During our second Ars Live event earlier this month, screenwriter/producer Ed Solomon (Bill & Ted franchise) joined physicists Sean Carroll (Johns Hopkins University) and Jim Kakalios (University of Minnesota) and Ars Senior Reporter Jennifer Ouellette for a rousing discussion on the science and logic of time-travel movies. The discussion was inspired by last fall's Ars Guide to Time Travel in the Movies, written with the objective of helping us all make better, more informed decisions when it comes to choosing our time-travel movie fare—and having a bit of fun while doing so. You'll find the entire discussion in the video above, complete with a transcript.

Not all time-travel movies are created equal. Some make for fantastic entertainment, but the time travel makes no scientific or logical sense, while others might err in the opposite direction, sacrificing good storytelling in the interest of technical accuracy. The best strike a good balance between those two extremes.

We started off by letting Carroll recap his fundamental rules for time travel in the movies: (1) You can't go back earlier than whenever the time machine you're using was built; (2) it's easy to travel to the future, and special and general relativity give us ways to get to the future faster; (3) it may or may not be possible to travel to the past BUT.... (4) if you do, you can't change the past. Whatever happened, happened.

Enlarge / A stack of 60 Starlink satellites launched in 2019. (credit: SpaceX / Flickr)

SpaceX began launching operational Starlink satellites five years ago this month. Since then, the company has been rapidly developing its constellation of broadband satellites in low-Earth orbit. SpaceX has now launched about 6,000 satellites with its Falcon 9 rocket and has delivered on its promise to provide fast Internet around the world. Today, the company is the largest satellite operator in the world by a factor of 10.

But is this massive enterprise to deliver Internet from space profitable?

According to a new report by Quilty Space, the answer is yes. Quilty built a model to assess Starlink's profitability. First, the researchers assessed revenue. The firm estimates this will grow to $6.6 billion in 2024, up from essentially zero just four years ago. In addition to rapidly growing its subscriber base of about 3 million, SpaceX has also managed to control costs. Based upon its model, therefore, Quilty estimates that Starlink's free cash flow from the business will be about $600 million this year.