String theory – the other way of interpreting our universe

artistic interpretation of element string which may make up our universe, pic from medium

The string theory is a very interesting idea that propose another totally different way of thinking what made up our universe. Although this theory is very arguable because it can never be tested in our labs so that a lot of physicist do not like this theory, but it indeed gives a very unique prospective and interpretation about parallel universe and dimensions.

The theory proposes that everything in the universe is made of the tiny element called string. It is itself one dimension particle. String theory proposes a unique solution trying to figure out the ultimate equation of the universe, that equation that unifies all four major forces in the universe: gravity, strong nuclear force, weak nuclear force and electromagnetic. This is the equation that Einstein seemed for his whole life.

Before string theory, people think this equation does not exist because of the quantum physics. Modern quantum physics thinks that things happen in the quantum field can not be predicted. We can only predict the possibility of something happen in the quantum field. In other words, there are no equations related to quantum physics since everything is about probability.

Einstein believes that there must be an ultimate equation that explain everything from large scale to quantum physics. String theory provides one possible answer for seeking this equation. String theory proposes that every forces are generated because of specific particles. For example, the one that is responsible for gravity, also the special one, is called graviton. This is a theoretic particle that has never been found by human.

Other things that are very interesting about string theory is that it tries to explain the parallel universe by the idea of membrane. Basically our universe is like a membrane where there are a lot of other parallel universe. Only graviton can sometimes travel through membranes.

cross section of a quintic Calabi-Yau manifold, pic from wikipedia

Introduction to various spacecraft propulsion methods

ion-accelerating engine, pic from wikipedia

Space propulsion is different from regular propulsion methods that deal with situations on the ground or in the air. It’s also different from launch propulsion which space propulsion methods exclusively deal with propulsion systems used in vacuum of space.

Before introducing different propulsion methods, there are couple of concepts that are really important related to the effectiveness of a propulsion system. The purpose of propulsion in the space is to change the velocity of a spacecraft. Since obviously that the more massive an object is, the harder to change its velocity, designers of spacecraft propulsion usually use the amount of change in momentum per unit of propellant consumed to compare the effectiveness. This is called specific impulse. It’s a different concept from the thrust which is the force moving a rocket through the air.

SpaceX’s Kestrel engine, pic from wikipedia

Essentially all spacecraft propulsion systems are reaction engines. These engines provides propulsion by generating chemical or physical reaction and expelling reaction mass. The most common engines are internal combustion heat engines. These kind of engines combust either liquid, solid or gaseous fuel with oxidizer within a combustion chamber. These type of engine needs a large mass of fuels. Rockets propelled by this kind of engine, such as monopropellant or bipropellant rocket, have relatively low specific impulse but high thrust.

The other kind of system, which is actually more suitable for traveling in the space, is electromagnetic propulsion engine. They don’t rely on the high temperature and fluid dynamics to accelerate the reaction mass, but rather accelerating the reaction mass directly by electromagnetic forces. The ion accelerating engine is the most common one among this type. These kind of engines are good that they usually carry little mass of fuels and they gain much greater specific impulse in the space.

electric propulsion engine in lab, NASA, pic from wikipedia

Extrasolar planets — Earth-like planets

Extrasolar planets is one of the most interesting astronomical research topics. It can help us answer questions such as whether there are aliens, or is it possible for us to find another “Earth” to live on.

Artistic illustration of enormously amount of extrasolar systems in the universe, pic from wikipedia

An extrasolar planet is defined as a planet that is outside of solar system. The history of extrasolar planet can be traced back to 1917, but the first confirmed detection of extrasolar planet happened in 1988. As of April 1st, 2019, we have found 4023 confirmed planets with in 3005 systems.

Because of the bias in detecting extrasolar system due to the mythology of transit photometry and Doppler spectroscopy, most of the planets we found are huge in size and close to the star in the system. By in theory, there should be about 1 Earth-like size planet in the habitable zone in 5 sun-like star’s system. This number is amazing considered that we have 200 billion stars just in Milky Way — that’s about 11 billion Earth-like planets! Are we the only special one?

