Here's the basic idea, and it has to do with the speed of light in a vacuum being constant no matter the speed of the source. Suppose one person stays on Earth while another flies away on a spaceship going a significant fraction of the speed of light. Also suppose that they magically have a way of keeping an eye on each other instantaneously as the one travels.
To the guy on Earth, a day passes, but his monitor of the spaceship shows only a handful of minutes has passed there.
To the guy on the spaceship, a day passes, but his monitor of Earth shows years have passed there.
(The exact ratios depend on what fraction of the speed of light the spaceship is going, but you get the idea, I hope.)
Does that seem really bizarre and out there? It shouldn't. We run across the same thing all the time in our writing efforts.
(Yes, I just segued from Einstein to writing fiction.)
Sometimes you read a scene that's supposed to happen in a matter of seconds, but you feel like it takes hours. Or significant time is supposed to pass, but it feels like the blink of an eye.
Time passage mismatch = PROBLEM
The most obvious solution may be a matter of real estate on the page. Something that's supposed to happen quickly takes only a line or two. The passage of more time gets several paragraphs.
That might work in certain situations, but what if the details of that super-quick scene are significant? What if nothing of note happened in the passage of three months?
In the first situation, how do you get those details in there without getting that feeling of sluggishly trudging along? I've had some success with shorter, snappier sentences, particularly in fight scenes and the like.
In the second situation, how do you get across that passage of time without just a blink-and-you'll-miss-it statement of "Three months later..."? I think that's a matter of transitions. Those three words may be too little. Several paragraphs about a lot of irrelevant nothing happening during those months is too much. But a few carefully worded sentences in the transition can give that weight, get the reader in that feeling of time passing.
Those are the first solutions that came to my mind, but I'm sure there are more. What tips do you have for controlling your readers' perception of time?
And for those of you interested, here's one of my favorite clips explaining that whole time dilation concept.