An introduction to rocket science

The Impossible EM drive and Newtons laws


Here I will describe the basic principals of rocket science to show why the EM drive is considered impossible, but also why it is probably just an experimental error. 

Classical physics is based on Newton’s three Laws of motion which describe:
1) What happens to something when no force is acting on it.
2) What happens to something when there is a force acting on it.
3) That forces always come in pairs.

Newton’s first law states that when there is zero resultant force acting on an object it will stay at the same velocity. This means if it isn’t moving it will stay that way. It also means 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 is that Force is equal to the rate of change of momentum, F = dp/dt. However, in most cases can be expressed as the equation F = ma. Which means if a force (F) acts on a mass (m), it will accelerate (a). Since acceleration is a rate of change of velocity, this means 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, speed just has magnitude. So 50mph or 300m/s are speeds since they are just a number without direction. If I said 50mph north, or 300m/s straight up, they would be velocities, since they have direction.

This means whenever something speeds up, slows down or changes direction there is a change in velocity, so there must be a resultant force 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 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 Newtons laws only applies to forces. A better description of Newtons third law is:
“Of Object A applies a force to object B, then object B also applies and equal and opposite force to object A."

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

There is an easy way to construct a Newtons third law pair of forces. Since every force requires two objects, so 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 Newtons 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 achieve this by trying to push 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, it you want to turn to the left you do this by trying to push the floor to the right and the equal and opposite force is the floor pushing you to the left.

Now, this is not brain surgery, this is rocket science. After inflating a balloon, and without tying the end and then 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. Due to Newton’s third law, since the ballon is pushing the air out, the air pushes the balloon forwards. This is the basic principal of rocket science. The rocket pushes some propellent back and the propellent 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 extingiusher as propellent for a rocket chair:

The EM drive is apparently able to push its self forward without pushing anything else back, it is a closed system with no repellent, if this were true it would appear to be violating Newtons 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 some situations where Newtons Law’s don’t work, they work just fine when we look at large scale, slow speed objects. But if we look at the atomic scale, or high speed objects we need to use quantum mechanics or relativity instead of Newtons 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 Goldache

However the EM drive appears to even violate our deeper understanding, so this means 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 theory and our understand of the universe is 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 is the least wrong things that we know. The continuing idea of science is build an increasingly accurate picture of the universe, trying to eliminate things we know are wrong, so we can be less and less wrong as we learn and progress. (Note Quantum physics describes what happens in experiment, 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 scientist dream of. Brownian motion, the Michelson Morley experiment, emission spectra, double slit and photon electric effect are all examples of experiments that completely changed our view of reality. Because once 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 propellent. With he publication in 2016 the work has only been done by one group, they had published a peer reviewed paper, but peer review only review the paper, it does not review the experiment, and it is only the first step towards credibility. There is still a large possibility the either the experimenters themselves are in error, or there is something wrong with the equipment equipment. 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 are a major factor that needs to be taken into account for proper µN thrust measurements for these type 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. Hence I think we can consider the EM drive is now dead, and really it was never alive, and Newtons third law is still alive and well.

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

References:

(1) (http://arc.aiaa.org/doi/10.2514/1.B36120)
(2)https://www.researchgate.net/publication/325177082_The_SpaceDrive_Project_-_First_Results_on_EMDrive_and_Mach-Effect_Thrusters
(3) https://xkcd.com/1404/ - credit Randall Munroe aka xkcd

Dr Andrew Gibson BSc PGCE QTS MSc PhD 

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