We briefly talked about event horizon in our list of Black Holes facts. It is time that we undertake an elaborate discussion on event horizon and understand what exactly it is and why it is so important. So, here are 20 interesting facts about Event Horizon hand-picked and simplified for you.

**1**. In order to even understand Event Horizon, we need to explain spacetime. Spacetime or space time or space-time in physics is a mathematical model where both space and time (which are two different entities) are combined into a single continuum.

**2**. So, what is a continuum? A continuum is nothing but a continuous sequence in which elements adjacent to each other are not different perceptibly but at extremities, they are very distinct. So, in the spacetime continuum, space and time are not perceptibly different. However, at extremities, they are very different. For instance, space has three dimensions and time on the other hand has only one dimension which we call the fourth dimension.

**3**. Now that we have a brief idea of what spacetime is, we can move on with Event Horizon. Event Horizon is a boundary in a spacetime continuum. If a person is outside this boundary, the events taking place within the boundary will not affect the person.

**4**. Inside the Event Horizon, the events that take place will simply kill a person. In fact, the boundary is actually a point of no return. Vaguely speaking, anything outside the boundary is safe. Anything that steps inside the boundary will never return and will be destroyed completely.

**5**. Why the Event Horizon is called a point of no return? That’s because, the gravitational pull inside the boundary is so enormously strong that escaping that gravitational pull is in one word – ‘impossible’.

**6**. Event horizons are mostly found around black holes.

**7**. Even if light is emitted inside the boundary, it cannot escape the enormous gravitational pull and hence, it cannot reach an observer located outside the boundary.

**8**. Because light cannot escape the Event Horizon, black holes are literally invisible to human eyes.

**9**. Looking at the other side, if something approaches the Event Horizon from outside the boundary (the side where the observer is located) it will appear to gradually slow down and that object will never pass through the horizon.

**10**. As something approaches the Event Horizon, it image will gradually become redshifted with every passing moment.

**11**. The question is, what is a redshift? Redshift is a state where any electromagnetic radiation or even light from any object gradually increases in wavelength. This means that the light or the electromagnetic radiation is gradually shifting towards the spectrum’s red end. As redshift occurs, the frequency or the photon energy is gradually lowered, which is nothing but increase in wavelength.

**12**. So, what causes the redshift when something approaches the Event Horizon? When we observe a redshift, it means that light from the object is traveling back to us. This is possible until the object actually goes inside the Event Horizon where light cannot escape. Before the object enters Event Horizon, the photons in the light emitted by the object will try to escape the enormous gravity. This means that the photons will have to work harder to escape. The more work they do, the more energy they lose. In other words, the photons will have to release some energy in order to escape from the gravity. Now, the frequency of the photons is directly proportional to their energy. So, if a photon’s energy drops, its frequency drops. Drop in frequency means increase in wavelength. This explains why an object becomes redder and redder as it approaches the Event Horizon. This is called gravitational redshift as predicted by Einstein’s theory of General Relativity.

**13**. So, why does the object appear to slow down as it approaches the Event Horizon? That happens because of Gravitational Time Dilation. We will explain Gravitational Time Dilation in a different list.

**14**. Funny thing about Event Horizons is that even though a person located at a safe distance from the Event Horizon experiences or observes redshift and Gravitational Time Dilation, the object which actually approaches the boundary will not experience anything strange. It will not experience redshift or time dilation and will pass through the Event Horizon in a finite amount of what is known as “proper time”.

**15**. So, what is proper time? Let us assume that the observer outside the Event Horizon ties a clock to an object and throws it towards the boundary. This ‘throwing’ is an event when the clock was active and was measuring time. When the object approaches the horizon, the clock will also approach at the same time. So, this ‘approaching’ is another event. Thus, a proper time is nothing but the elapsed time measured by the clock between two events given that the clock itself passes through both events.

**16**. Because an object approaching Event Horizon will not even observe anything strange, it will not even know that it is actually approaching an Event Horizon because this boundary is nothing physical. It is only an imaginary boundary based on mathematical calculations.

**17**. Only after an object enters an Event Horizon it can experience changes. How? All attempts to return will actually fail.

**18**. Event Horizons of black holes give rise to a phenomenon known as Spaghettification or Noodle Effect. This is a phenomenon where an object that enters the Event Horizon will get stretched vertically and squeezed laterally until it takes the shape of a spaghetti or a noodle. This happens because of the enormous tidal forces that exist within the Event Horizon and are caused by the immense gravitational pull of the singularity.

**19**. Interestingly, this spaghettification can take place even before the object enters the Event Horizon. Why? In a broad sense, there are two types of black holes – Super Massive and Smaller black holes. Smaller black hole such as the Stellar Mass Black Hole will have its singularity very close to the Event Horizon and as a result, the tidal forces caused by the gravity of the singularity exist and spread outside the Event Horizon. So, any object that approaches a smaller black hole will experience spaghettification even before entering the Event Horizon.

**20**. In case of Super Massive black holes, the point of singularity resides far from the Event Horizon and hence, the tidal forces don’t exist outside the Event Horizon. So, spaghettification will not occur until the object enters the Event Horizon. So, one thing that can be concluded is that a distant observer will actually see an object spaghettified before it disappears if it approaches a small black hole. The observer will not be able to see the spaghettification at all in case the object approaches a Super Massive black hole.

Sources: 1, 2, 3, 4, 5, 6, 7, 8

## 1 comment

Thank you dear Sankalan for sharing these very interesting space facts! Love your writing style