They are synonyms. = If you expect something to happen or be true, it is 'probable' or 'possible' 'It is possible / probable that in the course of time a cure for cancer will be found.' However, 'possible' can also be used when something can be done: 'It is possible to get there by bus.'

Possible means something is capable of happening. Probable means something is likely to happen. Which also implies it's possible. However, if something is possible, that doesn't mean it's therefore probable! ;)

Probable = 很可能, more than 50% likely to happen Possible = 能, It can be done, it's not impossible Probable has to do with how likely something will happen, possible has to do with if it can be done or not.