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10 points · 7 days ago · edited 7 days ago

I'm fine with a text chat slur filter, but the current implementation which is already live and impacting players is far too blunt. I suggest the following improvements:

  • Introduce one warning message before moving on to actual match removals and bans. Further offenses are then more harshly treated, but this freebie warning should regenerate after say a month of good behavior. This allows for the occasional typo to not effect innocent players. E.g. forgetting the "r" in "frag".

  • Only include hard unambiguous slurs in your ban list. Do not include archaic slurs with multiple still in use meanings or slurs which have innocent usage in other languages. E.g. the nickname of the raccoon which is still common in the United States and "negro" meaning black in Spanish.

Unfortunately people are clever and can be toxic in plenty of ways without obvious hard slurs--I think the current report functionality is appropriate for players who are toxic by either altering the slurs to get around the filter or just don't use them all together. I'd like to add, even though Ubisoft hasn't done this or said they'd do this, but I don't think we should have a general profanity filter in this game. It is an adult rated mature game and sometimes coarse language is a fun part of online gaming from either just using expletives to emphasize things or engage in harmless trash-talk.

1 point · 14 days ago · edited 13 days ago

༼ つ ◕_ ◕ ༽つ GIVE BAN ༼ つ ◕_ ◕ ༽つ

Edit: I don't feel so good....

༼ つ ◕_ ◕ ༽つ

༼ つ ◕_ ◕ ::;:.::..:. . . . . . . .

༼ つ ◕_ :;:.::..:. . . . . . . . . . . . . .

༼ つ :;:.::..:::....:.:... . . . . . . . .

༼ ;::,':;:.::..:::....:.:... . . . . . . . . .

:,':;:.::..:::....:.:... . . . . .

;.::. .:. : ... . .

25 points · 1 month ago · edited 1 month ago

Your questions' premise is not right:

but in the scenario the force(weight) doesn't change

The gravitational force (manifested as weight) doesn't change, but the contact force (caused by the solidity of the tile and rock via electromagnetism) which only lasts a fraction of a second while the tile is stopping the rock is enormous.

Energy and force are deeply related ideas so asking which one is more important here is a bit like asking which is more important for a working car: The wheels or the engine?

Original Poster1 point · 1 month ago

Why would the expansion void the affect? It should still have an affect, because they’d both bear mass and have a distance.

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In essence, general relativity, GR, doesn't actually obey Newton's law for gravitation. In the situations we're used to dealing with (like planets) Newton's gravitation is the limit of GR when the masses, distances, and velocities aren't too crazy and you throw away all the effects which are astonishingly tiny.

In our current understanding, there are particles/stars/galaxies/etc out there who now no longer effect us, those who soon will no longer effect us, and those whose signals have yet to reach us, and those that have never or will never reach us.

The last two I mentioned above come with a big caveat "at least since inflation ended". The precise boundary and whether or not there are objects that have never or will never have signals reach us depends heavily on the large scale structure of the universe (current favorite is infinite and spatial flat) and the precise duration and behavior of inflation, if inflation occured.

20 points · 1 month ago · edited 1 month ago

The fundamental problem in combining pilot wave theory and relativity is that:

  • Pilot wave requires each particle's motion to be determined by the motions of every other particle's motions in the whole universe at that instantaneous moment. This is why pilot wave is called a deterministic and nonlocal theory.

  • In relativity, there is no such thing as a universal time, so there is no such thing as a shared instantaneous moment.

In essence this implies that for unmodified pilot wave to function you can't just know what the universe is doing "now", but also what the universe will be doing in the future to even solve for what "now" is supposed to look like.

To illustrate consider two observers trying to solve for the motion of an electron in a box. One observer A is at rest relative to the box, the other B is moving.

  • A can solve for the electron's motion if they know how the pilot wave is behaving now.

  • B can also do this, but their "now" relies on events that can change the pilot wave that haven't happened yet for A. The reverse is true as well.

It's actually even worse because neither A and B can agree on the order of many events, but wouldn't the pilot wave change if two external events happened in a different order? And on top of this difficulty, A and B must predict the same behavior for the electron. We must agree on what happens right?

This is a serious problem. There are efforts to fix this and make a relativistic pilot wave, but I'm not really qualified to comment on them, but from a light-reading on them, they do not look very satisfying.

-13 points · 1 month ago


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Overtaking light if it has a tiny tiny mass isn't a problem, but overtaking a massless particle completely breaks our understanding of physics and allows time travel. Since we have strong experimental and theoretical backing that light is massless, overtaking it is a huge deal.

