Is there any difference between antimatter, dark matter, dark energy, and degenerate matter?
Category: Physics Published: July 25, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Yes. Although the names sound vague and almost fictional, the types of matter called antimatter, dark matter, dark energy, and degenerate matter are all different, specific entities that really exist in our universe.
Antimatter is just regular matter with a few properties flipped, such as the electric charge. For example, the antimatter version of an electron is a positron. They both have the same mass, but have opposite electric charge. Antimatter is not as exotic as science fiction makes it out to be. For starters, antimatter has regular mass and accelerates in response to forces just like regular matter. Also, antimatter is gravitationally attracted to other forms of matter just like regular matter. For every particle that exists, there is an antimatter counterpart (some particles such as photons are their own anti-particles). What makes antimatter unique is that when antimatter comes in contact with its regular matter counterpart, they mutually destroy each other and all of their mass is converted to energy. This matter-antimatter mutual annihilation has been observed many times and is a well-established principle. In fact, medical PET scans routinely use annihilation events in order to form images of patients. Antimatter is therefore only distinct from regular matter in that it annihilates when meeting regular matter. For instance, a proton and a positron are somewhat similar. They both have regular mass. They both have a positive electric charge of the same strength. They both have a quantum spin of one half. But when a proton meets an electron, it forms a stable hydrogen atom. When a positron meets an electron, they destroy each other. The key difference is that a positron is antimatter and a proton is not.
Antimatter is very rare in our universe compared to regular matter, but there are small amounts of antimatter all over the place in the natural world, including inside your body. Antimatter is created by many types of radioactive decay, such as by the decay of potassium-40. When you eat a banana, you are eating trace amounts of antimatter-producing atoms. The amount is so small, that it does not really affect your health. But it is still there. Why doesn't antimatter build up in your body? The key is that our universe is mostly made of regular matter, so antimatter cannot stick around for very long. Very soon after antimatter is created, it bumps into regular matter and gets destroyed again. Antimatter is also produced by lightning and cosmic rays. It is well understood by physicists, and is predicted by standard particle physics theories.
Dark matter is matter that does not interact electromagnetically, and therefore cannot be seen using light. At the same time, dark matter does interact gravitationally and can therefore be "seen" through its gravitational effect on other matter. It is common throughout the universe and helps shape galaxies. In fact, recent estimates put dark matter as five times more common than regular matter in our universe. But because dark matter does not interact electromagnetically, we can't touch it, see it, or manipulate it using conventional means. You could, in principle, manipulate dark matter using gravitational forces. The problem is that the gravitational force is so weak that you need planet-sized masses in order to gravitationally manipulate human-sized objects. There remains much unknown about dark matter since it is so hard to detect and manipulate. Dark matter is not predicted or explained by standard particle physics theories but is a crucial part of the Big Bang model.
Dark energy is an energy on the universal scale that is pushing apart galaxies and causing the universe to expand at an increasing rate. Like dark matter, dark energy is poorly understood and is not directly detectable using conventional means. Several lines of evidence make it clear that our universe is expanding. Not only that, our universe is expanding at an increasing rate. Dark energy is the name of the poorly understood mechanism that drives this accelerating expansion. While dark matter tends to bring matter together, dark energy tends to push matter apart. Dark energy is weak and mostly operates only on the intergalactic scale where gravitational attraction of dark matter and regular matter is negligible. Dark energy is thought to be spread thinly but evenly throughout the entire universe. Dark energy is also not predicted or explained by standard particle physics theories but is included in modern versions of the Big Bang model. Dark energy may have a connection with the vacuum energy predicted by particle physics, but the connection is currently unclear.
Degenerate matter is regular matter that has been compressed until the atoms break down and the particles lock into a giant mass. Degenerate matter acts somewhat like a gas in that the particles are not bound to each other, and somewhat like a solid in that the particles are packed so closely that they cannot move much. A white dwarf star is mostly composed of electrons compressed into a state of degenerate matter. A neutron star is mostly composed of degenerate neutrons. Further compression of a neutron star may transform it to a quark star, which is a star composed of quarks in a degenerate state. But not enough is known about quarks to determine at present whether quark stars really exist or are even possible. These concepts are summarized in the list below.
Regular Matter
- Examples: electron, proton, neutron
- Main Role: forms atoms, molecules, objects, planets, etc.
- Reflects Light?: yes
Antimatter
- Examples: positron, antiproton
- Main Role: annihilates regular matter
- Reflects Light?: yes
Dark Matter
- Examples: unknown
- Main Role: adds mass to galaxies
- Reflects Light?: no
Dark Energy
- Examples: unknown
- Main Role: drives cosmic expansion
- Reflects Light?: no
Degenerate Matter
- Examples: neutron star
- Main Role: forms dense stars
- Reflects Light?: yes