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A new way of defining a metre using speed of light is also developed. The new 4-dimensions is also described, how different the path is seen when one changes reference from 3D to 4D or from 3D to 2D.
It is spacetime curvature , where light moves in a straight path in 4D but is seen as a curve in 3D. These straight line paths are geodesics. The Twin paradox , a thought experiment in Special relativity involving identical twins, considers that twins can age differently if they move at relatively different speeds to each other, or even at different places where spacetime curvature is different.
Special relativity is based upon arenas of space and time where events take place, whereas general relativity is dynamic where force could change spacetime curvature and which gives rise to the expanding Universe.
Hawking and Roger Penrose worked upon this and later proved using general relativity that if the Universe had a beginning then it also must have an end. The picture shows the Universe expanding over time. In this chapter, Hawking first describes how physicists and astronomers calculated the relative distance of stars from the Earth. In the 18th century, Sir William Herschel confirmed the positions and distances of many stars in the night sky. In , Edwin Hubble discovered a method to measure the distance using brightness of the stars.
The luminosity , brightness and distance are related by a simple mathematical formula. Using all these, he fairly calculated distances of nine different galaxies. We live in a spiral galaxy just like other galaxies containing vast numbers of stars. The stars are very far away from us, so we only observe their one characteristic feature, their light. When this light is passed through a prism, it gives rise to a spectrum. We use thermal spectra of the stars to know their temperature.
In , when scientists were examining spectra of different stars, they found that some of the characteristic lines of the star spectrum was shifted towards the red end of the spectrum. The implications of this phenomenon was given by the Doppler effect , and it was clear that some stars were moving away from us. It was assumed that, since some stars are red shifted, some stars would also be blue shifted.
When found, none of them were blue shifted. Hubble found that the amount of redshift is directly proportional to relative distance. So, it was clear that the Universe is expanding. Despite this, the concept of a static Universe persisted until the 20th century.
Moreover, many astronomers also tried to avoid the face value implications of general relativity and stuck with their static Universe, with one notable exception, the Russian physicist Alexander Friedmann. Friedmann made two very simple assumptions: the Universe is identical in every direction, i. Homogeneity , and that this would be true wherever we look from, i. His results showed that the Universe is non-static.
His assumptions were later proved when two physicists at Bell Labs , Arno Penzias and Robert Wilson , found extra microwave radiation noise not only from the one particular part of the sky but from everywhere and by nearly the same amount.
At around the same time, Robert H. Dicke and Jim Peebles were also working on microwave radiation. They argued that they should be able to see the glow of the early Universe as background microwave radiation. Wilson and Penzias had already done this, so they were awarded with the Noble Prize in His work remained largely unknown until similar models were made by Howard Robertson and Arthur Walker.
First, the Universe would expand for a given amount of time, and if the expansion rate is less than the density of the Universe leading to gravitational attraction , it would ultimately lead to the collapse of the Universe at a later stage. Secondly, the Universe would expand, and at some time, if the expansion rate and the density of the Universe become equal, it would expand slowly and stop at infinite time, leading to a somewhat static Universe.
Thirdly, the Universe would continue to expand forever, if the density of the Universe is less than the critical amount required to balance the expansion rate of the Universe. The first model depicts the space of the Universe to be curved inwards, a somewhat Earth-like structure.
In the second model, the space would lead to a flat structure, and the third model results in negative curvature, or saddle shaped. Even if we calculate, the current expansion rate is more than the critical density of the Universe including the dark matter and all the stellar masses.
This concept of the beginning of time was against many religious beliefs, so a new theory was introduced, "Steady state theory" by Hermann Bondi , Thomas Gold , and Fred Hoyle , to tackle the Big Bang theory. Its predictions also matched with the current Universe structure. But the fact that radio wave sources near us are far fewer than from the distant Universe, and there were numerous more radio sources than at present, resulted in failure of this theory and everybody finally supported the Big Bang theory.
Roger Penrose used light cones and general relativity to prove that a collapsing star could result in a region of zero size and infinite density and curvature called a Black Hole. Hawking and Penrose proved together that the Universe should have arisen from a singularity, which Hawking himself disproved once Quantum effects are taken into account.
Chapter 4: The Uncertainty Principle[ edit ] The uncertainty principle says that the speed and the position of a particle cannot be found at the same time. To find where a particle is, scientists shine light at the particle. The uncertainty principle disproved the idea of a theory that was deterministic, or something that would predict everything in the future. Here is a picture of a light wave. How light behaves is also talked more about in this chapter.
Light interference causes many colors to appear. Light waves have crests and troughs. The highest point of a wave is the crest, and the lowest part of the wave is a trough.
Sometimes more than one of these waves can interfere with each other - the crests and the troughs line up. This is called light interference. When light waves interfere with each other, this can make many colors. An example of this is the colors in soap bubbles. Chapter 5: Elementary Particles and Forces of Nature[ edit ] Quarks and other elementary particles are the topic of this chapter.
Quarks are very small things that make up everything we see matter. There are six different "flavors" of quarks: up , down , strange , charm , bottom , and top. Quarks also have three " colors ": red, green, and blue. There are also antiquarks, which are the opposite of the regular quarks. In total, there are 18 different types of regular quarks, and 18 different types of antiquarks.
Quarks are known as the "building blocks of matter" because they are the smallest thing yet discovered that make up all the matter in the Universe. A particle of spin 1 needs to be turned around all the way to look the same again, like this arrow. All particles for example, the quarks have something called spin. The spin of a particle shows us what a particle looks like from different directions. For example, a particle of spin 0 looks the same from every direction. A particle of spin 1 looks different in every direction unless the particle is spun completely around degrees.
A particle of spin two needs to be turned around halfway or degrees to look the same. The example given in the book is of a double-headed arrow.
All of these particles follow the Pauli exclusion principle. This is a proton. It is made up of three quarks. All the quarks are different colors because of confinement.
Particles with a spin of 0, 1, or 2 move force from one particle to another. Some examples of these particles are virtual gravitons and virtual photons. Virtual gravitons have a spin of 2 and they represent the force of gravity. This means that when gravity affects two things, gravitons move to and from the two things. Virtual photons have a spin of 1 and represent electromagnetic forces or the force that holds atoms together.
Besides the force of gravity and the electromagnetic forces, there are weak and strong nuclear forces. Weak nuclear forces are the forces that cause radioactivity , or when matter emits energy. Strong nuclear forces are the forces that keep the quarks in a neutron and a proton together, and keeps the protons and neutrons together in an atom.
The particle that carries the strong nuclear force is thought to be a gluon. The gluon is a particle with a spin of 1. The gluon holds together quarks to form protons and neutrons. However, the gluon only holds together quarks that are three different colors. This makes the end product have no color. This is called confinement.
Some scientists have tried to make a theory that combines the electromagnetic force, the weak nuclear force, and the strong nuclear force.
This theory is called a grand unified theory or a GUT. This theory tries to explain these forces in one big unified way or theory. Chapter 6: Black Holes[ edit ] A picture of a black hole and how it changes light around it.
Black holes are talked about in this chapter. Black holes are stars that have collapsed into one very small point. This small point is called a singularity.
Black holes suck things into their center because they have very strong gravity. Some of the things it can suck in are light and stars. Only very large stars, called super-giants, are big enough to become a black hole.
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