Why it is that of all the billions and billions of strange objects in the Cosmos -- novas, quasars, pulsars, black holes --

youare beyond doubt the strangest?-Walker Percy

When you watch someone fall into a black hole, what you actually *see* is pretty surprising. You see, a black hole's gravity distorts the space around it, and it does so *without* providing any light of its own, giving you a unique perspective on the Universe.

Well, if you watch someone else fall in, you'd see them approach the black hole normally, and then the bizarreness starts. As they go deeper and deeper into the gravitational field of the black hole, a few super bizarre things all start to happen simultaneously.

- The light coming from the person gets redshifted; they'll start to take on a redder hue and then, eventually, will require infrared, microwave, and then radio "vision" to see.
- The speed at which they appear to fall in will get asymptotically slow; they will appear to fall in towards the event horizon at a slower and slower speed, never quite reaching it.
- The
*amount*of light coming from them gets less and less. In addition to getting redder, they also will appear dimmer, even if they emit their own source of light!

But if you think that's bizarre, here's where it gets really weird: the person falling in notices *no difference* in how time passes or how light appears to them. They would continue to fall in to the black hole and cross the event horizon as though nothing happened.

What would you see if *you* fell into this black hole? Luckily, Andrew Hamilton and his group at Colorado have created a video (and an *accurate* video at that) to illustrate this:

- Tearing your extremities (head, arms, legs) from your torso,
- tearing the individual muscles, tendons, ligaments, etc., apart from your body,
- tearing individual cells apart from one another,
- tearing the organelles inside each cell apart, destroying cells themselves,
- tearing the individual molecules apart into atoms,
- tearing your atoms apart into nuclei and electrons, and finally
- tearing the individual nuclei apart into, eventually, quarks and gluons.

Fun stuff, yes? Perhaps someday, "death by black hole" will be commonplace, although it will take an infinite amount of time for you to see someone else experience it!

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Owie.

A few years back I was bored and did an order-of-mag calculation of the tidal acceleration across your body as you fall feet-first across the event horizon of a SMBH. I recall it being pretty small...

Plugging the Schwarzchild radius into the tidal accel eqn,

a_tidal = dr * c^6/(2GM)^2,

already showing that for big M, the accel is small. So for dr=2 meters (your height), and M=4*10^6 Msol (the Milky Way SMBH), you get

a_tidal ~ 10^-3 m/s^2 ~ 10^-4 g

which is about 1000x bigger than the tidal accel on the surface of the earth from the moon (~10^-7 g), but still *VERY* small!

For comparison to a stellar mass BH (M=3 Msol), a_tidal ~ 3*10^8 g.

However, I'm not really sure if this is all GR-valid. Anyone care to comment/correct?

So they don't see the external universe evolve at an ever-increasing rate into the future?

Also what about Hawking radiation? If an external observer requires infinite amount of time to see something cross the event horizon but the black hole evaporates in finite time, wouldn't an infalling observer just end up blasted to bits by the extremely energetic death throes of the black hole rather than cross the event horizon?

I don't approve of death by BH, but I'd certainly consider it for my funeral.

@andy: see http://en.wikipedia.org/wiki/Hawking_radiation

The power(energy/time) released by a BH through Hawking radiation is about P ~ 10^-28 Watts (M/Msol)^-2. Very small for a stellar-mass BH (P~10^-29 Watts), very very very small for a SMBH (P~10^-41 Watts). For a rather large micro-BH (~mass of the moon) it's still pretty small (P~10^-13 W) so I don't think you'd have to exactly worry about being "blasted to bits" on your way in.

If that "time stop" effect were true, we could never observe a black hole growing, it would never swallow anything. In fact, all the matter a black hole absorbs would - to our view - still hang around on the event horizon. Obviously, that's NOT what happens.

Instead, you'll get shredded to pieces and your matter will form a very faint accretion disk around the BH, spiraling downwards and then eventually passing into the horizon. Before you enter the event horizon, you will be accelerated very harshly, causing your emitted photons to immensely redshift. Once you pass through the event horizon, you will move away from the outside universe at or greater than the speed of light, effectively becoming invisible. Your mass will then be added to the black hole.

Brandon,

Ironically, the tiniest black holes have the largest tidal forces at their event horizons. If you're math-inclined, the size of a black hole rises in direct proportion to its mass, but the tidal force falls off as (1/distance)^3.