845 points · 1 month ago · edited 1 month ago

The only observed ways that matter is created is when an antimatter-matter pair of particles is created from energetic bosons. This process will never make more of one of the types of matter. Statistics has nothing to do with it in this process.

Since we know that there is more matter than antimatter, there must be some other process by which this apparent asymmetry came to be. Thus we have a baryon asymmetry problem.

An interesting note: if you take the Dirac equation and you calculate the "probability" of finding a particle or an anti-particle, when you approach the non-relativistic limit, the "probability" of finding an anti-particle goes to zero.

Edit: My previous statement was a slight misinterpretation. In the non-relativistic limit of the Dirac Equation,one of the two component spinors representing Antimatter solutions is significantly smaller than the other spinor. That's the most technically correct statement. The derivation can be found in the quantum mechanics book by Bjorken and Drell.

Edit 2: upon further review, that problem above doesn't really have anything to do with the prevalence of antimatter. Although it is still an interesting problem none the less.

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In regards to your edit, there is nothing stopping you from making the positron the dominant wave function, in the nonrelativistic reduction, in the Dirac basis, you pick the sign of the energy corresponding to which particle or antiparticle state you want.

Therefore the Dirac equation tells you nothing about baryon asymmetry. As a clear example, solve the Dirac equation in the chiral basis where the electron is a particle/antiparticle mixture.

13 points · 1 month ago · edited 1 month ago

Not a dumb question! It's perhaps one of my favorite questions to ask folks because it requires some deep thinking.

Evaporation must be treated as a surface effect. Water in the liquid or gas phase at some temperature T doesn't mean all those molecules moving around with the same energy, some are more energetic while others are less.

At the surface, the more energetic molecules can leave the liquid. As they leave (and make the air more humid), they must fight against the attractive intermolecular forces (the process is not unlike a baseball slowing down as it moves up in the air against gravity) leaving the remaining molecules with less kinetic energy and thus the bulk liquid temperature has decreased. This is called evaporative cooling.

The reverse happens too, gas molecules can be reenter the liquid which is called condensation. Whichever process dominates determines whether the liquid increases or decreases in volume. When the rates are balanced the air is as humid as it will get and we call this equilibrium.

Now here's the important part: Closed systems are relatively uncommon and air circulation means that newer dry air will pass over the puddles and we will not reach equilibrium, we'll just keep evaporating until the puddle is completely gone. However if we have humid air, the puddles may be persistent and stable or even grow if the air cools forcing condensation. An example of this is morning dew on leaves.

To get a feel for this under simplier circumstances take notice of how water droplets condense and vanish as the temperature of a sealed water bottle changes.

Edit: Changed some of the text to better reflect liquid physics.

2 points · 1 month ago

At the surface, the more energetic molecules can leave the liquid. As they leave (and make the air more humid), the remaining molecules are more likely to be less energetic therefore the temperature of the liquid decreases.

That is a highly limited, naive view of evaporation, and I wish it stopped being presented as an actual picture of why evaporation cools surfaces (looking at you, high school physics teachers!)

As an obvious counterexample, your liquid could be at a temperature where EVERY (or almost every) molecule in the liquid has enough energy to leave the surface, and yet the surface still gets cooled by the same amount. What's going on?

The answer is that an evaporating molecule performs work against attractive intermolecular forces, and thus can only leave the surface with a lower amount of kinetic energy (lower temperature). The situation is symmetrical from the point of view of the surface, meaning that the surface also ends up colder.

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2 points · 1 month ago · edited 1 month ago

You're completely right, a discussion of evaporative cooling without mentioning enthalpy of vaporization is woefully incomplete.

The evaporation process however is still a kinetic one otherwise vapor pressure (and evaporative rate) would not increase with temperature. I was hoping to impress that, but I see that now that what I've written makes liquids sound like gasses which is definitely not the case.

I've cleaned it up a bit.

i3 is -i. To realize this consider

  • i3 = (i2)i = (-1)i
9 points · 1 month ago · edited 1 month ago

The free proton is lighter than the free neutron therefore it cannot decay into it. Decay chains always run downhill in terms of mass with the daughters being lighter than their parent particle. This restriction is because of mass energy conservation in special relativity. Essentially the decay as a whole must obey the equation

  • E2=(pc)2+(mc2)2

Protons can be converted into neutrons, but these have to be at least two-body collisions, for example protons colliding in the pp-chain

Other examples include:

  • Heavy nuclei decay through electron capture.

  • Neutronization, the process of converting protons and electrons into neutrons when a neutron star is formed during a supernova.

In all three of these cases, neutrinos are emitted.

Still by no means conclusive evidence though.

