Expert Voices

Take a Fun Trip into a Black Hole: What's It Like Inside?

A simulated image of a black hole.
A simulated image of a black hole. (Image credit: NASA/ESA/Gaia/DPAC)

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of Your Place in the Universe.

You've managed to travel tens of thousands of light-years beyond the solar system. Bravely facing the depths of the great interstellar voids, you've witnessed some of the most achingly beautiful and outrageously powerful events in the universe, from the births of new solar systems to the cataclysmic deaths of massive stars. And now for your swan song, you're going big: you're about to take a dip into the inky blackness of a giant black hole and see what's on the other side of that enigmatic event horizon. What will you find inside? Read on, brave explorer.

Nearing the monster

First, we need to clear up some definitions. There are many kinds of black holes: some big ones, some small ones, some with electric charges, some without, and some with rapid rotations and others more sedentary. For the purposes of our adventure in this particular tale, I'm going to stick to the simplest possible scenario: a giant black hole with no electric charge and no spin whatsoever. Of course this is decidedly unrealistic, but it's still a fun story with plenty of cool physics to unpack. We can save a more realistic trip for another visit (assuming we'll survive this hypothetical journey into a black hole, which of course we won't).

From a distance the black hole is surprisingly benign. After all, it's just a massive object, pretty much like any other massive object. Gravity is gravity and mass is mass — a black hole with the mass of, say, the sun will pull on you exactly the same as the sun itself. All that's missing is the wonderful heat and light and warmth and radiation. But if you felt like orbiting it at a safe distance, you most certainly could.

But why bother orbiting it when you could go farther in?

The black hole itself is a singularity, a point of infinite density. But you can't see the singularity itself; it's shrouded by the event horizon, what we generally and wisely consider the "surface" of the black hole. To go farther, you must first pierce that veil.

Beyond the horizon

The event horizon isn't a real, physical boundary. It's not a membrane or a surface. It's simply defined as a particular distance from the singularity, the distance where if you fall below this threshold, you can't get out. You know, no big deal.

This is the distance from the singularity where the gravitational pull is so extreme that nothing, not even light itself, can escape the black hole's clutches. If you were to fall below this boundary and decided you had enough of this black hole exploration business, then too bad. As hard as you fired your rockets, would find yourself no farther from the singularity. You're trapped. Doomed.

But not instantly. You have a few moments to enjoy the experience before you meet your inevitable demise, if "enjoy" is the right word. How long it takes to reach the singularity depends on the mass of the black hole. For a small black hole (a few times the mass of the sun counts as "small") you can't even blink an eye. For a giant one, at least a million times bigger than our sun, you have a handful of heartbeats to experience this mysterious corner of the universe.

But hit the singularity you must. You don't get a choice. Within the event horizon, nothing can stay still. You are forever compelled to move. And the singularity lies in all your possible futures.

Outside the black hole's event horizon, you can move in any direction in space you please. Up? Left? A little bit of both? Neither? The choice is yours. But no matter where you do (or don't) go in space, you must always travel into your future. You simply can't escape it.

Inside the event horizon of a black hole, this common-sense understanding breaks down. Here, a single point — the singularity — lies in your future. You simply must travel toward the singularity. Turn left, turn up, turn around, it doesn't matter — the singularity always remains in front of you. And you will hit that singularity in a finite amount of time.

Clock's ticking.

A rendezvous with infinity

As you fall toward the singularity, you're not cloaked in blackness. Light from the surrounding universe fell in with you and continues to fall in after you. Due to the extreme gravity, that light is shifted to higher frequencies, and because of time dilation the outside universe appears sped up, but it's still there.

That's not to say it isn't weird.

Because all the mass of the black hole is concentrated into an infinitely tiny point, the differences in gravity are extreme. You are stretched head to toe in an aptly named process known as spaghettification. And what's more, you're squeezed along your midsection. This squeezing operates on the beams of light surrounding you as well, concentrating the infalling light into a bright band about your waist.

