“So Einstein was wrong when he said, ‘God does not play dice.’ Consideration of black holes suggests, not only does God play dice, but that sometimes he confuses us by throwing them where they can’t be seen.” – Stephen Hawking
On 10th April 2019, humanity reified a sensational idea that had been pitched by Einstein and achieved the extra-ordinary feat of moving beyond ‘selfies’ and capturing the first image of a black hole. After years of hard-work, the first image of a black hole is now a visual proof of the existence of black holes and another corroboration for Einstein’s theory of relativity. Einstein’s general theory of relativity states that mass can be seen as ripples in the fabric of space-time. When this mass tends to infinity and volume tends to zero, the object warps space-time in such a way that not even light can escape. The boundary of a black hole is known as the “event horizon” and objects that go beyond the event horizon are lost forever. The black hole, in the image taken, is said to be at the center of the M87 galaxy. So, how does the first image of black-hole prove Einstein’s theory? Let’s take a look at the picture:
As you can see, the image has myriads of discernible features, but the most evident and salient feature I see in this picture is the bright ring around the black hole. This ring in the image is known to be the “photon sphere”. The photon sphere is a spherical region of space where the gravity forces the photons to travel in orbits. Photons are particles that make up light, and when photons approach the event horizon with enough energy, it is not pulled into the black hole. Instead, the photons travel in a tangential direction known as the “exit cone”. Also, the bright patch at the bottom of the picture ascertains that the material around the black hole is moving at light speed, which is consistent with simulations derived from the theory of relativity. The disparate brightness of this ring evinces that some regions around a black hole are brighter compared to the rest. When photons from regions around black hole move towards us, these regions appear brighter than the rest. On the other hand, if photons move away from us, these regions appear dimmer than the rest.
For the photo to be seen, the photons must have travelled from the region around the black hole to us. In this particular galaxy, these photons were assisted by the black hole in two distinct ways. Firstly, the photons were assisted by an accretion flow. What is this accretion flow? A violent disk of the gases orbiting the black hole moves towards the event horizon, but by the time it is pulled into the black hole, the gas particles collide with each other and the disk collapses into itself. This makes the gas particles very hot and turns it into plasma at a temperature of hundred million degrees. The second assistance is provided by narrow astrophysical jet beams flowing out of the black hole and speeding away from it at the speed of light. Jets are powered by black hole’s spin and hence the black hole in this image is a spinning black hole. The black hole in this image has been discovered to be spinning clockwise. Rotating black holes drag magnetic fields and photons about themselves and are responsible for the astrophysical jets. In M87, one of these astrophysical jets is luckily pointed towards Earth and helps us view this ginormous black hole clearly. The Event Horizon Telescope was able to capture these images because of the emitting regions, rotating accretion disks, outflowing jets and gravity itself. There also is a dark shadow or silhouette cast upon the black hole’s event horizon because of the emission from the astrophysical jets. The size and shape of the shadow is determined by gravity alone. This is what the black hole image tells us.
How does this prove Einstein’s theory of relativity? Einstein clearly had vaticinated the features of this picture. According to the general relativity, the shape of the black hole shadow should be circular within 10 percent. The diameter of a black hole should be its mass multiplied by fundamental constants. After the calculations performed on this image by scientists, the image justifies both the predictions made by Einstein. The shadow exists and is nearly circular. The inferred mass from the shadow also matches the estimates from the general relativity. So, it is safe to say that the general relativity has passed another hurdle.
Now, what about the picture? How was the photo taken? The photo is a result of extra-ordinary coincidences on a cosmic level. The radio waves had enough energy to travel 60,000 years through M87 and then another 55 million years in intergalactic space until they ended up in Earth’s atmosphere where most of them were absorbed by water vapor. This prompted scientists to set up the event horizon telescope at dry sites on Earth. The Event Horizon Telescope uses a technique known as the “Very Long Baseline Interferometry”.
The photo demanded cosmic coincidences in order to majestically unveil itself to our generation. What are these coincidences? To begin with, a photon had to travel from the event horizon and through empty space in order to do so. While doing so, the photon had to resist the strong gravitational pull of the black hole and the photon had to constitute light rays of a millimeter wavelength. Then, the radio waves had to travel another 55 million and 6000 years through intergalactic space to reach Earth’s atmosphere, where it faced the wrath of our belligerent atmosphere. It was entirely possible for these radio waves to be absorbed by water vapor, and so the telescopes needed to capture these radio waves had to be set up in dry, remote areas. Using the time interval difference between the arrival of two radio waves, different telescopes were set up at particular distances all around the globe. Radio waves from the black hole hit these telescopes, where the arrival times were recorded using an atomic clock, which loses one second only every 10 million years. This data was compiled into hard drives and send to central facilities to combine them into an image. The algorithm for this process was developed Katie Bouman. There still are a few questions that remains unanswered, for example, what is the reason behind black holes ejecting enormous astrophysical jets that move at such high speed. Hopefully, these questions will be answered in time. Until then, it’s important to understand the significance of this discovery and appreciate it heartily. So, keep reading!!! And continue to speculate, innovate, till you constipate