Is it real?

According to the general theory of relativity by Albert Einstein, gravity is nothing more than an imagination or illusion defined by us. Let’s debunk the theories or prove ourselves wrong in this piece of writing. Albert Einstein once said that one of his most satisfying works was his thought of the man falling off of a roof (he was not a sadist though). His observations were quirky and appealing. He observed that when a man is falling from the roof, he wouldn’t feel his own weight and that while falling if he drops something that he was holding or observed something that was falling along with him, the items would be stationary with respect to him and that they will fall with same the speed as the man.

Seeing it the way Einstein saw it

Try imagining that you were in a space shuttle in the outer space. Since there is no noticeable gravity there, you will feel weightless ,and anything that you exert force on will continue to move with a uniform with velocity in a straight line indefinitely.

The fun fact is that you will perfectly suffice the soul boundaries of inertial observer, i.e.

• No acceleration
• No gravitational field
• All the laws of physics are applicable

Hence, no way to distinguish your inertial frame from any other. Here comes the climax. Einstein saw these two scenarios to be exactly the same, not only equivalent on papers, but also in physical terms. Stating this, he said that man falling from roof is under no gravitational field and that there is no gravitational field! He said that the man is not accelerating and that he is an inertial observer like the man in outer space.

Woah, what?

Wait, hold up! Are you crazy boy? The man is clearly falling from roof under the influence of gravity and that he is accelerating with 9.8 m/s². With every second passing by, the man would crash harder into the ground, making him realize that things like gravity are real!

I know that these two scenarios might seem way different, but Einstein’ Equivalence Principle points out the portion we need to look out for- The Experience Of The Observer. It is the restatement of the fact that if they feel weightless, then they surely are in the inertial frame of reference.

Now, let’s say that the space shuttle (which we talked about earlier) comes near a planet or any such planetary mass (with appropriate mass). The straight path of the rocket would be disturbed and it will change its course slightly towards the planet. The fun fact is that for the man inside there is no force experienced by him even though the shuttle would be moving with an increasing velocity as it moves closer to the planet. The man in it would be unconscious of this fact. The man still feels ‘weightless’ and for him there is nothing erratic about the journey. Even the accelerometer wouldn’t budge a bar and that the man wouldn’t notice until he crashes onto the planet (his frame is inertial up to the point he crashes). So, can you tell when the switch of frame from inertial to non-inertial happened.

But wait… So

How do you explain the deviation of path in first place?

Are you out of your brain cells? The rocket obviously swayed from its path due the gravitational field of the planet. (or not… maybe… let’s see.)

Here comes the fun, Einstein once explained the phenomena of ‘curved space-time’.  Initially, let’s concentrate on what the man in space shuttle felt – movement with constant velocity in a straight line. As he went nearer to the planet, the path curved, or did it? The crux lies in the fact that he was still in a straight line of the space-time plot, and that near such masses, the space-time graph is curved.

This is the sole reason his path appeared curved to a distant observer.

As Quirky as it could get

You can imagine it in an easier way. The plane tries to accomplish the journey with the shortest route possible, moving in straight line on the globe, but since our planet is a sphere (I know it’s geoid, but let’s keep it simple here), the path appears to be curved when seen on a 2-D representation of earth (basically on world map, written in a complex way to make the article seem nicer.)

These curved paths are called ‘Geodesics’, and the same notation is used to name the curved path followed by the shuttle in the inertial frame of reference. Imagine another scenario. You and your friends, who are 1000km apart in equator travel to north pole in a straight line.

You both will eventually bump into each other at north pole, coming closer as if there was a force acting on you, but you didn’t feel it at all. Gravity is analogous to that force. It doesn’t actually exist!

People think that the bent sheet experiment can explain the special theory of relativity, but it is pretty much misleading instead. It makes us resemble a lighter body falling in the space-time well created by a heavier body, but what we should be thinking is a bit different. It is due to the fact that the space-time is curved around heavy masses that the body starts moving circularly, and not because it is falling into the well.

