Universal Law

I wonder whether claims about the inviolability of physical laws (in general, rather than any specific law) actually mean anything... at a fundamental level the distinction between a law with an exception and just having a different law is pretty difficult to pin down. Either the "exception" is due to differences in circumstances that were previously ignored, or it's just random. In either case, you can probably always make a new "law" that accommodates the difference.

As far as we know, there has been not one single violation of conservation of momentum from the uttermost dawn of time up until now.

But the only reason we call the conservation of momentum a law, is that we do not know of any exceptions to it. A law is just a pattern in the behaviour of the universe we do not know of any exceptions to.

And when we do discover exceptions, we just call the old law a "special case" of the new law.

But the new law doesn't look like the old law plus a special clause exempting the exception. It looks like a single, universal, mathematically simple, coherent statement, and it is then very clear that the old law was simply the approximate behavior of the new law under certain special conditions, the way that Newton's old laws of motion are simply the approximate behavior of Special Relativity under conditions in which the relative motion of particles is very slow compared to lightspeed.

The universality of the law is not a sophistry; the universe really does look that way.

Two very different points. One is the whole "multiverse" theory. In that view, different physical laws could hold in different "universes." To the extent that we might ever be able to travel between them, would this not violate the "(multi)universality" of those laws?

Also, of course, there is the problem that the conditions under which such laws do not always hold, indeed may never hold completely. The law of gravity holds precisely only in a perfect vacuum. But there are no perfect vacuums. Now there are plenty where it holds very closely, but there are also many quite relevant to use where it does not even come close to holding, as when air resistance is sufficient to substantially change the rate at which a body's movement towards the earth accelerates, even possibly changing the sign for a period of time (watch the helium balloon go up into the air, dear).

I think that the social sciences seem to still be following the Greek paradigm. Exceptions are excused and generally ignored rather than studied in more depth. New theories are rarely asked to explain the findings that supported old theories. Outliers are dropped, partially to make ignoring exceptions easier.

Barkley, regarding the "multiverse" theory, it's not clear whether you're referring to (a) different bubbles in the inflationary scenario, which hypothetically would possess fundamental laws identical to our own but with potentially different constants "frozen out", or (b) Tegmark's Level IV multiverse, which has never been observed. Regarding (a), as I understand it with my rather limited expertise, the freezing-out process that sets the constants and keeps them stable is itself universal. Regarding (b), it gets into very woolly territory for obvious reasons, but I will remark that it is rather a coincidence that the particular universe in which we find ourselves seems to work according to absolutely stable and absolutely global fundamental laws.

As for your second point about air resistance, one must distinguish between fundamental laws and surface generalizations. If you phrase the law of gravity as "things fall down", it is a surface generalization with exceptions. Closer to your sensory experiences, yes, but that's not always a good thing. The law of gravity in its fundamental form describes a curvature of spacetime which influences all matter. Other forces also impinge upon matter. The sum of these influences is a surface phenomenon, which may or may not add up to "thing X falls down". But the contribution of gravitation to the sum is (so far as we know) a fundamental and absolutely universal law. It applies to the chair beneath you just as much as to a satellite in orbit.

"The law of gravity holds precisely only in a perfect vacuum."

This is another case of confusing the law with the scientific model of it, the equations. The law holds precisely, everywhere, the equations describing it are too difficult to solve except in the simplest cases.

pseudonymous.com says: "A law is just a pattern in the behaviour of the universe we do not know of any exceptions to."

No. The law is the putative explanation of the pattern. As for what a law actually is, I think David Armstrong's approach is promising (a law is a "contingent necessitation between universals"), but to really make progress we would have to be able to say what causation itself is.

Are you familiar with Nancy Cartwright's work, in particular "How the Laws of Physics Lie"? She argues that universality is traded off against truthfulness. Specific phenomenological laws can be shown to be strictly true, whereas the artificial assumptions and approximations needed to make the data fit with theoretical laws argues against the truthfulness of the latter.

Eliezer,

I would agree that there are different variations on the multiverse theory, more than the ones you listed, and that they will give different answers. Of course, within our sufficiently narrowly defined "universe" we like to believe that there are universal laws (including even some constants) that apply everywhere, although we do not know that for sure.

Regarding gravity more particularly, yes the examples I gave involved surface applications. I regularly tell students that the fact that a helium balloon floats upward does not prove that the law of gravity is false.

However, there is a higher level problem, that general relativity, and especially its parts dealing with gravity, remain unreconciled clearly with other parts of apparent physical law, notably quantum mechanics, with the clearest surface manifestation of the problem being the continued inability to "unite" gravity with the other three basic forces (which have been pretty much shown to be "unitable" within existing frameworks). Of course this is what string theory, quantum loop theory, and some other candidates have been trying to achieve, but the failure to clearly find such a GUT leaves a certain level of doubt at a very fundamental level regarding the complete universality of the "law of gravity" in its general relativistic conceptualization.

How careful do we have to be deriving laws from our empirical observations... deriving laws that we think must be true because we have observed them to always be true.

I believe this is analogous to Hempel's Paradox, otherwise known as the raven paradox http://en.wikipedia.org/wiki/Raven_paradox .

I wonder what, out of everything we think we know, must actually be true. Is there anything we can really say with 100% confidence? What truths can be derived by examining what happens when a proposed truth is not in fact true?

Only 80%? I hope you've brushed up on your physics in the past three years.

The speed of light isn't some arbitrary speed limit. The speed of light is the speed of masslessness. Everything without mass (prime example: photons), must travel at that speed. Further, anything traveling at that speed does not witness the passage of time, experiencing the entirety of its trajectory at once.

Stated even better, everything travels at the speed of light; it is merely that massive particles divert most of that velocity into traveling through time. There is an intimate connection between spacetime and mass; note that no amount of electric charge bends spacetime.

The speed of light barrier exists absolutely with a probability easily exceeding 99%.

I think unusual things have happened (things with a low probability that nevertheless occurred; things about which it might not be appropriate to say they actually had a 100% chance of occurring all along if only we knew enough). So unusual things have occurred, but nothing strange.

Does your 80% confidence that the speed of light has been a limit "throughout the entirety of [...] time" mean you assign less than 20% confidence to versions of the inflationary hypothesis that violated it, or do you really mean "throughout the entirety of time except perhaps in the first tiny bit when things might have worked differently"?

Ahem.

http://www.wired.com/wiredscience/2012/09/warp-drive-plausible/

This is likely what I will study in the future. And we've developed metamaterials that can create negative indexes of refraction and optical black holes, once again supposed impossibilities. It may be best to save assumptions of impossibility for when we know the exact underlying universal laws.

One of the reasons the ancient Greeks didn't discover science is that they didn't realize you could generalize from experiments.

Wow! That is a really cool thought.

As far as we know, there has been not one single violation of conservation of momentum from the uttermost dawn of time up until now.

And because we know that, any unusual reports that would seem to imply such a violation may have happened are obviously false...  Ties up the loose ends.