An 89-year-old man in the US has achieved a goal he spent two decades working towards and nearly a lifetime thinking about - earning his PhD and becoming a physicist.

Manfred Steiner, from Rhode Island, recently defended his dissertation successfully at Brown University in Providence.

Mr Steiner cherishes this degree because it is what he always wanted - and because he overcame health problems that could have derailed his studies.

"But I made it, and this was the most gratifying point in my life, to finish it," he said at his home in East Providence.

As a teenager in Vienna, Mr Steiner was inspired to become a physicist after reading about Albert Einstein and Max Planck.

He admired the precision of physics.

But after the Second World War, his mother and uncle advised him that studying medicine would be a better choice in turbulent times.

He earned his medical degree from the University of Vienna in 1955 and moved to the United States just a few weeks later, where he had a successful career studying blood and blood disorders.

Mr Steiner studied haematology at Tufts University and biochemistry at the Massachusetts Institute of Technology before becoming a haematologist at Brown University.

He became a full professor and led the haematology section of the medical school at Brown from 1985 to 1994.

Mr Steiner helped establish a research programme in haematology at the University of North Carolina, which he directed until he retired from medicine in 2000 and returned to Rhode Island.

He and his wife Sheila, who is 93, have been married since 1960.

They have two children and six grandchildren.

He will celebrate his 90th birthday this month.

Mr Steiner found medical research satisfying, but it was not quite the same as his fascination with physics.

"It was something like a wish that was never fulfilled, that always stuck in the back of my head," he said.

"I always thought, you know, once I'm finished with medicine, I really don't want to spend my life just sitting around and maybe doing a little golfing or doing something like that. I wanted to keep active."

At the age of 70, he started taking undergraduate classes at Brown, one of the Ivy League universities.

He was planning to take a few courses that interested him, but by 2007, he accumulated enough credits to enrol in the PhD programme.

Physics professor Brad Marston was sceptical when Mr Steiner entered his quantum mechanics class.

Prof Marston had taught graduate students in their 40s, but never in their 70s.

Then he realised how serious Mr Steiner was about the subject and how hard he worked.

Prof Marston became Mr Steiner's adviser for his dissertation.

"He has written many papers in medical science, more papers than I've written in physics. He already had a scientific way of thinking that younger students have to develop," Prof Marston said this week.

"And any research problem that's worth its salt, you're going to run into roadblocks. If you let obstacles discourage you, you won't get anywhere. One thing that's really true about Manfred is he perseveres."

Mr Steiner defended his dissertation in September after recovering from a serious medical condition.

In his dissertation, he explores how electrons within conducting metals behave quantum mechanically and how fermions can be changed into bosons in their behaviour.

He is working with Prof Marston on a paper on bosonisation that they aim to publish.

Mr Steiner now hopes to help professors he befriended during his studies with their research.

"I'm not looking for a paid job. I'm past that," he said, laughing.

Guinness World Records says a 97-year-old man in Germany in 2008 was the oldest person to earn a doctorate, while news reports describe even older people pursuing such degrees.

Though he is not the oldest, attention has been intense.

Brown University featured Mr Steiner on its website after he earned his PhD, and people across the country contacted him to ask for advice on pursuing their dreams later in life.

Mr Steiner told a 57-year-old aspiring mathematician: "You're still a youngster, by all means do math."

He said his advice is: Do what you love to do.

"Do pursue it because later in life you maybe regret it, that you didn't do that," he said.

"You wish you could've followed this dream."