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GRAVITY : Newton's and Einstein's perspectives

Hello readers, the topic of discussion today is..................ya, you guessed it right. Its gravity, that I want to unveil today. In my last post, if you remember, I had promised to tell what is Gregorian Calendar. So, here we go.

Gregorian Calendar is the calendar which we are using today. Initially, months were mostly 29 days long and the average length of a month was 29.5 days which is the time taken by the Moon to orbit the Earth. However, this resulted in a year of only 354 days while the orbital period of the Earth is 365.2422 days. As a result, the calender became out of syncronisation with seasons very soon, which was bad. This was initially corrected in an arbitrary way by adding a 13th month, but soon the calender drove itself into severe confusion.

In 46 B.C., Julius Caesar reformed the calender by ordering the year to be 365 days in length and to contain 12 months. This forced some days to be added to some of the months to bring the total from 354 up to 365 days. To account for the extra 0.2422 days, every fourth year was made a leap year. This made the average length of a year to be 365.25 days.

However, as the Julian year still differed from the true year and by 1582, the error had accumulated to 10 days. So, 10 days were dropped from the year 1582 so that October 4, 1582 was followed by October 15, 1582. In addition, a modification was made that century years that were not divisible by 400 would not be considered as leap years. For example, 2000 would be a leap year while 2100 would not. This made the year sufficiently close to the actual year and this calender is called the Gregorian calender.

Now let's come to the real topic of the post. Scientists have known for a long time that gravity is unusual. Take a bunch of wooden blocks, some big and some small, and sweep them off a table; they will all fall at the same speed and hit the ground at the same time. Glue a piece of metal to each and attract them with a magnet, though, and they will move at different rates; try to pull them with a rope, and you'll have to pull harder to get the bigger objects up to speed. Why is it that gravity, and gravity alone, is able to adjust itself to pull everything towards the Earth at the same rate?

Newton was the first to coin the term 'Gravity'. It is said that Newton was inspired by an apple falling on his head. I really don't know how truth this story is. But what is amazing in this discovery is the way Newton related the force on apple to be the same force which causes Earth to revolve around the Sun. So he instated his new theory of gravity according to which every object which has mass is attracted by another object having mass. So, he predicted the orbits of all the planets, satellites and all were soon found to be correct. But with the advancement of technology, it was found that the orbits predicted by Newton were not exact. The actual orbits of planets and satellites were slightly different from Newton's orbits. For example, in a day, there is double sunrise and double sunset on planet Mercury. Because of Mercury's elliptical egg shaped orbit and sluggish rotation, the morning Sun appears to rise briefly, set and rise again from some parts of the planet's surface. The same thing happens in reverse during sunset. Why does this happen, could not be explained with Newton's theory. Another big question was: why does Black hole trap light. A Black is an object that has so strong gravitational field that nothing, not even light can pass through it. Newtonian theory of gravity says that gravitational force is between objects which have mass. If any of the two objects has zero mass, then Force would be zero. So, the question is, if light has no mass, why is it trapped into Black hole. The answer is Einstein's General Relativity.

Einstein answered this question in a revolutionary way. According to Einstein, gravity is not a force which pulls on things; rather, it is a curvature of space and time caused by the presence of a nearby massive object (like the Earth). When something comes along and moves past the massive object, it will appear to be pulled towards it, but in reality, it isn't being pulled at all. It is actually moving along the same straight line that it was moving along in empty space, but this straight line will now look like it is curved, due to gravity's warping of the underlying space-time continuum.

Space-time is a four dimensional object. Gravity's warping has got something to do with curvature of four dimensional space-time (the space we live in plus one dimension of time) rather than two dimensions (the surface of the Earth). Space and time near a massive object are curved, but we are unable to perceive this directly since we are limited to seeing things in three dimensions. Our brains therefore assume that space is flat, and in the process of making this assumption, things get screwed up. Objects which are actually moving along straight lines appear to be traveling along curves in the map we construct inside our heads, and to be pulled by the massive object nearby.

