An Introduction to General Relativity

Hey! you there don't run away. I wont scare you with big equations or extremely theoretical stuff. Don't worry. Today I will discuss some fun facts about what most people like calling their nemesis, General Relativity. Remember, everything is easy just after you know what it is.

Lets start with Newtons laws of gravitation. Newton said that "If we have two masses separated by a distance R, then the force acting between them is denoted by 

where G is the universal gravity constant, M1 and M2 are the respective masses, and R is the distance between the two masses".

 

So when we are standing on earth. Our distance from the centre of the earth is R(which is constant). The mass of earth is constant and so is G. The only thing which is unpredictable is our mass (because of our periodic feeding frenzy, our mass keeps dangling). So if we substitute the respective values we see that this 

part of the equation becomes constant. This constant is commonly known as g=9.81m/s^2. Hence giving us the equation F=Mg.

This means, when I am standing on the surface of the earth, I am experiencing a force which is equivalent to my mass times acceleration due to gravity. So even if I don't actually move there is a force acting on me. 

This led Einstein to consider what was called as the Equivalence Principle.

What he said was, If you were in a box free falling under the action of gravity g, inside that box you would feel weightless. This was because the relative motion of the box and you are the same (i.e. the box and you are falling with the same speed). On the other hand, if you were in outer space, where there is no gravity, you would feel the same within the box. Hence the free falling box on earth is related to the box in outer space. 

Now, he went further.

He even said that, if you were in a box on earth stationary on the surface. You would feel force similar to the person who is in outer space travelling with an acceleration of 9.81m/s^2.

Technically speaking, this is not exactly true. This is where Tidal Forces come in. 

We all know from the formula, stated above, that the force acting on a body due to another body depends on the distance between them. In fact, when you are on earth's surface, the force experienced by your head is slightly less than the force experienced by your feet, because the earth is closer to our feet than to our heads. This causes us to stretch a little bit. This difference in forces between our heads and feet is called tidal forces. The tidal forces on us on the surface of the earth is very small because of our small size. But the tidal forces of the moon does make a difference on our oceans.

As we can see in the above diagram, R>r (i.e. the distance R is significantly greater than r). Hence the effect of gravity will be more on part A than at part B. Hence causing high tide at A and low tide at B. 

This is what causes high and low tides. 

Phew, that was quite something wasn't it? Well I wont saturate you with more physics. Stay tuned for my next post where I will shed light on a few more fun facts on relativity. Cya, and as always, criticism is welcome.