When Albert Einstein applied his equations of General Relativity to the observable universe, he found something that he didn't like. His equations were correct, but for some reason, his equations predicted that the universe was dynamic when he and everyone else though the universe was static. This was in 1917, before the Big Bang theory and before Edwin Hubble found that the universe was expanding. To account for what he felt was incorrect, he introduced a fudge factor to take away the dynamic universe solution. He called his fudge factor, the cosmological constant. He hoped and felt that in time, physics and astronomy would be able to allow the cosmological constant to go away. When Hubble found the expansion of the universe and the Big Bang theory were proposed, Einstein thought that his cosmological constant was his biggest blunder. But was it?
Now, with the introduction of dark energy to help explain the expansion of the universe, the cosmological constant was reintroduced. As explained last time, if the dark energy density is constant, the universe will be open and expand forever. With a constant dark energy density, this implies that the universe is homogeneous in both space and time. Remember that this is referred to the Perfect Cosmological Principle which was briefly mentioned here. In other words, the universe appears static and therefore, the cosmological constant may be a physical quantity describing the dark energy density of the universe. Unfortunately, we still don't know what the dark energy density is doing and it may be centuries or millennia before we know.
Showing posts with label Einstein. Show all posts
Showing posts with label Einstein. Show all posts
10 March 2015
09 March 2015
Dark Energy
What is dark energy?
First, what isn't dark energy? It isn't energy used by either Sauron or Lord Voldemort. They use dark magic, which isn't the same. You also shouldn't confuse it with negative energy, which is strange phenomena in itself. We will look at negative energy later.
Dark energy is defined as the energy that permeates space and drives the expansion of the universe. At the present time, the amount of dark energy in the universe is seeming to accelerate the expansion rate of the universe. It should be noted, however, that this does not mean that the universe is open. Just because the universe's rate of expansion is accelerating now, it does not mean that sometime in the future, the rate of expansion can slow, stop, or reverse.
Dark energy is thought to be one of two things: a constant energy density over time and space (static) or a scalar field density that has a value that can change with time or space (dynamic).
If the energy density is constant over time and space, that means as the universe expands, the amount of energy (not including mass) must increase. The only way this happens without violating the conservation of energy is that mass must be converted to energy in some way. This could be done in the normal way (matter-antimatter collisions) or in some way that we don't know. This constant energy density is referred to as the Cosmological Constant, and was first introduced by Albert Einstein. We will discuss this more later. If this is what dark energy is, then we live in an open universe.
The other is that dark energy density is a changing quantity which in the future could either slow down (but not stop - open universe), stop (flat universe), or reverse (closed universe) the expansion, depending on how the density changes over time.
At our current knowledge, dark energy is 68.3% of the total mass-energy density, dark matter is 26.8%, and ordinary matter is only 4.5%. So you can see, what we don't know about the universe is a heck of a lot more than what we do know.
First, what isn't dark energy? It isn't energy used by either Sauron or Lord Voldemort. They use dark magic, which isn't the same. You also shouldn't confuse it with negative energy, which is strange phenomena in itself. We will look at negative energy later.
Dark energy is defined as the energy that permeates space and drives the expansion of the universe. At the present time, the amount of dark energy in the universe is seeming to accelerate the expansion rate of the universe. It should be noted, however, that this does not mean that the universe is open. Just because the universe's rate of expansion is accelerating now, it does not mean that sometime in the future, the rate of expansion can slow, stop, or reverse.
Dark energy is thought to be one of two things: a constant energy density over time and space (static) or a scalar field density that has a value that can change with time or space (dynamic).
If the energy density is constant over time and space, that means as the universe expands, the amount of energy (not including mass) must increase. The only way this happens without violating the conservation of energy is that mass must be converted to energy in some way. This could be done in the normal way (matter-antimatter collisions) or in some way that we don't know. This constant energy density is referred to as the Cosmological Constant, and was first introduced by Albert Einstein. We will discuss this more later. If this is what dark energy is, then we live in an open universe.
The other is that dark energy density is a changing quantity which in the future could either slow down (but not stop - open universe), stop (flat universe), or reverse (closed universe) the expansion, depending on how the density changes over time.
At our current knowledge, dark energy is 68.3% of the total mass-energy density, dark matter is 26.8%, and ordinary matter is only 4.5%. So you can see, what we don't know about the universe is a heck of a lot more than what we do know.
