The year is 2241. You are an astronaut on-board the ‘Orion Interstellar Space Module’, or OISM for short. You contemplate what your journey has in store, but know that it is of little use to ponder over. The hatchet doors lock into place, its metallic joints fully lubricated. You fall into a deep slumber as the door seals your tomb, or hibernating chamber if you wish, for the next 37 years….

The conciousness that seemed to fade at least an aeon ago slowly creeps back into your mind. Your senses are dulled. A warm, muffled sound is permeating the background. The light that creeps in from the sides of the now open chamber becomes increasingly brighter. Suddenly everything hits you at once – the now screaming alarm and flashing orange module lights snap you from your comatose state. Something is wrong.

You scramble over to the control deck. The year now reads 2253. That is 17 years shy of when you should have been woken. The interstellar map highlights your trajectory. Your are significantly off-course, floating in an uncharted interstellar region. At once you realise that floating is far from what you are doing. A more precise description would be… falling. Falling into a Black hole.

— Falling into a Black Hole —

Firstly, what is a black hole?

A black hole is a region in space of immense gravitational attraction. The gravity is so strong that not even light can escape it! Black holes form when a very large star reaches the end of its lifetime. When the star uses up all of its fuel it can no longer support the nuclear interactions which keep it burning, and so it collapses in on itself from its own large internal gravitational attraction. This subsequently causes one of the most violent explosions in the universe – a supernova. And at its centre lies a very tiny point with an extremely dense amount of matter. This is often referred to as the singularity inside the black hole, which produces an extreme amount of gravitational attraction.

Orbiting a black hole contains a more detailed understanding into what a black hole actually is and how it came about for interested readers.

A singularity* (that predicts a point of infinite density) arises from a particular Star collapsing past its own Schwarzchild* radius (radius of event horizon). As was explained in the above article, the reason why any object that falls within the Schwarzchild radius cannot escape is because the warping of space-time forces it towards the singularity. But that doesn’t mean that the photons themselves cease to exist inside the black hole. No, light itself still exists as photons past the event horizon – they just become forever trapped within it (well not exactly, but let’s leave quantum tunnelling for another time). This light would only be able to travel inwards towards the singularity, and therefore if you were already within the black hole then you would not be able to see the approaching point of death (singularity). The universe disappearing behind you however should still be very visible (but will be greatly warped). Here is a great video demonstrating it [1]:

Freaky stuff, eh.

Let us evaluate it in a stepwise manner. Make sure to keep this picture in mind of how the gravity stemming from a black hole acts.

Gravity of a Black Hole [1]
  1. As you fall towards the black hole, you won’t actually feel any gravity (as you are in free fall). As you approach an orbit of two times the Schwarzchild radius, you will go into an unstable circular orbit. At this orbit you could go around the black hole in a mere 10 minutes. The problem is, however, that even a slight burst with your retro thrusters would send you straight into the black hole. It also happens to be your last chance to get out alive – if you manage to gain some additional thrust then you would be slingshot back out into the universe. If you have seen the film Interstellar then this is the orbit they enter the black hole at. The amount of time you spent in orbit may not be what you think it may be! More on this later [1].
  2. The next significant event would be crossing the Photon Sphere at 1.5 Schwarzchild radii. This is the closest distance that light rays can enter circular orbits around the black hole. The corresponding GIF shows how cool it would look [1].
  3. Crossing the event horizon,  contrary to what how important it may seem, would be unrecognisable. However, as we mentioned above, while inside the black hole we would NOT be engulfed in darkness. This is what it would look like if you were looking away from the singularity [1].

 Obtained from [9]

But things surprisingly become even stranger! Let us imagine there is another astronaut observing you fall into the black hole from a safe distance. Now as you head towards the singularity, you actually fall towards it faster than the speed of light! But how is this possible, isn’t it breaking Einstein’s Relativity!? Well not quite. The reason behind this is because you yourself aren’t travelling through space faster than light, rather it is the space itself which is moving faster than light! This does not violate any laws and is perfectly possible!

Moreover time would actually stand still, but not for you. To understand this you must be aware of Einstein’s Special Theory of Relativity. What the astronaut watching you fall in would see is that the second hand on your watch ticks much slower than their own [2]. This means that time for you has dilated and as a result they will see your speed asymptotically reduce towards zero as you approach the event horizon. On top of this, the light being reflected off your body becomes increasingly red shifted, and your astronaut counterpart would eventually see you fade away (this is because light becomes stretched in the increasingly warped space).

However Relativity specifically states that two observers in relative motion will generally not agree as to whether two events are simultaneous [2]. Therefore, as far as the person falling into the Black Hole is concerned, time is passing by at its normal rate unlike what the observing astronaut witnessed. So who see’s the correct version of events? Well, they both do, and that’s the point of Relativity. So while the observing astronaut see’s you fade away at the event horizon, from your perspective it only take you ~ 16 seconds to fall from the horizon to the singularity of a 5 million Solar Mass black hole irrespective of what you ‘perceive’ [1]. All of this can be more simply expressed in Einstein’s General Theory of Relativity, where time dilates in a region of high gravity (which is what is seen in the movie Interstellar).

Okay, so let’s get back to the real question. What would you experience while falling into a Black Hole?

