Thread by @anderssandberg

#FridayPhysicsFun - the EmDrive has failed some fairly rigorous tests. So no neat reactionless space propulsion. But why were most people so confident that it really did not work even before this?
popularmechanics.com/science/a35991…

The EmDrive purports to produce thrust by reflecting microwaves inside a conical cavity, producing an uneven force on the device that would make it move - in violation of momentum conservation.
en.wikipedia.org/wiki/EmDrive

Momentum in Newtonian mechanics is just defined as the product of mass and velocity. The change of momentum is equal to the net force on it. In relativistic mechanics one often takes the force law as the definition.

Historically, people had been groping towards a theory of inertia or impetus long before Newton nailed it mathematically. John Wallis noted that momentum was conserved in 1670. en.wikipedia.org/wiki/Momentum#…

In a closed system where no external forces act the total momentum is constant. This follows from Newton's laws of motion (or their relativistic and electrodynamic counterparts). But *why*?

The deep answer is Noether's theorem: every differentiable symmetry of the action of a physical system has a corresponding conservation law. Translation invariance leads to momentum conservation. en.wikipedia.org/wiki/Noether%2…

The action is a pretty abstract thing that describes how the various kinds of energy in a system changes across time; what matters is that one can derive the equations of motion from it. In a sense it describes the laws of physics in a bundled up form. en.wikipedia.org/wiki/Action_(p…

The symmetry part is about what changes leave the form of the action unchanged. Gravitation and electromagnetism do not care if they are acting east-west or up-down in some coordinate system, or where or when it happens.

Noether's theorem basically says that unless physics works differently in different places there should be something conserved - momentum. This is a very strong theorem that underlies much of modern physics. It is more about math than physics. hackaday.com/2016/06/14/sym…

So if the EmDrive were to accelerate without any forces from the outside, that would require some *very* weird physics. Normal mechanics, relativity, electromagnetism and quantum mechanics after all obey the theorem.

It is always possible that physics really doesn't obey Noether's math (e.g. maybe it is actually a discrete cellular automaton pretending to be continuous space). It would be a truly fundamental break with what we have seen work so far - lots of evidence.

https://twitter.com/anderssandberg/status/1223307332081405958

On the other hand there is a *long* history of devices purporting to produce reactionless force. After all, it would be so desirable! Unfortunately this history is crammed with wishful thinking, fraud, and measurement errors. en.wikipedia.org/wiki/Dean_drive
projectrho.com/public_html/ro…

If you propose some clever mechanism that does this, follow the advice of Jerry Pournelle and demonstrate that it has a sizeable real effect. No amount of theory will be convincing without a solid demonstration.
jerrypournelle.com/science/dean.h…

My prediction is that the EmDrive will not go away, since there will always be people thinking it is being suppressed by the Powers That Be just like cold fusion. No evidence will convince them otherwise, but the rest of us can focus on other weird things.

There are *some* ways of moving without external forces or ejecting stuff with momentum. My favourite is gravitational swimming: by deforming a body in curved spacetime one can translate it.
web.mit.edu/wisdom/www/swi…
repositorio.unicamp.br/jspui/bitstrea…

What is happening here is that different places *are* different because of the curvature, and Noether doesn't conserve momentum. In fact, general relativity doesn't have any global conservation of energy in expanding universes since curvature changes over time. Awkward!

You can also move by releasing your microwaves behind you, but this is a normal photon rocket (& inefficient). If you had a directional neutrino source you could fake a reactionless drive. If you could bend spacetime you're golden. But you better demonstrate it practically.

Noether's theorem and thermodynamics are super-powerful constraints on what happens. There are ways of cheating, but they require much cleverness. Let's search for them.