Problems with Classical Physics around the year 2000

Some selected pioneers in the development of Physics
Leucippus (first half of 5th century BC) first proposed atomism
Archimedes (287 BC – 212 BC) derived many correct quantitative descriptions of mechanics, statics, hydrostatics, levers etc.
Of the modern physicists, the most famous is Isaac Newton (1642 - 1727) who built on the works of Galileo Galilei (1564 - 1642) and Johannes Kepler (1571 - 1630)
James Clerk Maxwell’s (1831 – 1879) theory of electromagnetism unified the fields of electricity, magnetism and optics
The work of Albert Einstein (1879 – 1955) marked a new direction in physics that continues to the present day.

Between 1600-1900 AD, great progress was made in explaining the way the world is organised. Classical physics was largely based on observations at macroscopic scale and at modest values of velocities - the laws of physics were very successful in explaining a large body of empirical data.
However, the world operates on the atomic/molecular level.
Speeds at which electrons and other particles move are comparable to the speed of light. Physics works differently for these conditions.

Quantum Physics and Relativity provide the theoretical basis to understand the new physics. The results of quantum physics and relativity reproduce those of classical physics for macroscopic systems and for low velocities.

By the end of the nineteenth century, the general belief was that science and technology are fully developed. In 1897 Charles H. Duell, Director of the US Patent Office, advised President McKinley to close down the Patent Office, because it no longer serves a purpose since:
“Everything that could ever be invented had already been invented”!

but physicists were worried...
Serious cracks had begun to appear in Classical Physics!

By the year 1900, Classical Physics started to have serious difficulties with the experimental results from black-body radiation, photoelectric effect etc. Results were in contradiction with the fundamental laws that had served well for the past two centuries.
Additionally, the nature of light and of the newly discovered X-rays and nuclear radiation just did not make sense. Probably most serious of all, physics could not define a reference frame with respect to which laws could be stated – the whole framework on which physics was based was in doubt!