Recent content by bcrowell

Our political system is supposed to provide a system of checks and balances that prevents the president from being an autocrat. Trump's *style* is very much that of the Mussolini-style strongman, but it remains to be seen what would happen if he got into office. A not-so-disastrous scenario...

There are two different definitions of an inertial frame, one in Newtonian physics and one in relativity. Of the four forces of nature, three can be shielded against, so a particle can be isolated from them in a well-defined sense. However, we don't have gravitational shielding, and the...

I came across this very interesting podcast interview with Robert Paxton on the parallels and non-parallels between Donald Trump and 20s/30s fascism. This is a topic on which we've had some previous discussion here. Paxton was a historian of fascism at Columbia, is now emeritus. The following...

Discussion of string theory would be more on topic in a separate thread in the BTSM forum.

I find it really helpful to use Penrose diagrams when thinking about this kind of thing. My book Relativity for Poets has a nonmathematical introduction to Penrose diagrams in section 11.5.

I don't know what Orodruin intended when he said, "To a good approximation, the stress energy tensor is zero[...]" Doesn't make sense to me. Maybe he could clarify. The second half of his sentence is this: "[...] and you can consider Mercury a test planet moving on a geodesic world line in the...

If you want a book at the level of the Feynman Lectures but that's more practical as an introduction, the classic choices would be Kleppner and Kolenkow for mechanics and Purcell for E&M. If you want something that's easier than the Feynman lectures, there are quite a few possibilities out...

I wouldn't overstate this. For example, we have pretty good models of the interior of Jupiter. I don't think there's any realistic prospect of ever testing those models directly and in detail, since we can't use seismic waves as we do on earth, but that doesn't mean we can't believe the models...

If technology is such that you can accelerate continuously at 1 g for 20 years of proper time, then the technology is pretty godlike -- many centuries more advanced than anything we have. The amounts of energy involved are insane. Given that, I can't imagine that exposure to ionizing radiation...

Are you claiming that nothing inside the event horizon is covered by current physics, or only that what happens at the singularity (or at the Planck scale) isn't covered? If the former, then I don't know why you would say that. Same question.

This is very interesting, but India has specific characteristics that don't translate to other places. Big parts of their educational system are completely dysfunctional (K-12 teachers who draw a paycheck but don't show up, business students in college who spend their classes chanting...

Your example is not really a good one IMO. I would tell them it's a handheld device containing a camera and radio transmitter and receiver, and I think they would immediately understand that. It also has the ability to let you read books, watch moving pictures, and play arcade games. 100 years...

Such a switch doesn't mean they suddenly discovered they had a previously unknown talent for math. They could be switching from physics to chemistry, or from history to English.

This isn't specifically a GR issue. An object can't make a nonzero net force on itself, because of conservation of momentum. Therefore its motion isn't affected by its own field. This is true for a test mass in Newtonian gravity and also for a test charge in electricity and magnetism. The part...

Any method of interstellar exploration requires technology that's multiple centuries ahead of what we have now. With that kind of technological advance, it doesn't seem reasonable to me to imagine that our species would remain bound by the physiological limitations of a Homo sapiens body...