Gravitational Singularity

Gravitational Singularity, or spacetime singularity, is a point with infinitely small size and infinitely big density, gravitational force and time curvature. At that point, no physicals laws we know right now can be applied. According to the big bang theory, our universe is expanded from a universe singularity.

According to current theory, when an object falls into the blackhole and approaches to the central singularity of the blackhole, the object will be flattened and elongated because of the larger gravitational force. As it will be longer and longer, the object will finally lose its dimension and disappears at the singularity. Besides, some mathematical inductions state that materials in three dimension world will be destroyed by the singularity and lose its third dimension. Instead it will exist in the blackhole in the form of two-dimensional object, while its two-dimensional data can be restored as three-dimensional object. Therefore, some scientists believe that our world is actually a two-dimensional world, and the third dimension is actually a projection of the two-dimensional data.

Because of the nature of singularity, it is possible that we will never be able to describe singularity with full detail. Nor will we truly understand the singularity in the center of the blackhole. Even though spectator might be able to send signal into blackhole, the spectator might never get any response back. Therefore, we can only induce that singularity do exist by theorems, while we are lack of any data to provide its existence.

Singularity, pic source

The Dark Forest Theory

If you are interested in aliens and Fermi paradox, there is a must-read novel “The three-body problem”.

The novel was written by a Chinese writer Liu Cixin, and in the novel Liu introduced a theory called “The Dark Forest Theory”. The theory has two axioms. The first one is that “survival” is the first need of a civilization, which means each will do whatever it can do to ensure it is able to survive. And the second axiom is that the civilizations will always grow, develop and expand, needing more and more resources, while the total amount of resource in the universe is finite.

Liu then introduced a concept called suspicion chain. If two civilizations meet each other, they would like to know if the other side is “friend” or “enemy”. Even though the other side behaves friendly, they would not know for sure if the other side is genuinely friendly or just faking. Therefore, the bottom line is every civilization other than us is a potential threat, and the best way for our civilization to survive is to hide ourselves and destroy any potential threat we could find (without revealing ourselves).

The theory farther refers the universe as a dark forest, while each civilization is a hunter inside it. Each hunter should be extremely careful since there are a lot of hunters like him in this forest, and when he find another people in the forest, he should kill them as quick and insidious as possible.

As Fermi paradox is the apparent contradiction between the high possibility estimates for the aliens and the lack of existence of aliens, the dark forest theory might be a possible response as it states that all the civilizations will try to protect themselves by not revealing themselves for the outer world.

The Dark Forest written by Liu, pic source

What will black hole be eventually?

Black hole, pic source

Among all the astro bodies, black hole is unique. We all know that black holes have strong gravitation effect that nothing, even including light, is able to escape from black hole. Then, a question arises and attracts me a lot – What will black hole be eventually after it absorbs a lot of matters? Will it be heavy and small enough to become another singular point? When I am lying on the bed, I sometimes think about this question and imagine that a blackhole gradually develops to another world.

However, sadly in the real world, a blackhole is not able to develop to a singular point and then become a new world. Instead, it will gradually radiates itself. In 1974, Stephen Hawking found that black holes do not only absorb matters from outer world but also radiates its matter towards the outside world. According to Hawking’s theorem, if particles are too close to the blackhole, it will radiates some energy called Hawking radiation. Also, Hawking also stated that small blackholes have already done radiation and disappeared, while black holes with larger mass are still alive but they are also accelerating towards their death. In order to obtain enough mass to make up the loss of radiation, black holes have to absorb enough matter. If there is enough matter near the blackhole, it will be able to stay alive and grow. On the other hand, the black hole will stop growing and gradually goes to its death.

Halley’s Comet

Halley’s Comet is well-known to people and it might be the most famous comet in the world. However, people know it but most of people do not know what makes Comet Halley famous. In fact, Comet Halley is famous because it is the only short-period comet which regularly comes close to Earth and be visible to the naked eye from Earth. Every 74-79 years, Comet Halley comes visiting our Earth, which means a person might be able to observe the comet twice in his/her life time.

The high frequency of visiting makes Comet Halley easily observable compared with the other comets, and in fact since 240 BC, people began noticing and recording this comet, which is a reason why Comet Halley is so popular.

Nowadays scientists are studying Comet Halley and able to give us more information about it besides its regular visiting. Thanks to the Giotto and Vega mission, scientists found that Comet Halley’s nucleus is small – 15km long, 8km wide and perhaps 8km thick. What is more interesting, the comet are made of many many small pieces. These small pieces hold loosely, which makes the density and mass of Comet Halley quite small. However, even though the nucleus of the comet is small, when it goes near the Sun, it will be observed much larger by people. The reason is that when Comet Halley comes near the sun, its violate compound such as water, carbon monoxide, etc., will begin sublime from the nucleus and form a coma.

Comet Halley, pic source

Fusion vs. Fission

The nuclear energy can be generated in two ways – fusion and fission. Both fusion and fission energy are generated by altering atoms. What is the difference between fusion and fission? And which way will generate more energy?

As the word “fission” means separate a thing into different parts, nuclear fission means releases energy by splitting atoms. Nuclear fission happens when an unstable isotope is contacted by some high-speed particles. At this time, the unstable isotope is accelerated and then break into small particles.

Unlike “fission”, nuclear fusion means “fusing” several particles into “one”. Nuclear fusion happens when several low-mass particles compress together and release the neutron they do not need anymore. By uniting different particles and releasing neutron, a huge amount of energy is released.

