I have been wondering lately... Physicists are very certain that as far as they can see (meaning the observable universe which is estimated to be about 93 billion lightyears across), physics are the same everywhere. This excludes bubble universes (i.e. the multiverse scenario). That means, for as far as we can see, we believe that E = mc2 applies on Earth just as it applies to the galaxies in the Hubble Extreme Deep Field, as does Entropy, as does F = G(m1m2/r2) and all the other theorems we came up with.
My question is, how can we be sure? Let me give you an analogy I was thinking of yesterday.
Assume we are all small retroviruses living in the ocean. We live in the Pacific Ocean, so our average SG is about 1.025. Since we are about 80nm in length, scaling the sea : microbe ratio, if a microbe was 1.6m tall (like us), then the ocean with a length of approximately 19,800km at the longest end would be 0.05 ly across. That does not seem like much, but think about this - the Voyager 1 which is the furthest man made object from our planet is currently about 0.00207 ly from us. That is 1/10th of the distance the retrovirus is from land in our ocean - universe analogy. So it is safe to say that this retrovirus (located in the middle of the pacific ocean) has not yet had any means to visit the boundaries of the universe to prove the physical properties are the same as in its current location.
The retrovirus performs various physical experiments and measurements. Lets look at specific gravity. He measures that to be 1.025. For him, it seems to be a fundamental constant as everywhere he looks the SG seems to be 1.025. Now he wonders whether this is truly universal. Since he cannot really measure this value at the boundaries of his galaxy (the Pacific Ocean) without making assumptions, he assumes it is constant because of the following reasoning. If the SG would have been significantly different than 1.025, say 1.020, life as he knows it could not readily exist. Osmotic pressure would become troublesome for living organisms (those that have evolved and adapted to SG of 1.025). So he theorises that if SG had not been 1.025 or very close to it, life (as he knows it), would not be possible. Without an example, he cannot fathom how life would look like at a different SG. So SG must be constant.
The problem with this reasoning of course is that it is false. Not only is SG not 1.025 in all oceans - for instance, the average SG in the Red Sea is about 1.032 - some places have fresh water - an SG of 0. And, some places do not even have water at all - just air. Some other places do not even have air - just the cold vacuum of space. The question is - how can this retrovirus in its ocean galaxy detect that not only is his SG not constant, some places the concept of SG does not even exist? Remember, the probe he sent out in 1977 is only 0.002 ly from him. It still has to go 10 times that distance (approximately 342 years) to hit land / a different ocean. And even if it did, it would take an additional 380 years to get the signal of the measurements back to him. So soonest he can directly probe the outer rims of his world, is 722 years from now.
Physicists have long tried to work around this issue by using proxies and inferring data from known facts. For instance, the only way we "know" how big our universe is, is by using a proxy. Nobody can take an actual ruler and measure the universe. So they use a concept called redshift to measure the distance to distant galaxies. However, this redshift is based on certain assumptions. I call them assumptions because as long as we do not fully understand our universe, our best theorems and axioms will always be vulnerable to be proven false one day. Redshift assumes our knowledge of electromagnetic waves and space is universal. That atoms here are the same as atoms there. So if atoms at the boundaries of our observable universe behave differently, or if spacetime behaves differently, we cannot really use redshift and thus our calculations of the size of the universe is incorrect.
Point is, proxies are no substitute for direct measurement. For the time being they are our best alternative, but they cannot be considered infallible. Because of that, I sometimes wonder just how much we really do know about our universe.