What is inside a Black Hole?
They are one of the most mysterious things in space. Theorized from Einstein’s theory of general relativity, they were thought not to actually exist for much of the 20th century.
Today not only do we know they exist, but in 2019, we were able to see the first actual photograph of a black hole through a collection of synchronized network of telescopes around the world called the event horizon telescope.
Black holes are mysterious because they are utterly impenetrable. No one can ever peek inside one of these things and come back out to tell us what they saw. Black holes are like the Las Vegas of celestial objects – what happens inside a black hole stays inside a black hole.
Yet, they may hold the key to not only the formation of our galaxy, but to our understanding of the true nature of the universe itself.
Although we can’t see inside, we can calculate what’s inside. Let’s take a trip there because what lies inside a black hole just might make you rethink reality itself. What would you see as you fell in? Would everything go dark, or would you find yourself in a new universe? The preposterous fantastical trip inside a black hole is coming up right now!
First, let’s define what a black hole is. A black hole is really not a celestial body like a planet or a star. There is really no substance there other than a severely curved space-time.
It’a a region in space where matter is condensed to a theoretical infinitely small point – so small in fact that this point effectively disappears from our universe. This is called the gravitational singularity. This is like a hole in space time and is the final destination for anything that enters beyond the event horizon.
The singularity is a region where none of our equations or notions of reality applies.
So if there is nothing there, what does it mean when we talk about different size black holes? When we talk about the size of a black hole, we are really talking about the size of its event horizon. This is like a sphere around the singularity. It is completely dark because light and no other form of energy can escape beyond it. There is a hypothetical energy called Hawking radiation which does escape from the event horizon, but that is a subject for another video.
The event horizon is like the edge of a waterfall beyond which space is actually falling in faster than the speed of light.
The radius of this event horizon is called the Schwarzschild radius, and it is defined by this equation. Karl Schwarzschild calculated using Einstein’s equations that anything with mass can theoretically become a black hole if the mass was compressed enough. If you compressed earth into the size of a marble about one and a half centimeters in diameter, it would become a black hole. Similarly, If you compressed the sun to a sphere about 6 kilometers in diameter, it would become a black hole.
What happens beyond the event horizon can be ascertained to some degree by general relativity, but what happens at the singularity – the hole in space time is anyone’s guess. And there is at least one competing theory – loop quantum gravity, that says that no singularity actually exists. Something even more fantastical happens there.
Let’s explore this right now by taking a video journey to a black hole and beyond.
To do this we have to choose the black hole very carefully. All black holes are not the same. Ironically the more massive a black hole is, the less its “density” and the less dangerous it is from the perspective of its gravity ripping your body apart. This is because the Scharzschild radius doubles as the mass double according to his equation. But if the radius doubles, then proportionately the volume increases 8 times, so the density decreases quite a bit.
Very large black holes like the one at the center of the milky way called Sagittarius A* has about the same density as that of water. So if want to go inside a black hole, we should choose this one or bigger.
𝑅𝑠=2𝐺𝑀/𝑐2
If the black hole had only mass of a few times our sun, you would be torn apart, or spaghettified, long before you reached the event horizon because gravity would be so large that if you fall feet first, your feet would be pulled much harder than your head.
You would likely not get spaghettified near Sagittariu A. You could go inside probably just fine.
We will use NASA’s hypothetical IXS enterprise with Warp drive to travel to Sagittarius A. It will take us almost 3 years to get there at 9000 times the speed of light.
When we arrive, we see that its event horizon is about 24 million kilometers in diameter, which is about 17 times the diameter of our sun.
For the sake of simplicity we will presume that Sagittarius A is a non-rotating, uncharged black hole. This is also called a Schwarzschild black hole.
So if there is nothing there, what does it mean when we talk about different size black holes? Well what we are really talking about regarding size of a black hole is the size of its event horizon. This is like a sphere around the singularity. It is completely dark because light and no other form of energy can escape from beyond it. There is something called Hawking radiation which is thought to escape from the event horizon, but that is a subject for another video.
