According to Einstein, a lot of energy is needed as a spacecraft nears the speed of light. Therefore, > 1. If Einstein is correct, it will be impossible for humans to travel at the speed of light. > > 2. If Einstein is correct, it would be impossible for humans to travel at the speed of light. > What are the differences among the above sentences if both are correct? Because I wonder if it is possible to use "would" in the main clause while the conditional clause is a present tense?