superposeur

This is true — I take the upshot of this observation to be that only proper times are ever directly physically measurable. Coordinate times and concepts that rely on them (such as what you call one-way velocity of light) are always merely a bookkeeping device for organizing the results of different proper time measurements and will always entail some amount of arbitrariness. General relativity makes this more explicit: the arbitrariness is that of the choice of coordinates on spacetime, and one can choose any old coordinate system or none at all.

Of course, if you already assume special relativity then this is baked in: taking the inverse of the Lorentz transformation matrix effectively sends v to -v. And velocity addition for sure already assumes you have the Lorentz transformations.

The point is that in the derivation of relativity from elementary postulates, the “reflectivity of velocities” assumption is used. For instance the linked paper uses it between equations (12) and (13). All such derivations use it at some stage.

Intuitively, “reflectivity of velocities” is why time dilation leads to length contraction with same gamma factor.

I don’t think it follows from absence of a preferred frame since one could imagine some complicated group structure relating the velocities of the various observers relative to each other in such a way that all are on equal footing and yet v_AB is not exactly -v_BA.

As mentioned, this is certainly a reasonable assumption but who knows what whacky alternatives are out there? After all, in the different context of quantum mechanics, such “obvious” relationships as xp = px no longer hold true.

On the subject of implicit assumptions (like linear trajectories transform to linear trajectories), it has always struck me that there is another assumption: if A sees B at speed v, then B sees A at the same speed v. It’s hard to imagine this symmetry not being true but who knows what’s possible in theory space?

Thanks and great diagram — I’d imagined a tiny orbit around L2, not one the size of the entire earth!

Thanks — is there a short answer to what can wide angle get you?

Do you know what advantage Euclid offers over Webb?

Also, the Lagrange point is where Webb and some other stuff is located — is it getting crowded up there?

Drinking diet sodas daily also linked to being Donald Trump: https://www.washingtonpost.com/news/to-your-health/wp/2017/1...

“Elementary” is a relative term!

The Forward party is the latest manifestation of Americans’ misguided hope of “rising above” politics (to use the cliched phrase from the Obama era).

Unfortunately there is no way out but through. Talking to your political opponents reasonably and performatively evincing civility and empty slogans such as “there is no red and blue just Americans” ain’t gonna get us there. The filibuster and gerrymandering and the unreasonable power of the Supreme Court and the electoral college and the unfettered influence of money will still be things at the end of all these slogans.

The system is broken and the horrible truth is that unbreaking it (if possible) will require positively advancing specific reform measures and a painful, unglamorous fight.

I think what drives much conspiracy-theorizing are two facts: 1) people have terrible intuition about physics outside a very narrow range of everyday conditions, 2) people are viscerally unaware of fact 1).

For instance, I recall sitting next to a table of 9/11-truthers meeting up at a coffee shop. Their discussions kept circling back to the sheer implausibility of a fire causing structural beams to buckle. People don't have intuition for jet-fuel-hot fires; nor skyscraper-high gravitational potential energy. Similarly, the dynamics of a high-speed bullet is so far outside people's intuitions that they derisively call its behavior "magic."

I'd like to hope that at least problem 2) could be ameliorated if good laboratory-based educational experiences were widespread enough; the terribleness of everyday intuitions smacks you in the face quickly when you try to do even a basic experiment. On the other hand, maybe the old dictum holds true that education benefits those most who need it least.

As a physicist, fluid mechanics was the most glaring gap in my undergraduate preparation, despite its centrality to most physics applications. Somehow it is always a “time permitting” topic at the end of an already-cramped curriculum.

I first encountered the Euler equation in the context of GR — absurd. In another decade or two, I suspect its rightful place early in the physics curriculum will be emphasized.

Actually, the terminology and ontology are astonishingly simple. Electromagnetic waves are ripples in the electromagnetic field, which is a collection of 4 real numbers attached to every point in space and time (the four components of the "4-vector potential"). Gravity waves are ripples in the metric tensor field, which is a collection of 10 real numbers attached to every point in space and time (the independent components of a symmetric 4x4 matrix). The laws of gravity and electromagnetism are a differential equation satisfied by these components governing how they fluctuate from place to place and time to time.

Photons are merely quantized ripples in the electromagnetic field (according to quantum mechanics, the momentum and energy in these ripples come in discrete chunks).

It's instructive to wonder what could possibly be a simpler ontology. Perhaps one imagines a "medium" like that of sound, consisting of little atoms of some sort spread throughout spacetime. But then immediately this would beg way more questions: what are the atoms themselves made out of? When neighboring "atoms" are displaced why and how do they affect a given "atom" in order to propagate disturbances? What other properties might these atoms have and what would such a word as "properties" even mean since they are supposed to be the elemental constituents of light/gravity itself?

According to modern physics, none of this: just 4 + 10 = 14 numbers, no substructure whatsoever.

(Of course, there are other fields too, like matter fields and gluon fields and one a necessary part of the whole story is a declaration of what these numbers look like according to different observers, but these features are just elaborations on the basic picture of a collection of numbers attached to events.)

TC’s questions are far from random, but designed to be just adjacent to things the guest has commented on previously, or which he suspects they have may have given thought. If a guest gives a long discursive answer, he lets them go on or asks follow ups. Otherwise, he keeps moving forward to the next question, like drilling for oil. PG is never discursive in his answers, so this results in a rapid fire question-answer that nonetheless I found interesting.

PG kept saying how hard the questions were, but I wish he’d instead been more willing to just concentrate and go with the flow as the results would have been even more interesting.

