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Physics facts not always taught in school

90 点作者 monort超过 6 年前

15 条评论

Xcelerate超过 6 年前
I think the coolest fact that I didn&#x27;t grok until undergrad is that gravity relates to the stress-energy tensor and is not just due to rest mass alone. Which means that anything that has energy associated with it (i.e., everything we&#x27;ve ever observed) affects the gravitational field.<p>Examples: A compressed spring weighs more than the same spring in an uncompressed state. A box of light (e.g., a box with perfectly reflective mirrored walls) weighs more than the same box without light, despite the fact that photons have zero rest mass. The bulk of a proton&#x27;s &quot;effective mass&quot; is due to the kinetic energy of the quarks that comprise it. The joint earth + moon system weighs less than if you added up each component weighed in isolation.<p>The other interesting fact is that the Heisenberg Uncertainty Principle is commonly misunderstood to mean that you can&#x27;t simultaneously measure the position and momentum of a particle. In fact, you can indeed obtain <i>partial</i> information on both properties at the same time, and there&#x27;s quite a few papers out there describing how to perform joint measurements of incompatible observables.<p>What HUP more accurately entails is that, for a quantum state corresponding to a specific system, you cannot obtain complete information about the two properties — no matter what you do. This is because performing a position measurement destroys your ability to perform a subsequent momentum measurement (on the same system) and vice versa. One of the postulates of quantum mechanics is that measuring a system collapses it into an eigenstate of the observable corresponding to the type of measurement that was performed. Since the position and momentum operators don&#x27;t commute, it&#x27;s not possible to put the two into a joint eigenbasis (they can&#x27;t be simultaneously diagonalized).
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wodenokoto超过 6 年前
What kind of school? I have a masters degree, and I fail to see where these facts would fit into to my education.<p>Quantum mechanics and Feynman diagrams in first year of highschool?
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TheCoelacanth超过 6 年前
&gt; Wormholes are science fiction<p>It would be more accurate to say the wormholes are hypothetical than to say that they are fictional. They are an idea someone came up with of something that might really exist, we just don&#x27;t have any evidence of them.
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dschuetz超过 6 年前
The title is misleading. It&#x27;s an opinion from some blogspot blog without sources.<p>&quot;<i>Aspects of theoretical physics</i> not always taught in school&quot; would be more appropriate.
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noobermin超过 6 年前
I like the first part, entropy is essentially a measure of degeneracy for a state (the elementary definition is the log of the number of states multiplied by a unitful constant). Technically, if you&#x27;re a supposed super-intelligence, that is good at remembering detail, every state of a system can be distinguishable and thus have small entropy. For example, consider a finite number of legos in a room. A computer could potentially remember where every lego is placed, while person can&#x27;t do as well but could distinguish between a state where the legos are strewn about or built into a castle. So, the person would lump all the disordered states into just one state (the &quot;mess&quot; state) and give it a high entropy compared to the number of organized states (castles or ships made out of the legos).<p>I guess I always knew this, and this is sort of what we mean when we say &quot;high entropy&quot; but it&#x27;s kind of fun to say it out explicitly like this. Most of the others seem like conflating the strict applicability of a theory vs. it&#x27;s practical limits, like that QM doesn&#x27;t really mean &quot;small&quot; or (equivalently) high energy, it just means when you&#x27;re near the Heisenberg uncertainty limit for the observations you&#x27;re making, which in most cases means small.<p>Allow me to add another one: &quot;Special Relativity means when you go faster, time slows down for you!&quot;<p>And another (controversial may be): &quot;In Schrodinger&#x27;s cat, the cat is both dead and alive at the same time!&quot;
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mehrdadn超过 6 年前
&gt; You can fall into a black hole in finite time. It just looks like it takes forever.<p>This one is really interesting! I&#x27;ve definitely never seen anyone emphasize this before.
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sunstone超过 6 年前
I, for one, thought that conservation of energy was iron clad. It turns out that &quot;except for the expansion of the universe&quot; proviso, it is. As a religious adherent of the conservation laws this makes me unhappy. And brings up new questions. I&#x27;ll have to sleep on this one.
platz超过 6 年前
&gt; the idea that any system tries to minimize its energy is just nonsense.<p>Very interesting, I feel somewhat misled if this is true.
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dschuetz超过 6 年前
To evaluate the validity of those facts, whom can I ask for advice? Because, I cannot afford years of study in physics to understand and assess the relevance of those facts to graduate school.<p>Besides that, when we talk about <i>facts</i>, why is that I cannot find any solid scientific sources to those claims in that article?
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avmich超过 6 年前
The very first item - disorder vs. likelihood. Can somebody explain the crucial difference?<p>Why the dough is of higher entropy than the (random) distribution of dough components?
kgwgk超过 6 年前
&gt; Which state is more orderly, the broken egg on flour with butter over it, or the final dough?<p>&gt; I’d go for the dough.<p>Strange.
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asimpletune超过 6 年前
Wait, there’s no conservation of energy?
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craftyguy超过 6 年前
&gt; If the original title begins with a number or number + gratuitous adjective, we&#x27;d appreciate it if you&#x27;d crop it. E.g. translate &quot;10 Ways To Do X&quot; to &quot;How To Do X,&quot; and &quot;14 Amazing Ys&quot; to &quot;Ys.&quot; Exception: when the number is meaningful, e.g. &quot;The 5 Platonic Solids.&quot;<p><a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;newsguidelines.html" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;newsguidelines.html</a>
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chasing超过 6 年前
This is interesting, but I&#x27;m going to be one of those people for a second...<p>I hate headlines that assume what I do and don&#x27;t know. Just call &#x27;em 10 physics facts that most people aren&#x27;t aware of and are pretty interesting. Because I actually did know a bunch of those facts from school and I was a liberal arts major...
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throwaway2048超过 6 年前
Please keep click-bait title crap off HN.