Big Ideas

The night sky presents the viewer with a picture of a calm and unchanging Universe. So the 1929 discovery by Edwin Hubble that the Universe is in fact expanding at enormous speed was revolutionary. Hubble noted that galaxies outside our own Milky Way were all moving away from us, each at a speed proportional to its distance from us. He quickly realized what this meant that there must have been an instant in time (now known to be about 14 billion years ago) when the entire Universe was contained in a single point in space. The Universe must have been born in this single violent event which came to be known as the "Big Bang."

The question of what caused the Big Bang is one of the most difficult facing humanity. We may never find an answer, and even if we do, we probably won't understand it. It's difficult to imagine an event occurring without a cause, and yet, that is precisely the prospect we are faced with when it comes to this question.

During the 1920's and 30's, Edwin Hubble discovered that the Universe is expanding, with galaxies moving away from each other at a velocity given by an expression known as Hubble's Law: v = H*r. Here v represent's the galaxy's recessional velocity, r is its distance away from Earth, and H is a constant of proportionality called Hubble's constant.
The exact value of the Hubble constant is somewhat uncertain, but is generally believed to be around 70 kilometers per second for every megaparsec in distance, km/sec/Mpc. (see e.g. the online proceedings of How Far Can You Go?. A megaparsec is given by 1 Mpc = 3 x 10^6 light-years). This means that a galaxy 1 megaparsec away will be moving away from us at a speed of about 70 km/sec, while another galaxy 100 megaparsecs away will be receding at 100 times this speed. So essentially, the Hubble constant sets the rate at which the Universe is expanding.

Additionally, the present age of the Universe can be assessed vis-a-vis the Hubble constant: the inverse of the Hubble constant has units of time. By substituting in kilometers for Mpc in the Hubble constant, we find that upon inverting H we get a quantity with units of seconds (kilometers canceling out in the denominator and numerator). For a Hubble constant of 100 kilometers per second per Mpc, we get 3 x 10^17 seconds, or about 10 billion years.

Hubble's law is the name for the theory in physical cosmology that: all objects observed in deep space are found to have a Doppler shift observable relative velocity to Earth, and to each other; and that this Doppler-shift-measured velocity, of various galaxies receding from the Earth, is proportional to their distance from the Earth and all other interstellar bodies. In effect, the space-time volume of the observable universe is expanding and Hubble's law is the direct physical observation of this process. The motion of astronomical objects due solely to this expansion is known as the Hubble flow. Hubble's law is considered the first observational basis for the expanding space paradigm and today serves as one of the pieces of evidence most often cited in support of the Big Bang model.

In the sixteenth century, most people believed in the ideas of the ancient astronomer Ptolemy, that the planets, Moon, and Sun all orbited around the Earth. Then in 1543, Nicolaus Copernicus proposed the idea that the planets and the Earth orbited around the Sun. However, Copernicus' new theory was no better at predicting the positions of the planets in the sky than the older, Earth-centered theory. There was still something missing.....

Half a century later, Johannes Kepler sought to refine the Copernican system and truly understand how the planets move around the Sun. He studied observations of Mars recorded by his mentor, Tycho Brahe. Rather than trying to force the data to support a pre-determined view of the Universe, Kepler used Tycho's observations to guide the creation of his theories. This was a radical departure from the thought processes of his era, and it is a signal of the beginning of our modern scientific age.

In 1609, Kepler published his first and second laws of planetary motion, The Law of Ellipses and The Equal-Areas Law. Ten years later he published a third law, The Harmonic Law. He had succeeded in using a scientific method to create a simple, elegant, and accurate model to describe the motion of planets around the Sun...

Isaac Newton compared the acceleration of the moon to the acceleration of objects on earth. Believing that gravitational forces were responsible for each, Newton was able to draw an important conclusion about the dependence of gravity upon distance. This comparison led him to conclude that the force of gravitational attraction between the Earth and other objects is inversely proportional to the distance separating the earth's center from the object's center. But distance is not the only variable affecting the magnitude of a gravitational force.

Newton knew that the force that caused the apple's acceleration (gravity) must be dependent upon the mass of the apple. And since the force acting to cause the apple's downward acceleration also causes the earth's upward acceleration (Newton's third law), that force must also depend upon the mass of the earth. So for Newton, the force of gravity acting between the earth and any other object is directly proportional to the mass of the earth, directly proportional to the mass of the object, and inversely proportional to the square of the distance that separates the centers of the earth and the object.

Newton explains the Universal Law of Gravitation

Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1727) was an English physicist and mathematician who is widely regarded as one of the most influential scientists of all time and as a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica ("Mathematical Principles of Natural Philosophy"), first published in 1687, laid the foundations for most of classical mechanics.

An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This law is often called "the law of inertia".