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19 points · 2 months ago · edited 2 months ago

I'll bite. The CMB is very good evidence that tells us that roughly 13.4 billion years ago the universe was densely packed with hot radiation at the temperature of the surface of the Sun today. The big bang model 100% predicts that this should be the case.

At this point - I'm not sure what "conclusive evidence" would look like beyond what we've observed. A notarized statement from God?

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Must be signed in triplicate or it doesn't count!

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I though planck constant over momentum was the De Broglie Wavelength.

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Notice I used h/mc rather than h/mv. The latter is the De Broglie wavelength.

I was wondering what you like about electrodynamics and if you have specific pedagogical resources on the topic that you recommend? Sounds like an interesting field.

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My main interest is really in the different ways quantum fields can interact, though by accident this has led to me spending most of my time studying E&M. Two books that get at this are "The Character of Physical Law" and "QED: The Strange Theory of Light and Matter" both by R.P. Feynman. Also I can recommend Asimov's "Atom: Journey Across the Subatomic Cosmos" which is wonderful, Weinberg's "The First Three Minutes" is also excellent though it's focus is cosmology. Hawking's "A Brief History of Time" is great for gravitation and cosmology as well.

While not specifically about physics, Carl Sagan's "The Demon-Haunted World" explains a lot about how to think scientifically which is important. Non of the books mentioned has mathematics.

If you're interested in getting into the mathematics, then I recommend both D.J. Griffiths introductory books "Intro to Electrodynamics" and "Intro to Quantum Mechanics". These are both undergraduate level books which are pretty standalone and cover a lot of the required math in the text itself.

Feynman's Lectures on Physics are also excellent and freely available here:

Lastly, I think Penrose's "The Road to Reality" is also great because you can treat it like a grab bag and make the reading experience as simple or complicated as you want.

For myself, when I study a topic I try to found 3-4 sources and read them all concurrently because I rarely find a single author explains things in a way I understand completely, I need multiple perspectives.

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3 points · 2 months ago

But they remove every comment in the entire thread?

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2 points · 2 months ago · edited 2 months ago

All of them except for u/AsAChemicalEngineer 's comment that was unfortunately cut off. Maybe they can answer some questions about this missing thread or u/ImQuasar the OP of the thread might have some answers.

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I didn't do any removals in that thread, but basically we're really strict on what a top level comment should be (e.g direct replies to OP) and to keep the thread clean we regularly remove the subsequent discussion around a bad answer even if the child posts aren't bad. This is less common in small threads which are just easier to manage.

Some of the removals were just trolls/junk which is common in popular threads.

1 point · 2 months ago · edited 2 months ago

Tagging /u/Skuzzyloki for interest.

Some are junk/trolls, but a lot of it are bad answers with incorrect info or info irrelevant to OP's question. In smaller threads this is easier to manage by just responding with corrections, but in threads that blow up it's harder to deal and the approach is to hard remove bad top answers and their child posts. Unfortunately good comments get caught up in this.

But this keeps the thread focused on the question and its answer. AskHistorians is another sub that does this like us.

Full disclosure: I did no moderating in that thread, so there wasn't an attempt to just make my post the visible one, we do occasionally have to even remove posts done by other mods or flaired folk if it benefits the thread as a whole.

3.7k points · 2 months ago · edited 2 months ago

There are number systems which do just as you describe. Here are two (I don't know of others) such examples of this:

The latter is the extension by defining z/0 in the complex plane.

A lot of the math rules are the same as you're used to, but there are important differences. For example in the projectively extended reals statements such as

  • a > b

  • a set of all numbers between -4 and 7 is [-4...-1...0...7]

are no longer meaningful without extra context. I can always pass through infinity to just as easily write

  • a < b

  • a set of all numbers between -4 and 7 is [-4...-10...infinity...7]

With some added assumptions of what "a" and "b" are and where infinity is on your interval if it's included, you can restore the idea of order and intervals.

Til that the science subreddit removed other posts to keep their AMAs artificially at the top.


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The reason this was done was because >90% of users never actually visit the subreddit, they click on posts in their personal front page.

Overwhelmingly only the top post in a sub reddit would appear in people's front pages and once it did it would stay there because the voting population overwhelmingly votes from their personal front pages. While the quality of a post is a determining factor in its success, this algorithm heavily leaves success up to pure chance--essentially success was statistically determined by which post would be the top spot randomly when America woke up and checked reddit in the morning.

This means that for a time sensitive post, like a special event AMA, any reasonably popular post would keep it effectively hidden for hours--even if you knew the post would be a hit if people could just see it. Normally this isn't a big deal because you're just posting a cat picture, it might blow up or it might not, but in this case you were playing Russian roulette with a post that you spent hours setting up and whose success determined your reputation and the reputation of whoever was doing the AMA.

