What he means by block time is this: Our common sense view of time is that the past no longer exists and the future is not yet. All that exists is the present moment. According to the idea of block time, however, every moment - past, present, and future - exists now. Your future marriage is happening now and has been happening as long as the universe has existed.
Think of all these moments as the moments in a movie on a dvd. The universe is like the dvd and every moment in the movie exists simultaneously on the dvd. Or think of a movie on a reel of film. The whole movie exists simultaneously, but only the frame passing in front of the projector lens is experienced.
In any case, if the block theory is correct then the future exists now which means that it is already established, which means that we're not free to change it, which means that there's no free will.
Here's Johnson:
The first and most obvious “threat” that the block universe theory raises is to our free will. If what I will do exists before I even do it, it doesn’t seem I freely choose to do what I do....if the future already exists, only one action is possible. And on the block universe view, the future already exists. It’s as set as the end of a movie I am already watching....So, my choice to do the action is not free.Johnson fleshes this argument out in his article and, if the block universe is an accurate description of reality, then I think most of what he says is correct, but he nevertheless draws a couple of conclusions that I think are mistaken. For instance he writes:
Now, it would still be possible to judge a person’s character—to recognize the difference between morally good and morally bad kinds of persons. But it wouldn’t make sense to judge them for having a bad character. Why? Because they didn’t freely choose to have the character they do; you could only judge (evaluate) the character itself. And if you recognize that it’s the character, not the person, that is the problem, you must approach reproaching them in an entirely different way.The mistake here, in my opinion, is to assume that in a deterministic universe there are morally good and bad characters in the first place. If everything is determined, what does it mean to say that someone is morally bad? There can only be morality if people can choose between acts, but in a block universe there is no genuine choice and therefore no moral good or bad.
Further on he invites us to, "Imagine if people started doing good actions merely because they recognized them as good rather than because they fear they will be punished if they don’t."
Yes, but how is he defining good? And good for whom? However we define good, my good may conflict with the good of others so why should I not put my good ahead of theirs? Why is what's good for me not better and more important, at least for me, than what's good for someone else?
The block theory has its attractions, as does the common sense view (not the least of which is that we can't really escape thinking that time really is how our common sense pictures it), but one question it raises for me is this: If it's true and if, as philosophers like Johnson assume, we've evolved to fit our environment, why did we evolve in such a way as to completely misperceive the nature of the temporal world? Why did we not only evolve the illusion of past, present and future but also the illusion of free will?
After all, we certainly didn't need the common sense view of time in order to survive. Many other creatures probably have no sense of a past or a future and they survive pretty well.
Anyway, there's much else that's interesting in Johnson's essay. If you're fascinated by the philosophy of time you should check it out.