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In all of space, there are few things as mind-meltingly strange as black holes. These objects, cloaked in blackness, are places where space becomes time, and time becomes space, where our models of physics simply cease to work. To enter one would, of course, be fatal in a host of different ways, but I can't help find myself fascinated by them. What mysteries would you see in your final moments as you cross the event horizon before you are irrevocably claimed by the singularity? An unending black void? Another universe entirely? Yourself from the past or the future? I've heard all these claimed, but the truth might be stranger still. Forget wormholes to other universes or paradox-creating time distortions.
Within some black holes might be a surprise even more impossible our minds to comprehend. A second horizon. An event horizon beyond the event horizon. And beyond this boundary, known as the Cauchy horizon, time and space may not just swap. Physics might break down completely. That way madness lies. So, how can we not go take a look? I'm Alex Mackolgan, and you're watching Astrum. Join me today as we journey into a black hole in search of this new second horizon, assuming we can survive long enough to find it. Somewhere in a distant galaxy, a black hole is being born. This process is over in milliseconds, but does not go the way you might expect.
This star is at least 20 times the mass of our sun, and as it dies, its inner core runs out of the fuel that powers its fusion reactions, its incredible gravitational pull drags all that mass down towards its collapsing core in a devastating blow. But if we could watch this process in slow motion, we would see that as the core compresses, that process seems to slow. Time itself is slowing down, at least from an outside observer's perspective. As mass is being packed into a smaller and smaller area, which in turn is increasing the local gravity. Light is finding it harder and harder to escape and is redshifting. Its wavelength gets longer and longer, making things appear slower and slower. And the part of the
core that's closest to you seems to redshift ahead of the rest as its light has the least distance to travel. As the core size gets closer to the mathematical threshold known as the Schwarzschild radius, the time at the core for someone looking from the outside grinds to a halt entirely. And its redshift stretches to a level beyond the visible spectrum, vanishing into blackness. Although the singularity is still being formed as the core continues to shrink within, a new feature has been created in space. This is the event horizon. The point beyond which gravity is so powerful not even light can now escape from within. But from this we see that the event horizon is never truly visible.
It wears a snapshot of the star's last ever moment wrapped around it like a shroud. A millisecond of light stretched out across eternity. This is your first indicator that black holes are very difficult to know. It is thought that after this point, the core continues to shrink down into an infinitely small singularity. All of its mass packed into a single place. Our models struggle to describe infinities like this, and we can't just take a look because of that event horizon. We simply can't see past it, and nothing in localized space can go faster than light, so nothing can escape the event horizon created by a black
hole. There is no way to confirm what exactly things look like inside. Unless, of course, you go in. Entering a black hole is more than possible. We see that with Sagittarius A* the supermassive black hole at the center of our own Milky Way, which is lazily eating mass from its accretion disk at a admittedly meager rate of a few hundredths of an Earth mass a year. But, what would entering a black hole look like? Well, there's a short answer and a long answer. The short answer, you would certainly die. But, the long answer is what I want to look at today. Particularly, what lies at the end of your journey should you survive.
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And now, back to the video. Approaching a black hole is a tricky process. Black holes do not suck in the way a vacuum cleaner does, but they are very strong sources of gravity and as such approaching them can get a little intense. Matter nearby is swirled around the black hole in an accretion disk where the deeper towards the black hole you go, the faster things get and the sheer friction of this process can heat up matter to 12 million degrees Celsius. Trying to get into a black hole with an active disk will inevitably leave us fried. Not to mention all that heat and radiation will actually serve to push us away from the black hole itself. If we want to enter a black hole, best to find one without a queue.
Fortunately, within the Milky Way alone, there are an estimated 100 million stellar mass black holes, a majority of which will likely not have accretion disks. But sadly, these are not good black holes to try to enter either for one unsurprising reason. They're too small. This may seem an odd mark against them, but it becomes very relevant when we start thinking about something known as tidal forces. Let's imagine ourselves in a spacesuit heading feet first towards an ordinary stellar black hole. Let's say one 20 times the mass of our sun. As we get closer, we don't actually have to do anything to enter the black hole. As we saw earlier, the mass of a black hole warps space time around it, and
this curvature causes all free falling objects nearby to drift towards the center of that mass. In our space suit, we find ourselves gradually accelerated towards the black hole. But, like a car sliding down a steeper and steeper cliff, the rate at which we accelerate towards the black hole gets faster and faster the closer we get. At first, this is bearable. Looking around, we might sense we are moving faster, but there's no friction for us to worry about in space. So, we just see the black hole getting larger and larger as it rises up to meet us. From an external watcher's point of view, further away from the black hole, we appear to move closer and closer to the event horizon.
However, as we saw with the collapsing core when the black hole was born, the light bouncing off us as we get closer and closer to the black hole, suddenly finds it has to travel uphill against a greater and greater curvature of space time. Its wavelength stretches, which makes us look redder, and eventually blacker to the external person's perspective. To them, we would never actually reach the event horizon, and instead would simply vanish, our time seeming to have slowed down to a complete stop. Us appearing as a perpetually fading smear across the event horizon surface. From where we are, however, we are experiencing a very different story.
