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# Relativity & Particle Physics FAQ

What is light?

Light is a phenomenon that has particle and wave characteristics. Its carrier particles are called photons, which are not really particles, but massless discrete units of energy.

What is the speed of light?

The speed of light is 299,792,458 m/s in a vacuum. The symbol used in relativity for the speed of light is "c", which probably stands for the Latin word "celeritas", meaning swift.

Is the speed of light really constant?

The speed of light is constant by definition in the sense that it is independent of the reference frame of the observer. Light travels slightly slower in a transparent medium, such as water, glass, and even air.

Can anything travel faster than light?

No. In relativity, c puts an absolute limit to speed at which any object can travel, hence, nothing, no particle, no rocket, no space vehicle can go at faster-than-light (=superluminal) speeds. However, there are some cases where things appear to move at superluminal speeds, such as in the following examples: 1. Consider two spaceships moving each at 0.6c in opposite directions. For a stationary observer, the distance between both ships grows at faster-than-light speed. The same is true for distant galaxies that drift apart in opposite directions of the sky. 2. Another example: Consider pointing a very strong laser on the moon so that it projects a dot on the moon's service and then moving the laser rapidly towards earth, so that it points on the floor in front of you. If you accomplish this in less than one second, the laser dot obviously traveled at superluminal speed, seeing that the average distance between the Earth and the Moon is 384,403 km.

What is matter?

The schoolbook definition would be: Matter is what takes up space and has mass. Matter as we know it is composed of molecules, which themselves are built from individual atoms. Atoms are composed of a core and one or more electrons that spin around the core in an electron cloud. The core is composed of protons and neutrons, the former have a positive electrical charge, the latter are electrically neutral. Protons and neutrons are composed of quarks, of which there are six types: up/down, charm/strange, and top/bottom. Quarks only exist in composite particles, whereas leptons can be seen as independent particles. There are six types of leptons: the electron, the muon, the tau and the three types of neutrinos. The particles that make up an atom could be seen as a stable form of locked up energy. Particles are extremely small, therefore 99.999999999999% (or maybe all) of an atom's volume is just empty space. Almost all visible matter in the universe is made of up/down quarks, electrons and (e-)-neutrinos, because the other particles are very unstable and quickly decay into the former.

How fast does an electron spin?

An electron in an hydrogen atom moves at about 2.2 million m/s. With the circumference of the n=1 state for hydrogen being about 0,33x10-9 m in size, it follows that an n=1 electron for a hydrogen atom revolves around the nucleus 6,569,372 billion times in just one second.

Are quarks and leptons all there is?

Not really. Fist of all, quarks always appear in composite particles, namely hadrons (baryons and mesons), then there is antimatter, and finally there are the four fundamental forces.

What is antimatter?

The existence of antimatter was first predicted in 1928 by Paul Dirac and has been experimentally verified by the artificial creation of the positron (e+) in a laboratory in 1933. The positron, the electron's antiparticle, carries a positive electrical charge. Not unlike a reflection in the mirror, there is exactly one antimatter particle for each known particle and they behave just like their corresponding matter particles, except they have opposite charges and/or spins. When a matter particle and antimatter particle meet, they annihilate each other into a flash of energy. The universe we can observe contains almost no antimatter. Therefore, antimatter particles are likely to meet their fate and collide with matter particles. Recent research suggests that the symmetry between matter and antimatter is less than perfect. Scientists have observed a phenomenon called charge/parity violation, which implies that antimatter presents not quite the reflection image of matter.

What are the four fundamental forces?

The four fundamental forces are gravity, the electromagnetic force, and the weak and strong nuclear forces. Any other force you can think of (magnetism, nuclear decay, friction, adhesion, etc.) is caused by one of these four fundamental forces or by a combination of them.

What is gravity?