Your view of the singularity becomes grotesque and distorted as well. It's pitch black — you can't see it, because it lies in your future, and just like your future you don't know what it looks like until you get there. But instead of appearing as a tiny point, the huge gravitational differences stretch that point to engulf most of your vision.

As you approach the singularity, it appears as if you're landing on the surface of a vast, featureless, empty, black planet.

When the singularity stretches completely from horizon to horizon, then you've made it.

And what do you find there? We don't know. It would be nice if you could tell us, but like I said, nothing escapes a black hole, including you.

Learn more by listening to the episode "What happens when you fall into a black hole?" on the Ask A Spaceman podcast, available on iTunes and on the Web at www.askaspaceman.com. Thanks to Steve B., Martin N., Julius S., Joyse S., Randy W., and John W. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom or Facebook

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Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

  • rod
    Good video. I am looking forward to next week discussion. How many primordial black holes were created in the universe during the Big Bang event? My understanding, when you fall through the event horizon of a black hole, time slows down for you and you may never reach the singularity at the bottom. From your black hole frame of reference - time is running differently in the universe so the universe - evolves rapidly. I hear when you fall through the event horizon - you may never fall to the bottom of the black hole because of time dilation :)
    Reply
  • gproietti
    A question : While you are approaching to the event orizon your time dilatates asintotically to infinite, so for an external observer looks you will never reach the orizon because you will need an infinite time to touch it.
    Reversely, you, falling towards the orizon will see the universe time accellerating till infinite while touching the orizon.
    How this can match the statement in the article where is said you will see rest of the universe accellerating once you are inside the orizon ?
    Universe should appear having already reached the "end of the time" (whatever sense this definition could mean) while you where crossing the Orizon.
    Reply
  • Dwight Huth
    Im going to have to say that the center of a black hole is round.

    Take the horizon of Earth right when light and darkness meet. We can see the event horizon of the black hole much the same that we can see the horizon of Earth.

    We know that the horizon of the Earth is not flat but if we continue to approach the horizon of a black hole we know that it wraps around to the point that we started at.

    Otherwise the Event Horizon of a black hole would consume all of known space the moment it's star became a black hole.
    Reply
  • AlisterG
    rod said:
    when you fall through the event horizon of a black hole, time slows down for you
    Not quite, for you, time will appear to tick away at the same rate, while you will perceive things outside the event horizon to be sped up.
    Reply
  • AlisterG
    Dwight Huth said:
    Im going to have to say that the center of a black hole is round.

    Take the horizon of Earth right when light and darkness meet. We can see the event horizon of the black hole much the same that we can see the horizon of Earth.

    The event horizon is indeed spherical, but what lies at the centre is a mystery. Since quantum mechanics, and the introduction of the Planck length, the classical picture of an infinitely dense singularity has fallen somewhat from the realm of physics to simple mathematical terminology. A range of new ideas such as
    spin foam networks and loop quantum gravity try to describe what a black hole may be.

    As for seeing the event horizon, this won't occur unless there is a light source (which could be an accretion disk of glowing plasma) behind the black hole. One could encounter a big black hole and simply not see its event horizon (Hawking radiation is too faint), but that wouldn't make for a good Hollywood visual.
    Reply
  • tudormon
    "
    For a small black hole (a few times the mass of the sun counts as "small") you can't even blink an eye. For a giant one, at least a million times bigger than our sun, you have a handful of heartbeats to experience this mysterious corner of the universe.

    But hit the singularity you must. You don't get a choice. Within the event horizon, nothing can stay still. You are forever compelled to move. And the singularity lies in all your possible futures."

    This must be wrong on some level.
    If everything in the supermassive black hole falls into the singularity in a handful of heartbeats, then how come there is matter at the event horizon - mind you, water-density kind of matter. How about the different experience of time in the presence of such gravity? In those heartbeats you may get to see a billion years.
    At the Cauchy horizon, do you still experience time? Even more, at the Cauchy horizon determinism breaks down so whatever you think you're saying can simply be... not there.
    Reply