MBBS final year
Continued from the earlier issue….
However, even this degree of certainty, seems to be threatened by further developments in science. The problem arises because gravity can warp space-time so much, that there can be regions that we don’t observe. Interestingly enough, Laplace himself wrote a paper in 1799 on how some stars could have a gravitational field so strong that light could not escape, but would be dragged back onto the star. He even calculated that a star of the same density as the Sun, but two hundred and fifty times the size, would have this property. The same idea had been put forward 16 years earlier by John Mitchell. Both Mitchell and Laplace thought of light as consisting of particles that could be slowed down by gravity, and made to fall back on the star. But a famous experiment, carried out by two Americans, Michelson and Morley in 1887, showed that light always travelled at a speed of 3*10^8 meters per second, no matter where it came from. How then could gravity slow down light, and make it fall back? Light is a wave, why is it behaving like a particle which is affected by gravity? Wave-particle duality later came into existence explaining everything has a dual nature- particle and wave. Light and everything else have now two forms.
In 1915, Einstein put forward his revolutionary General Theory of Relativity. In this, space and time were no longer separate and independent entities. Instead, they were just different direction sin a single object called space-time. Time was 4th dimension of the space-time continuum. This space-time was not flat, but was warped and curved by the matter and energy in it.
Let us consider a sheet of rubber as the fabric of space-time with a weight placed on it, to represent a star. The weight will form a depression in the rubber, and will cause the sheet near the star to be curved. If one now rolls marbles on the rubber sheet, their paths will be curved, rather than being straight lines.
In 1919, a British expedition led by Arthur Eddington to West Africa, looked at light from distant stars that passed near the Sun during an eclipse. They found that the images of the stars were shifted slightly from their normal positions. This indicated that the paths of the light from the stars had been bent by the curved space-time near the Sun. General Relativity was confirmed.
Consider now placing heavier and heavier, and more and more concentrated weights on the rubber sheet. They will depress the sheet more and more. Eventually, at a critical weight and size, they will make a bottomless hole in the sheet, which particles can fall into, but nothing can get out of. Similar things happen in space-time. A star will curve and distort the space-time near it; the more massive and more compact the star is, greater will be the distortion.
Stars have a strong gravity and the energy from nuclear fusion keeps the star from collapsing onto itself due to its gravity. If a massive star, which has burnt up its nuclear fuel, it cools and hence the nuclear energy is now not enough to counter-balance the inward pull of gravity. The star collapses due to its own gravity, shrinks below a critical size as given by Schwarzschild radius R=2GM//c^2. It becomes more and more dense and it will quite literally make a bottomless hole in space-time, that light can’t get out of. Such objects were given the name Black Holes, by the American physicist John Wheeler, who was one of the first to recognize their importance, and the problems they pose. Now such objects have been identified in various binary star systems and at center of galaxies. Black Holes are Out of Sight. Their presence can be felt; can be made up by evidences only, by thee gravity they exert on their surroundings.
Newtonian mechanics explained actions would happen at an instant. His laws of Gravity that explains how Gravity of Sun holds Earth and other planets in its orbit It also says that if Sun were to disappear at once, light from earth would take 8 minutes and 20 seconds to vanish but Earth would immediately be out of its orbit, for then there would be no force of Gravity acting on Earth to keep it in the orbit. Einstein came up with special theory of relativity sand said- Wait, not so fast. Actions can’t happen at an instant. Everything has a finite speed and the speed of light in vacuum is the maximum attainable speed. Even the force of Gravity has a speed therefore, a maximum of 3*10^8 meters per second. Now it is presumed the force of Gravity is mediated through force particle known as Graviton. This proposed particle, yet to be found is presumed to be massless and hence can attain the speed of light which is the maximum speed of the Gravitational force too. This means that when sun disappears, it would take 8 minutes and 20 seconds for light on earth to vanish as well as the same amount of time to sway off the original orbit. The earth would continue to remain in its orbit for 8 minutes and 20 seconds amazingly.
With this view, if light cannot escape the pull of Black Hole for it has escape velocity greater than the speed of light, no any other information from that region of space would escape. Not only do the particles and unlucky martyr astronauts that fall into a black hole, never come out again, but also the information that they carry, is lost forever, at least from our region of the universe. You can throw television sets, garbage or even your worst enemies into a black hole, and all the black hole will remember, is the total mass, and the state of rotation. You could argue this loss of information didn’t seem to matter too much. You would say that the information still existed inside the black hole. It is just that you can’t tell what it is, from the outside. But wait!
Previously it was thought that black holes would continue to exist forever. The situation changed, when Stephen W. Hawking discovered that black holes aren’t completely black. Quantum mechanics causes them to send out particles and radiation at a steady rate- The Hawking Radiation. What we think of as empty space is not really empty, but it is filled with pairs of particles and anti-particles. These appear together at some point of space and time, move apart, and then come together and annihilate each other. These particles and anti-particles occur because a field, such as the fields that carry light and gravity, can’t be exactly zero. That would mean that the value of the field, would have both an exact position (at zero), and an exact speed or rate of change (also zero). It would be against the Uncertainty Principle, just as a particle can’t have both an exact position, and an exact speed. So all fields must have what are called, vacuum fluctuations. Because of the quantum behavior of nature, one can interpret these vacuum fluctuations, in terms of particles and anti-particles. These pairs of particles occur for all varieties of elementary particles. They are called virtual particles, because they occur even in the vacuum, and they can’t be directly measured by particle detectors. However, the indirect effects of virtual particles, or vacuum fluctuations, have been observed in a number of experiments, and their existence confirmed. If there is a black hole around, one member of a particle anti particle pair may fall into the hole, leaving the other member without a partner, with which to annihilate. The forsaken particle may fallinto the hole as well, but it may also escape to a large distance from the hole, where it will become areal particle, that can be measured by a particle detector. To someone a long way from the blackhole, it will appear to have been emitted by the hole and hence Hawking had to say- Black Holes ‘aint so black.
The emission will depend on the size of the black hole, and the rate at which it is rotating. But, the radiation will be otherwise independent of what went into the hole. It doesn’t matter what goes inside a black hole, what comes back out will be the same provided the mass that entered the black hole is same. John Wheeler called this, ‘A Black Hole Has No Hair.’ It shows is that there are many initial states depending on what entered the black hole, all ended up to the same final state, at least as it seems from outside the black hole which is the only way we have; we can’t look inside a black hole. So, what has all this got to do with making you stop thinking that your information is still inside the black hole, wondered, are you not?
The emission of particles, and radiation by the black hole, will cause the hole to lose mass, and get smaller. Eventually, it seems the black hole will get down to zero mass, and will disappear altogether. What then will happen to all the objects that fell into the hole? They can’t come out again, because there isn’t enough mass or energy left in the black hole, to send them out again. Even the information that fell into the hole could not come out again when the hole finally disappears. Information can’t be carried free. Information requires energy to carry it, and there won’t be enough energy left when the black hole disappears. What all this means is, that information will be lost from our region of the universe, when black holes evaporate.
You may now want to say the information that entered into black hole might have found some exit at other end to……..
To be continued in the next issue….