The h Bar

12:00 AM

Relativity of simultaneity is not due to a time delay between the event and us seeing the light of that event

12:48 AM

@WhitePrime it describes reality better than the incorrect low-velocity + instantaneous interaction intuition humans have developed, your/our intuition is wrong!

Secret

5:54 AM

plato.stanford.edu/entries/time-travel

Solving the compatibility problem between presentism and time travel

According to some cognitive science and psychology models, our brains get a sense of linearity due to the linear way it file memories one by one

Thus a presentist time travel is one where these archives are altered nonlocally by some other events such that the classical or quantum correlations set up will be such that given any linear history filing device recording it, it produces an interpretation consistent with an alteration of history

For self consistent time travel, this will corresponds to only nonlocal correlations between events and memory states are never altered in any fashion once formed

This also means, if time travel to the past is possible, then there will be classical nonlocal correlations,

or in other words, superluminal signalling phenomenon even in the absence of tachyons

It also mean that, even if JD's or more generally, the "time is illusion" philosophy camp is correct, there will be nothing stopping time travel to become possible

because it can emerge from the way events are correlated as viewed by any history filing device

Slereah

7:30 AM

morgen

PM 2Ring

7:51 AM

Happy Equinox! :)

Slereah

Finally

Get rid of summer, says I

PM 2Ring

Or vice versa, since I'm in the southern hemisphere.

Slereah

why aren't you writing upside down thne

PM 2Ring

Well, I am. But the http protocol automatically compensates for that. ;)

Slereah

Moon Pie...what a time to be alive.

►

PM 2Ring

8:20 AM

I still can't decide what to do about that eternalism vs presentism question. I already posted links to the relativity of simultaneity & the Andromeda Paradox, and I really don't understand the OP's response: it just seems illogical to me. I suppose he just doesn't get that stuff about relativity of simultaneity.

I'm tempted to VTC as non-mainstream, but nobody else has done that; Ben Crowell even posted an answer. I guess I could VTC as unclear...

Slereah

there's very little we can say about time structure from the point of view of philosophy using GR

There are some links to it relating to causal structures or foliation

But assuming a globally hyperbolic spacetime, whether spacetime is presentist or eternal just isn't a scientific question

PM 2Ring

It does sound like a philosophical question, except he's asking for empirical tests.

Slereah

Well the most you can do to rule out presentism is show that the universe admits no spacelike hypersurface

Which is something you could theoretically test for

DanielSank

@EmilioPisanty I downloaded Mercurial.

Slereah

But that's about the extent of it

DanielSank

8:25 AM

After two minutes of playing with it, it seems similar to Git. Could you remind me why you were saying that Mercurial is much easier to use than is Git?

PM 2Ring

I must confess that I considered similar stuff when I was younger. These days, my attitude is that the flow of time is an illusion. There are events, and there's temporal ordering for events with timelike separation. Our primate brains want to impose a flow of time and a universal now, but it just doesn't work.

Slereah

Well

You know my opinion of this

It happens to be the same as Dr. Jones

"Fact not truth"

►

Emilio Pisanty

9:00 AM

@DanielSank mercurial itself probably isn't

TortoiseHg definitely is

7

Q: How do we know neutrons have no charge?

We observe that protons are positively charged, and that neutrons are strongly attracted to them, much as we would expect of oppositely charged particles. We then describe that attraction as non-electromagnetic "strong force" attraction. Why posit an ersatz force as responsible, rather than descr...

Boy, I wish more HNQs were like this

Rick

11:55 AM

Hi guys. I have an question about sampling and aliasing. I am not sure if it would be a stupid question so in case of disturbing the main site. I come here. Sorry in advance. : 0

I was trying to learn some basic knowledge about audio and I watched a comprehensible video.

Sampling, Aliasing & Nyquist Theorem video (https://youtu.be/yWqrx08UeUs?t=387).

But I still have a question after that.

The video says when sample rate is not good/high enough, the original sound wave will get aliased, resulting in a lower freqency. Then here the Nyquist Theorem comes into play.

For example, I use 44.1kHz sample rate in order to record a maximum frequency of 22kHz. But what if there are sounds/waves that are higher than 22kHz? (Do we always have ultra sonic surrounded? I don't …

danielunderwood

12:07 PM

I can't speak to how it's done in practice, but you could use a band pass (or at least low pass) filter on the output of the microphone to keep it within human hearing range. I imagine some microphones are engineered with that in mind as well.