Relativity is weird, and it is, perhaps unfortunately, impossible to think about black holes without considering how time and space work. Despite our day-to-day experience, one person's perception of the flow of time does not always match another person's. So, while an external person watching from a safe distance never actually sees us cross the event horizon, we have no such problem. We do not have to worry about relativistic illusions like this. Our problem is that we are actually getting far too long.
The issue is the rate at which gravity is increasing the closer we get to the event horizon. The closer you get to an object, the stronger gravity from its mass becomes. We technically experience this on Earth with our gravity at our feet being 0.00007% stronger than the gravity at our head, assuming we're about 6 ft tall. This is so small a difference as to be unnoticeable. However, next to a black hole, that rate becomes much higher. How high?
Millions of times. A person 100 km above a small black hole, the mass of our sun, would experience 6 million more times the force of gravity on Earth in their feet compared to their head. Imagine trying to hold on to a pull-up bar while 6 million people grabbed your legs and dangled there. Pull-ups are hard enough already. This would not be a nice time. Every atom in your body would be ripped apart from the others, stretching you down in the direction of the black hole in a process famously known as spaghettification, caused by the tidal forces of the gravity involved.
Our journey into the black hole has ended before it even began. But actually, there is a surprising workaround. When it comes to black holes, bigger is actually better. Supermassive black holes are, counterintuitively, much safer to enter than black holes of only stellar mass, if you are thinking in terms of spaghettification, that is. This unexpected fact becomes easier to understand when you consider the direction gravity is pulling you in these cases.
Imagine for a moment that a black hole about the size of a golf ball opened up in your chest. Ouch. Well, tragically, this would be lethal. The direction the black hole would tug you would be different depending on the different parts of your body. Mass beneath the black hole would be tugged up. Mass above it would be tugged down. Compare that to a supermassive black hole with a surface area that is so large as you approach it, it is essentially flat to you.
Here, gravity is pulling you essentially the same way. So, there is no need for you to deform. It turns out that the difference between supermassive black holes and stellar black holes is astonishingly vast in terms of their tidal forces. Stellar black holes are small enough that tidal forces are powerful around them, but supermassive black holes can span millions of kilometers, which means their tidal forces only get powerful deep inside them, past their event horizons. So, while you might still spaghettify if you entered a supermassive black hole, at least you would get across the event horizon. And that is what we want to look at next. As you fall towards the event horizon, space starts to appear increasingly
warped. There are actually two main relativistic effects that are playing on you at this point. The first is your motion. Objects that move at speed see things behind them as red-shifted. As you accelerate towards the black hole, on its own, that red shift would make the external universe seem to redden and slow down, just like the external observer sees you doing as you enter the black hole. Like I said, relativity is weird. However, you are also entering a strong gravitational field, and this too has an effect on the wavelengths of light, the opposite effect. Light coming into a gravity well blue shifts.
It's when light comes out of the gravity well that the wavelengths become longer. For the external viewer watching you go in, these two effects are in alignment. You are traveling away from them into the black hole, so you red shift and light from you red shifts as it exits the black hole's gravitational field. For you, these effects fight each other. You are traveling away from the universe behind you, so that red shifts. But that light also blue shifts as it enters the gravity of the black hole. When it all shakes out, that means that from your perspective, the outside universe might not look all that
different. But as you might expect, that depends on a lot of complex factors. As you move towards the event horizon, the darkness of the black hole would rise up to meet you. Assuming you could somehow see the black hole's edges, perhaps thanks to the framing of stars behind it, it would feel like entering into a giant hole. Within, there is no light. But you have not entered the event horizon yet. At this point, you are simply at the photon sphere, the point where light traveling by the black hole enters orbit around it. Photons do not remain in the photon sphere for long as any slight perturbation sends them flying one way or another, making their orbits unstable, which is good for you as otherwise you'd be passing through a
burning firewall of photons at this point, which would have gradually accumulated to searing levels over the black hole's existence. Instead, all you see is yourself sinking into the darkness. And if your eyesight is really good and you could wait there long enough, you might just spot the back of your own head as light from your head orbits around the black hole and arrives at your eyes. Beneath here is all about angles. As the darkness rises above the horizons around you, it's still technically possible to escape. Light you shine from a torch towards the universe could still get out, but as you continue to fall, even
that soon becomes impossible. We don't know 100% what goes on inside a black hole, but according to our best models, here is where things start to get really strange, as if they weren't weird enough already. By this point, the curvature of space has reached a level where all paths now lead towards the singularity, but not because the singularity is ahead of you in space. No, the curvature is such that now the singularity is ahead of you in time. To understand this, we might need to look at a grid of 3D space. Normal space can be described like this. It is approximately flat, and we can move left or right, forwards or backwards, or diagonally through it. When you add time as the fourth
dimension, your freedom changes because in this space-time diagram, up is the direction of time, and it's only as time goes by that we move up towards the future and away from the past. Given that we are limited in this universe to a maximum speed, the speed of light, we often draw a 45-degree angle cone at our location. This describes all the locations we can go at different points in time and space. Anything within the cone we can get to. Anything outside it is forever outside of our reach. But gravity has the effect of curving this space-time. Effectively, our light cone gets reoriented the closer we get to a black hole, and once our light cone
is tilted 45°, which happens at the event horizon, there is no path we can take in our cone that will take us further away from the black hole singularity. We can stave off the inevitable, but eventually we will arrive in only one place. But look, it's thanks to this curve, the singularity is no longer just ahead of us in space. As we said, the singularity now exists ahead of us in time. As we go deeper and our cone tilts closer and closer to 90°. Our direction of time and space becomes the opposites of what they used to be outside the black hole. Remember the 4D space-time diagram? Now, moving inward toward the singularity is just as unavoidable as moving forward in time was before.
