Musk's response was that it isn't possible and using a law of physics to make it look like he knows something. Which he doesn't since he doesn't have any formal qualifications.
he literally has a physics degree lol. a real one. Thats not even a question.
and an economics degree (though not relevant to this point)
and the law he used is 100% the relevant factor - His third law states that for every action (force) in nature there is an equal and opposite reaction
. If object A exerts a force on object B, object B also exerts an equal and opposite force on object A.
heres NASAs take on it
the law doesn't state that it must always result in a movement, just that forces have an equal and opposite reaction. In the case of an electrically driven force, that force simply isn't strong enough in any currently known or developed technology to achieve moving a rocket. The force occurs, the opposite reaction occurs and nothing happens. Technically possible? sure. SO is building a space elevator, or stopping the earths rotation. The maths says it can be done. But this is the realm of dyson sphere science fiction right now, so for all intents and purposes, no, electric rockets cant be done, literally dues to newtons third law.
i love how the internet is full of people who are just like "aha, gotcha" like they're the ones putting rockets into space because some dude who is literally on the forefront of cutting edge tech, barely sleeps didn't take a more concerted effort to respond in a more technical way that accounted for all the hypotheticals on twitter (in 140 characters) to some dumbass that could have saved himself the question by thinking about it or just googling it.