Tag Archives: Lawrence Krauss

What are “laws of physics”?

In Sam Harris’s interview with Lawrence Krauss, regarding Krauss’s new book “A Universe from Nothing” (I haven’t read the book yet, it’s on order), Sam asks:

“You have described three gradations of nothing — empty space, the absence of space, and the absence of physical laws. […] Might it not be easier to think about the laws of physics as having always existed?”

Thinking about physical laws like this, as “entities” that can have existence in their own right, is widespread, but in my opinion fundamentally misguided. It seems to regard “laws of physics” as an underpinning “structure” that directs and controls physical matter. If this were true then it would make sense to ask whether “physical laws” have always existed. But if physical laws “exist” in this sense then what are they made of? How do they interact with matter? How do they effect their actions?

That train of reasoning is ill-founded. Physical laws are not entities with existence in their own right, they are simply descriptions of how matter behaves. The “laws” governing a fundamental particle are simply a summary of the nature of that particle and its behaviour when interacting with other particles. Oxford Dictionaries defines the scientific use of “law” as meaning:

Law: (3) a statement of fact, deduced from observation, to the effect that a particular natural or scientific phenomenon always occurs if certain conditions are present.

You can no more have “laws of physics” existing independently of matter than you can have a “description of X” independently of “X”. Thus if matter exists then you can have a description of it (= “laws”). But if there were no matter there could not be a description (and thus one could not have pre-existing “laws”).

To illustrate this let’s take a fundamental physical law, namely conservation of momentum, the fact that summed over all particles the total momentum never changes. Since a force can produce a change in momentum (Newton’s Second Law), this requires that every force be matched by an equal and opposite force (Newton’s Third Law), such that overall forces cancel and momentum stays constant. Continue reading