Lots of people could and did design and/or build personal computers in the 70s. Magazines published designs and sold mail order kits pior to the Apple I. There were also a bunch of pre-built home computers contemporary with the Apple I/II, and several of them were more popular.

Neither Steve was really the singular genius people like to retrospectively paint them as. Together they did good work and were in the right place at the right time with the right motivation.

Which is also mass density. Multiply by c to keep the units straight. (If you're a physicist, you just set c=1, and the units don't matter.)

I guarantee, if you have a mass density of M kg/m3 and you put it in Einstein's field equations, you get gravitation.

No, the box is an essential part of the system. In the thought experiment neither the box nor the photon has mass but both possess momentum since the photon can bounce off the walls of the box. The photon and the box, together, form a system that has mass.

You can see how systems can have mass by rearraning the mass-energy equivalence equation to solve for mass:

m = sqrt(E^2 - (pc)^2) / c

The energy and momentum of a fundamental particle are related so you can't manipulate them independently. But if you have more than one particle it's pretty easy to manipulate the momentum of the system without changing the energy, and thus make m non-zero.

Many teachers, including the pop science variety, like to appeal to your intuition. Sometimes that's okay, sometimes it just stunt's your understanding. Your idea of mass is rooted in pre-20th century physics. The end of the 19th and beginning of the 20th century changed how we think about a lot of things, mass included. You're trying to take your idea of mass as an axiom and make everything else fit. It doesn't. Photons are massless. If they weren't, they would either not travel at the speed of light or have infinite energy. The "thing" that gravitates is energy and various types of energy flux, including momentum. That explains, consistently, how photons can travel at the speed of light, massless particles can interact via gravity, and adding energy to a system can increase its mass.

Energy density, yes.

https://en.wikipedia.org/wiki/...

People used to say "relativistic mass." We've realized that was a bad idea.

A fun movie, but the movie wasn't in any way hard science fiction. I don't think there was any real science in it, other than the distances to various stars. It was hand-wavium from beginning to end.

But, heck, that's par for the course for film science fiction. If you try to find the science in "Dune" or "The Last Jedi", good luck.

The UK has a head of state (a king) they've spent a thousand years learning should stand around a look pretty with medals and things but that's about it. The US has a head of state (a president) they've spent at least the last hundred years turning into a cult of personality and giving more and more power to.

France has spent a few hundred years violently oscillating back and forth between the two. They've demonstrated it doesn't really matter whether you call it a king or a president, it's how much power you give them.

The first fission reactor in space, the American SNAP-10A had an experimental ion thruster.

Yes, but it didn't go to Mars. That's why I said we didn't have good ion thrusters. The one on SNAP lasted a whole hour and apparently had quite a few problems even then. Getting the things to last long enough and produce enough thrust to be useful even for station keeping is a fairly recent thing.

There most certainly is. Density-- mass per unit volume-- is the (0,0) term of the stress-energy tensor.

The conops says that the reactor doesn't get turned on until after it's successfully placed in a high orbit.

A good feature of nuclear reactors is that they aren't dangerously radioactive until after you turn them on.

I even wonder why they haven't done it much sooner.

We didn't have good ion thrusters back in the 50s, 60s and 70s and after that launching nuclear reactors into space was considered a bad idea, not without reason. A nuke plus ion engines isn't a slam dunk either, ion engines produce very little thrust and reactors are heavy even if you don't have to bother shielding them much, so there's an efficiency threshold you need to hit before it's worthwhile.

NASA has realized that beating, or at least competing with, the Chinese to a moon base is probably going to require a reactor, so why not demonstrate it as part of a drive too?

the original formulation of relativity and physics in general did not distinguish between rest mass creating gravitation and light speed particles generating gravitation

Maybe you have access to some early draft notes of Einstein's, but in his actual papers on relativity mass does not "create gravitation." Energy, momentum and some off-diagonal terms like stress and pressure gravitate. There is no mass term in the stress-energy tensor, nor anywhere else in the Einstein Field Equation. Mass is not fundamental in relativity, it's a property of a system. That property is the product of energy and momentum (and the other stuff) in particular configurations within the system so in many situations it can be used as a surrogate for the underlying energy, momentum and other stuff.

Physics prior to relativity did indeed say a lot of different, confusing things about mass, gravitation and light speed particles.

"Creating fusion" isn't hard. Kids do it for science fair projects. Here's a guy on Youtube making a fusion reactor.

Making a fusion reactor that produces more electricity than it uses is hard. That's what you're thinking of. Rocket engines famously do not usually produce electricity, and if they do they do it extremely inefficiently, so it's a completely different problem.

we don’t have massless drives

Reactionless drives. A massless drive would be an engine that didn't have any mass, I guess. We have lots of drives that don't involve throwing mass out the back, including solar sails, magnetotorquers, electrodynamic tethers, flashlights, etc. Hard drives have a few. Your car has at least one big one and a bunch of others besides, as does your body. None of them are reactionless though.

Reactionless drives are called that because they violate Newton's third law, which is really a statement about the conservation of momentum.

Individual photons don't have mass, abstract or ortherwise, they have energy and momentum. Individual fundamental particles of any kind don't really have mass because mass is a property of a system.

A system of multiple photons can have regular old non-abstract mass if they're configured properly.

Here's a case of a very experienced journalist getting caught by including made-up quotes that had been hallucinated by the AI he'd used to summarize research information: https://www.theguardian.com/te...

Vandermeersch added: “It is particularly painful that I made precisely the mistake I have repeatedly warned colleagues about: these language models are so good that they produce irresistible quotes you are tempted to use as an author. Of course, I should have verified them. The necessary ‘human oversight’, which I consistently advocate, fell short.”

When even experienced journalists fail to find AI hallucinations, you really can't expect unpaid volunteers to do better.