The quotes by John Wheeler best define the situation, “Matter tells Space-Time how to curve, and the Space-Time tells matter how to move.”

Going Shallow:

Imagine the space shuttle once again. Let’s say that the shuttle starts acceleration with an acceleration of 10m/s². To a distant observer it would seem as if the floor of the shuttle is accelerating into the objects inside it which would presumably be stationary to the observer. Different is the case with the one inside, the observer inside the shuttle would feel a force against that accelerating floor. Isn’t it similar to what we feel on earth? Let me tell you the answer… it is.

As a reader, do you believe that you are an inertial observer, or that you are in an inertial frame of reference? No. The situation you face, or I face is pretty much like the one faced by an observer on that accelerating shuttle, and by pretty much alike, I mean exactly the same, gravity doesn’t exist and you are or we are accelerating! Wrap your head around as it is going to get even crazier.

What The Force!

According to the Newtonian Physics, each body faces an equal and opposite normal force in reaction to the one applied by the gravitational acceleration. The theory of special relativity defies it. According to theory of relativity, there is no such force by the gravity and that there is only a normal, which accelerate you upwards. But I am not moving up or going anywhere around? So, let’s say for a moment that you were moving around due to this acceleration, but relative to what? Everything else around you, say table, fan, floor etc. are in the same frame of reference and that is not inertial. (Relative to everything in the train, you are not accelerating while sitting in it.) So who can see this acceleration? Remember that guy who was falling from the roof, he was in the inertial frame of reference and he could see you moving up with an acceleration of 10 m/s².

So if everything is moving up with us, am I implying that the earth is expanding? No, absolutely not. Here comes the theory of relativity to fill the loop-holes, according to which, it is possible that you can accelerate without changing your spatial co-ordinates. According to the double derivative equation of position for space-time, you need to accelerate, just to stand still in curved space-time.

All of this must be mind boggling, and yes Newtonian physics is easier, but this theory of relativity can explain one of the most basic questions that why all objects fall with the same rate. The way of understanding this was that the gravitational force equals the external force (force = mass*acceleration).

This way we cancel out the mass of the body from both sides of the equation as you can see and the accelerations remains the same. The twisting thing here is that the mass on the left side of the equation, is the gravitational mass (mass that leads to the experience of a gravitational field), while on the right is the inertial mass, then why these two conceptually different properties are numerically equal.

With years of experimenting, scientists have proven that these two types of mass are really the same. Here is the thing about the general relativity theory, there are no such mysteries, all the things fall the same way because they are just following a straight line in the space time and not accelerating.

Einstein really had something wrong in his head. This was as much shocking in 1915 as it is to you guys now. Being a super-human as he was, he came up with a measurable prediction to prove his works. He claimed that if we shone a lit ray from one end of the shuttle to the another (while shuttle was accelerating), the light must bend (the photon leaving at this instant would reach after a certain time to the other end, making up to only the lower point on the other side, hence not a straight line). Although this observation was really microscopic, but it baffled scientists. This showed that an accelerating frame of reference will bend light! Pretty cool, isn’t it ?

So, Einstein reasoned that light must also bend while it passes a large mass. Does it ring any bell? (supposedly with the name ‘black hole’ it might) Well we can talk about it in the next article.

What’s Next with theory of relativity?

The theory of relativity has answered almost all the virtually asked questions, but there are some which haven’t been answered, which aren’t yet logistically feasible to carry out. Let’s talk about one, and let others know in the comments what you think (who knows, maybe you are the one to crack this later). We know that accelerating charges radiate electromagnetic radiation. So here is one conceptually simpler test – comparing the behavior of a stationary in a gravitational field to a free-falling one. According to the theory of relativity, the free-falling charge is moving in a straight line and hence, no acceleration is there; while the one that is stationary must be acceleration to be standing still, and hence it must radiate radiation. So, what do you think will radiate electromagnetic radiation, the stationary one or the free falling one?

So, what do you think? Is gravity an illusion?

Report By: Sashit Vijay