The curved space and time predicted by Einstein have some astounding consequences, many of which have been confirmed by experimental tests. Perhaps the most famous of these involves gravity's ability to bend light as it passes through the warped space near a massive object; this effect was first observed by Arthur Eddington in 1919, an event which rocketed Einstein to international fame. Eddington's original results are now considered controversial, but improved technology has spectacularly demonstrated that Einstein's prediction was correct. In recent years, astronomers have not only confirmed gravity's ability to bend light but also found very strong circumstantial evidence for the existence of black holes, objects which bend light so much that it cannot even escape.

Another major success of Einstein's theory was that it fixed some serious problems that astronomers of his day had in understanding the orbit of Mercury, the closest planet to the Sun. Some people thought that there must have been another, unseen planet (which they called Vulcan) whose gravitational pull was affecting Mercury's orbit, but Einstein showed that all the problems went away once the theory of relativity was taken into account.

There are also interesting effects having to do with the curved time predicted by the theory of relativity. This effect manifests itself by causing time to go slower near a massive object, so much so that if you watched someone fall into a black hole you would see their time stop completely, and they would appear to freeze and fade away and to them the earth would seem to be evolving faster as if someone had pressed the fast forward button.

In the 19th century, physicists interpreted the laws of electromagnetism to require a preferred reference frame for the universe, one in which light traveled. Physicists thought they would see light move slightly faster (or slightly slower) depending on how the Earth's motion through space coincided (in the same direction or opposite) with the invisible medium, or ether, in which light traveled.

In the 1880's, however, experiments by Albert Michelson and Edward Morley showed something remarkable - the ether didn't seem to exist at all! As the Earth moves around the Sun, its direction changes, so its speed with respect to the ether should also change. But when Michelson and Morley made careful measurements of the speed of light in different directions at different times during the year, they found that it was always the same.

These results have truly bizarre implications.

Imagine trying to measure the speed of a truck on a highway while driving in the lane next to it. The truck is driving a little faster than you are, so you see it creeping by - first it catches up with your rear wheel, then with your rear door. Suddenly, you decide to slam on the brakes. Instead of passing by you very fast, the truck continues to creep up - now it's in line with your front door. You hit the accelerator, and the truck doesn't fall behind - it continues to creep up past your front tire. Finally, you stop your car entirely and get out - still the truck creeps by.

It looks like the truck has been shadowing your every move, but then you compare notes with a friend who was driving in the third lane, on the other side of the truck. She thinks that the truck has been shadowing her, even though she was driving completely differently than you were - zooming along at the same time you were stopped, slowing down at the same time you were accelerating! Seems impossible? It might, but the Michelson-Morley experiment proved that if trucks behaved like light beams, this is exactly what they would do.

Many physicists looked for complicated ways to dismiss the results of Michelson and Morley, but Einstein did something different - he simply accepted them and asked what the consequences would be if light really did behave in such a bizarre way.

Einstein realized that in order for the speed of light to remain constant as seen by all observers, other things which everyone had always assumed to be constant would have to change i.e., length and time. The variation of distance with respect to different observers is called Length Contraction and the variation of time with respect to different observers is called Time dilation. These two assertions seemed to suggest that everything in the universe has its own measure of time. A moving observer has his clock slowed down as compared to one who is at rest. Even more surprise, the two phenomenon have been observed to exist. Observations of particles with a variety of velocities have shown that time dilation is a real effect. In fact, the only reason cosmic ray muons ever reach the surface of the Earth before decaying is the time-dilation effect.

May be these results seem too bizarre, but they do not produce any contradictions with other laws of physics, and in fact enhance our understanding of them. Now the question which really seems mystery is: How is light trapped by Black hole if it has no mass?

Photons (which are the particles that make up light) have zero rest mass. To understand why photons "fall" into a black hole, you need to know a bit of general relativity. What general relativity says is that any massive object warps the space-time around it. You can think of this with a simple analogy. Imagine a stretched rubber sheet that is completely flat. This represents the space-time when there is no mass. Now, if you put a heavy ball in the rubber sheet, it will cause a distortion in the sheet. This is exactly what happens in space, except that it is in 3 dimensions instead of two.

Further, a photon always travels by the shortest distance between two points. As space-time is warped, the light appears to bend around a massive object. In reality, it is not that the object is attracting light, but it is just that the photons are traveling by the shortest distance in a curved space-time.

Around a black hole, the distortion of space-time is extreme. At the event horizon of a black hole, the space-time curves into itself and as a result, light cannot escape from a black hole.