Changing the Rules

There are two important differences between Laplace’s idea of a dark star and the modern idea of a Black hole. The differences are due to two important discoveries of modern astrophysics. The first discovery is that nothing can travel faster than light. This means that if light cannot escape from a massive star, nothing can. This also means that a dark star is a hole, because nothing that is pulled in by the star’s gravity can never escape. The second difference is that Laplace used Newton’s law of gravity as the basis of his work. Modern astrophysics says that these laws do not work in and around Black holes.

Dark Stars

Pierre Laplace (1749-1827), a French mathematician and astronomer, was one of the first people suggest that black holes could exist. As an expert on celestial Mechanics, he knew how the planets and moons move around the sun. In his book, Exposition of the System of the world, published in 1795, Laplace made an amazing prediction. Laplace realized that if a star were massive enough, its escape velocity would be greater than the speed of light. So he calculated how big a star would have to be, if it had the same overall density as Earth, to have an escape velocity equal to the speed of light. He calculated that the star would have to be 250 times the diameter of the Sun. Laplace predicted that a star of this size would have such an enormous gravitational pull that light particles would have such an enormous gravitational pull that light particles would never leave its surface. The star would be invisible.

Light and gravity

When Sir Isaac Newton devised his Law of universal Gravitation, he also suggested that light was subjected to the pull of gravity. On Earth, light always travels in a straight line, unless it hits an object. But light does bend as it passes close to bodies with very strong gravitational fields, such as very massive stars. This shows that the things we take for granted on Earth do not necessarily apply when super strong gravity is at work.

Black Hole

A black hole is a place in space where gravity is so super strong that nothing can escape it not even light. When scientists call a black hole a giant cosmic vacuum, they do not mean it sucks up anything that comes near it. However, anything passing close to a black hole will be affected by its strong gravitational pull. Astronomers think that events become very strange both near and inside black holes. The known world ceases to exist, time slows down, space is warped, and the accepted laws of physics no longer apply. No one should go into a mysterious black hole to investigate, because they would never return.
In most great mysteries, such as UFOs, science tries to explain the strange things that people claim to have seen. In the case of black holes, however things are the other way around. Scientists predicted the existence of black holes long before there was any real evidence that they existed. In fact, the very nature of black holes means they cannot be seen to be believed!

Varying Heights

Astronauts become 1-2 inches taller while they are in space. Without Earth’s gravity tugging on their bodies, the vertebrae in their spines stretch out. The extra height is only temporary, though. Astronauts shrink back to their normal size after returning to Earth.

Life in the slow lane

Time slows down when traveling at great speed in space, so space travelers age more slowly than they do on Earth. Albert Einstein figured this out
In 1905, long before we started flying in space. Einstein knew the speed of light never changes as it is constant. Time however, is relative said Einstein. It can change. It changes according to the speed of what is measuring it. The faster the speed, the slower time passes. In fact, a very accurate clock aboard a space shuttle was measured after its return to Earth it lost 2.95 x 10^-10 seconds for each second of the trip. If the shuttle had been traveling near the speed of light and had been gone for several years, the time loss would have been bigger. So if you went on a very long space trip and our space craft could travel close to speed of light we would be younger than our current same-age friends when we returned to Earth.

Weight on different Planets

Planet Earth Weight Multiply by Our Space Weight
Mercury ----- x .38 =
Venus ----- x .91 =
Earth ----- x 1.00 =
Mars ----- x .38 =
Jupiter ----- x 2.34 =
Saturn ----- x .93 =
Uranus ----- x .79 =
Neptune ----- x 1.14 =
Pluto ----- x .04 =

Relative theory of Einstein

Einstein made other predictions in his General Theory of Relativity. He said that in super strong gravitational fields, time passes more slowly than it does outside the field. And as gravity becomes infinitely strong, time actually slows to a stop. He also said that all laws of geometry would no longer be true because three-dimensional space would be changed and distorted. For example, the geometry rule that the area of a square is equal to its length multiplied by its width would not be true in super strong gravity.
Einstein’s other great theory, the special Theory of relativity, predicted that the passing of time and the measurement of distance change as movement becomes faster and faster. The effects of this become noticeable only as the speed of light is approached. This would have the consequences for black holes, since objects that fall into black holes would begin to go as fast as the speed of light.

Mass and Weight

Mass and weight are often confused. In physics, the mass of an object is the amount of matter in it. Mass is measured in kilograms (kg). Weight is the pull of gravity on an object. Weight is a force. It is measured by both the mass and gravity. It is measured in units called Newton (N). The mass of an object is always the same, but its weight can change.

Escape Velocity

Imagine throwing any object straight up. The Earth’s gravity would gradually slow it down and make it fall back to Earth. But if the object was thrown upward fast enough, gravity would not be able to stop it escaping into space. The minimum speed needed for this to happen is called Escape Velocity is 25,000 miles per hour. Space Rockets must accelerate to a minimum of 17,000 miles per hour to stay in orbit around Earth. They must travel faster to escape from Earth.

Astrophysics

Astrophysics is the branch of astronomy that describes the physical characteristics of things in the Universe. Two of the characteristics that astrophysicists study are Gravity and Light

Nature of light

Light travels in waves of particles called photons. Eyes detect light. Some objects, such as stars, can be seen because they make light. Other objects, such as planets of this book, can be seen because light bounces off them. Light travels very fast. In the vacuum of space, the speed of light is 186,282 miles (300,000 kilometers) per second. Its sped decreases when it passes through substances, such as air and glass.