Stump the Scientist: What started motion?
Michael Seidel sent in the following question:
If everything is in motion, and we are to believe Newton’s 3rd law to be true, what started the motion?
We can make this question hard or easy, depending on how we interpret it. The hypothesis “everything is in motion” is certainly approximately true, since, if we examine things around us closely enough, even seemingly stationary objects are composed of atoms which are moving because of thermal excitation (heat). For the reader, Newton’s 3rd law says “for every action, there is an equal and opposite reaction”, and this means that the force needed to place anything into motion will also place into motion the thing which applied the force. So we can answer this question by saying that there are 4 forces (gravity, electromagnetism, weak, and strong) which act on all known matter and energy in the universe, and therefore it is natural that these combined forces will have caused “everything to be in motion”, since they act on everything. So the 4 forces started the motion, in the sense of Newton’s 3rd law.
We can make the question harder philosophically by asking whether there is any net motion in the universe as a whole, combining all its parts, and how the 4 forces got started. While physics is theorizing about how the 4 forces arose from the “Big Bang”, we have no generally accepted answer. Such questions also require speculation about whether there is any other thing outside our (known) universe to act as a reference for motion, another unknown.