Very good points -- but there's still a way to do it in GR, dropping all references to lightcones. The below figure is from my Rev Mod Phys piece with Nathan Argaman. Consider the screening region in (a) rather than (c). One can define no-action-at-a-distance to mean that S screens off 1 from 2.

Heading to Alaska soon, where apparently even the landslides are beautiful.

Here’s a relevant slide from a talk I’m giving at a Foundations conference at Chapman next week.

Tonight I got to see the first-ever spacecraft image of Mars, from Mariner 4's flyby. Apparently, before the raw image was processed, people at JPL printed out paper strips of numerical pixel values and -- coloring by hand! -- used pastels to distinguish different brightness levels.

It'll be a week before we are allowed back in to see what’s left of the neighborhood, not to mention the rest of Altadena. That makes it hard to process the larger scales of the tragedy, especially as we’re all dealing with personal issues.

But Altadena, as we knew it, is gone.

It’ll be missed.

I’m not even totally sure that they’re okay – I don’t know their last name, and quite a few people are still missing. I hope they got out in time and they’re now hunkering down somewhere safe like the rest of us.

4/

Another shot of Feynman’s house. The couple who lived in the house are extremely nice, and when they found out I was a physicist, they invited me inside for a “Feynman history tour”. Sadly, I never got around to taking them up on it, except for a dog-play-date in the backyard.

3/

It was a beautiful and friendly neighborhood. When Feynman lived here, I’m told that he went by “Dickie” to the neighbors, and often went running in these hills above Altadena.

2/

Richard Feynman’s former house in Altadena burned to the ground last week in the Eaton Fire. So did thousands and thousands of others -- including mine.

1/

So whatever happens to Marty 2.0, after he goes back in time? We never find out. Presumably, he was unable to get Back to the Future. It brings tears to my eyes every time I watch it.

You may now weep.

(end)

But then, what to make of the scene where Marty watches his doppelganger go back in time? Who is he? Not the original Marty, that’s for sure. This other “Marty 2.0” was raised in a well-adjusted household. He’s probably a much nicer person, perhaps even the true hero of the story.

6/7

Evidently, “Back to the Future” is a T2-type story. Marty McFly’s activities in 1955 somehow have led to substantial changes to the original block universe, not just in 1985, but through the entire intervening history.

5/7

But most time travel movies are “T2”-type stories, like Terminator 2. It’s not logically coherent to have a nuclear war in 1997 and to not have a nuclear war in 1997. In order to carefully analyze such a scenario, we need two universes, each with their own block-universe events.

4/7

Not all time-travel movies fit in a single block universe. The few that do -- i.e., the first Terminator movie -- I call “T1”-type stories. Even though nothing is changing on this diagram, we can still talk about causal arrows (using the modern interventionist view of causation, of course).

3/7

First, a primer on how to properly think about time. Put relevant events on a spacetime diagram, the so-called “block universe” perspective. Nothing moves or changes on a spacetime diagram (time is represented by a spatial axis, so it’s incoherent to also consider ordinary temporal changes).

2/7

We’ve started a tradition in our department where the faculty give 5-minute “silly talks” for the last physics seminar of the semester. Here was my contribution:

The Hidden Tragedy of Back to the Future

1/7

Pro Tip: When you use ChatGPT to compose an email to your professor, it's a good idea to proofread it first...