There are two reasons for this. Firstly, present-day technology is greatly limited by the lack of power inherent in existing neutron sources. Some experiments are impossible to perform because weak signals are drowned in static. Other experiments take so long to carry out that, in practice, only a handful of parameters can be studied.
It is often impossible to study material under the correct conditions using present-day neutron sources. Instead, the studies are often limited to model systems. These are less complex and detailed than the systems researchers really want to study.
Today, researchers have the knowledge to construct neutron sources with the power to open up entirely new research opportunities. Two such neutron sources have now been built in the USA and Japan. ESS will become the third and the most powerful. If neutron scattering using present-day technology can be compared to taking a photograph in the glow of a candle, the technology behind ESS can be described as like using a flash.
Secondly, Europe will need a new, large neutron source around the same time that ESS should be in full operation, about 2020. Today, there are 15 laboratories based on neutron sources in Europe. However, the technology that they are based on was developed several decades ago. Many of the laboratories will need to be dismantled over the course of the next 15 years. ESS is needed to replace them and to ensure that Europe does not lose its leading position within neutron research.