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.

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.

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.

Could you translate this phrase to English?

Yes?

As for the one in New York, I'll give you three guesses where La Liberté éclairant le monde was made.

Damn science believers, always messing up people's fantasies.

Have you ever seen a shiny new penny versus an old tarnished one? Or the Statue of Liberty? Or an old building with one of those weird green roofs?

They're all copper, with varying amounts of oxygen. Oxygen free copper is expensive copper that's specially made to get rid of as much of the oxygen as possible: https://en.wikipedia.org/wiki/...

If the article got something so simple as THIS completely wrong, one can easily presume that the REST of the article is incorrect gibberish.

"oXyGEn-fREE cOppER", lmao

Indeed.

You can of course have it delivered to your door as well. If you can't receive it at home, as is the case being discussed in this thread, then you can have something delivered to a locker. When you go to pick it up you tell Amazon you're there and they pop open the correct door.

It's interesting how practically everything is "the most convincing proof we have."