How can we be confident about universal properties, or anything for that matter, in astrophysics when we can’t actually do experiments on stars, distant planets, etc. ?
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We can *see* them though, and this can tell you a tremendous amount about how physics works there.
The sun has hydrogen gas absorption lines in its spectrum because the hydrogen gas absorbs very specific wavelengths of light.
We see these same lines in every galaxy in the observable universe. This is very telling – all the chemistry and quantum physics and electron behavior and blackbody radiation required to produce this very specific property is exactly the same everywhere in the universe.
The way they orbit eachother, the light they generate and absorb, speeds, spectrums, everything. We’ve yet to see any distant objects that *don’t* follow the exact same physics rules our own planets follow.
When you have something you can only observe, and not do experiments directly on, you can still do “experiments” to test hypotheses.
For example, if you’re looking at visible light data from some stars, and you think you have an idea about what’s going on — you can try and predict what you’d see in infrared.
The idea is that a good, simple scientific explanation can usually be based on only a subset of observations: the rest fall in line with those predictions.
There are a lot of stars out there, in every direction and at all sorts of distances.
We can’t, and that makes astronomers just slightly nervous all of the time. It’s a conscious *assumption* of astronomy that any given place in the Universe is broadly similar to any other place in the Universe, and this assumption has been very helpful in piecing together a picture of what’s going on elsewhere. It seems consistent enough to be reliable. But if any evidence ever came out that this wasn’t the case we’d have to rethink everything we know about the Universe.
Enter the **Axis of Evil.**
This is a peculiar observation about the Cosmic Microwave Background. Specifically it’s the observation that under certain kinds of data analysis the CMB appears to have a “hot” side and a “cold” side rather than being entirely uniform, and the line that divides the two is disturbingly aligned with the orbital plane of our own Solar System.
It’d be *really* improbable that this would happen by chance, and that suggests some rather troubling implications for that assumption of uniformity in the Universe and our own unprivileged position within it. It’s *probably* just a quirk of our measurements or something but no one’s ever been able to conclusively explain it. And, absent a solid explanation, it lurks on the periphery of all astronomy and threatens to upend the whole apple cart.
The same laws of physics that explain the motions of planets, moons, stars – which we can directly observe- also govern man-made satellites, like what makes your GPS work. Everything agrees very well.
We can recreate all sorts of conditions of all the physical phenomena we understand here – high temperature, low temperature, vacuum, high pressure, electromagnetic fields…things being different on other planets/stars would require some fundamental new phenomenon.
Plus, we haven’t ever seen evidence that universal properties don’t hold elsewhere, so the assumption that they do continues to gain momentum.
Because 2+2=4 everywhere in the universe as far as we can figure, maybe not inside black holes but I’m not going there now.
“Confident” isn’t “certain.” Ultimately it’s predictive power that showd whether a theory has explanatory power.
As long as we can make enough correct predictions, we can assume for the time being that the theory generating the predictions is close enough to true. If we starting making wrong predictions, we adjust the theory.
When you study physics, you’re assuming that there is physics to be studied. If we’re just noticing coincidences in our measurements and calling them physical principles, the problem is a lot closer to home than stars and distant planets.
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