Warping of Space-Time: Credit

‘Death by Black Hole:  And Other Cosmic Quandaries’ is the title of a book written by famous Astrophysicist Dr. Neil deGrasse Tyson. In it he describes what would happen to you, if for some unknown reason, you decide to take a plunge into the pit of no return (that is, a Black Hole). Looking back at the definition of a Black Hole, we know it can be calculated as a singularity (an infinitesimally small point that contains a tremendous amount of matter). This means that the tidal forces* (which were explained in my previous post) experienced by an object falling in are HUGE [3]. Perhaps this description given by Dr. Tyson explains it terrifyingly well;

As you fall in, the gravity at your feet becomes rapidly greater than the gravity at your own head. As a consequence, your feet would start to fall faster than your head, and initially it will feel like a good stretch. But eventually you will begin to stretch beyond comfort levels, and the tidal forces (gravity acting within your body) will become greater than the intermolecular bonds holding you together. You can imagine what comes next – your body will snap into 2 pieces. The good news is that it actually turns out you will SURVIVE this initial snap because there are no vital organs beneath your waist. The bad news – you will continue to snap in half to the point where you become a stream of atoms descending toward the abyss. And it turns out that’s not the worst of it. According to General Relativity, the fabric of space and time funnels down towards a black hole, so in fact while you’re getting stretched, you will also be getting squeezed. There is actually a word, invented for just this purpose – spaghettification[4].
(Image credit: Katherine Streeter, NPR)

A really good ‘real-time’ visualisation of this can be found here [5]:

Fall into a Black Hole

There is one last thing to mention, something that could upset the balance of what we thought we knew about black holes.

In recent years, a new theory of what could happen to a person falling into a Black Hole has emerged. This is the infamous Black Hole ‘Firewall’ paradox. One of Stephen Hawking most famous postulates was the idea of Hawking Radiation. That is, some of the particles that have crossed the event horizon manage to escape the black hole via a method known as quantum tunnelling (article coming soon). According to widely accepted research, the outgoing particle must be entangled* with all the Hawking Radiation previously emitted. This creates a paradox as quantum entanglement allows for a maximum of only two mutually entangled particles (also coming soon). The firewall resolution suggests that the entanglement must somehow get immediately broken between the infalling and outgoing particle, releasing inconceivable amounts of energy in the process. In turn this creates a ‘firewall’ that lies just inside of the event horizon. This, in effect, would mean that anybody passing through the event horizon would be burnt to a crisp. And the scariest thing about it – you wouldn’t even see it coming [6].

blackhole_pastabarbecue_2_0

However this Black Hole information paradox that has led many physicists to postulate the existence of the Firewall resolution is still up in the air. Without a doubt though, General Relativity and/or Quantum Theory need some serious revisions if it is going to overcome this issue plaguing Astrophysicists. Either that, or our understanding of Black Holes is simply wrong [7].

Coincidentally, (or perhaps not so coincidental) earlier last year Stephen Hawking released a scientific paper claiming that we currently do not understand Black Holes as they exist. This arose from the incompatibility with the event horizon and quantum theory – that is, information is not preserved. Instead, he postulates that Black Holes have an ‘apparent horizon’ (meaning that this horizon only holds matter and energy temporarily, before being released). Consequently, this would provide a solution to the ‘Firewall Paradox’ presented above [8]. The paper provides no mathematical framework behind the idea, and a lot of work has yet to be done before any conclusions can be made. It is still early days.

So the moral of the story – don’t fall into a black hole any-time soon. Have a nice day!

Definitions:

Event Horizon:  the boundary that separates the inside of a black hole and the universe that exists outside of it.

Scwarzchild radius: radius of the event horizon.

Singularity: a point where the curvature of space-time becomes infinite.

Tidal Forces: the difference between the pull of gravity at one end of an object compared to the pull at the other end.

Entanglement: Particles which are fundamentally entangled across space-time (they can communicate instantaneously).

References:

[1] – Journey into a Schwarzschild black hole. 2014. Journey into a Schwarzschild black hole. [ONLINE] Available at: http://jila.colorado.edu/~ajsh/insidebh/schw.html. [Accessed 20 September 2014].

[2] – Halliday, Resnick, Walker,  2014, Fundamentals of Physics: 10th edition, WILEY, Chapter 37: Relativity

[3] – Neil deGrasse Tyson, Death by Black Hole:  And Other Cosmic Quandaries, 2007. W. W. Norton & Company.

[4] – Neil deGrasse Tyson speaking at the Morrison Planetarium about his book: Death by Black Hole:  And Other Cosmic Quandaries. Neil DeGrasse Tyson: Death by Black Hole – FORA.tv. 2014. Neil DeGrasse Tyson: Death by Black Hole – FORA.tv. [ONLINE] Available at: http://fora.tv/2008/02/19/Neil_DeGrasse_Tyson_Death_by_Black_Hole. [Accessed 21 September 2014].

[5] – HubbleSite: Black Holes: Gravity’s Relentless Pull interactive: Encyclopedia. 2014. HubbleSite: Black Holes: Gravity’s Relentless Pull interactive: Encyclopedia. [ONLINE] Available at: http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q16.html. [Accessed 21 September 2014].

[6] – Gauge/Gravity Duality and the Black Hole Interior, 2013, http://arxiv.org/pdf/1307.4706.pdf [Accessed 21 September 2014].

[7] – Black Hole Information Paradox: An Introduction | Of Particular Significance. 2014. Black Hole Information Paradox: An Introduction | Of Particular Significance. [ONLINE] Available at: http://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/black-holes/black-hole-information-paradox-an-introduction/. [Accessed 21 September 2014].

[8] – According to Stephen Hawking, black holes as we currently understand them do not exist | IFLScience. 2014. According to Stephen Hawking, black holes as we currently understand them do not exist | IFLScience. [ONLINE] Available at: http://www.iflscience.com/physics/according-stephen-hawking-black-holes-we-currently-understand-them-do-not-exist. [Accessed 21 September 2014].

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