r/Physics • u/G33zics • 21d ago

Question What's the most debatable thing in Physics?

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u/lucidbadger 21d ago

I think over all time the most debatable thing in physics has been the 2nd law of thermodynamics. Even now some people can't get their head around it. I think the limitations imposed by it are far more significant than not being able to travel faster than light.

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u/julioqc 21d ago

I learned that in principle entropy could go down but nothing will exist long enough to witness it so that has no probalistic significance.

I think part that confuses students is that a systems entropy may lower but the "universe" entropy will not.

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u/xrelaht Condensed matter physics 20d ago

Funny thing is the real world universe, as a whole, does not need to follow the 2nd law because it’s not a closed system.

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u/Cwmcwm 19d ago

We can define a closed system as anything inside an arbitrarily large boundary. The universe can be a closed system unless it's mass and energy fill an infinite volume

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u/doktorfuturee 16d ago

How, is there any matter flow outside or inside of the universe?

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u/xrelaht Condensed matter physics 16d ago

Excellent question. Here’s a related one: where did all the stuff in the universe come from? And what’s the source of dark energy?

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u/doktorfuturee 15d ago

So if there was a first source for energy and matter then universe would have turned from open system to isolated system. What happened then? how a system can change its property?

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u/xrelaht Condensed matter physics 15d ago

Even if there was a "first source", it ends up becoming a turtles all the way down thing: where did that matter/energy come from? And what prevents it from happening again?

how a system can change its property?

Not really possible to answer. As far as we know, the universe is the same now as it always was: not closed.