By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory
There are hardly any astronomical bodies that are as misunderstood as the black holes. From notions such as black holes are remnants of gigantic stars which collapsed in a cataclysmic supernova to black holes are powerful cosmic monsters that gobble anything in their gravitational field, we hear a lot of stories. Some of these are true while others are myths. Let’s examine two of the greatest myths about black holes.
Do Black Holes Grab All Matter Near Them?
It is a common misconception that black holes grab all matter near them. To answer this, we need to understand that the event horizon is a sphere with a special radius around a black hole from which nothing can escape, not even light. And this special radius is known as the Schwarzschild radius. Based on the distance from the center of the black hole, the regions near the black hole can be divided as the distances much larger than the Schwarzschild radius, distances close to but outside the Schwarzschild radius, distances inside the event horizon but not close to the singularity, and finally areas near singularity.
The closer someone is to the black hole, the greater would be the speed required to escape the massive gravitational pull. However, far outside a black hole gravity would have little effect. To illustrate with a hypothetical example, if the sun were to be swapped with a black hole of exactly the same mass, it would have no effect on the rest of the solar system. The time taken by the Earth to orbit the black hole would remain unchanged and the rest of the planets, asteroids, and comets in the solar system would also experience no change. Black holes hence are, after all, not the ravenous monsters that gobble everything around them. At large distances, they are just heavy stars that don’t emit light, contrary to common belief.
Learn more about why black holes get such a bad rep.
Falling into a Black Hole and Spaghettification
As we get closer to the event horizon, the gravitational pull becomes stronger. In such circumstances, Newton’s law of gravity ceases to be accurate and the Einstein’s theory of general relativity comes into play. According to the general relativity theory, time slows down in strong gravitational fields. So, it is expected that falling near event horizons will slow down the clock.
However, the slowdown of time depends on the position of the person. In this case, one of them is far from the event horizon while the other is approaching the event horizon. The person far away from the event horizon will see the other person move slower and slower, frozen forever at the event horizon. But the person falling into the singularity at the center of the black hole would not feel any difference in time. This would be just the same as in the case of special relativity where a fast-moving person experiences a slower time, but the slowed-down person would just feel that time was normal for them. Theoretically, the effects of general relativity are defined for tiny objects, i.e., objects with zero size. However, that doesn’t mean they are not applicable for real objects and real gravitational fields.
We know that he effect of gravitational field experienced on the head and feet are different on the Earth. Similarly, at distances greater than a kilometer above the surface of an event horizon, the force exerted on the head would be much greater than that on the feet. Mathematical calculations prove that the difference in the force exerted at a distance of just two meters is as much as ten million tons. So, the strength of the gravitational pull on various parts of the body would differ as in the closer parts get pulled harder than the ones further away. A living being falling into the black hole would actually experience a force that would rip the body apart in a process called spaghettification. Thus, a person who falls into a black hole may not feel a difference in time but would experience a very violent fall.
This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, Wondrium.
The Singularity Misconception about Black Holes
Wormholes are two black holes that are attached to each other somehow. The idea that someone could fall into one and come out of the other is another misconception. As we have already discussed, falling into a black hole stretches and tears apart the body. So, the person who fell into the black hole is not going to survive the fall, let alone coming out of the wormhole.
Theoretically speaking, farther inside an event horizon, gravity will be much stronger. Applying Einstein’s theory of relativity all the way down to singularity and assuming that the person falling into the black hole survived spaghettification, the gravitational pull experienced would be much stronger. However, as always, the general relativity applies to zero size, so for a person falling into the event horizon, body parts closer to the center of the black hole can’t interact with parts further away. So, while approaching a black hole, there is no escape but things are different at the center of a black hole.
It is a myth that the center of a black hole is the singularity, a place with infinite density where all of the mass of the hole is concentrated into a sphere of zero mass and radius. This misconception originates from the equation of general relativity but when equations cannot connect the world around us, they need to be discarded. As we get closer to the center, gravity keeps increasing; mathematically approaching infinity. But, practically it is not possible to have infinite energy as this is against nature. General relativity predicts gravitational waves when moving massive objects emit energy. These waves are emitted in quantum situations and as a result objects would lose energy. The concept of loop quantum gravity contradicts the common belief that space and time can be forever subdivided, meaning as per this theory there is a smallest bit of space and a shortest duration of time. So, finally, no one knows in detail what happens at the center of a black hole. Probably, it is bubbling, simmering bits of space and time but definitely not singularity.
Learn more about is there a theory of everything?
Common Questions about Busting the Biggest Myths about Black Holes
A black hole is a place in space where matter is compressed in a tiny space and the gravitational pull is so strong that even light cannot escape.
Apart from the most common misconceptions that black holes have a singularity at their center and that they are monstrous cosmic bodies that will capture anything in their gravitational field, it is also a myth that black holes are eternal and act like cosmic vacuum cleaners and spit out of white holes.
The stretching of objects when they experience extreme gravitational force is called spaghettification. To graphically elucidate, the human body will be stretched out like spaghetti inside a black hole before being torn into pieces as there is a limit on the force that can be tolerated by the human body.