What would

happen if you fell into a black hole?

One of the biggest paradoxes in physics

today is one that sounds straight out of a science fiction novel. What would

happen if you fell into a black hole? Rest assured,

the answer to this bizarre question is that you would die – that is not up for

discussion. But it is how exactly you would die that is keeping physicists up

at night. There are

currently two major theories fighting over this horrifying scenario and the

outcome of this battle could revolutionize the fundamental laws of our universe.

To begin to

understand this controversy, we need to first understand what a black hole is.

A black hole is a region in space where the force of gravity is so strong that even light is not able to escape. Although some black holes are thought to have formed

in the early universe, soon after the big bang, most medium-sized black holes form

when the center of a very massive star collapses in upon itself.

For most of the past century,

the scientific community thought that the extreme gravitational pull would

crush all the matter that made up the black hole into a one-dimensional point,

called a singularity which is not only incredibly massive, but also incredibly

dense. The closer you are to this point, the stronger the gravitational

attraction is.

An analogy

inspired by William G. Unruh of the University of British Columbia, one of the

pioneers in black hole quantum mechanics, helps to explain the significance of

this pull. Imagine you are fish, swimming downstream a river that leads towards

a waterfall. If you are significantly far away from the cliff, you can easily

swim away to safety. But once you get far enough downstream, no matter how fast

you swim in the opposite direction, you cannot escape the pull of the water.

For black holes, this ‘point of no return’ is called the event horizon and it

is the place beyond which nothing, not even light can escape.

So, what

would happen if you fell into a black hole? For years scientists thought they knew how you would

meet your end. Imagine falling into the black hole feet first. As your feet are

closer to the singularity, they would feel a stronger gravitational force and

will thus start to move faster than the rest of your body, causing you to get

stretched into a long noodle. Physicists call this process ‘spaghettification’.

The

Information Paradox

The

spaghettification idea satisfied scientists until the 1970s,

when Hawking dropped a bombshell with the proposal that black holes radiate particles. The

so-called Hawking radiation causes

black holes to shrink in size and eventually evaporate completely. What has now become a

widely accepted idea about the nature of black holes raised a lot of questions, one of which still concerns

physicists today – Where did the information go? If the information about everything that went into the black hole disappeared along with its evaporation, that

would lead to the violation of one of the fundamental principles of quantum

mechanics – information cannot be destroyed. Maybe

the information came back out with the Hawking radiation? The problem is that

the information in the black hole simply cannot get out due to the intense gravitational field it has to

overcome to do so. One might argue that the problem could be solved if the

information inside the black hole is copied onto the Hawking radiation, but

having copies of information also disobeys the laws of quantum mechanics. This gave rise to a paradox, that physicists refer to

as ‘The Black Hole Information Paradox’.

The ‘information paradox’ has drawn

attention to a potentially serious con?ict between quantum mechanics and the

general theory of relativity, leaning towards the idea that one, if not both,

of the theories is incomplete. This battle polarized the scientific community.

Some scientists, such as Stephen Hawking believed that the quantum theory is

incomplete and that it needs to be extended, just like Einstein extended

Newton’s laws of motion in his theory of relativity. However, others felt that

it was the general theory of relativity, not quantum theory, that needed to be

changed.

Complementarity: Saving Quantum Theory

In search of a flaw in the general theory

of relativity, in 1992, Leonard Susskind, a professor of theoretical physics at Stanford

University, and his younger co-workers developed a proposal, called the ‘Complementarity

Principle’. It suggested that the inside and outside of a black hole can be

thought of as two different realms and the position of the information depends

on the point of view of the observers. Observers that remain outside of the

black hole would see the information of everything that is falling into the

black hole accumulate at the surface of the event horizon and then fly out in

the Hawking radiation. However, observers that fall into the black hole would

see the information located inside it.

This can further be explained with the aid

of the special theory of relativity. Einstein’s gravitational time dilation has

shown that clocks run differently depending on the strength of the

gravitational field they are in. Clocks that are in a stronger gravitational

field will run slower than those in a weaker gravitational field. Therefore,

clocks that are closer to the singularity of a black hole will run slower than

those that are further away.

