatter are brought together, in theory, all the mass will be converted to energy. If there happens to be more matter than antimatter, there will be some matter left over after the encounter. Conversely, if there is more antimatter than matter, there will be some antimatter remaining.
In a nuclear reaction, only a tiny fraction of the mass of the constituents is liberated as energy; plenty of matter is left over, although its form has changed. You might push together two chunks of uranium-235, the isotope of uranium whose atomic mass is 235 atomic mass units (amu), and if their combined mass is great enough, an atomic explosion will take place. However, there will still be a considerable amount of matter remaining. We might say that the matter-to-energy conversion efficiency of an atomic explosion is low. Of course, this is a relative thing. Compared with dynamite, an atomic bomb is extremely efficient at converting matter to energy. Compared to an antimatter bomb, however, should one ever be devised, an atomic bomb is inefficient.
In a matter-antimatter reaction, if the masses of the samples are equal, the conversion efficiency is 100 percent. As you can imagine, a matter-antimatter bomb would make a conventional nuclear weapon of the same total mass look like a firecracker by comparison. A single matter-antimatter weapon of modest size easily could wipe out all life on Earth. A big one could shatter or even vaporize the whole planet.
Where Is All The Antimatter?
Why don’t we see antimatter floating around in the Universe? Why, for example, are Earth, Moon, Venus, and Mars all made of matter, not antimatter? (If any celestial object were made of antimatter, then as soon as a spacecraft landed on it, the ship would vanish in a fantastic burst of energy.) This is an interesting question. We are not absolutely certain that all the distant stars and galaxies we see out there consist of matter. We do know, however, that if there were any antimatter in our immediate vicinity, it would have long ago combined with matter and been annihilated. If there were both matter and antimatter in the primordial Solar System, the mass of the matter was greater, for it prevailed after the contest.
Most astronomers are skeptical of the idea that our galaxy contains roughly equal amounts of matter and antimatter. If this were the case, we should expect to see periodic explosions of unimaginable brilliance or else a continuous flow of energy that could not be explained in any way other than matter-antimatter encounters. No one really knows the answers to questions about what comprises the distant galaxies and, in particular, the processes that drive some of the more esoteric objects such as quasars.
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