Physics, Ontology and the Burden of Proof

Posted on October 13, 2008
Filed Under Philosophy | 6 Comments

For some reason Askimet is treating all my comments everywhere as spam today. I’m not sure why. I’d made a comment over at Philosophy Sucks in his 4-Dimensionalism post. Unfortunately my post never shows up. So I figured I’d post the comments here because I think they’re relevant to various discussions I’ve had here at times.The question is over how relevant physics is for philosophy. In this case the question is whether one can reject how physics considers properties like velocity in place of an alternative ontology.

Richard was making an argument that there was no “instantaneous velocity” because velocity calculated from the difference in position over time. Thus it intrinsically requires more than a moment to have a velocity.

My argument was that physics does treat velocity as a state variable both in quantum mechanics but also in traditional classical physics with the idea of v = dx/dt where one can think in terms of a point.

Richard’s response was:

I think you raise an interesting point, but I am not convinced. Physics often employs simplifying assumptions (like that mass is an intrinsic property of an object in Newtonian mechanics) but that isn’t any reason to think that that is the way that things are. Why can’t I make the same response to your point about velocity being treated as a state variable? It is useful to do so, in the context of some physical theory, but why think it is any more than that, especially given the above argument?

I think this ends up being a burden of proof question. While one can always take an instrumentalist view of physics rather than a realist view there is I think a burden of proof on those who adopt an ontology different than the straight faced reading of physics. Certainly physics can be wrong. Anything can be. But there is a certain degree of evidence with the physics that I think you’re discounting too easily. To say physics is only about utility (the instrumentalist view) while simultaneously arguing ontology seems odd. If one is going to be an instrumentalist about physics why not an instrumentalist about metaphysics in general?

The other problem is of course that Richard’s view of velocity was offered as an argument against 4-dimensionalism. But if Richard’s response to physics is a “just so” argument why can’t the physicist who accepts 4-dimensionalism make the same argument against Richard’s view of the ontology of physics? That is, doesn’t Richard’s response make the argument he appeals to even weaker than it already was?

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Comments

6 Responses to “Physics, Ontology and the Burden of Proof”
1 Stephen, son of David on October 13th, 2008 3:45 pm

Actually, from a physical rather than a philosophical point of view, there can be no such thing a “instantaneous velocity” because (1) spacetime is quantized, not infinitely divisible, and (2) the Heisenberg uncertainty principle states that it is only possible to measure any particle’s location and momentum within certain well-defined limits.

Therefore, it is only reasonable to speak of a particle’s average velocity over some finite distance and time, and then only within a particular range of uncertainty.

On the philosophical subject of “burden of proof”, however, I believe that each party in an argument has a responsiblity to define, at the beginning of the discussion, just what *level* of certainty he or she is willing to accept as “proof”. Only in mathematics is there absolute proof; everywhere else there is only evidence.

For example, Richard Dawkins asserts that he considers the existence of God to be so unlikely, that literally all the evidence in the Universe could not convince him that God exists– that there would still be, in his mind, a simpler and more likely explanation. Specifically, he argues that God is more unlikely, statistically speaking, than the entire Universe.

Quantum mechanics does not define the limits of our mechanical abilty to measure quantities, but the theoretical universal limits of our ability to measure the universe.

In quantum mechanics we can talk about definite values of velocity even if we can’t talk about a particular time. In general relativity we can also talk about velocity, albeit in a more complex fashion.

Also note that QM doesn’t quantize space-time. Indeed QM doesn’t really have a concept of space-time per se. Now various attempts at GUT do have quantized space time with varying degrees of failure.

I don’t think physics necessarily says that velocity is intrinsic to an instantaneous physical state. Jeremy Butterfield makes this point in Against Pointillisme About Mechanics:

the particle’s instantaneous velocity v at t codes a lot of information about what its velocity and location are at nearby times—but not ‘‘categorical information’’. The information is conditional or hypothetical information about average velocities (and consequently, locations). [...] for nearby times the collection of average velocities must be so ‘‘well-behaved’’ as to have a single limit, v, as the times get closer to t; or in spacetime terms, by saying that the nearby history of locations (the local segment of the worldline) must be smooth enough to have at t a tangent vector (a 4-velocity determined by v).

Note that he’s arguing in terms of Newtonian mechanics.

To add, to assume this is average velocity seems mistaken to me, but I don’t have time to argue that in the context of classical physics and I can see folks taking either position. Once you accept QM though things are very different since you’re asking what an operator picks out from a given system after you calculate your eigenvalues and eigenvectors. Uncertainty of course enters in but in a bit trickier way.

Whoops. Sorry. That should have read momentum which is fundamental and not velocity. But you get the idea.

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