21 March 2015

The Vernal Equinox

On March 20th, 2015, the northern hemisphere's spring season officially began. This time of year is called the vernal equinox, or spring equinox. We've discussed equinox before and we learned that equinox comes from the Latin for "equal night". It just means that there are the same number of hours when the Sun is above the horizon (day) as there are when the Sun is below (night).

We have discussed the autumnal equinox before, and we learned that this is when the Sun appears to travel from north to south along the ecliptic across the celestial equator. In this case, the vernal equinox occurs when the Sun travels across the celestial equator from south to north along the ecliptic. You should note that when the spring starts in the northern hemisphere, fall or autumn begins in the southern hemisphere.

Another fun thing to note is that on either equinox, the Sun rises due east and sets due west. At the equator, the Sun is directly overhead at noon. As the Sun travels farther north on the ecliptic and we get deeper into the northern spring, the Sun will be farther north in the sky at noon until it reaches its farthest point north at the summer solstice (which will be discussed as we approach that time of year).

18 March 2015

The Fine Structure Constant

The fine structure constant, also known as the coupling constant for electromagnetism, is used to define the strength of the electromagnetic force between two charged particles. It is symbolized by α and is approximately 7.2974e-3 (or approximated as 1/137) and is unitless.


In terms of other physical constants, in this case the elementary charge e, the Planck constant h, the speed of light c, the permittivity of free space (electric constant) εo, and the permeability of free space (magnetic constant) μo, we can relate α to these quantities.





The amazing thing about the fine structure constant is that any change in one of the fundamental constants will change α. A change of just 4% of the constant will not allow carbon to form, and hence no carbon-based life. An even larger change to α > 0.1 would mean that stellar fusion can't take place and therefore no warm planets, no liquid water, and again no life.


Fun note: the product of the permittivity of free space εo and the permeability of free space μo is the reciprocal of the speed of light squared.








17 March 2015

The Anthropic Principle

There has always been a question about why our universe is the way it is. Physics tells us that the laws of physics and the constants we measure are perfect for life to exist. If any laws were slightly different or any constants were a little smaller or larger than what they are, we would not be here to question any of this. This idea is called the Anthropic Principle and in a way, it's a circular argument. The universe is the way it is because we are here and we are here because the universe is the way it is.


Because we are here, we are able to question what makes the universe unique to allow stars, galaxies, planets, and life itself to form. Martin Rees surmised that there are six fundamental numbers that dominate the cosmos to allow the universe we see to exist. We will go over these six numbers in the following posts, but the numbers can be broken down into a variety of the four fundamental forces: the strong nuclear force, the weak nuclear force, the electromagnetic force, and the gravitational force.
  1. The strong nuclear force is the force that hold the nucleus of an atom together. Without it, nuclei would not be stable because nuclei are made up of protons, which are all positively charged. This is the strongest of the four forces, but acts on the smallest scale.
  2. The weak nuclear force is the force responsible for nuclear decay, the decay of subatomic particles (like the proton and neutron) and fission of atoms.
  3. The electromagnetic force is the force responsible for the electron orbiting the nucleus of an atom, the existence of electromagnetic radiation, even the force between magnets.
  4. The gravitational force is the force that interacts between masses. It is what keeps our feet on the surface of the Earth and what allows bodies to orbit one another. In terms of size, it is actually the weakest of the four forces, but acts on the highest scales. Because it is weaker than the electromagnetic force, it is the reason why electrons orbit the nucleus, rather than falling into the nucleus.
The six numbers that Martin Rees surmised control the cosmos are:
  1. The ratio between the fine structure constant (electromagnetism) and gravitational coupling constant
  2. The fraction of the mass of four protons to one helium nucleus
  3. The mass density ratio
  4. The energy density ratio
  5. The energy required to break up the largest structures in the universe
  6. The number of spatial dimensions
We will talk more about these as we continue.

16 March 2015

Solar Eclipse of 20 March 2015

There is a solar eclipse this week that will be visible in a small portion of the world. So for my readers in Europe and northwest Asia, you will get to see a partial eclipse. If you live in the archipelago of Svalbard, you will be able to see the total eclipse.

All information below is from Time and Date.com
Area seeing the total solar eclipse.
More than 90% of the sun is covered.
Up to 90% of the sun is covered.
Up to 40% of the sun is covered.
Eclipse is not visible at all.
Note: Percentage values (%) relate to moon coverage of the sun and depends on location. Visibility is weather permitting.

Here are the times the eclipse will be visible.
EventUTC Time
First location to see partial eclipse beginMar 20 at 7:41 AM
First location to see full Eclipse beginMar 20 at 9:09 AM
Maximum EclipseMar 20 at 9:45 AM
Last location to see full Eclipse endMar 20 at 10:22 AM
Last location to see partial Eclipse endMar 20 at 11:50 AM

11 March 2015

Olbers' Paradox

In the early 1800's, there was a quandary. Scientists did not know about the size of the universe or the speed of light. There was an assumption that the universe was infinite in size, age, and mass. Not only that, they assumed that light itself travelled instantaneously from place to place. They did not know about the Big Bang, or Hubble's Law, or even Relativity. So from these assumptions, there arose a question: why is the night sky dark?


They believed the universe was infinite and static. Because of this, they wondered where all the stars were. Imagine looking in any direction in the sky. If the universe is infinitely populated with stars, no matter where you look, you will see a star, despite the great distances involved (remember, light is instantaneous). Therefore, you should see light from that star. Because of this, at night, when the Sun is not dominating the sky, the sky itself should be lit up like daytime. Though this question has been posed before, Heinrich Olbers was the first to formulate the question and try to answer it, and we call this question, Olbers' Paradox.






He came up with a few reasons why the night sky is not dark. I'm only going to mention two for simplicity.
  1. The universe is not infinite. There is a limit to the size as well as the age of the universe, though the size may be larger than the age of the universe. We still don't know exactly how big the universe really is since we can only see about 14 billion light-years in any direction.
  2. The speed of light is not infinite. He didn't know how fast it was, but he could conclude that light travelled at a finite speed. Therefore, light from stars that are farther away from us than the age of the universe have not had enough time to reach us since the star/galaxy/whatever was formed.
From these two reasons, we can see the night sky is dark because the light from most of the stars in the universe has not had enough time since the light left the star to reach us.

10 March 2015

Cosmological Constant

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.

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.