Light travels at about 186, 000 miles per second. Now, by looking at this number you may conclude that light travels really fast. Sure it does compared to the fastest that we can go or anything that we have ever made can go. But if you look at how long it takes light to travel distances you probably won't think it's so fast anymore. The distance from Earth to the Moon is about 239, 000 miles. That means that light takes 1.
2 seconds to travel that far. Yeah that is still fast you may say. But light takes 8. 5 seconds to travel from the sun to the Earth. That is still fast compared to how fast we can drive or run. But light takes 100, 000 years to go from one side of the Milky Way Galaxy to the other side.
That is a very long period of time. If someone wanted to get from one side of the galaxy to the other they would have to be going about 1000 times the speed of light to make it in one lifetime. And that number is correct only if they were to live at least 100 years. It took humans a long time to calculate the real speed of light.
Different scientists tried many times over many years and most of them were way off with their calculations. The first real measurement of the speed of light came about in 1676 by a Danish astronomer, Ole Romer while he was working at the Paris Observatory. He was studying one of Jupiter's moons, Io at the time. It was eclipsed by Jupiter at regular intervals while it went around Jupiter at a steady rate.
But after several months, Romer found that the eclipses lagged more and more behind the expected time, until they were finally running 8 minutes late and then over time it changed all the way to 8 minutes early. After observing and studying the pattern Romer concluded that the reason for this was because sometimes the Earth was farther away and it took longer for the light to reach the earth at that time. He then concluded that light took about 22 minutes to cross the earth's orbit. This estimate was a little bit high because Romer had underestimated the speed of light.
To accurately find the speed of light what was needed was the distance from earth to the sun. Many attempts were made to measure the parallax of Mars, or how far it shifted against the background of distant stars when viewed simultaneously from two different places on earth at the same time. This very slight shift could be used to find the distance of Mars from earth, and hence the distance to the sun, since all relative distances in the solar system had been established by observation and geometrical analysis. According to Crowe, the distance to the sun from the Earth was somewhere between 40 and 90 million miles. The correct value for this distance is 93 million. Once again the scientists were wrong.
Now Romer estimated the speed of light to be 125, 000 miles per second. That is pretty far off the real value of 186, 300 miles per second. The reason for this error is because Romer said that it took 22 minutes to go across the earth's orbit but it really took 16 minutes. The next substantial improvement in measuring the speed of light took place in 1728, in England.
An astronomer named James Bradley was sailing on the Thames with some friends and noticed that the little pennant on top of the mast changed position each time the boat put about, even though the wind was steady. He thought of the boat as the earth in orbit, the wind as starlight coming from some distant star, and reasoned that the apparent direction the starlight was "blowing" in would depend on the way the earth was moving. Another possible analogy is to imagine the starlight as a steady downpour of rain on a windless day, and to think of you walking around a circular path at a steady pace. The apparent direction of the incoming rain will not be vertically downwards. More will hit your front than your back. In fact, if the rain is falling at, say, 15 miles per hour, and you are waling at 3 miles per hour, to you as observer the rain will be coming down at a slant so that it has a vertical speed of 15 miles per hour, and a horizontal speed towards you of 3 miles per hour.
Whether it is slanting down from the north or east or whatever at any given time depends on where you are on the circular path at teat moment. Bradley reasoned that the apparent direction of incoming starlight must vary in just this way, but the angular change would be a lot less dramatic. The earth's speed in orbit is about 18 miles per second; he knew from Romer's work that light went at about 10, 000 that speed. That meant that the angular variation in apparent incoming direction of starlight was about the magnitude of the small angle in a right-angled triangle with one side 10, 000 sides longer than the other, about one two-hundredth of a degree.
So he found the speed of light to be 185, 000 miles per second. The next big step toward finding out the speed of light with great accuracy was made in 1850 by two rivals, Fizeau and Foucault, using only slightly differing techniques. In Fizeau's contraption a beam of light shone between the teeth of a rapidly rotating toothed wheel, so the "lantern" was constantly being covered and uncovered. Some distance away from the toothed wheel Fizeau placed a mirror, which reflected the beam back, where it passed a second time between the teeth of the wheel. The idea was, the blip of light that went out through one gap between teeth would only make it back through the same gap if the teeth had not had time to move over significantly during the round trip time to the far away mirror.
Foucault's method was based on the same general idea, but instead of a toothed wheel, he shone the beam on to a rotating mirror. At one point in the mirror's rotation, the reflected beam fell on a distant mirror, which reflected it right back to the rotating mirror, which meanwhile had turned through a small angle. After this second reflection from the rotating mirror, the position of the beam was carefully measured. This made it possible to figure out how far the mirror had turned during the time it took the light to make the round trip to the distant mirror, and since the rate of rotation of the mirror was known, the speed of light could be figured.
Both of these techniques gave the speed of light with an accuracy of about 1, 000 miles per second. After humans found out the correct speed of light we then tried to expand our knowledge on the speed of light and if we could ever travel it. Many people still debate today whether or not we can ever travel at a speed of light or even close to the speed of light. Many scientists have different theories regarding us going the speed of light. Einstein's Theory of Relativity is probably the most well know theory about the traveling the speed of light. The first thing that the Theory of Relativity tells us is that light always travels at the same speed relative to some observer, no matter what the relative motion of the observer is.
Take an airplane for example: The light emitted from a moving airplane does not travel with the speed of light plus the speed of the airplane, it travels with the "speed of light", no matter what the speed of the airplane. This is unlike other things that happen all the time. For another example take a car that is traveling at 60 miles per hour down the highway. You are sitting in the car with the window rolled down and a ball in your hand.
If you were to throw the ball out the window at 15 miles per hour then it would really be traveling at 75 miles per hour because the car is moving too. This is basically the exact opposite of what the speed of light does. The next part of the Theory of Relativity is about traveling the speed of light. According to Einstein's Theory of Relativity nothing can exceed the velocity of light or even come close. As the speed of an object increases it's mass also increases.
So if an object is going 50 percent the speed of light its mass is much bigger than it usually is which means that the object would need even more power to go faster and so as it goes faster it's mass increases and it needs even more power. By the time an object reaches the speed of light its mass is infinite and to go any faster you would need infinite energy, which does not exist to our knowledge. Another part of the theory deals with time and what happens when the speed of light is achieved. If someone is going the speed of light slows down greatly for them and the rest of the world stays the same. So if I was going the speed of light for 5 years when I returned to regular speed, all of my friends may have aged about 20 years. When you are going the speed of light your pulse, blood movement, and everything else slows down which makes you go slower.
Now if the speed of light was reachable then we may be able to achieve time travel. It is still being debated but there are ways that could work providing we could go the speed of light. If we could travel back in time then there are many things that could happen that are bad. If you went back in time 300 years and killed your great-great-great-grandfather then you would cease to exist and no one knows what would happen then.
But we don't have to worry about any of that unless somehow we are able to reach the speed of light. Right now we do in a way have a way of seeing the past. The farthest away object in the universe that astronomers can see is about 18 billion light years away. So when we look at it now we are really seeing what it looked like 18 billion years ago. So if we were to observe a star in the sky it would be really what it was years back but while you are looking at it may not even be there. You would just be seeing a star that doesn't even exist.
All of this time travel talk is most likely never going to happen but it is pretty neat to dream. Many scientists have devoted their entire life to the speed of light and the result is what we know today which isn't even that much. There are still so many questions that we want to know the answers to that can never be answered. And some will always remain a question with no answer.