Black holes are not eternals. They evaporate via the Hawking radiation. But what exactly is Hawking radiation?
If you are a science guy, then you must have probably know the explanation of Hawking radiation. Virtual particles are popping in and out of existence everywhere annihilating each other. When the particle pairs get close to a black hole, one of the particle pair get suck in by the black hole. So, the other particle pair cannot get annihilated and it takes energy from the black hole and escape, reducing the mass of the black hole and eventually evaporating it.
But this explanation leaves a lot of unanswered questions. Shouldn’t the falling particles add to the mass of the black hole? How can the other particle escape the black hole? And how do the other particle even have enough energy to escape?
Well, the above explanation for Hawking radiation is partly correct and not the full explanation. It has been simplified to make the general public understand and it misses some of the core ideas. Before we start explaining about Hawking radiation, we need to understand black hole and quantum field. So read this blog first if you haven’t:
What exactly are black holes?
What is a Quantum Field?
Quantum Field Theory is a tough topic. Here is a quick summary. Space is not empty. In QFT, the fields exist everywhere. And the field is hovering around zero. Because of the Heisenberg uncertainty principle, the energy of the field is not exactly zero. Particle is just energy concentrated in a part of the field.
Also, the field is filling space with quantum properties. What makes these properties ‘quantum’ is that they are probabilistic. Every point in space is teeming with everything that could possibly happen. We only see a particle somewhere when we reduce those possibilities to one. But it is possible for these non particle points to become particles.
Now, imagine a string placed between two heavy blocks. If you tap the string, it will vibrate.
There are different frequencies available to the strings – one loop, two loops, three loops, etc.
There are an infinite number of frequencies available to the string. But the first few ones are the most dominating. These are the natural modes. These frequencies are all available to the string unless we pinch the string in the middle.
Pinching the string in the middle means that the middle point of the string must remain stationary. Some of the frequencies which were available are now unavailable.
This is exactly what black holes do to quantum fields. Now, rather than having a flickering box represent the point in space, we will take snapshots of it across time.
This pattern can be expressed by a combination of positive and negative wave modes. The positive modes go forward in time and the negative modes go backward in time. That is why quantum field is kind of like a wave on a string. Usually, the positive and negative modes tend to cancel out and this is what people mean by virtual particles.
But when a black hole forms, it kind of pinches the strings of the quantum field.
So certain frequencies which were available to the string before the black hole’s formation are now unavailable. And the modes don’t cancel out. The black hole has turned the vacuum into a particle-filled region of space.
The reason for the use of ‘region’ is that the waves are not localised. They arise from a region which is around a few times the event horizon.
Hawking radiation are exclusively photons and by the time, they escape the black hole, that photon (light) has a wavelength of about 80 times the radius of the black hole. Remember, a black hole is not a material thing. It is extreme curvature in spacetime. More curvature means more energy while less curvature means less energy. So, as the black hole loses energy, its curvature will decrease. The process will start slowly but speed up as it shrinks.
As the black hole gets smaller and smaller, the electromagnetic wave(photon) will shift towards visible light and then towards gamma rays. In the last instant of the black hole, the black hole will explode with huge amount of matter and photons. And there will be no curvature in spacetime.
But, it takes extremely long time for the black hole to evaporate. For a black hole that is around 3 to 150 solar masses, it takes around 10^67 years. And for a supermassive black hole, it takes around 10^106 years.
Watch this video for better understanding:
This is it for today. I write fascinating blogs about science and other exciting topics today. So please visit my website everyday and learn amazing things.
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