26 February 2015
Antimatter
Many of you have probably heard of antimatter, a strange form of matter that seems to be opposite of what we call "normal" matter. If you've seen Star Trek, the Enterprise and all the rest of the Starfleet fleet are run by matter-antimatter collisions. We will see why this is a very efficient way to power starships, cities, almost anything.
First, we need to know what antimatter really is. Antimatter is a form of matter that has the same physical properties as normal matter, but with one huge difference. For a charged particle, the corresponding antimatter particle has an opposite charge. So for an electron, its corresponding antimatter particle is an antielectron, or more commonly, a positron. A positron has the same mass and spin, but is positively charged. In the same way, the antimatter equivalent of a proton is the antiproton with a negative charge. A neutron has the corresponding antineutron. Wait a minute, neutrons have no charge, so how does an antineutron differ from a neutron?
To look at an antineutron, we should know a couple of things about protons and neutrons. They are a part of set of particles known as baryons, which are particles that can be subdivided into smaller particles called quarks. A proton is made up of three quarks: two up quarks and one down quark. A neutron is composed of two down quarks and one up quark. Quarks come in six flavors, have a electric spin of +1/2 and they have unique charges.
Quarks are particles known as fermions, while electrons belong to a family of particles called leptons (which also include neutrinos, muons, and tau particles as well as all their corresponding antiparticles). But I digress.
The antiquarks have opposite charges: anti-up quark (u) has a -2/3e charge and an anti-down quark (d) has a +1/3e charge. (The line above the letter distinguishing the particle as an antiparticle). This means that an antiproton is made up of two antiups and one antidown while a neutron is made of one antiup and two antidowns.
A cool thing about matter and antimatter (or weird, depending on how you look at it) is that when matter and antimatter interact, they completely annihilate each other. In other words, 100% of the mass is converted to energy via Einstein's energy-mass relation, E=mc². If an electron and a positron collided, the energy produced would be 1.022 MeV (1.67e-13 Joules) which is really not a lot. But when there are trillions upon trillions of interactions like this, the energy can add up. Compare this to the proton-proton chain, which is the process in the core of the Sun where four hydrogen nuclei fuse into one helium nuclei where only 0.07% of the total mass of the four protons is converted to energy (one proton has a mass of 938.3 MeV/c² or 1.673e-27 kg). Four protons have a mass of 3753.2 MeV/c² which only 2.63 MeV/c² is converted to energy, or 2.63 MeV (4.21e-13 Joules). One proton and one antiproton would create 1.877 GeV of energy (3.01e-10 Joules) - almost 1000 times as much energy. By the way, one kg of matter reacting with one kg of antimatter would equal 1.8e17 Joules (43.02 millions of tons of TNT - 4 times the energy released by one hydrogen bomb).
So why don't we use matter-antimatter reactors to power the world? For one thing, antimatter is rare. There is not a repository of antimatter near to us in the universe. However, antimatter can be created in the lab, but only in small quantities. But once it is created, you have to find a way to store it without allowing it to interact with normal matter. That means placing it in a vacuum in a magnetic field so it does not touch the container it is in. Air itself would interact with antimatter.
Anything we need to know, is why do we have more matter than antimatter? Nothing in the laws of physics says that their should be an imbalance between the two, but we know that there is. Early in the universe, just a difference of one quark or one electron in a billion quark-antiquark or electron-positron pairs would lead to the imbalance we see. But we don't know why there was an imbalance in the first place.
One theory is the idea of virtual particles. Virtual particles are particle/antiparticle pairs that are created when a photon with the correct energy spontaneously is turned into a particle and an antiparticle (as long as their masses equal the energy of the photon). In most cases, virtual particles don't last very long because they immediately react with each other and annihilate. However, if a photon spontaneously decayed into a particle/antiparticle pair near a black hole, it is possible that the antiparticle can fall into the black hole. However, this still can't explain why there are more particles than antiparticles as random matter/antimatter creation should not choose one over the other.
Another theory is that matter and antimatter are subject to anti-isotropy, meaning that our region of the universe is dominated by matter while there could be regions that are dominated by antimatter. The only way we could be able to confirm this is to travel to that region of the universe since observation alone won't tell us that antimatter is the dominate matter. However, since we are made of "normal" matter, travelling to a region of space dominated by antimatter would not be healthy.