Gravity is the force that causes objects on earth to fall down and stars and planets to attract each other. Isaac Newton quantified the gravitational force: F = mass1 * mass2 / distance². Gravity is a very weak force when compared with the other fundamental forces. The electrical repulsion between two electrons, for example, is some 10^40 times stronger than their gravitational attraction. Nevertheless, gravity is the dominant force on the large scales of interest in astronomy. Einstein describes gravitation not as a force, but as a consequence of the curvature of spacetime. This means that gravity can be explained in terms of geometry, rather than as interacting forces. The General Relativity model of gravitation is largely compatible with Newton, except that it accounts for certain phenomena such as the bending of light rays correctly, and is therefore more accurate than Newton's formula. According to General Relativity, matter tells space how to curve, while the curvature of space tells matter how to move. The carrier particle of the gravitational force is the graviton.

What is electromagnetism?

Electromagnetism is the force that causes like-charged particles to repel and oppositely-charged particles to attract each other. The carrier particle of the electromagnetic force is the photon. Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth. Residual electromagnetic force allows atoms to bond and form molecules.

What is the strong nuclear force?

The strong force acts between quarks to form hadrons. The nucleus of an atom is hold together on account of residual strong force, i.e. by quarks of neighboring neutrons and protons interacting with each other. Quarks have an electromagnetic charge and another property that is called color charge, they come in three different color charges. The carrier particles of the strong nuclear force are called gluons. In contrast to photons, gluons have a color charge, while composite particles like hadrons have no color charge.

What is the weak nuclear force?

Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. It is the primary reason why matter is mainly composed of the stable lighter particles, namely up/down quarks and electrons. Radioactivity is due to the weak nuclear force. The carrier particles of the weak force are the W+, W-, and the Z particles.

How are carrier particles different from other particles?

Carrier particles, such as the photon, gluon, and the graviton are hypothetical. They are thought to be massless and having no electrical charge (except W+ and W-). Force carrier particles can only be absorbed or produced by a matter particle which is affected by that particular force. They allow us to explain interactions between matter.

How old is the universe?

Today's most widely accepted cosmology, the Big Bang theory, states that the universe is limited in space and time. Current estimates for the age of the universe are between ten and twenty billion years; the exact figures are quite controversial. It is assumed that the universe began in a violent explosion called the Big Bang.

What came before the Big Bang?

The Big Bang model is singular at the time of the Big Bang. This means that one cannot even define time, since spacetime is singular. In some models like the oscillating universe, suggested by Stephen Hawking, the expanding universe is just one of many phases of expansion and contraction. Other models postulate that our own universe is just one bubble in a spacetime foam containing a multitude of universes. The "multiverse" model of Linde proposes that multiple universes recursively spawn each other, like in a growing fractal. However, until now there is no observational data confirming either theory. It is indeed questionable, whether we will ever be able to gain empirical evidence speaking in favor these theories, because nothing outside our own universe can be observed directly. Hence, the question can currently not be answered by science.

How big is the universe?

The universe is constantly expanding in all directions, therefore its size cannot be stated. Scientists think it contains approximately 100 billion galaxies with each galaxy containing between 100 and 200 billion star systems. Our own galaxy, the Milky Way, is average when compared with other galaxies. It is a disk-shaped spiral galaxy of about 100,000 light-years in diameter.

What is the universe expanding into?

This question is based on the popular misconception that the universe is some curved object embedded in a higher dimensional space, and that the universe is expanding into this space. There is nothing whatsoever that we have measured or can measure that will show us anything about the larger space. Everything that we measure is within the universe, and we see no edge or boundary or center of expansion. Thus the universe is not expanding into anything that we can see, and this is not a profitable thing to think about.

Why is the sky dark at night?

If the universe were infinitely old, and infinite in extent, and stars could shine forever, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers's paradox, named after Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826. Absorption by interstellar dust does not circumvent this paradox, since dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the universe. However, the universe is not infinitely old, and the expansion of the universe reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers's paradox, but they both act at once.

If the universe is only 20 billion years old, how can we see objects that are 30 billion light-years away?

This question is essentially answered by Special Relativity. When talking about the distance of a moving object, we mean the spatial separation now, with the positions of us and the object specified at the current time. In an expanding universe, this distance is now larger than the speed of light times the light travel time due to the increase of separations between objects, as the universe expands. It does not mean that any object in the universe travels away from us faster than light.

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