DontThinkSo,

There is some correct physics in what you've said mixed up with some very incorrect statements. The thing that you say "obviously" doesn't happen is, in fact, what we would see. But we don't have an infinite number of photons; we have a finite number spread out over an infinite time, so the actual photon density drops to zero. Once that happens, it appears to you as though the black hole has grown by the mass that fell into it.

The formation of an accretion disk is not a necessity, and is highly dependent on what your angular momentum is. Your acceleration towards the event horizon has almost nothing to do with the redshifting of your photons; that is a gravitational effect. And inside of a black hole, nothing moves faster than the speed of light, just like outside of a black hole.

DontThinkSo: Say you dropped a watch into a black hole from a safe distance. In your rest frame, as the watch approached the event horizon, it would approach the speed of light. The rate at which the watch was ticking would approach 0. It would never reach the horizon, it would just keep falling faster and faster and ticking slower and slower.

But you can't see the matter that's "hanging around the event horizon" because it is moving away so fast it appears very dark and red shifted.

Even stranger: If you toss someone in to a black hole, then follow them in, it still appears to take an infinite time for them to reach the event horizon.

Jonathan,

Crossing the event horizon occurs at infinite coordinate time in the Schwarzschild solution, but that's just a coordinate singularity. I wouldn't place too much significance on it. I think you'll see your friend at the horizon after a nice finite (proper) time, it's just that when you do you'll be at or inside the horizon yourself.

Now, if you're hanging around just outside the black hole it's a different story. You won't see your friend go in in that case. But there will also be other differences - high acceleration to keep from going in, and a blue-shift to the incoming light.

Can any object in this universe actually fall through the event horizon? If there is enough mass to form a black hole then light can't escape because the escape velocity exceeds the speed of light, so shouldn't anything trying to enter from the other side also try to accelerate past C? I should think all the infall, because general relativity, if I read it right, dictates that an object gets shorter in the direction of travel. At the very extreme of this phenomenon anything should become two dimensional, and become part of the shell that is the event horizon. Forever falling into the singularity. Now please chew me up and spit me out so this will stop bugging me. Thanks.

I agree with everything Ethan says about the outside of a black hole. And I must add that everything he says about the experience of someone falling into a black hole is correct if you believe the standard theory. Which I don't. I simply ask, why do you assume that the event horizon of a black hole is a classical boundary across which a coherent classical object like a human being can cross intact?

Back to the black hole at the center of the Milky Way galaxy, if we get a good enough superHubble telescope; we should be able to identify the event horizon by seeing astronauts from advanced civilizations (i.e other star systems) who are getting redder and dimmer and of course are lost in space.

Ethan, yes, the tidal accel goes as ~M/d^3, so at the Schwarzchild radius, where d=R~M, the tidal accel goes as ~1/M^2. Yes, like I had also said, smaller/less massive black holes have a stronger tidal force at their surface.

Still polling the audience to see if it's one of those magic situations where the classical calculation is still valid after all the index juggling...

Thomas:

If you don't believe the standard theory, which indicates that an object can cross the event horizon of a black hole massive enough to weaken the tidal forces, then isn't it your job to tell us why you think an object can't? When you refute the standard theory then ask everyone who doesn't to explain a phenomena already described by the standard theory, then you either don't understand the standard theory or you're setting up for a circular discussion.

So what do you think happens when an object crosses the event horizon of a black hole with negligible tidal forces at the event horizon?

"...finally tearing the individual nuclei apart into, eventually, quarks and gluons."

What makes you so sure that's the final step?!

The way it was explained to me, an external observer not only sees the falling object as becoming infinitely-slowed near the horizon, but also essentially destroyed by the high-energy particles also falling inward, all while the object itself notices nothing different as it passes the Sch. radius (shortly before being torn apart by tidal forces). A true dual-reality.

I was listening to a BBC radio show about black holes a couple of days ago, and a physicist dropped a little gem in that still has my eyes popping. He said that once you cross the event horizon, space and time swap - i.e. space becomes time and time becomes space. There was no more detail about that and I am really struggling to understand it. I love your excellent and informative blog and am hoping you might be able to explain it. :)

Jude: space time swapping,

In just my thinking of trying to rationalize the swapping, I have come to one conclusion. In normal everyday life, space is finite, but time keeps ticking or expanding if you will. When you cross the event horizon, time slows to almost stopping... But not quite, and space keeps expanding. Thats ,y theory anywAys.