# Length – From Electrons to Galaxies

Even compared to a hydrogen atom (with its diameter of 1 A = 1 angstrom), the electron is microscopic. It is about 0.00006 A in diameter, or in other words: you would need 16,700 electrons to fill the length of a hydrogen atom. A water molecule, which consists of two hydrogen atoms and one oxygen atom, measures roughly 3 A. The largest naturally occurring atom, uranium, goes up to 4 A and a common glucose molecule is around 9 A.

This is where we leave the realm of atoms and molecules, that can only be penetrated by hi-tech electron microscopes. With these, resolutions below 1 A can be achieved, making images of individual atoms possible. The good old light microscope can go as low as 2000 A and no further. But this is enough to observe individual bacteria (10,000 A) and human cells (100,000 A). The latter is already about one-tenth the width of a human hair (1,000,000 A = 0.0001 m). This means that now we are nearing the length scales we are familiar with.

The thickness of a credit card is around 0.0008 m, the average red ant is about 0.005 m long and one inch measures 0.025 m. From the length of a cigarette (0.1 m) over the height of a person (1.7 m) and the wingspan of the Boeing 747 (64 m), we quickly approach the high end of the length scale.

The tallest man-made structure is Burj Khalifa, a 830 m tall skyscraper in Dubai. While already mind-boggling, it dwarfs in comparison to the highest mountain on Earth, the mighty Mount Everest, with its height of 8848 m. From this height, it would take about two minutes to reach sea level in free fall. From the International Space Station (400,000 m) however, the Mount Everest is just a small bump in an enormous sphere of diameter 12,700,000 m.

Going into space, the distances quickly grow beyond our comprehension. The Apollo astronauts had to travel 380,000,000 m = 1.3 light-seconds to get to the Moon. Any mission to Mars has to travel one-hundred eighty times that (4 light-minutes). Multiply that by another factor of ninety, and you get to the former planet Pluto (350 light-minutes).

This is where things get crazy. To reach the next star, Alpha Centauri, you’d have to travel 4.2 light-years or about 550,000 times the distance Earth-Mars. The center of our home galaxy is roughly 10,000 light-years away, the nearest galaxy Canis Major Dwarf adds another factor four to that (42,000 ly). In the grand scheme of things though, even this is not that much. The light we we observe coming from the nearest spiral galaxy, Andromeda, has been traveling for a mind-blowing 2.5 million years.

Where does it all end? Nobody knows for sure. The farthest galaxy is z8_GND_5296, discovered 2013 by the Hubble telescope and Keck Observatory in Hawaii. It is 13.1 billion light-years away. This means the light we see has been sent into space long before Earth came to be. Maybe the galaxy does not even exist anymore, maybe all the stars within it are dead by now. We’ll have to wait another 13.1 billion years to see if that’s the case. I’ll update the post then.

# The Fourth State of Matter – Plasmas

From our everyday lifes we are used to three states of matter: solid, liquid and gas. When we heat a solid it melts and becomes liquid. Heating this liquid further will cause it to evaporate to a gas. Usually this is what we consider to be the end of the line. But heating a gas leads to many surprises, it eventually turns into a state, which behaves completely different than ordinary gases. We call matter in that state a plasma.

To understand why at some point a gas will exhibit an unusual behaviour, we need to look at the basic structure of matter. All matter consists of atoms. The Greeks believed this to be the undivisible building blocks of all objects. Scientists however have discovered, that atoms do indeed have an inner structure and are divisible. It takes an enormous amount to split atoms, but it can be done.

Further research showed that atoms consist of three particles: neutrons, protons and electrons. The neutrons and protons are crammed into the atomic core, while the electrons surround this core. Usually atoms are not charged, because they contain as much protons (positively charged) as electrons (negatively charged). The charges balance each other. Only when electrons are missing does the atom become electric. Such charged atoms are called ions.

In a gas the atoms are neutral. Each atom has as many protons as electrons, they are electrically balanced. When you apply a magnetic field to a gas, it does not respond. If you try to use the gas to conduct electricity, it does not work.

Remember that gas molecules move at high speeds and collide frequently with each other. As you increase the temperature, the collisions become more violent. At very high temperatures the collisions become so violent, that the impact can knock some electrons off an atom (ionization). This is where the plasma begins and the gas ends.

In a plasma the collisions are so intense that the atoms are not able to hold onto their outer electrons. Instead of a large amount of neutral atoms like in the gas, we are left with a mixture of free electrons and ions. This electric soup behaves very differently: it responds to magnetic fields and can conduct electricity very efficiently.

(The phases of matter. Source: NASA)

Most matter in the universe is in plasma form. Scientist believe that only 1 % of all visible matter is either solid, liquid or gaseous. On earth it is different, we rarely see plasmas because the temperatures are too small. But there are some exceptions.

High-temperature flames can cause a small volume of air to turn into a plasma. This can be seen for example in the so called ionic wind experiment, which shows that a flame is able to transmit electric currents. Gases can’t do that. DARPA, the Pentagon’s research arm, is currently using this phenomenon to develop new methods of fire suppression. Other examples for plasmas on earth are lightnings and the Aurora Borealis.

(Examples of plasmas. Source: Contemporary Physics Education Project)

The barrier between gases and plasmas is somewhat foggy. An important quantity to characterize the transition from gas to plasma is the ionization degree. It tells us how many percent of the atoms have lost one or more electrons. So an ionization degree of 10 % means that only one out of ten atoms is ionized. In this case the gas properties are still dominant.

(Ionization degree of Helium over Temperature. Source: SciVerse)