Space.com: "Physicists have an excellent grasp of the concept of space-time at the classical levels provided by Einstein's two theories of relativity, with his general relativity theory being the magnum opus of space-time theory," [Eric] Davis said. "However, physicists do not yet have a grasp on the quantum nature of space-time and gravity." by Nola Taylor Redd
'[Luca] Amendola agreed, noting that although scientists understand space-time across larger distances, the microscopic world of elementary particles remains less clear.
'"It might be that space-time at very short distances takes yet another form and perhaps is not continuous," Amendola said. "However, we are still far from that frontier."
'Today's physicists cannot experiment with black holes or reach the high energies at which new phenomena are expected to occur. Even astronomical observations of black holes remain unsatisfactory due to the difficulty of studying something that absorbs all light, Amendola said. Scientists must instead use indirect probes.
'"To understand the quantum nature of space-time is the holy grail of 21st century physics," Davis said. "We are stuck in a quagmire of multiple proposed new theories that don't seem to work to solve this problem."'