How do we know the rules of physics are the same waaay over there...

I was trying to talk to a man at church the other day - nice guy - drug rep - not the most scientific person in the world, shall we say.

He’s got an obvious mistrust of science, since it conflicts with his view of how God & the universe are related. (I’m stating things carefully because I’d like to avoid the theological argument).

Anyway, he asked me how can we know that what we use as the laws of physics here are the same laws of physics way the hell over there. We estimate the age of the universe and all sorts of other things based on how that galaxy a million light years away is acting, but is it acting within the same physics rules as our local galaxy?

My answer was that we can measure the light coming back from it and see what we expect. We can watch it rotate and know that gravity seems the same over there.

My argument seems weak, though.

Any other insights on this?

You’ve got it. We don’t know the way he knows. He knows things that he accepts on faith. Science knows things that it can measure, analyze, predict, and replicate. When analyses based on measurements taken locally accurately predict behavior seen elsewhere, science becomes comfortable that those analyses are correct. When they are able to be replicated over and over again, that confidence grows. But it will never satisfy someone who takes things on faith, because magical thinking is inherently immune to that kind of process.

We don’t know that the rules are the same everywhere, but we can observe many things from far away, and if there were considerable differences we’d probably see inconsistencies. So far, no inconsistebncies not otherwise explicable have turned up (at least as far as I know). The physical scaling that begins with parallax and continues with observations of variable stars and continues on to red-shifting seems to be consistent within the three different methods 9where they overlap) and with theories of cosmology.

If I understand correctly, one of the beliefs of the ancients was that the “heavens” and the earth were different kinds of places, where different laws applied; and one of Isaac Newton’s key insights was that the same force (gravity) that caused an apple to fall to the ground causes the planets to “fall” toward the sun, keeping them in orbit.

But really, we don’t “know” that the same laws of physics that apply here also apply throughout the universe, any more than we know that the same laws that applied yesterday will still apply tomorrow. It’s an assumption that we make, and we accept it because it’s such a fruitful assumption and has worked well for us.

(By the way, KneadToKnow, how do you know so much about the man’s thought process from what little the OP told us?)

I analyzed based on the measurements I was given and predicted. If my analysis fails to predict accurately, I will modify it.

I’ll just throw in the official term for this assumption or hypothesis - the Cosmological Principle, which can be used to find more info. (I’ve been interested in the idea ever since I heard it mentioned on ‘the Science of Superheroes’ hehe.)

At this point we seem to have some evidence for it applying at certain ranges and for certain laws and properties of the universe, but it’s so wide-reaching that it will probably never be ‘proven’ as more than a philosophical stance. :slight_smile:

It was one of the two postulates (or axioms) Einstein used for both relativity theories (the other being the constancy of C in a vacuum). Like any axiom, it is just assumed to be true because the nature of logic is that you must start somewhere to avoid arguing in circles.

And it’s not just visually looking at the light from things and saying they appear to be behaving the same. We can study the light through spectroscopy (putting the light through a prism and analyzing the rainbow it makes) and find the chemical composition of other stars. Recently, they found that the mass ratio between protons and electrons are the same six billion light years away as they are on earth.

It might be interesting to point out to the fellow that the very products he is selling - pharmaceuticals - are discovered/created/isolated by scientists working on a molecular level. A level of existence so minute that images of it can only be detected by a complex and expensive apparatus. In other words waaaaay down there. And those scientists, using the assumption that the same Laws of Nature apply even where they can’t see, are able to do their work and provide him with his livelihood.

I can promise no enlightenment will ensue, but feather-ruffling can be amusing.

I’ve wondered about this assumption too, but bigger leap I think is our position in the universe.

As I understand it when we started looking out into space we found it looked the same in every direction.

Rather than the natural assumption that we’re in the middle of the universe, instead we assume that everyone is in the middle.

No matter where we go in the universe you would always seem to be in the middle.

Seems quite a leap.

Well, the physics of very small things has progressed well past the stage of “Let’s assume that things there are the same as at much larger scales.”

As opposed to assuming we’re in the middle?

We already know the Earth isn’t the middle of the Solar System. And the Solar System isn’t in the middle of the Milky Way. And the Milky Way isn’t in the middle of our local group of galaxies. So if some grand universal force was trying to center everything on humanity it did a pretty poor job of it.

It’s true that when we look beyond our local group of galaxies everything seems to be receding from us. So maybe the universe is really centered on some planet in the Andromeda Galaxy and we’re just an afterthought … . :wink:

Seriously, though … the red shifts of distant galaxies are consistent with space stretching uniformly in all directions. That means that aliens in a distant galaxy will observe exactly the same expanding universe as we do. If we see the same thing they do, how can either spot claim to be the center?

Aren’t the Laws of Nature actually quite different below a certain scale? Or has someone reconciled quantum mechanics and relativity without telling me?

The fundamental laws of nature are invariant (we assume) everywhere and on all scales; it is our limited interpretation of them that causes them to appear to be “different” at different scales. It’s like looking at the television set; up close, all you see are colored dots; pull away and you don’t see any dots, just an (apparently continuous) image.

There is no middle, except insofar as as you are “in the middle” of your own inertial frame of reference. You might as well talk about being in the middle of the surface of a sphere.

As for the question posed by the o.p., we assume the invariance of basic physics because its the only way to make a coherent theory. In General Relativity this is known as “general covariance”, an extension of global Lorentz invariance. How do we know it is true? Because our observations are consistent with that theory. Some kind of pan dimensional deity could be having a bit of sport with us, of course, making it look all neat and tidy while actually stretching space here and folding it there, but we’d never know it. That seems sort of unlikely, though; most gods have better things to do besides hanging and messing with the basic physics or smiting people for eating the wrong thing and that sort, but there’s always one slow kid over in the corner who can’t keep up with the class.