This system wouldn't be needed if a users frontpage was more of a grab bag of several of a sub's top posts rather than only the first. Things would be more consistent.

The admins offered to promote AMAs on Reddit's social media accounts. As far as cross posting obviously they didn't do a good enough job. As for the vote manipulation that's definitely a no go from me and do not care that it hurt their AMAs

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The admins offered to promote AMAs on Reddit's social media accounts.

That just doesn't lead to tangible attention. It's a false solution that just sounds like a good idea, but doesn't actually work.

As far as cross posting obviously they didn't do a good enough job.

They already spend hours setting these things up and they cross-post everywhere. Like a dozen cross posts were common.

As for the vote manipulation that's definitely a no go from me and do not care that it hurt their AMAs

Done for years with admin knowledge and tacit approval.

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Reddit admin, speaking officially1.6k points · 2 months agoGilded4 · edited 2 months ago

The decision for r/science to no longer host AMAs is disappointing, and blaming us at Reddit is counterproductive.

u/nallen, having met you personally a number of times and after personally trying to work through this issue with you over the past months, I'm disappointed you've taken this approach to mislead your community about what's going on.

So here's what's really going on:

How it used to work

r/science used to be a default community, which means it was one of one hundred communities that made up the front page of Reddit for most of 2011–2016. As a result, r/science and the other defaults had high visibility at the expense of non-default communities.

r/science used to promote AMAs by removing other more popular posts so that the AMA could be top of r/science without the votes. This, combined with being a default community, sent a lot of traffic to these AMAs.

How it works today

We replaced the defaults with r/popular, which is basically a SFW version of r/all. This puts all communities on an equal footing.

We don't allow the post manipulation for obvious reasons. Here is a discussion we had with u/nallen on this topic months ago.

We are indeed testing new sorting algorithms, but if anything they should help communities like r/science get more visibility. One of our engineers recently wrote a pretty good post about it.

Going forward

Regardless of u/nallen's decision, we will continue to work to improve our onboarding and sorting so that users get to see more of what they love, and we have in mind some specific features that will help promote "event" posts (AMAs, game threads, episode threads) in the future.

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I think your comment misses the point. Nothing you state, except future promises of better sorting algorithms, addresses the key issue: Event posts just don't have good visibility.

Until that is fixed, what do you want the mods to do?--Continue pouring hours into setting up complicated events that nobody will see?

Promotion is incredibly hard and from my limited view the only solutions (including the ones I've seen you guys discuss with mods) require massively more volunteer hours from the mods. Frankly the solution needs to come from within reddit, perhaps simply as an algorithm change that applies to any post as you mentioned, or a dedicated event algorithm which a sub's mods can use to artificially get more eyeballs above that of the regular front page algorithm.

...You removed legitimate user submissions in order to artificially increase visibility for preferred posts?

Not cool. :-/

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That's how it worked. The posts were never permanently removed, they would be put back up soon after the AMA gained traction. The system worked and allowed successful AMAs as per the viewership stats.

It wasn't really a secret as the practice was discussed on public mod-centric subs, it just was not advertised. The admins didn't like it, but they tolerated the practice for years.

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Radiation. It is generally in the form of photons/light, but heat transfer in vacuum can also occur from the transfer of matter radiation too.

To add to the answers here, he's the starting reference for those interested in the topic:

  • Tolman, Richard C., Paul Ehrenfest, and Boris Podolsky. "On the gravitational field produced by light." Physical Review 37.5 (1931): 602.
66 points · 3 months ago · edited 3 months ago

I have never even heard of uniformitarianism, but after a quickly skimming the Wikipedia article, I'll now pretend to be an expert on it. I won't comment on its relation to geology where the idea seems to have gotten more attention, but from a physics perspective I can say this:

Yeah, science isn't invincible. It cannot defend itself against Last Thursdayism and it cannot defend itself against perfect and completely deceptive coincidence which reminds me of a bit in the play "Rosencrantz and Guildenstern are Dead" where the two titular characters bet on coin tosses using a coin which comes up heads nearly every time. The universe R+G live in is utterly unfair because Hamlet was already written therefore R+G cannot survive their own play. In a similar fashion we have to assume that the universe is being honest and fair with us when it reveals behavior through experiment. It we cannot do this, we can't move forward and do things like build computers and radio telescopes.

If it's true isn't that a problem?