Although nuclear fission can generate higher energy than nuclear fusion, we can hardly found any nuclear fission in our sun. The reason is that nuclear fission requires a particle with enough mass so that it can be “split” into several particles. However, the major element inside Sun are Hydrogen and Helium, which are both low-mass particles. Therefore, the major nuclear energy inside the sun is generated by nuclear fusion.

Nuclear Fusion, pic source 

Moon and Tides – The Magic of Gravity

Tides, picture source

Our sea has a periodic rising and falling phenomena. At a time, the sea will go up and reach to the peak, and at another time, the sea will go down and leaves us an island or a beach. This phenomena is called tides.

The tides are mainly caused by gravitational force from Moon. When we are at the near side of Moon, we are closer to Moon and therefore we will experience a larger gravitational force from Moon. Thus, as the seawater near us will also experience a larger gravitational force, it will be pulled a little bit closer towards Moon and then a rising tide occurs. Besides, when we are at the far side of Moon, we will experience the least gravitational force, which means we will experience the most centrifugal force. Therefore, we the gravitational force from Moon cannot hold the water at the far side of Earth from it, and the seawater at that part will also experience a rising tide. On the contrary, when we are at middle way between the far side of Moon and near side of Moon, the gravitational force from Moon will be neither too large nor too small. Therefore, neither will the seawater be pulled towards Moon nor will it loosed by the Earth. The seawater will come closer towards the earth surface and then a falling tide occurs.

Besides the liquid tide such as tide of seawater, there are some other kinds of tides. An interesting one needs to be mentioned is solid tide. Solid tide occurs on Moon due to the gravitational force from Earth. Unlike Earth, Moon always faces Earth from one direction, that is – one part of Moon will always be the near side of Earth and another part will always be the far side of Earth. As Moon is orbiting Earth and experiencing the gravitational force from Earth, its near side and far side of Earth gradually stretched out, which slowly makes it transit from sphere towards a ellipsoid. Scientists call this phenomenon “Tidal Locking”

Archeoastronomy – Armillary Sphere and Horolgoe in China

Through out the history of China, people are curious about how Sun, Moon and stars move according to Earth, and astronomers have created different tools to assist observing stars.

Armillary Sphere

The earliest well-known tool is Armillary Sphere (Chinese: 浑天仪), which is designed by Zhang Heng in 117 AD and completed in 125 AD. An Armillary Sphere is a spherical model of object in the sky, with Earth centered in the middle of the sphere. (At that time, Chinese people still believe in geocentric.) The spherical model contains multiple rings and each ring is movable. Different ring represent different solar objects –  the 28 Mansions (28 important constellations in Chinese Myth), north/south pole, the equator, the ecliptic, the 24 Solar Terms (Chinese: 节气). There are two water clock in the Armillary Sphere. In the bottom of each water clock, there is a hole.  As water leaking from the hole, the Armillary Sphere will gradually move by itself.

Zhang Heng, pic source

Armillary Sphere, pic source

Horologe – Equatorial Sundial

According to the Science Civilizations in China, the emergence and improvement of Horologe happened both in Eastern and Western at almost the same time. However, there is some difference between the Horologes made by the east and by the west.  The horologes designed by the Western Countries are Vertical Sundial and Flat Sundial. On the other hand, the horologes designed by China are the Equatorial Sundial. Although each type of sundials has its own advantage, Joseph Needham mentioned in Science Civilization in China that Equatorial Sundial has always been the most accurate one.

Chinese Equatorial Sundial, pic source

Historical Astronomers in Context – Nicholas Copernicus

Nicholas Copernicus

Nicholas Copernicus was important to astronomy because his heliocentric theory. At that time, people believed that Earth is the center of the universe and all the other astro bodies were revolving around Earth. The appearance of the heliocentric theory from Nicholas Copernicus helped people correct their false assumption, which hugely helped scientists at that time to understand the universe better. 

Nicholas Copernicus – pic source

Major Historical Event at that Time

1. Christopher Columbus discovered a new continent – America in 1492

2. King Ferdinand II of Aragon married Isabella I of Castile in 1479. The Aragon and the Castile combined together to form a new nation – Spain

Major Historical Figure at the Time

Michelangelo di Lodovico Buonarroti Simoni, known by his first name Michelangelo, is one of the most famous sculptor in the world. His work David is considered as the most balanced and beautiful men sculptor in the world. 

Michelangelo – pic source

David – pic source

Brief Reflection

I think it is interesting as learning this context, I can understand how people thought in the past and how understood the planet and world they lived step by step. To some extent the process people understand the world is like the growth process of a child. When I was child, I also once believed that the earth is the center of the world and no matter the sun or the moon were revolving around the earth. And when I grew older, I gradually understood that the earth is only a normal planet among the numerous planets and stars. Therefore, I think this context is pretty interesting.

 

 

How Fast Light Travels

picture sourse

We all know light travels fast, but do you know how fast it can be? The answer 299,792 kilometers in a second. The fact is, if you can travel as fast as light, you could go around the Earth seven and a half times in just one second. Theoretically, there is nothing runs faster than light.

Canis Major Dwarf Galaxy. Picture Source

From the universal perspective, it is hard to use normal distance unit to measure the distance between stars or galaxies. Therefore we use the light travel distance to represent it. The special unit is light-year – the distance a light can travel in a year. Even though the light runs fast enough, it still takes a long time for light to travel between galaxies. For example, the distance between our Milky Way Galaxy and its closest galaxy – Canis Major Dwarf Galaxy is about 42,000 light-years. It means light need to run 42,000 years without any rest from our sun to Canis Major Dwarf Galaxy!

 

First Post

This is the very first post. Below is a picture of my hometown.

Nanjing, Jiangsu, China. Pic Source