Space Time

Frequently Asked Questions

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

Spacetime, 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

Spacetime,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. The current estimate for the age of the universe

is 13.7 billion years. This figure was computed from the cosmic microwave

background (CMB) radiation data that the Wilkinson Microwave Anisotropy Probe

(WMAP) captured in 2002.

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

Spacetime, 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 13.7 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.

Source:- thebigview.com

Flatland

In Flatland, a "jail" is a circle drawn around a person. Escape from this circle is impossible in two dimensions. However, a three-dimensional person can yank a Flatlander out of jail into the third dimension. To a jailer, it appears as though the prisoner has mysteriously vanished into thin air.

If we peel a Flatlander from his world and flip him over in three dimensions, his heart now appears on the right-hand side. All his internal organs have been reversed. This transformation is a medical impossibility to someone who lives strictly in Flatland.

A Flatlander can visualize a cube by examining its shadow, which appears as a square within a square. If the cube is rotated, the squares execute motions that appear impossible to a Flatlander. Similarly, the shadow of a hyper- cube is a cube within a cube. If the hypercube is rotated in four dimensions, the cubes execute motions that appear impossible to our three-dimensional brains.

In Flatland, Mr. Square encounters Lord Sphere. As Lord Sphere passes through Flatland, he appears to be a circle that becomes successivley larger and then smaller. Thus Flatlanders cannot visualize three-dimensional beings, but can understand their cross sections.

A Mobius strip is a strip with only one side. Its outside and inside are identical. If a Flatlander wanders around a Mobius strip, his internal organs will be reversed.

The mystic Henry Slade claimed to be able to change right-handed snail shells into left-handed ones, and to remove objects from sealed bottles. These feats are impossible in three dimensions, but are trivial if one can move objects through the fourth dimension.

Source : Michio Kaku's Parallel Worlds

Fourth Dimension

Flatlanders cannot visualize a cube, but they can conceptualize a three-dimensional cube by unraveling it. To a Flatlander, a cube, when unfolded, resembles a cross, consisting of six squares. Similarly, we cannot visualize a four-dimensional Hypercube, but if we unfold it we have a series of cubes arranged in a cross like tesseract. Although the cubes of a tesseract appear immobile, a four-dimensional person can "wrap up" the cubes into a Hypercube.

In Christus Hypercubus, Salvador Dali depicted Christ as being crucified on a tesseract, an unraveled hypercube. (The Metropolitan Museum of Art. Gift of Chester Dale, Collection, 1955. © 1993. Ars, New York/Demart Pro Arte, Geneva)

Cubism was heavily influenced by the fourth dimension. For example, it tried to view reality through the eyes of a fourth-dimensional person. Such a being, looking at a human face, would see all angles simultaneously. Hence, both eyes would be seen at once by a fourth-dimensional being, as in Picasso's painting Portrait of Dora Maar. (Giraudon/Art Resource. ® 1993. Ars, New York/ Spadem, Paris)

The shadow of a hyper-cube is a cube within a cube. If the hypercube is rotated in four dimensions, the cubes execute motions that appear impossible to our three-dimensional brains.

Source : Michio Kaku's Parallel Worlds

Fifth Dimension

If we lived in a hyperdoughnut, we would see an infinite succession of ourselves repeated in front of us, to the back of us, and to our sides. This is because there are two ways that light can travel around the doughnut. If we hold hands with the people to our sides, we are actually holding our own hands; that is, our arms are actually encircling the doughnut.

If a rocket disappears off the right side of a video-game screen, it re-emerges on the left. If it disappears at the top, it re-emerges at the bottom. Let us now wrap the screen so that identical points match. We first match the top and bottom points by wrapping up the screen. Then we match the points on the left-and right-hand sides by rolling up the screen like a tube. In this way, we can show that a video-game screen has the topology of a doughnut.

Source : Michio Kaku's Parallel Worlds

Worm Holes

If we insert our hands into the window from two different directions, then it appears as though our hands have disappeared. We have a body, but no hands. In the alternative universe, two hands have emerged from either side of the window but they are not attached to a body.

In this purely hypothetical example, a "window" or wormhole has opened up in our universe. If we look into the window from one direction, we see one dinosaur. If we look into the other side of the window, we see another dinosaur. As seen from the other universe, a window has opened up between the two dinosaurs. Inside the window, the dinosaurs see a strange small animal (us).

Source : Michio Kaku's Parallel Worlds

View from Voyager 1