My aristocratic friend

I once had a friend. Every second year he wrote me a letter, hoping to receive acknowlodgements for his achievements and developments on behalf of all of human kind.

He was born at an Imperial hospital.

He had three friends. One of them founded Red Cross half-life. Peter and Mary was curious about life.

Once he travelled with another friend to Africa, to commemorate the church founder, singing Hallelujah without 'o'!

Einstein's forefather plays golf.

My friend's favourite object was a brown radio.

In what city did he meet his emperor again?

Hint:

My friend's birthday is in the 111th period.

• @Gamow If I just tagged it wordplay, science and sequence straight away, it would not been an enigma to the same extent that it became... – Enigma Mar 30 '16 at 16:39
• I found this puzzle pretty difficult. (But not as difficult as your Cuba Gooding jr puzzle from last year.) – Gamow Mar 30 '16 at 16:41
• Well, I am also not sure that I would have made it through the aristocratic friend puzzle without your strong hints... – Gamow Mar 30 '16 at 16:47
• @Gamow My philosophy is that if it is too easy, it's no fun:-) – Enigma Mar 30 '16 at 18:32

I think that I now understand most of the puzzle.
I would guess that the answer to "In what city did he meet his emperor again?" should be

Vienna (= Wien).

Explanation:

The main theme are

Nobel prize winners of physics in odd-numbered years, starting from the year 1901;
this is indicated by "Every second year he wrote me a letter";
The puzzle covers 1901, 1903, 1905, 1907, 1909; hence the answer should fall into the year 1911.

He was born at an Imperial hospital.

Wilhelm Conrad Röntgen; Nobel prize for physics 1901
Imperial=Wilhelm (Emperor Wilhelm II); hospital=Röntgen (X-rays)

He had three friends. One of them founded Red Cross half-life. Peter and Mary was curious about life.

Henri Bequerel, Pierre Curie, Marie Curie; Nobel prize for physics 1903
Red Cross = Henri (Henri Dunant); half-life = Bequerel;
Peter=Pierre; Mary=Marie; curious=Curie

Once he travelled with another friend to Africa, to commemorate the church founder, singing Hallelujah without 'o'!

Philipp Lenard; Nobel prize for physics 1905
The Apostle Philip is regarded as the founder of the African church.
Leonard Cohen sang "Hallelujah"; Leonard without 'o' = Lenard

Einstein's forefather plays golf.

Albert Abraham Michelson; Nobel prize for physics 1907
Einstein=Albert; forefather=Abraham; golf=Phil Mickelson

My friend's favourite object was a brown radio.

Karl Ferdinand Braun (brown); Nobel prize for physics 1909
Guglielmo Marconi (radio); Nobel prize for physics 1909

In what city did he meet his emperor again?

Wilhelm Wien, Nobel prize for physics 1911
German physicist; the first name Wilhelm is the name of the German emperor Wilhelm II, the family name Wien is the German name of the city Vienna.

My friend's birthday is in the 111th period

Wilhelm Conrad Röntgen; Nobel prize for physics 1901
The chemical element with number 111 is Roentgenium, called after Wilhelm Röntgen.