If you were to go into your regular high school physics class, and you were asked by your teacher what is gravity, your answer would most likely be that it is the force one mass asserts onto another, with the force being proportional to the masses. And if this question were on a test, you would have to better answer it this way, or else that’s one point off. This is Newton’s definition of gravity.
In my last post, I said that I would be showing why Newton’s definition of gravity is wrong. So one might ask, if it is wrong, then why do they even teach it? Well, my answer is that it is not entirely wrong. The calculations and formulas under Newton’s theory of gravity are actually pretty accurate most of the time, and thus we use them. But notice the phrase “most of the time;” that is because there are things that Newton’s gravity cannot account for.
One of them is Mercury’s orbit. Although Newton did not know it at his time, in the 19th century, when advanced telescopes had been made, it had been observed that there was a small discrepancy in its orbit. Under Newton’s theory, the only way to have explained this perturbation was that there was somehow a planet closer than Mercury to the Sun that existed. However, no planets have been or will ever be found that are closer to the Sun than Mercury. Therefore, something was wrong with Newton’s gravity in this case.
Another problem was the fact that black holes could suck in light. Light is made of photons, and photons have no mass. However, according to Newton’s theory, black holes could only assert a gravity onto another mass. However, photons again have no mass, so the fact that photons could still be sucked in is contradictory to Newton’s gravity.
Overall, another theory of gravity was needed. And that’s where Einstein’s theory of gravity came in, something that you don’t usually get taught about at high school. To explain Einstein’s theory, one must use an analogy. Suppose there is a blanket, that stretches across infinitely, with no limits and boundaries. Now, suppose I were to put a rock in the middle of this blanket. You would probably notice that this rock creates a depression in the blanket as seen below.
Now visualize this: suppose I were to fling a small marble into this depression. The ball would simply go round and round and round in the depression. Now if this were in the real world, the marble would eventually stop moving, due to friction and resistance. However, remember that this is space, in which there is no resistance whatsoever. So if it were in space, this marble would actually never stop, unless acted upon by something else.
With this in mind, suppose now that this big rock is the Sun and the little marble is Earth and this blanket is the fabric of space-time. Under Newton’s definition, the Earth is rotating because the thrust/tendency of the Earth to just speed away is counter-balanced by the force of the Sun’s gravity. Thus, it must stay in one orbit. However, under Einstein’s theory, the real reason why the Earth orbits the sun is because it is simply following this depression, or this curvature of space-time. In essence, gravity is the result of the geometry of space-time (as opposed to what Newton thought- that gravity was just this instantaneous force). The video below helps illustrate this concept:
This theory of Einstein does not totally reject all of Newton’s ideas, but rather explains it in a different way. For instance, let’s look at the case of elliptical orbits. Suppose there was a star and an orbiting planet. If we were to follow Newton, the reason such orbits occur is because at some points, the star’s gravity is stronger, pulling the planet more in, and at some points the star’s gravity is weaker, letting the planet be more far out. Thus, the end shape is an elliptical orbit. (In an ellipses, some parts are closer to the center than others. And if you don’t know what an ellipses is, it is pretty much an oval.) With Einstein, elliptical orbits were the result of the orbit of the object that is being orbited. Take the example of the Earth and Moon. We all know that the Earth orbits the Sun. So while the Earth is orbiting in this depression caused by the Sun, so is the Moon orbiting in this depression caused by the Earth. However, as the Earth is orbiting, the Moon has to move along with it while orbiting all at the same time. To see an illustration of this, go to time 0:29 in the the video above and play it. Notice that the depression of the Earth is constantly changing, due to it orbiting the Sun. Since the orbit of the Moon is based upon this depression, its orbit will not be perfectly circular, but rather elliptical.
Another example of how Einstein’s theory is just explaining gravity in a different way than Newton’s theory can be seen in mass. Newton said that the bigger the mass, the bigger the gravity, and he used a formula to show this. Einstein, on the other hand, showed that the reason this is true is because the bigger the mass of an object, the bigger and deeper the depression in this “blanket” of space-time, and thus it is more liable of things to fall into this depression, thus giving the impression that the object’s gravitational force is strong. It seems as if this object is pulling more things to it because of it’s bigger mass, when in reality it’s just that this depression is really big and deep so more objects are rolling around in this depression.
And speaking of strong gravity, let’s go back to the black hole I mentioned earlier. You have already seen that Newton’s theory doesn’t work here. Now I will show how Einstein’s theory works. First of all, black holes have gravity that is exceedingly high. Why is that? Well, just as I compared a Sun to a rock, compare the black hole to a bowling ball. Not only is it larger, but it is also much more heavier. The depression will be extremely deep; thus, anything that goes near this bowling ball aka black hole will have little chance of “bouncing” out of this depression, and will easily slide in given that the deep depression gives a steep incline for it to fall. In a sense, this depression is not a depression anymore but a super deep hole where it is very easy to fall into. All of this can explain the property of why so few things can escape a black hole.
If we look at black holes this way (in other words Einstein’s way), we can see that the fact photons have mass or not doesn’t matter. The photons will slide in no matter what into the black hole because they are simply rolling along the curvature caused by the black hole. That’s all. As you can see, this is much more efficient, given that Newton couldn’t have explained this using his theory.
I will conclude my post here. Of course, there are much more other things that can be examined, but I believe that I have covered at least the basics and in fact a little bit more. But I do hope that your mindset to physics has been broadened. Let me tell you, the physics out there can get more interesting than even this.