Dear Muktomonas, 

Stephen Hawking has proposed a new model of a black hole, which does not completely swallow an object but can send information out of the hole. Here is the story as understood by me presented in non mathematical language. 

With regards, 

Ajoy Roy 
Dhaka 
18. 07. 04

 

 

Stephen Hawkins revises his opinion about black holes.

-Ajoy Roy



The world-renowned theoretical physicist and cosmologist Stephen Hawkins in an address of international conference on Wednesday, July 11, 2004 gave an indication to revise his thirty-year-old earlier opinion that black holes destroy everything that enters into them. In a Dublin seminar he would disclose his new thinking of black holes which might solve the so called ‘black hole paradox’, a crucial puzzle of modern physics, the problem that was generated from his own work.

In simple language we may describe the formation of a celestial object called as black hole. It is believed that when a star is sufficiently massive, gravitational collapse may continue until the density is so high and gravity predominates to such an extent that a black hole results. Objects believed to be massive black holes have been found at the center of some galaxies- they appear to power very luminous objects such as quasars and active galactic nuclei. Matter and radiation cannot escape from a black hole due to its immense gravitational field, but as material falls towards it intense radiation is emitted.

Although black holes are assumed to be some kind of monsters in the space-time swallowing every thing matter or radiation approaching near them in a frenzied cosmic feast. The physicists now a days think that rather than being a space menace these objects may be fundamental to the creation of galaxies. Black holes are regions of space where gravity is so strong that not even light can escape, making them impossible to see. However, the staff that is being sucked into those massive cosmic vacuum cleaners could be ‘seen’. Anything that approaches a black hole is first torn apart by its immense gravitational and then forms a flat rotating disc that spirals into the hole. As the debris gets closer and closer to the mouth of the black hole it speeds up and the bits start to smash together. The material heats up due to this friction. When this happens around a black hole, x-rays are given off which we can detect. If the black hole is really very large and has lots of debris in the disc, then it can reveal itself as one of the brightest objects in the universe- a quasar.

There are two main types of black holes- stellar and supermassive. Stellar sized black holes are the remnants of massive dead stars that have imploded. It is estimated that our galaxy contains millions of these stellar black holes. Supermassive black holes can have initial mass millions of times that of the Sun. It is believed that these massive holes lurk in the center of every galaxy and be integral to the way galaxies evolve. There may be mini black holes, smaller than atoms in volume but as massive as whole mountains. There are some black holes lying between stellar and supermassive. It is quite likely that some kind of objects might found in the universe having opposite characteristics of a black hole, which spewing out every matter and radiation into the universe. This is just another of the cosmic mysteries still awaiting a solution. Hawking’s new thinking The great physicist of our time Stephen Hawking recently said that what he thought about the behaviour of black holes 30 years back is based on a key wrong argument. He said, “ I have been thinking about this problem for 30 years, but I now have the answer. What was the key argument at that time ? In 1976, he showed that when a black hole is formed, it radiates energy and starts loosing mass. This radiation gives no information about the matter inside the black hole, and once the hole disappears – all the information are lost with it. This brings in a serious paradox known as ‘black hole information paradox’. If information is totally lost it arises important practical and philosophical consequences. If this really happens, then ‘we could never be certain of the past or predict the future precisely. A lot of people therefore wanted to believe that information could escape from a black hole but they didn’t know how it could get out.’, commented the Lucasian Professor of mathematics at Cambridge to the BBC. He further continued that once a material body or radiation had fallen into a black hole ‘it has gone and lost for ever and the only information remained was its mass and spin.’ The professor claimed saying, “… some time ago I discovered that black holes are not that black after all. They give off what has been called ‘Hawking radiation’. Because of this emission black holes will lose mass and eventually evaporates completely. The Hawking radiation seemed to be random and featureless so it appeared that all information about what fell into a black hole was lost.” However this contradicts the laws of quantum physics, which describe the behaviour of the Universe at the smaller scale. These dictate that information can never be completely lost ! Hawking has not yet revealed the detail mathematical calculations of his new thinking, but some points have been come out in a seminar talk he gave at Cambridge. Gary Gibbons, a physicist of the University of Cambridge, who attended the seminar, said that Hawking’s newely defined black holes did not have a well defined “event horizon” that hid everything in them from the outside world. “It’s possible that what he presented in the seminar is a solution.”, Gary Gibbons told New Scientist magazine. This remind us of controversy between Chandrasekhar and Edington- who claimed that within the dense celestial bodies like white dwarf the quantum laws break down including the Pauli’s exclusion principle.

For a long time astrophysicists including Hawking argued that the extreme gravitational fields of black holes somehow overturned the quantum laws. But now Stephen has dropped the idea.

According to his new concept of black hole model these holes never completely destroy everything that falls in. Instead, they continue to emit radiation for extended periods, eventually open up to reveal the information within them. He explained ‘The black holes only appears to form but later opens up and releases information about what fell in, so we can be sure of the past and we can predict the future.’

It may be recalled that Einstein’s general theory of relativity predicts that the collapse of starts can cause great distortion of space-time, resulting in the formation of black holes with such intense gravitational fields that even light cannot escape. The general theory further predicts that heavy moving objects (like collapsing stars) emit gravitational waves in the form of massless gravitons traveling at the speed of light – just like photons in case of electromagnetic waves. Their detection is one of the greatest challenges of experimental physics. If these are detected, the gravitational field can be explained in terms of an exchange mechanism and hence to confirm the unified theory of all forces. But still we have a long way to go !


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