Very large black holes like the one at the center of the milky way called Sagittarius A* has about the same density as that of water. So if want to go inside a black hole, we should choose this one or bigger.
You could likely go deep inside Sagittarius A*’s event horizon before tidal forces eventually tore you apart near the singularity.
F = G M m / r 2 F=GMm/r2, and you can see that the force blows up at r = 0 r=0
A highly curious and brave member of our crew, Adam, has volunteered to enter the black hole while the rest of us watch from a safe distance away. Adam’s spacesuit by the way is made of a futuristic material that is super strong and immune to high energy radiation.
As Adam gets closer to the event horizon, we notice that he speeds up, then something strange happens, he slows down and his white space suit appears to get redder and fainter until he disappears from our view. He is still there, but the light reflecting off of him is so red shifted that it is invisible even to our infrared cameras. But if we had some futuristic technology that allowed us to view Adam at close to the event horizon, he would appear to be completely stationary, not moving at all.
This is because at the event horizon, from the perspective of the ship, time stops completely. This is due to gravitational time dilation, as defined by Einstein’s special relativity. Time slows as gravity gets stronger relative to someone not subject to the same gravity. So Adam stays completely stationary for all of eternity from the perspective of the Enterprise.
From Adam’s perspective his time is running just fine. But he is moving faster and faster, and close to the speed of light as he nears the event horizon. He may see some things in front of him that have fallen before him that are experiencing time dilation more severely than himself. But he will never catch up to them. We presume that nothing is falling behind him.
The image you are seeing now are not artists impressions but actual computer renderings based on calculations by Dr. Andrew Hamilton of the University of Colorado who generously agreed to let us show them. They are some of the most realistic available anywhere.
The event horizon appears to get much larger far faster than he expected, as the curvature of space gets severe. Around the event horizon, space is so distorted that Adam sees multiple images of the outside Universe. spacetime curves more and more severely as he gets close to the black hole. His “lines-of-light” from the stars in the Universe that surround him are bent severely.
But Adam sees nothing beyond the event horizon. It is completely dark. As he peers back, all the light reaching him is being blue shifted. In fact, as he gets closer to the event horizon, light that was infra red, not visible to him before, is now in the visible spectrum. And light that was visible before has blue shifted to x-rays, and even gamma rays. If it weren’t for his indestructible suit, he would have been incinerated from this radiation.
Since Adam is experiencing severe time dilation near the event horizon, would he be able to see the entire future history of the universe just before he enters the event horizon? Theoretically yes, but in reality no, unless he had a very powerful rocket attached to his suit and he could hover over the event horizon, then he could view this. But it would only be viewable as a small dot of light directly overhead, and it would be so blue shifted and bright that he would not be able to make out much of anything.
Why only a small dot? Because the intense gravity causes light to fall directly in the direction of the center of the black hole. Very little light from his peripheral vision reaches him diagonally. But looking at this nearly infinitely blue shifted light would damage his eyes, so he probably would want to use some other instrument.
You might ask, how is it possible that the people on the ship see Adam as not moving, but Adam is moving and doing just fine. This is one of the crazy things about reality that Black holes reveal. Both perspectives are correct. There is no timeline in our universe where we would see Adam crossing the event horizon. Yet for Adam, he is moving and about to go inside.
How can this be? That two realities are equal?
This is because the laws of quantum mechanics requires that Adam remains on the outside of the event horizon because otherwise it would violate one of its fundamental conservation laws – that information can never be lost.
On the other hand, Einstein’s theory of relativity requires that relative to Adam, nothing is different. Time for him ticks normally, and he goes straight on through the event horizon.
So does this mean that Adam enters a different universe when he crosses over. This is speculation, but that is entirely possible, because the event horizon serves as a kind of cosmic firewall that keeps two different timelines, in perhaps two different universes apart.
Let’s get back to Adam – he is now inside the event horizon. But not much changes for him. The view of the outside universe remains visible. But he can also see some parts of the universe that have fallen behind him as well.
But he cannot see the light that fell before him, and he can never actually sees the singularity because all light is headed towards it, no light is headed away from it, so the inside is likely mostly dark.
