# Light Speed, the Universal Speed Limit

What Is the Speed of Light?

In a vacuum, light travels at a constant and finite speed of 186,282 miles per second (299,792 kilometers per second) which translates to about 670,616,629 miles per hour. This is so fast that a person travelling at the speed of light can go around the Earth at the equator 7.5 times in one second.

The Speed of Light over the Years

The speed of light has fascinated astronomers for ages. Even the ancient astronomers knew that light travelled at an extraordinary speed.

The first known discussion on the speed of light was between Aristotle, the ancient Greek philosopher and Empedocles, a Greek scientist. Empedocles believed that the fact that light moved meant that it took time to move while Aristotle believed that light travelled instantaneously.

Various astronomers attempted to determine the speed of light achieving varying degrees of accuracy. Here are the measurements of the speed of light over the years.

Galileo’s Attempt

Galileo, the famous Italian astronomer, attempted to measure the speed of light as early as 1667. He positioned two people on a hill at a distance of less that a mile with each holding a covered lantern. The first person uncovered his lantern and when the second person saw the light also uncovered his lantern.

Galileo hoped to calculate the speed of light by determining how long it took the person holding the first lantern to see the light. He even took into account the reaction time.

However, he was not able to measure the light speed because the distance between the two persons was too small for him to notice a difference. He was only able to determine that light travelled at least ten times faster than sound.

Ole Romer’s Light Speed

Ole Romer, a Danish astronomer, gave the speed of light its first number in the 1670s. He determined the speed of light to be 125,000 miles per second (200,000 km/s).

He determined this number by observing the eclipses of Jupiter’s moon, lo, as it passed behind Jupiter. He noticed that light took time to travel from lo to Earth and used his observations to conclude that light took ten to eleven minutes to travel from the sun to the Earth. In reality, it takes 8 minute and 19 seconds. Though Romer’s number was an overestimate, scientists at last had a number to work with.

In 1728, James Bradley an English physicist achieved 1 percent accuracy when he determined the speed of light to be 185,000 miles per second (301,000 km/s).

He came up with this number by monitoring the change in the apparent position of the stars caused by the Earth travelling around the sun.

Hippolyte and Foucalt

In the mid-1800s, French physicist used a rotating wheel and mirrors to measure the speed of light while a French physicist, used a rotating mirror.

Both scientists missed the speed of light, as it is known today, by just 1,000 miles per second.

Albert Michelson

Albert Michelson refined Foucalt’s technique in 1879 by using superior quality of mirrors and lenses and determined the speed of light to be 186,355 miles per second (299,910 km/s). This was taken as the most accurate measurement for 40 years until Michelson re-measured it.

Einstein’s Special Relativity Theory

Einstein’s speed of light theory is known as the special relativity theory. In it, he states that the speed of light does not change with time or place.

It was in 1905 that Albert Einstein published his first paper on special relativity. He established that light moves at the same speed whether the observer is standing still, moving slowly or moving fast.

He illustrated that the speed of light originating from the sun travels at the same speed whether measured by someone standing inside or outside the galaxy.

Is the Speed of Light Constant All the Time?

The speed of light is generally used to refer to the speed at which light travels in a vacuum. If light is travelling in a vacuum, nothing can slow it down and it travels at a constant speed all the time.

Other than this, light speed will vary depending on what it is travelling through. Just like sound, light travels in waves. If it is travelling through an area that contains particles such as dust or other matter, light can bend in its path when the waves encounter these particles and the speed of light will be reduced.

Light is slowed down when travelling through transparent media such as air, water and glass. Jean Foucault discovered that light is slowed down at a ration that is always greater than one. This ratio is known as the refractive index of the medium.

What is a Light Year?

A light year is the distance that light travels in a year. This concept measures both time and distance.

It is used to describe the distances of objects in space. For example, since light takes 1 second to travel from the moon to our eyes, the moon can be said to be 1 light-second away. The light from the sun takes about 8 minutes to travel from the sun and reach our eyes. The sun can be said to be 8 light minutes away from Earth.

Stars and other objects that exist outside the solar system are light-years away. Some are just a few light-years away from us while objects in the distant universe are a few billion light-years away from Earth.

The Universe is Kind of a Time Machine

In a way, the universe functions like a time machine. All we see of the distant universe is history not the present. The present will probably be viewed in the future.

When scientists study objects and space-based telescopes such as the Herschel space observatory and Hubble space telescope capture objects that are light-years away from us, they see and study the objects and the universe not as it is currently but as it was when the light left it.

This is why astronomers can study the universe as it was immediately after the Big Bang, which happened about 13.7 billion years ago. They are able to study the young universe as it was then rather than how it is today.

Is the Speed of Light Attainable?

According to Albert Einstein and other scientists, the speed of light is a theoretical limit. It cannot be attained by anything that has mass. It is like the universal speed limit. This means that nothing can travel faster than light.

Is It Possible To Travel Faster Than Light

Science fiction movies like to portray warp speed, which is faster than light travel, as possible. While the idea of travelling faster than light has not been proven impossible, the idea is still extremely farfetched and scientists have differing views on this topic.

Einstein’s general theory of relativity states that as an object moves faster, its mass increases while its length reduces. If an object were to achieve the speed of light, it would have an infinite mass and a length of zero, which is impossible. Einstein’s theory therefore states that nothing can achieve the speed of light.

This theory does not stop speculations that warp speed is attainable. Some scientists are positive that people will one day be able to travel between stars just as they travel between cities. In fact, scientists at the Johnson Space Center are working hard every day conducting research and experiments to make space travel at a speed faster than that of light a reality.

One scientist who believes that it is possible to travel faster than light speed is Miguel Alcubierre, a Mexican scientist. One way he says this could be achieved is by manipulating space-time so it propels a spacecraft at faster than light speed.