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.

Not yet but I’ll go see it soon What makes me really sad though is that being a “non franchise movie” is now enough of a thing for it to be pointed out specifically. All the big productions these days are in some “universe”, part of a franchise, a sequel or prequel or reboot. God forbid a studio dares to allot a blockbuster budget to an original work.

Once they come of age, they can verify their age by whatever means provided, and flip the adult flag.

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.

s'okay. Biology is the only scientific discipline where division and multiplication are the same thing.

Don't care. If your shit shows me ads, it's not getting into my kitchen. Note to self: Don't buy appliances from Samsung anymore.

Yes, I am vocal in how much I hate ads. I believe the CEOs of advertising companies should get one hit with a stick for every time their ad bothered someone even in the slightest.

Exactly what I'm saying.

The fact that users and enterprise customers are not demanding better software from Microsoft with the same fervor their ancestors demanded that the witch be burnt speaks volumes.

And I'm specifically talking about operating systems here. Software can crash for all I care. I'm fine software quality being all over the place, the market can sort that out. But operating systems are natural monopolies and the foundation for everything else. We should not accept shoddy quality there.

Gardyloo wants you to follow the laws, not Gardyloo.

uh, no. You didn't win.

Places like Bell Labs were more like university research centers than corporate dressing on mandatory-overtime grind. They were not expected to directly turn a profit as business units of the company, because what they did was to lay the groundwork for technology that the other business units could then adapt into products. The return on the investment paid into running them took years or even decades to realize. Without the pressures of needing to turn quarterly or even annual profits they weren't working their researchers to the bone and they were fostering a culture of internship for college students into joining their ranks as researchers to perpetuate the institutional knowledge.

The latter, I think. For instance, Philips was by no means a monopoly, but they had a similar long-term vision. Philips Research generated a similar number of patents, and their researchers were very much encouraged to publish papers and engage with the wider scientific community.

Why use a date field, which introduces all manner of privacy and anonymity issues? Instead, you could use flags: unverified, verified-minor, verified-adult. (and for further protection you could opt to leave minors at the unverified state). It might need some refinement since age restrictions vary with jurisdiction. But recording whether someone is at least over a certain age beats recording their exact date of birth.

And the fridge will be locked until you finished watching them. Resume viewing --- Resume viewing --- Resume viewing ---

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?