He falls with the faster-than-light flow of space-time. But time for Adam is ticking just like it was outside the event horizon.
He experiences a very weird sensation though. He still has a sense of upness and downness inside the black hole, but every direction feels downwards. Even looking up feels like looking down.
Space-time is bent so drastically towards the singularity that there is literally nowhere to go but towards the singularity. If Adam tries to fire his rockets to go back up towards the event horizon where he entered, he will actually be accelerating towards the singularity. Any effort to go in any direction will take him faster towards the singularity at the center of the black hole.
Time becomes space inside the black hole. Like time is always moving in our universe, space is always moving inside the event horizon. Inexorably moving towards the singularity. The same way that all possibilities move towards the future. All of space moves towards the singularity.
And what happens once Adam reaches the singularity? He will be ripped to shreds, and so will all the atoms in his body. He will be a soup made of all the fundamental particles that will get crushed. And what was once Adam will become one with the singularity.
At least, This is what MOST physicists think will happen to Adam inside the black hole.
Is there any way that Adam can escape this grim fate? Believe it or not, there are two theoretical scenarios in which he may actually survive, if of course his spacesuit or spaceship was strong enough.
First, if this black hole is charged, called a Reissner-Nordstrom black hole, it’s math would be similar to a spinning black hole, as most Black Holes are believed to be. In this case the singularity would take the shape of a one-dimensional ring. And this spinning singularity creates such a high centrifugal force, that space near it becomes gravitationally repulsive instead of attractive. And this spinning singularity creates an inner horizon that is a worm hole or an Einstein-Rosen bridge. Once you hit this inner horizon, you would see an infinitely bright point of light which is an image of the outside universe reflected by the repulsive singularity. Then you would be catapulted out through a white hole.
What is a “White Hole?” It is like a black hole reversed in time. Nothing goes in, everything comes out. This would mean that if Adam’s suit was strong enough (which may be unrealistic), he would be catapulted to another universe.
And in the second case, if the theory of Loop Quantum Gravity is correct, which is a competing theory to string theory, then there would be no singularity in a black hole at all. This case would be very similar to the rotating black hole, except there would be no infinitely bright light. But Adam would be transported to another universe. See my video on Loop Quantum Gravity if you want to know more about it.
All the scenarios I highlighted here are possible. But are they realistic? Probably not, because these simulations don’t take into account the tremendous amount of matter and light that is likely falling into the event horizon. What would a trip inside the black hole look like that had matter and light. Dr. Hamilton also created what is considered the most realistic trip inside a black hole available anywhere. And here is what that would look like if you were to fall into Sagitarrius A* – the 4 million solar mass black hole at the center of our galaxy:
Here’s what it would likely look like as you approached the event horizon. Notice that nothing special happens when we cross the outer event horizon, the point of no return. The tidal forces here would not be enough to rip you apart so could make it through here just fine. Then you near the inner event horizon. Notice how the outside universe grows brighter and more blue shifted. Now you go inside the event horizon. Notice no wormhole. Because something called mass inflation instability, due to the tremendous energy and apparent gravity caused by incoming positive energy and outhoing negative energy, makes wormholes impossible. And this energy also instantly vaporizes you to your fundamental particles. And you become one with the singularity. This happens pretty quick, so your demise will be pretty painless.
And how long would yours or Adam’s trip inside the black hole last? Well in all scenarios it will take only about 16 seconds. So not much time for sight-seeing.
The black hole is where quantum mechanics and Relativity collide. Gravity becomes a dominant force at the quantum scale at the singularity. I’ve been describing he singularity as a point or a ring. That’s hypothetical, because it is really a 3 dimensional boundary where relativity dies, and some kind of new physics, like quantum gravity takes over. What quantum gravity is, is probably the biggest question in physics. Just about anything can happen as far as we know. I see one scenario in which Adam come out of a white hole, and becomes Matthew Mcconaughey in the real life version of Interstellar.
The great secret that black holes may reveal to us is that there is no objective reality. Reality depends on whom you ask. It seems to be observer dependent. Ultimately Einstein’s equations may lead us to an understanding that not only is time relative, but reality itself may be relative.
