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Comment by swayvil | Hacker News Reader

Would it be upsetting if we sent a probe a lightyear away and found that some constant, I dunno, Planck's constant or whatever, had changed?

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Not sure if upsetting is the right word, but people are looking for things like that. It would be a remarkable discovery for sure. There are markers in the spectra of distant stars that could give us a hint, so sending probes is not necessarily a requirement. So far, potential variances have been constrained to a very high degree.

Excuse me, to a very high degree? Can you elaborate?

To put another way, the amount of change in plank’s constant that could potentially happen is highly limited (based on our current methods of measuring and inferring), and only getting more limited as we improve accuracy.

Not OP, but this is talking about bounds on how different such constants can be across time and space. For example, wikipedia.org/wiki/Time-variation_of_fundamental_constants

Here's one example, where they measure the separation of spectral features from the OH molecule in a fairly distant galaxy and use that information to put a limit on how much variation there could be in the fine structure constant over the past 2.9 billion years: https://ui.adsabs.harvard.edu/abs/2018PhRvL.120f1302K/abstra...

I'm confused - is this essentially saying we haven't ruled out fluctuations but we are putting a tighter bound on the variance? But not that we actually detected variation right?

My curiosity led me down the rabbit hole of all the eras that happened before the first microsecond and to consequently learning about when the forces diverged. What is the intuition on the four forces being unified? What does that mean? Is this even a question that makes sense to ask?

Yes. If there are no variations, it's impossible to rule them out. You can only put tighter and tighter constraints on them.

Very. One consequence is that, for example, momentum might not be conserved, since according to Noether’s theorem momentum conservation is directly linked to the translation invariance of fundamental physical laws. One consequence of that in turn would be that Newton’s 3rd law, “every action has an opposite and equal reaction”, would not always hold since it’s equivalent to momentum conservation.

Stuff I've read is spectroscopy can detect molecules from billions of light years away. And if something like Planck's constant varied with time or distance the spectral properties of those would change radically.

Upsetting? Hardly, it would be exciting.

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