First, we need to know what antimatter really is. Antimatter is a form of matter that has the same physical properties as normal matter, but with one huge difference. For a charged particle, the corresponding antimatter particle has an opposite charge. So for an electron, its corresponding antimatter particle is an antielectron, or more commonly, a positron. A positron has the same mass and spin, but is positively charged. In the same way, the antimatter equivalent of a proton is the antiproton with a negative charge. A neutron has the corresponding antineutron. Wait a minute, neutrons have no charge, so how does an antineutron differ from a neutron?
To look at an antineutron, we should know a couple of things about protons and neutrons. They are a part of set of particles known as baryons, which are particles that can be subdivided into smaller particles called quarks. A proton is made up of three quarks: two up quarks and one down quark. A neutron is composed of two down quarks and one up quark. Quarks come in six flavors, have a electric spin of +1/2 and they have unique charges.
- up quark (u) - charge of +2/3e, mass of 2.3 MeV/c²
- down quark (d) - charge of -1/3e, mass of 4.8MeV/c²
- charm quark (c) - charge of +2/3e, mass of 1.275 GeV/c²
- strange quark (s) - charge of -1/3e, mass of 95 MeV/c²
- top (or truth) quark (t) - charge of +2/3e, mass of 173.07 GeV/c²
- bottom (or beauty) quark (b) - charge of -1/3e, mass of 4.18 GeV/c²
Quarks are particles known as fermions, while electrons belong to a family of particles called leptons (which also include neutrinos, muons, and tau particles as well as all their corresponding antiparticles). But I digress.
The antiquarks have opposite charges: anti-up quark (u) has a -2/3e charge and an anti-down quark (d) has a +1/3e charge. (The line above the letter distinguishing the particle as an antiparticle). This means that an antiproton is made up of two antiups and one antidown while a neutron is made of one antiup and two antidowns.
A cool thing about matter and antimatter (or weird, depending on how you look at it) is that when matter and antimatter interact, they completely annihilate each other. In other words, 100% of the mass is converted to energy via Einstein's energy-mass relation, E=mc². If an electron and a positron collided, the energy produced would be 1.022 MeV (1.67e-13 Joules) which is really not a lot. But when there are trillions upon trillions of interactions like this, the energy can add up. Compare this to the proton-proton chain, which is the process in the core of the Sun where four hydrogen nuclei fuse into one helium nuclei where only 0.07% of the total mass of the four protons is converted to energy (one proton has a mass of 938.3 MeV/c² or 1.673e-27 kg). Four protons have a mass of 3753.2 MeV/c² which only 2.63 MeV/c² is converted to energy, or 2.63 MeV (4.21e-13 Joules). One proton and one antiproton would create 1.877 GeV of energy (3.01e-10 Joules) - almost 1000 times as much energy. By the way, one kg of matter reacting with one kg of antimatter would equal 1.8e17 Joules (43.02 millions of tons of TNT - 4 times the energy released by one hydrogen bomb).
So why don't we use matter-antimatter reactors to power the world? For one thing, antimatter is rare. There is not a repository of antimatter near to us in the universe. However, antimatter can be created in the lab, but only in small quantities. But once it is created, you have to find a way to store it without allowing it to interact with normal matter. That means placing it in a vacuum in a magnetic field so it does not touch the container it is in. Air itself would interact with antimatter.
Anything we need to know, is why do we have more matter than antimatter? Nothing in the laws of physics says that their should be an imbalance between the two, but we know that there is. Early in the universe, just a difference of one quark or one electron in a billion quark-antiquark or electron-positron pairs would lead to the imbalance we see. But we don't know why there was an imbalance in the first place.
One theory is the idea of virtual particles. Virtual particles are particle/antiparticle pairs that are created when a photon with the correct energy spontaneously is turned into a particle and an antiparticle (as long as their masses equal the energy of the photon). In most cases, virtual particles don't last very long because they immediately react with each other and annihilate. However, if a photon spontaneously decayed into a particle/antiparticle pair near a black hole, it is possible that the antiparticle can fall into the black hole. However, this still can't explain why there are more particles than antiparticles as random matter/antimatter creation should not choose one over the other.
Another theory is that matter and antimatter are subject to anti-isotropy, meaning that our region of the universe is dominated by matter while there could be regions that are dominated by antimatter. The only way we could be able to confirm this is to travel to that region of the universe since observation alone won't tell us that antimatter is the dominate matter. However, since we are made of "normal" matter, travelling to a region of space dominated by antimatter would not be healthy.
Subscribe to:
Posts (Atom)