An Indian theoretical physicist  who questioned the existence of black holes and thereby challenged Stephen Hawking of Britain at last feels vindicated. But he is sad.

Abhas Mitra, at the Bhabha Atomic Research Centre (BARC) in Mumbai, was perhaps the first and the only scientist who had the guts to openly challenge Hawking of Cambridge University who is regarded by many as the modern-day Einstein.

For over 30 years Hawking and his followers were perpetuating the theory that black holes -- resulting from gravitational collapse of massive stars -- destroy everything that falls into them preventing even light or information to escape.

Mitra, four years ago, in a controversial paper in the reputed journal, Foundations of Physics Letters, showed that Hawking's theory was flawed. He proved black holes couldn't exist because their formation and existence flouted Einstein's general theory of relativity.

Except a handful, the majority of mainstream scientists dismissed Mitra's conclusions even though, till now, no scientist has contradicted him in writing. Mitra invited several notable black hole theorists including Hawking and Jayant Narlikar of India to criticise his work but no one replied.

Naturally, Mitra now feels vindicated following Hawking's own admission two weeks ago at a conference in Dublin, Ireland, that there isn't a black hole "in the absolute sense."

In essence, Hawking's "new" black holes never quite become the kind that gobble up everything. Instead, they keep emitting radiation for a long time -- exactly what Mitra showed in his paper.

Hawking's about-turn has vindicated Mitra. But, in retrospect, he feels sad about the treatment he got at home while trying to take on Hawking all by himself.

Too "embarrassed" to be associated with a man who challenged Hawking, even Mitra's close colleagues avoided him and he became an outcast. To add insult to injury, BARC authorities removed Mitra from the theoretical physics division on the excuse that this division was meant only for those doing "strategic research."

"The ironic element in this whole exercise," Mitra told  PTI, "is that the person who actually dared to show that there cannot be any black holes was completely ignored both by the academicians and the media."

A black hole is characterised by an imaginary boundary called the "event horizon" that shuts everything within. But in 1976 Hawking introduced quantum mechanics into the problem and claimed that black holes do radiate energy -- although at a low rate -- and ultimately vanish into nothingness.

The vanishing act, however, destroys all the trapped information as well - directly conflicting with the laws of quantum physics that say that information can never be completely wiped out. This is the "information loss paradox" associated with black holes that, in a way, was created by Hawking's own work.

One logical resolution of this paradox would have been to realise that black holes did not exist. But Mitra says that such sweeping, yet logical thinking "was never undertaken by either party involved in this prolonged debate and they kept on debating effectively to make the paradox more popular and perpetuating."

It was then that Mitra published his seminal paper showing that gravitational collapse of massive star can at best produce an "Eternally Collapsing Object" but not an "event horizon" or a black hole in the strict sense. "Since no event horizon is formed, there is no paradox at all in the first place," Mitra argued.

In a subsequent work Mitra showed that the "Eternally Collapsing Objects" that he proposed are actually the massive compact objects now referred to as Black Hole Candidates (BHCs).

Motivated by Mitra's work, American physicists Stanley Robertson and Darryl Leiter have confirmed in 2002 that BHCs have intense magnetic fields as predicted by Mitra and therefore are not real black holes which cannot have magnetic field.

Mitra says that in the light of new developments, "the supposed black holes are not really black holes and it would be intellectual dishonesty to still call them as black holes and keep the debate alive."

Though his own colleagues had sidelined Mitra after his first paper, he is solaced by the encouraging e-mails he had received from several physicists around the world.

One from Salvatore Antoci, University of Padova, Italy, a noted relativist says: "Let me express to you my great joy in seeing your much-disputed paper eventually accepted for publication by Foundations of Physics Letters. Convincing the community of relativists about the mythical nature of black holes will remain a tremendous task, but it is a little less desperate thanks to your success."

Peder Norberg, of the Department of Physics, Durham University, UK, said he carefully read through Mitra's paper and found "that most of the results presented there are more than impressive" while Stanley Robertson, a relativist of South Oklahoma State University, USA said: "On first becoming acquainted with your work, I was dubious, thinking it unlikely that something as profound as belief in the existence of black holes could become erroneously established in the literature. In the meanwhile, I have found no errors in your work. It is fascinating."

The only Indian who praised Mitra's work was relativist Pankaj Joshi of the Tata Institute of Fundamental Research in Mumbai.

The BARC scientist recalls the episode in the 1930s when Subramanian Chandrasekhar's work on the upper mass limit of white dwarfs was considered incorrect by celebrated astrophysicists like Sir Arthur Eddington even though no one could precisely point out any error in Chandra's work.