A brief video for children explaining Newton's laws of motion in an easy & fun way. The first law states that 'Things want to keep on doing what they are already doing'. The 2nd law states that' Force is directly proportional to mass and acceleration. Newton's 3rd law states that there is a action & reaction act in a pair.

Thermodynamics is the study of energy, the conversion of energy between various forms and the ability of energy to do work. Initially, three laws of thermodynamics were posited. There seems to be a fourth, called the Zeroth Law, because Laws 1, 2, and 3 were spoken for.

C.P. Snow, the British scientist and author has offered up an easy and funny way to remember the Three Laws. He says they can be translated as: (1) you cannot win (you can’t get something for nothing because matter and energy are conserved. (2) You cannot break even (you cannot return to the same energy state because entropy always increases (3) you cannot get out of the game (because absolute zero is not attainable).

So, what do these laws really say and why are they important? In simple terms, the Laws dictate the requirements for heat and work. They were generated during the 19th century as the industrial revolution took hold and grew. Physicists became involved with studying the flow of heat from machines as well as the chemical changes that accompanied the actual work. They were interested in gaining maximum efficiency. In other words, they wanted to create a perpetual motion machine, or one that could run off its own heat it created during the process of work, so it could do more work, create more heat and …you get the point.

In this About.com video, learn the Four Laws of Thermodynamics and examples of their practical applications.

Kings don't like to be tricked. I'm sure you know this from all the stories you read when you were small. King Hieron II of Syracuse, was no exception. He worried that the people who made his crown charged him the price of using solid gold but instead they tricked him and used gold mixed with silver which costs less.

Sometimes Archimedes got so busy thinking about mathematics that he forgot to take a bath and his servants would have to force him to go to the public baths.

Legend has it, although some people doubt this, that one time when Archimedes was at the public bath he noticed that when he climbed in to a soaking bath the water level went up. Have you noticed the same thing when you climb into your own bathtub at home?

This video explains how to calculate the weight of a horse using Archimedes' Principle.

Includes a demonstration with digital scales and overflow apparatus.

The forces acting on hot air balloons, cargo and cruise ships is explained by this principle from the ancient Greeks.
Marine architects and engineers use this basic principle to design floating structures - ships, submarines and oil rigs.
Suitable as a learning resource for an introduction to buoyancy and Archimedes in physics and general science.

Most educated people in Europe and the Americas during the 19th century had their first full exposure to the concept of evolution through the writings of Charles Darwin click this icon to hear the name pronounced. Clearly, he did not invent the idea. That happened long before he was born. However, he carried out the necessary research to conclusively document that evolution has occurred and then made the idea acceptable for scientists and the general public. This was not easy since the idea of evolution had been strongly associated with radical scientific and political views coming out of post-revolutionary France. These ideas were widely considered to be a threat to the established social and political order.

Introduction to evolution, variation in a population and natural selection

Albert Einstein, in his theory of special relativity, determined that the laws of physics are the same for all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no matter the speed at which an observer travels. As a result, he found that space and time were interwoven into a single continuum known as space-time. Events that occur at the same time for one observer could occur at different times for another.

As he worked out the equations for his general theory of relativity, Einstein realized that massive objects caused a distortion in space-time. Imagine setting a large body in the center of a trampoline. The body would press down into the fabric, causing it to dimple. A marble rolled around the edge would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space.

A clip from the series 'The Elegent Universe' regarding some aspects of General Relativity and gravity.

In 1927, Werner Heisenberg was in Denmark working at Niels Bohr's research institute in Copenhagen. The two scientists worked closely on theoretical investigations into quantum theory and the nature of physics. Bohr was away on a skiing holiday, and Heisenberg was left to mull things over himself. He had a shocking but clear realization about the limits of physical knowledge: the act of observing alters the reality being observed. At least at the subatomic level. To measure the properties of a particle such as an electron, one needs to use a measuring device, usually light or radiation. But the energy in this radiation affects the particle being observed. If you adjust the light beam to accurately measure position, you need a short-wavelength, high-energy beam. It would tell you position, but its energy would throw off the momentum of the particle. Then, if you adjust the beam to a longer wavelength and lower energy, you could more closely measure momentum, but position would be inaccurate.

This principle punctured the centuries-old, firmly held belief that the universe and everything in it operates like clockwork. To predict the workings of the "clock," one needs to measure its qualities and parts at a specific point in time. Classical physics assumed that the precision of measuring is theoretically unlimited. But Heisenberg stated that since you could never with great certainty measure more than one property of a particle, you could only work with probability and mathematical formulations. (Heisenberg called this matrix mechanics, soon shown to be equivalent to Erwin Schrödinger's more visualizable wave theory.)

The Heisenberg uncertainty principle - in a nutshell!