An introduction to rocket science

velocity. This means if it isn’t moving, it will stay that way. It also means that if I were in intergalactic space (where there is no air and negligible gravity to get in the way) and I threw a ball away from me. The ball would continue moving along that same straight line at the same speed, for all time. Or at least until a force came along. Which brings us to the second law;

Newton’s second law is that force equals the rate of change of momentum: F = dp/dt. However, in most cases, it can be expressed as the equation F = ma. This means that if a force (F) acts on a mass (m), it will accelerate (a). Since acceleration is the rate of change of velocity, this means that if a force acts on something, its velocity will change.

Note that I am using the term velocity, not speed. This is because velocity is a vector quantity; it has magnitude and direction, while speed has only magnitude. So 50mph or 300m/s are speeds since they are just a number without direction. If I said 50mph north, or 300 m/s straight up, those would be velocities because they have direction.

This means that whenever something speeds up, slows down, or changes direction, its velocity changes, so a resultant force must be acting on it. When you are in a car or just walking around, every time you change your velocity, there must be a force acting on you to make that happen.

Newton's third law is often expressed as “for every action there is an equal and opposite reaction.” Some people think this means revenge (an eye for an eye), but Newton's laws only apply to forces. A better description of Newton's third law is:
“If object A applies a force to object B, then object B also applies an equal and opposite force to object A."

This means if you push the desk, phone, or tablet in front of you, you can feel it push back on you. This means a force can only act between two objects, with a force of the same type, equal size, but opposite direction acting on each object.

There is an easy way to construct a Newton's third law pair of forces. Since every force requires two objects, we can write every force in terms of the two objects; then all you need to do is take the two objects in each sentence and swap their positions. For example:

The hand pushed the ball. -> The ball pushed the hand
My feet push the floor -> The floor pushes my feet
The horse pulls the cart -> The cart pulls the horse
The Earth pulls you down (gravity) -> You pull the Earth up

Notice all you need to do is swap the two objects in the sentence (maybe a slight change in wording) and this will always get you a Newton's third law pair of forces. A Force is never lonely.

Let us consider how this works. Just walking forwards is something most of us are lucky enough to be able to do without much thought. You do this by pushing the floor backwards with your feet. So your feet push the floor backwards; the equal and opposite force is that the floor pushes your feet forwards. Every time you change velocity, you do this. If you want to turn left, you do this by pushing the floor to the right; the equal and opposite force is the floor pushing you to the left.

Now, this isn't brain surgery; but it is rocket science. After inflating a balloon, don't tie the end; let it go. It will fly across the room and make a funny noise. It does this because the balloon is pushing the air backwards. If you hold the balloon still, you can feel the air blowing out. Because of Newton’s third law, since the balloon pushes the air out, the air pushes the balloon forwards. This is the basic principle of rocket science. The rocket pushes some propellant back, and the propellant pushes the rocket forwards.

If you happen to be on an office chair, you can try this: throw something away from you, and if the friction is low enough, you should be propelled in the opposite direction. Also see using a fire extinguisher as propellant for a rocket chair:


The EM drive is apparently able to push itself forward without pushing anything else back; it is a closed system with no propellant. If this were true, it would appear to be violating Newton's third Law. (If you break the law, you go to jail; however, if you break the laws of physics, go to Sweden for a Nobel Prize)

We already know of situations where Newton’s laws don’t work; they work just fine for large-scale, slow-speed objects. But at the atomic scale, or with high-speed objects, we need to use quantum mechanics or relativity instead of Newton's laws. Newton’s laws work as a simple model, but the universe is not simple, and like most things, the deeper you look into it, the more complexity you will find.

“I Think You'll Find It's a Bit More Complicated Than That”
- Ben Goldacre

However, the EM drive appears to violate our deeper understanding, so either the experiment is wrong, or physics is wrong. The former is far more likely.

The late, great Richard Feynam said:
“It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.”

Since this experiment disagrees with our theory, surely that means our theories and our understanding of the universe are wrong. But wait a second: nearly all scientific ideas, theories, and even a huge amount of published papers turn out to be wrong. Even our current best theories are wrong to some extent. Quantum mechanics and the Standard Model are the most precise and accurate descriptions we have of reality, but we know they are wrong, or at least incomplete, because they cannot describe gravity. General relativity is an excellent theory which describes gravity, but we know it is wrong since it is incompatible with quantum mechanics. However, the Standard Model and general relativity are the least wrong things that we know. Science continues to build an increasingly accurate picture of the universe, eliminating what we know is wrong so we can be less and less wrong as we learn and progress. (Note: Quantum physics describes what happens in experiments; it does not explain itself. Our attempts to explain how Quantum relates to reality will have to go in another post)

An experiment that flies in the face of our understanding, and things that we find in nature that we cannot describe, are what scientists dream of. Brownian motion, the Michelson-Morley experiment, emission spectra, the double-slit experiment, and the photoelectric effect are all examples of experiments that completely changed our view of reality. Because when we find something unexpected, we can learn something completely new and improve our understanding of the universe. So part of me hopes that this EM drive does work.

However, before we start rewriting the textbooks, we need to check that this EM drive really does create thrust without propellant. Since the 2016 publication, only one group has done the work: the published, peer-reviewed paper, but peer review only reviews the paper; it does not review the experiment, and that is only the first step towards credibility. There is still a strong possibility that the experimenters themselves are in error, or that something is wrong with the experiment. This is especially true since they are using a lot of power to measure a very small force (1.2 /pm 0.1 mN∕kW.); see XKCD cartoon below. Another group has since shown that lack of shielding can cause forces of this size:

“Our results show that the magnetic interaction from not sufficiently shielded cables or thrusters is a major factor that needs to be taken into account for proper µN thrust measurements for these types of devices.” (2)

So they put a massive amount of power into a small chamber, and the Earth's magnetic field made it twitch a bit. So I think we can consider the EM drive dead now; it was never alive, and Newton's third law is still alive and well.

Dr The Online Physics Tutor Gibson
The Online Physics Tutor
BSc MSc PGCS QTS PhD