"Mr. Rideout, we wander at the present moment through a sort of vorticalist twilight, holding up the lantern of the Maxwell Field Equations and squinting to find our way. Michelson's done this experiment before, in Berlin, but never so carefully. This new one could be the giant arc-lamp we need to light our way into the coming century. I don't know the man personally, but I'll write you a letter of introduction anyway, it can't hurt."
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"Mr. Rideout"
That'd be Merle! He's the guy with whom Heino is speaking! Try to keep up!
"wander at the present moment"
To wander: "walk or move in a leisurely, casual, or aimless way." Vanderjuice surely means to invoke the aimless choice in that definition due to science's current lack of knowledge about Æther (what is known at "the present moment").
"The present moment" can be seen as a boundary or a dividing line between the past and the future. It contains all movement, all discovery. Pynchon sees this particular present moment (the end of the 19th Century and, probably the beginning of the 20th Century, judging by the size of the book!) as a critical juncture between many old academic ideas and what they will be replaced by. Or, perhaps, what new ideas will be killed by those who have gotten rich off the old ideas.
"a sort of vorticalist twilight"
The darkness of a deadly storm (vortex/tornado/hurricane) with a hint of danger due to the high winds and debris of cast aside or outdated theories. That's probably what he means! "Look out! Hollow Earth theory coming right at your head!"
"the lantern of the Maxwell Field Equations"
Published in 1861 and 1862, they proposed that light was an electromagnetic phenomenon, unifying theories on light, magnetism, electricity, and electromagnetic radiation, which had all been described previously as separate phenomena. Anyway, that's the simple gist of the Wikipedia entry and even that summation is threatening to crack open my brain. I suppose I should understand how light works much better than I currently do, seeing as how this book (at least this first section), is really focusing on the light theme.
This phrasing is clever because the lantern is what is needed to make our way through the darkness and the lantern uses light and the Maxwell Field Equations provide answers to how light works to help us see our way to understanding light. And electricity. And, um, magnetism. Reading the rest of the Maxwell Equations Wiki entry has provided little help except to remind me how dumb I am.
"Michelson's done this experiment before, in Berlin, but never so carefully"
The problem with Michelson's previous experiment was that his device was subject to experimental errors too large to provide any meaningful results.
"This new one could be the giant arc-lamp we need to light our way into the coming century"
Fitting words as this experiment will, more or less, destroy science's fascination with Æther, allowing it to move forward in different directions, and eventually leading to Einstein's theory of relativity. Also fitting because he compares it to a giant arc-lamp needed to light the way. Again, he uses the metaphor of light to describe our ever-increasing knowledge of light itself. Also knowledge is always said to illuminate. I can't do all the math needed to understand how magnets create electricity which produce light (or whatever is going on there!), so instead I just look for dumb metaphors and apt analogies! I haven't taken a calculus class in 30 years but I've re-read like twelve Xanth novels in the last few years so that probably explains my intelligence level.
"it can't hurt"
When somebody says, "To tell the truth" or "To be honest," they've either just previously lied to you or are about to lie to you. In the same vain (vane? vein?), when somebody says, "It can't hurt," you better believe it fucking will.