Thanks.

Question for the cognoscenti: supposing that LK-99 superconducts with T_c = 110 K, how significant a discovery would this be? I.e., does it establish a brand new class of high-T superconductors, or is it merely one unremarkable member of a known class? Further, does it offer advantages over other high-T superconductors such as ease of fabrication or cost?

Well-respected scientists taking a weird, cranky turn late career is a thing (for whatever reason). It makes me wonder: are there examples where the weird turn actually winds up being correct in the long term? I.e. where the mainstream consensus eventually endorses the late-career turn?

Indeed time is relative. But, the observed distribution of matter in the universe turns out to single out one particular frame of reference (= large scale spacetime coordinate system). Namely the one in which the CMB radiation is isotropic (same in all directions, neither red shifted nor blue shifted). It is with respect to this particular reference frame that the “age of the universe” is defined.

Indeed an “entertainer” who “challenges our relationship with the truth” would be a good description of Trump.

This isn’t a glib comparison, as it gets to the heart of what is so wrong with letting public figures slide by with the excuse that they shouldn’t be taken literally. I can attest that many close relatives of mine literally believe the 2020 election was stolen b/c Trump says so. And as for Geller, he’s nothing special as a magician — his blurring of the actually true line is the very essence of his undering appeal.

The message of the article seems to be that everyone should just chill out about whether Geller is really performing magical feats or not — he himself persists in saying that he is and lots of people persist in believing him.

I strenuously disagree with this attitude and agree with James Randi that chilling out about what is actually, literally true is a dangerous step. When this mushy mentality sets in, bad people will find a way to use this to their advantage.

This is especially true in our era of competing narratives, misinformation, and deepfakes.

Do you know if there are any arguments in the literature to the effect that actually useful quantum supremacy may never be possible in principle (not merely in practice)?

What I mean is: decoherence with environment must be held at bay long enough for the final answer to appear in the qubits with high probability. Based on examples from thermodynamics (such as impossibility of extracting useful work from a heat reservoir without a second, lower-temperature reservoir into which to dump heat), I could imagine a scenario where decoherence would always foil the computation at some point, perhaps at some intermediate time or in the final measurement. In particular examples, this would show up as irksome experimental limitations, but in fact these limitations would all stem from a deeper prohibition. And to the extent that decoherence could be managed (such as in the cited experiment), the resulting computation would not be "useful".

This makes sense, but then again my question is motivated by the astronomical example, which still seems mysterious.

Naively the various arguments and theorems about thermal equilibrium (at least macroscopic equilibrium as you point out) would seem to apply to the situation of 10^24 bodies bouncing around and interacting gravitationally (with some feedback mechanisms into other modes)… basically like a box of billiard balls interacting mostly electromagnetically.

Yet there is no “gravitational temperature” that allows one to predict the expected speed of a rock of a certain mass floating in the solar system. (Whereas knowing the temperature of a gas in a box gives good statistical information about the speed of a molecule of a certain mass in the box.)

Incidentally I’ve put some version of this question to four different physicists and gotten four different replies! (“Chaos” being one of these :) )

Thanks for this information — I’ll give the built in Zotero annotation took a closer look.

I like to annotate pdfs by hand, so import from Zotero into Notability on my ipad. Does anyone know of a good way to automatically reintegrate hand annotated pdfs back into Zotero? Ideally a given Zotero reference entry would have two pdfs, original and annotated, as well as a .note file. (The various plug in options seem only to manage text annotations.)

Why is it that some systems come to equilibrium quickly, effectively erasing the imprint of initial conditions, while others do not?

For instance, if we look at the distribution of astronomical bodies in the universe, almost nothing can be understood without knowing the initial conditions. Or, to take the other side of the coin: by studying the present day state of astronomical bodies, we can infer quite a bit about the initial conditions of the universe. Whereas, knowing a later state of the balls in the video tells us almost nothing about where the initial ball drop occurred.

Perhaps the answer is “chaotic dynamics”. Yet gravitational interactions can certainly be chaotic. And some chaotic systems still don’t “come to equilibrium”.

I suppose I’m asking how one might look at the dynamical laws at a glance and say one system will come to equilibrium and another will not.

TBP itself is bad, agreed, but the other books in the trilogy are some of the best ever. Dark Forest in particular is amazing —- for me the actual moment the series takes flight is when the Wallfacer Project is introduced in DF.

I use a variation of this I call now-and-next: write down a “now” task and “next” task, do the “now” task, switch “next” to “now”, write a new “next” task, and repeat.

My spouse saw me doing this and adopted the technique. When things get overwhelming, we say: “let’s now-and-next it”.

Yes, and music and all other parts of culture too: the lone genius model of human achievement, suffering=greatness, the primacy of emotion, and many other tropes that are now our cultural “defaults”. It’s hard to see clearly because we’re still inside the romantic movement.

Ha, no, I'm old(ish) and long term means 30 years for me too :)

Notice, I did not use the phrase "common interests"; my use of "interests" is in the sense of exchange: you get something and I get something in return. Equivalently, this same exchange can be described as an "alignment of values" or "alignment of outlooks" since sharing a value or a lens on the world with someone is a special pleasure and is something you get out of a relationship.

I think two people committing to each other and valuing their commitment certainly qualifies as "alignment of values" contributing to the positive side of the relationship ledger.

No I’m thinking of my spouse of 20+ years and my friends from high school.

I guess, but reflecting further, the scar tissue/injury model is a little odd in that it focuses only on the negative, not on what the two parties actually get out of the relationship.