Imagine two observers, Bob and Charlie that

are on a spaceship, orbiting a black hole. While Bob remains in the ship, Charlie

takes a jump towards the black hole. As Charlie falls towards the singularity,

the gravitational field he is in starts to get stronger and thus his clock

starts to run slower and slower compared to Bob’s clock. Therefore, according

to the Complementarity Principle, Bob will observe Charlie fall towards the black

hole, but then gradually slow down and accumulate at the surface of the event

horizon. Even though in Bob’s frame of reference Charlie does not fall through

the event horizon of the black hole, does that mean that Charlie does not pass

through it in his own reference frame? No! In

Charlie’s reference frame, Charlie will pass through the event horizon and will

continue falling towards the singularity of the black hole. The two observers, Bob and Charlie, would therefore see

the information in a different location, but since they cannot communicate, the

principles of quantum theory are not violated and thus there is no paradox.

This solution to the information paradox requires

that all events happening in the interior of a black hole can be described as

though they were just outside of the black hole. It involves ‘holography’, an

idea that was developed by Gererd’t Hooft, a Dutch theoretical physicist and

professor at Utrecht University, and further by Susskind. The idea is that the

information about the 3D interior of a black hole, which is greatly affected by

gravity, is stored in a 2D from just above the event horizon, where it is

described by two-dimensional equations that do not include gravity at all.

Remarkably, significant evidence emerged in

the late 1990s in support of the holographic principle. Theoretical physicist

Juan Maldacena of Princeton University hypothesized that under the right

circumstances, string theory is equivalent to a quantum theory but without

gravity and with fewer dimensions.

This success of the holographic principle

brought more faith into the Complementarity Principle idea and by 2005, Stephen

Hawking had come to agree that black holes do not cause information to be

destroyed and that the general theory of relativity, rather than the quantum

theory, needs to be modified.

The

Firewall

Until recently, many scientists satisfied

their frustration with the information paradox with the aid of the

Complementarity Principle. However, in search of equations to describe this

idea, the AMPS – Almheiri, Marolf, Polchinski and Sully, discovered that the

Complementarity Principle contains a self-contradiction. They imagined what

would happen if the two classes of observers, one outside and one inside of the

black hole, were replaced by a pair of entangled particles; i.e. one of the

entangled particles was tossed inside the black hole, while the other one was

kept outside. However, before we explain their argument, let’s first talk

entanglement.

Quantum entanglement is a quantum

mechanical phenomena that occurs when two particles are generated such that the

quantum state of one of the particles cannot be described independently of the

other. The two entangled particles are linked in such a way, that a change in

the properties of one of them will cause a change in those of the other,

regardless of the distance between them. In addition, making a measurement of

the entangled particle pair would destroy the entanglement between them. According

to the Complementarity Principle, the regions inside and outside of the black

hole can be thought of as two different realms that cannot communicate, but if

we tossed an entangled particle inside a black hole, while keeping its twin

outside, that would create a problem, simply because the very nature of

entangled particles is to be able to respond to one another.

In this way, the AMPS totally threw away

the idea of the two realms. So, is the paradox back? Only for a while.

The AMPS took the idea

about entangled pairs across the event horizon even further and got rid of the

idea of spaghettification.

But first,

let’s talk more entanglement. articles can be entangled but entanglement itself

does not require the presence of particles to exist. In fact, even empty space

is entangled. The quantum properties of empty space tell us that it is not

really empty. Quantum fluctuations rive rise to particles that constantly pop

in and out of existence.

Imagine

dividing empty space into two halves.

Even

though, according to the principle of monogamy, entanglement cannot occur

between more than two systems, that does not mean that it is restricted to

small systems. On the contrary; imagine you had a machine that continuously produces

pairs of entangled particles and is at the same time connected to two boxes. Every

time the machine produces a pair of entangled particles, one of them goes into one

of the boxes, while its twin goes to the other. As this process is repeated over

and over again, the systems in the two boxes will become more and more entangled

with one another. Taking this idea even further, imagine we could hypothetically

compress each of these two systems with so much force that they collapse into black

holes. We would then end up with two black holes that are entangled with each other.

Just as with an entangled particle pair, the two black holes will have this property

that we can find anything we want about one of them by making a measurement on the

other.

which shows up about halfway through the

evaporation of a black hole.

? Entanglement is not restricted to small systems;

? Entanglement does not require the presence of

particles;

? If a black hole is created, entangled with another system,

a ‘firewall’ will occur, causing the whole interior of the black hole to be

wiped out, creating ‘nothingness’;

? About half way through the evaporation of a black

hole, a ‘firewall’ will occur, contradicting the principle of complementarity.

For over a

century, the two planks of modern physics, the theories of relativity and

quantum mechanics have been the best …, but nevertheless they face

Since

Einstein

String Theory

The Current Turmoil