That he may sometimes have missed the target in his speculations…

 

"The idea that light is made from particles – that is to say that ‘the electromagnetic field is quantized’ – was deeply controversial and not accepted for decades after Einstein first proposed it. The reluctance of Einstein’s peers to embrace the idea of the photon can be seen in the proposal, co-written by Planck himself, for Einstein’s membership of the prestigious Prussian Academy in 1913, a full eight years after Einstein’s introduction of the photon:

'In sum, one can say that there is hardly one among the great problems in which modern physics is so rich to which Einstein has not made a remarkable contribution. That he may sometimes have missed the target in his speculations, as, for example, in his hypothesis of light quanta, cannot really be held too much against him, for it is not possible to introduce really new ideas even in the most exact sciences without sometimes taking a risk.'

In other words, nobody really believed that photons were real. The widely held belief was that Planck was on safe ground because his proposal was more to do with the properties of matter – the little oscillators that emitted the light – rather than the light itself. It was simply too strange to believe that Maxwell’s beautiful wave equations needed replacing with a theory of particles."