Exoplanet population distribution by type, pic from wikipedia

One example of a likely Earth twin is Kepler-452b. It’s one of the most Earth-like exoplanet we have found so far. It lies at 1,400 light year away from us, and it’s about 60% larger than Earth, orbiting a sun-like star that is 10% bigger and 20% brighter. Kepler-452b’s orbital period is about 385 days, making it lying in the habitable zone. Kepler-452b is likely to process thick atmosphere, lots of water and volcanos. The planet and its sum have been around for about 6 billion years, and is there any life on Kepler-452b? We expect future researches can reveal us the answer.

Comparison between Kepler-452b and Earth, pic from space

The Mysterious Black Hole

A simulation of a black hole, picture from popsci

The black hole is one of the most amazing and mysterious object in our universe. In 1916, Karl Schwarzschild first provided the solution to general relativity that characterize a black hole.

Black hole is the remnant of super massive star’s explosion, and it has such a strong gravitational pull that nothing can escape, not even particles and light (electromagnetic radiation). And this is why it gets the name “black hole”.

The model of black hole is supported by the theory of general relativity that a super massive object can deform the space-time to form a black hole and the boundary that nothing can escape is called event horizon. The movie “Interstellar” has a very famous part about the horizon event and falling into blackhole.

View of black hole in “Interstellar”, picture from YouTube

The black hole has three independent properties: mass, charge and angular momentum. The center of black hole, according to general relativity, is the singularity where the curvature of space-time is infinite.

Another interesting thing about black hole is called gravitational time dilation. It’s describing that if something is falling into black hole, as it’s closer to event horizon, the time will be slower according to general relativity. And it will take infinite amount of time to reach event horizon.

On February 11th 2016, LIGO announced the first detection of gravitational waves, which also represent the first observation of black hole merging.

Composition of Solar System

Composition of Solar System, picture from wikipedia

The solar system is a planetary system consisted of star Sun, planets, comets and other objects orbiting around the Sun, and Earth is one of them. The major mass of solar system is from the Sun. The sun is a G2 main-sequence star at the center of solar system which contains about 99.86% of total mass in solar system. The sun generates its heat and light through nuclear fusion. It’s 4.6 billion years old as the rest of the solar system is, and it will continue to last for about 5 billion years.

The next thing in solar system are planets. Mercury is the planet that is closest to sun, followed by Venus, Earth, and Mars. These four planets are called terrestrial planets as they are rocky, small in size with little moons.

Between the terrestrial planets and Jovian planets is the asteroid belt. It’s a Circumstellar disc occupied by numerous asteroids between Mars and Jupiter.

Overview of Kuiper belt, picture from NASA

Jupiter, Saturn, Uranus and Neptune are called Jovian planet which are gaseous, giant, with a lot of moons. Beyond Neptune, it’s the Kuiper belt which is also a Circumstellar disc but it’s much larger in size than asteroid belt — 20 times wider and 20 to 200 times more massive. It’s occupied by asteroids which are remnants from when the solar system formed.

The window of universe — Various types of Telescope

NGC 6357 under X-ray telescope by NASA. Image from Gizmodo

The science of astronomy is about the telescopes. Without them, our naked eyes can only see thousands of stars in the night sky away from city lights. We get to see galaxies, stars, nebulas and other celestial objects that are light years away from us through telescopes. Furthermore, as the technology improved over the last century, we developed various kinds of telescopes beside optical telescope. Here is some short description about them:

It is a kind of telescope to detect high energy gamma ray photons in the universe range of 50 GeV to 50 TeV. They are usually not in the space but on the Earth.

Infrared image from NASA’s Spitzer Space telescope. Image from Space

All celestial objects with a temperature above absolute zero emit some form of electromagnetic radiation. Astronomers use infrared telescope to detect celestial objects.

