Gravity, the attractive force between two masses, is what keeps us close to the earth, given the gravitational effect it exerts on us. To get rid of Earth’s gravity, you need an opposite force. It is the escape velocity what’s required to make it
Newton initially wondered how the Moon stayed in an orbit around the Earth, he analyzed this and deduced it by means of an analogy with a parabolic shot, like the one you have when firing a cannon bullet; in this analogy he considered that, by increasing the firing force, it would generate greater acceleration and therefore a greater distance. So, there would then be a moment when the force could be enough for the bullet to circle the earth and reach the cannon again. And if it accelerates even further, it would cause the bullet to keep spinning around the planet; in a continuous “free fall” around the earth. This is the so-called “orbital velocity”, and it is used in aerospace engineering to launch and maintain artificial satellites in orbit; the international space station and yes, the Moon are also in continuous free fall. If this bullet is accelerated even further, the “escape velocity” can be reached, completely leaving earth’s gravitational influence. This is what was done in Apollo missions that traveled to the Moon, with the Trans-Lunar Insertion maneuver (TLI).
Newton’s theories describe the motion effects, as well as of those from gravity; but they do not describe its cause. It was Einstein who, with his theory of special relativity explained the cause for Newton’s laws of motion, and with his general relativity, those to explained gravity. And together with Einstein’s theories, certain gravitational effects that were considered weird and unlikely, were deduced.
Effects, such as time dilation, gravitational waves and black holes, were for a long time considered only interesting exercises derived from Einstein’s theories. Yet, all of them were later confirmed experimentally.
Black holes and escape velocity.
Newton’s experiment about how to escape Earth’s gravity is pushed to the limit when a black hole is considered. Black holes are a singularity in space-time described by Einstein, where the mass of an object is so large that its matter cannot withstand the force of its own gravity and collapses into itself, giving rise to a black hole, and these objects have interesting implications:
Making an analogy with Newton’s experiment, to escape from a black hole requires extreme speed, but speed has a limit, which is that of light. As described by Maxwell and Einstein, everything has a speed limit, which is the speed at which light moves, equivalent to 186,000 miles/sec (300,000 km/sec). This speed is enough to escape the gravity of a black hole… but up to a certain distance or limit. Once this limit is crossed, the gravity force is so strong that it is impossible to escape from it, even for light itself, being inexorably trapped in the black hole. This limit is called the “Event Horizon”.
If the event horizon limit is crossed, whatever crosses it remains inexorably trapped in the black hole, and this has several implications, among others:
Since the action of gravity affects space time, in the presence of a force of greater gravity, time runs slower, and as we move closer to the black hole’s center we would notice that time would run even slower; and in its center time simply stops, so from that perspective, theoretically it would be possible to witness all the development of the universe in an instant, because even if it takes billions of years, for us this time would be only a moment , since time would be stopped. And for someone who saw us from the outside of the event horizon it will seems that we would be totally still and frozen in time.
A not-so-pleasant effect, is what an object would suffer entering into the black hole; for example our body, crossing the event horizon and getting closer to its center; given the intense gravity, if we are getting closer to its center with our feet going forward; the gravity force on our feet would be much greater than that on our head, and this difference would end up ripping-up our body in half; this effect would be repeated again and again, cutting each piece into two more parts, until these reach a level where the atoms would be separated, after all this process our body would end up as a flow of atoms. This is called “spaghettification” because the body is literally extruded, like a spaghetti.
The name of black holes might seem inconsistent, because according to simulations, and now even pictures indicate that they aren’t completely “black”, this is because they are surrounded by matter that is being absorbed by them, which becomes incandescent due to the intense friction it suffers when turning around the black hole, thus causing the matter’s warm up which resulting in the emission of several types of radiation, including ultraviolet rays, rays X or gamma.
Black holes are not spaces of solace; quite the opposite, they are bodies with extremely violent effects; many of them, the biggest in particular, rotate at extremely high speed; and in the case of some supermassive black holes this speed of rotation comes close to the speed of light itself.
Gravity is by far the weakest of the four forces in nature, but this is far-reaching, and in cases such as those from black holes, can accumulate so much force that causes these strange effects. Gravity may not be the strongest and may even be considered a gentle force in certain cases; but it is undoubtedly relentless.