Not really. I'm emphatically uninterested in the myriad of 'gotcha' carefully constructed what-ifs which do nothing but make sure that the natural laws are somehow not what they appear to be to plain, but careful eyes. Nobody else has even come close to making a formal system of generating knowledge about the natural world that works as well as science.

I do not believe so, but here's some information on how such an orbit would work and why it's unlikely or at least unstable and temporary:

21 points · 3 months ago · edited 3 months ago

That quote is fine until the "one physicist has suggested..." part which then becomes wild speculation which contradicts our current understanding. While trained physicists aren't immune to pulling ideas out of their rear, that article you link doesn't provide names or other resources... might just be made up even.

Anyway, the core idea here we're discussing is that different observers will disagree on how much time has passed between events. In plain and old special relativity, this disagreement arose because different observers were moving relative to one another. To preserve the speed of light as being the same for everyone, moving observers can no longer assume they'll measure the same distances and times as other observers.

General relativity makes things a bit more interesting, not only do observers moving relative to one other disagree on times and distances, but observers in different places now disagree as well. Curvature in spacetime (e.g what we call gravity) now becomes the cause for this. The classic example is the portion of time dilation we experience compared to global positioning satellites because we're deeper in Earth's gravitation field than the satellites are. We don't even have to leave our solar system to see this effect.

Now back to the galaxies accelerating away... Our universe is indeed expanding and that expansion is accelerating. This is pretty surprising considering matter is attractive! You would think that gravity's attraction means that the universe would either: slow in expansion until it "falls back" into itself like a big crunch (think of a baseball being thrown up and eventually falling back to Earth) or slow in expansion to some residual expansion rate and just coast (think of a baseball being thrown so hard it escapes the Earth).

You can check out the FAQ for more info, but in short our current model of the universe requires we fill all of space with a substance with negative pressure we call "dark energy" which is responsible for this accelerating expansion. We have no idea what it is or why it's there, but essentially it has a constant density which doesn't change even if you increase the amount of space. This is very different from say the behavior of a gas which decreases in density as you increase the volume. Like any form of energy, this "dark energy" curves spacetime. This means those far away accelerating galaxies seem to be working in slow motion; time seems slower over there. This isn't anything to freak out about too much though because to aliens in that galaxy it is our galaxy that seems to be operating in slow motion and time is normal for them.

Edit: That quote is actually not fine until the "one physicist" part. It's wrong much sooner. Even with a constant (not accelerating) expansion, it is still true that father galaxies are moving faster away than closer galaxies! Blow up a balloon halfway and then draw a grid of dots onto it equally spaced out. Now blow up the balloon some more. You'll see that even though the distance between every adjacent dot has increased the same, the n-th far away dot has moved n-times as far as the closest dot in the same amount of time!

The shock to physicists around 20 years ago is that the expansion rate itself is becoming larger than just the situation described above.

It is hard to deny that animal testing has produced enormous benefits (though it has also been misleading and counterproductive, too). But having benefited from animal research in the past doesn't, in my mind, commit us to an eternal future of animal research. We should begin phasing out the use of live animal models and work hard to find replacements. With the incredible advances in computer technologies, I would hope that we could innovate our way to a more ethical future. In the meantime, as we make our way toward animal-friendly utopia, I would like to see a halt to animal studies that simply replicate already existing data (e.g., the proposal by U of Wisconsin researchers to re-do Harry Harlow's nasty experiments on maternal deprivation in monkeys), that don't make important contributions to the literature (there is far too much junk research), or that cause profound suffering to animals.

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Thank you for your reply!

The strong force doesn’t decrease in strength with distance like the other forces

Other comments suggest that the strong force has a significant decrease in strength as a function of distance. Is this another simplification taught to students: the net force is negligible even if the force itself is still strong?

I ask because you later mention that the force can be neutralized and "screened". If I am understanding correctly, this means that the force does apply at longer distances, but that it usually cancels out with negligible net effect? Also, how do hadrons "block" the force?

particles will actually be created to prevent that from happening

Can you elaborate on this?

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The residual strong force, aka nuclear force, has a significant drop off with distance. The strong force itself, also called the color force, which involves quarks and gluons does not drop off with distance.

This manifests in color confinement, or why we don't see free quarks. By the time you've spent the energy pulling two quarks apart an appreciable distance, the system spontaneously decays into more quarks-antiquark pairs and preventing the free quark from happening.

The reason the residual strong force drops off with distance so quickly is that the particles which mediates the interaction are massive mesons (quark-antiquark pairs). Unlike the photon which is massless and has essentially infinite reach, massive bosons can't propagate so readily. Mathematically it means the potential energy exponentially decreases with the exponent containing the mass,

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