Radio telescopes are used to receive electromagnetic radiations from stars in the radio wave range. They are usually huge parabolic antennas. Radio telescope is essential to radio astronomy study.

Parkes CSIRO radio telescope. Image from wikipedia
  • Ultraviolet telescope, X-ray telescope and Submillimeter telescope

These are just other kinds of telescopes that astronomers used to detect electromagnetic waves in ultraviolet range, X-ray range and Submillimeter range edited by celestial objects in the universe

These various kinds of telescope allow us to picture and study universe in different perspective and enable us to see much more stuff than optical telescope can. Our development in astronomy is all based on these telescopes and that’s why people are saying the astronomy is the science of telescope.

Historical Astronomers in Context – Johannes Kepler

Portrait of Kepler from Wikipedia

Johannes Kepler. (Dec.27.1571 – Nov.15.1630)

He was important to astronomy for his revolutionary discovery of laws of planetary motion, and his books Astronomia novaHarmonices Mundi, and Epitome Astronomiae Copernicanae. His laws of planetary motion describe how planets orbit around the Sun and the relationship between their period and their orbit length. His works contribute to Newton’s theory of gravity too.

Contemporary events:

In 1614, John Napier discovers logarithms. Logarithms is a new function which extended the world of math and analysis beyond the scope of algebraic methods. The method of logarithms was discovered and published first by John Napier in 1614 in his book called Mirifici Logarithmorum Canonis Descriptio(Description of the Wonderful Rule of Logarithms).

In 1610, Galileo sees the moons of Jupiter (Galilean moons) through his telescope. Galileo’s discovery marked the importance of telescope as a tool for astronomers as telescope allowed him to find objects in the sky that could not be detected by naked eye. More importantly, the discovery of moons of Jupiter disproves the Ptolemaic world system, which held that everything has to orbit around the Earth. 

Contemporary person:

Jan Baptist van Helmont. (Jan.12.1580 – Dec.30.1644)

Portrait of J.B van Helmont. Picture from Wikipedia.

He was a Flemish chemist, physiologist, and physician. He is sometimes considered to be “the founder of pneumatic chemistry”. Van Helmont is important to science development and chemistry for his discovery of CO2 and ideas on spontaneous generation. He was one of the first people realizing there are gases different from normal air in our atmosphere and furthermore he introduced the word “gas” (from the Greek word chaos) into the vocabulary of science.


It’s very interesting to see that Galileo Galilei has lived entire life of Kepler and they are both huge astronomical figures to today’s science and astronomy. Also 16thcentury and 17thcentury was a huge revolutionary period in science in which a lot of great scientists are born and discoveries are made, just like Kepler’s laws of planetary motion, Newton’s law of gravity and discovery of CO2 and the word “gas”. All of these histories make me feel that the history of science is step-by-step with discoveries made based on achievements of our predecessors. And our scientific discoveries today will become the bricks for our future science development.

Picture the enormous universe

A logarithmic illustration of the universe, with solar system in the center and CMB at the edge. Image from Business Insider.

The vast size of universe is so hard to picture in one image because its size is unbelievably enormous compared to human and Earth. Our planet Earth has the size around 12,742 km diameter. And the nearest object to us is moon which is approximately 384,400 km from us. The number is already huge in our sense. But the size of our solar system is about 287.46 billion km and the Milky Way is around 950,000,000,000,000,000 km wide, or 105,700 light-years as the number is too large to use km as unit. It will take light 105,700 years to travel across the whole Milky Way! Consider that human has been around for only 200,000 years. And the size of Milky Way is not unique. In fact, some large galaxy can have size way bigger than Milky Way. IC1101, the largest galaxy has been found, is 6 million light-years in diameter.

Amazingly, astronomers estimated that there are around two trillion galaxies in the observable universe which is 92 billion light-years wide. So the only way to picture such enormous size in an image is to use logarithmic scales. As the picture above shows, each chunk of the circle represent a field of view several orders of magnitude larger than the one before it.  So the whole observable universe is now in a single image.