We will get to the black hole center by waiting. By moving, we will still get to the black hole center. That is inevitable, but it might take a little longer. We won't necessarily notice this within the black hole. All we will see is the universe shrinking behind us as an increasingly small ring of stars and galaxies. Around us, we would see mostly blackness. If anything is ahead of us in the black hole, we might not see it as its light would all travel inward. At this point, we can choose to simply freefall and head towards the singularity and our eventual demise, or
we can attempt to move around in this space. Perhaps push back on our arrival time at the singularity for a little longer. Given the curved nature of space-time within a black hole, some scientists have theorized that you could orbit around the black hole's edge and arrive roughly back where you started, albeit a little further back in time. This is known as a closed time-like curve and was theorized by Willem Jacob van Stockum in 1937 and Kurt Gödel in 1949. If true, it could result in scenarios where you meet your past self, which could lead to some strange conversations.
Maybe you could commiserate about why this trip was a bad idea. Of course, this opens the door to paradoxes as you could conceivably attempt to stop your past self from departing on that trip around the black hole on the closed time like curve, which would mean that you were stopped from taking the trip and thus couldn't be around to stop yourself from taking the trip. No wonder many physicists like Stephen Hawking believe or believed that time travel through such closed time like curves must be impossible and a sign that we have got the models wrong somewhere. And indeed, our models of exactly what takes place inside a black hole do tend to break down, particularly if we come face-to-face with the second
horizon within the black hole. Not all black holes would have a second horizon. When you were choosing which black hole to fall into, you would have needed to pick a black hole like a Kerr black hole, named after New Zealand mathematician Roy Kerr. Effectively, these are spinning black holes rather than stationary ones where the singularity whizzes around in a little ring shape, the only way an infinitely small point can rotate. Some call this a ring singularity or ringularity for short. The math of what happens when such black holes spin in their little loops can create a very strange effect. Namely, deep within the Kerr black holes exists a horizon beyond
which causality itself breaks down. This is called the Cauchy horizon and scientists have debated its existence almost as endlessly as your journey into this black hole has been. A Cauchy horizon occurs when light entering the black hole continues to blue shift to higher and higher levels while energy from within, thanks to curving space-time getting thrown about by that spinning top singularity, also scatters back into the Cauchy horizon from the inside. Within the Cauchy horizon, the solutions to physics equations no longer produce just one result. The rules of physics that described cause and effect, for instance, how far a ball would fly if you throw it with a certain amount of
force in a certain direction, could no longer be used to say where that ball would end up by the end. Reality could no longer be predicted based on what happened before it. The present had no impact on the future. This is true freedom and true madness. In the presence of the naked singularity within a Cauchy horizon, all rules that seem to govern physics go out the window. So, Nobel Prize winner Roger Penrose proposed a principle known as cosmic censorship, which said that if such mathematical impossibilities exist, they could not be accessible by the outside universe. Unfortunately, when it comes to black hole Cauchy horizons, there are mechanisms by which this could happen.
I mentioned that light blue shifts towards the Cauchy horizon and energy comes out at it from within. When these two things merge, the energy at the Cauchy horizon starts to rise exponentially. The effective mass of the black hole, as seen by the observer, blows up infinitely in a process known as mass inflation. Such a horizon might itself then collapse into a singularity that would destroy anything that tried to cross it. For you, traveling towards the black hole singularity, this might be your final fate. To be clear, all of this is very
hypothetical, and scientists are still struggling to understand what it all means, assuming any of it is real to begin with. And it leads to still more questions. What exists on the other side of this barrier? Or taking things one step further, beyond a black hole's singularity, could you pass the singularity itself and enter some kind of white hole leading to an entirely new universe? Penrose certainly thought that was a possibility. But such a place is certainly beyond our reach. I find it fascinating to think about black holes. They are the extreme cases of physics, where reality is bent to levels seemingly beyond its breaking points.
But the fact of their existence means that there must be an explanation somewhere. They are real. They work. Whatever impossibilities lie in their hearts, we only perceive them as impossible. And yes, they might warp time and space to such extremes that we cannot possibly hope to enter them, and time seems to freeze or reorient, and space itself rips you apart if you get too near. But one day we will understand them. And for the person bold enough to jump inside the event horizon, you will have the opportunity to study them for the entire rest of your life. Not that will be very long. Cool as they are, I think I'll stick to admiring them from a distance.
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