You can also occasionally find audio files with higher sampling rates, though I'm not really sure what the purpose of that rate is if it surpasses twice the upper range of human hearing.

Sir Cumference

Although if your question is simply "why don't we hear ultra sonic sounds as low-pitched sounds", my guess is that the reason is biological in some way

Rick

@danielunderwood filter on the output? I think it would be filtering the input. Ultra sonic -> recording -> aliasing -> lower freqency human can hear -> unexpected sound /noise.

Slereah

"At this point we must beg the reader’s indulgence. We are about to do something which is (quite properly) considered to be in poor taste."

danielunderwood

Well I guess I should specify output as whatever is collecting a digital signal from the microphone. I'm assuming that a microphone is just an object that creates an analog signal from sound waves and as long as you filter before sampling and digitizing, you won't have to worry about the Nyquist limit. That could be a bad assumption on how a microphone actually works though.

Rick

12:24 PM

@danielunderwood Ok I get the idea. Thank you

Sir Cumference

The Nyquist theorem definitely does apply to biology though, e.g. when you see a car wheel spinning faster and faster, then suddenly slower and eventually in reverse. It's a good question how it'd apply to human hearing though.

Perhaps the main site would be better for this than the chat

danielunderwood

Yeah I think it could be something that gets an interesting answer

Rick

Really? I am afraid that I will get terribly downvoted...

Sir Cumference

@Rick In my opinion it's a good question, though either way a few internet points shouldn't deter you from the pursuit of knowledge :P

Rick

12:39 PM

Thank you for your encouragement. I would go ask a question later. QAQ

danielunderwood

And who doesn't love a little SE roulette?

Ryan Unger

@Loong this one stackoverflow.blog/2019/06/18/… ?

Loong

@RyanUnger this one: stackoverflow.blog/2019/09/20/…

Sir Cumference

@RyanUnger SO is trying to please everyone these days huh?

Well then again they keep making controversial decisions

Loong

@RyanUnger And that's the same guy who started one of the twitter fiascos: twitter.com/real_ate/status/989908814290972672

Ryan Unger

1:16 PM

@Loong lmao is this a different scandal?

@Loong what am I missing here

This doesn’t seem like acceptable behavior

Loong

1:37 PM

Yeah, it's hard to keep track of all the PR debacles of SE during the last two years.

Secret

2:35 PM

arxiv.org/abs/quant-ph/0507052

You now have 50/50 of end up with the following surreal events:

1. You kill grandfather and you travel back

2. You kill grandfather but you have not travelled back

If reality is continuous in events, then this basically equal to a dramatic restructuring of the universe in order to match these two as boundary conditions

Secret

2:48 PM

If these surreal events can occur without any smearing, logic will no longer be able to explain events and there will be discontinuous phenomenon in physics

Ezze

So religion can finally prevail!

Ryan Unger

3:25 PM

@Loong how does this keep happening

It’s just a Q&A site

People care way too much

rob

@SirCumference Note that this only happens when the illumination is strobed, as a streetlight that is secretly flickering at 60 Hz with us power line. In sunlight or in an incandescent light, you don't observe the effect. It's interesting to set it up with a box fan and a fluorescent light, the turn on an incandescent and watch the "backward spin" vanish immediately.

Semiclassical

3:44 PM

just noticed I've got an Alex Fischer in one of my sections and Alec Fisher in another.

Let's see how long it takes me to confuse them.

JMac

@Semiclassical So it's about a week until they become Alex Fisher and Alec Fischer?

Rick

@danielunderwood The author of the video replied my email and he says what exactly you told me. Use low pass (though I don't know what it is) to filter the input. Thank you 💖 . Have a good day ~

Semiclassical

4:00 PM

@JMac pretty much

I guess mentally i'd have $\text{Ale}(|\text{x}\rangle+|\text{c}\rangle)\text{ Fis}(|\rangle+|\text{c}\rangle)\text{her}$

Sir Cumference

@rob Hmm, I was under the impression that this occurs because our eyes sample the position of the fan every ~10 ms, so a fan spinning at 1 rev per 10 ms would seem stationary to us

Semiclassical

though upon observation of one of the students, it only projects onto $\text{Ale}|x\rangle\text{ Fis}|\rangle\text{her}$ and $\text{Ale}|c\rangle\text{ Fis}|c\rangle\text{her}$

Sir Cumference

@Semiclassical Huh, are you a TA?

ACuriousMind

4:20 PM

@SirCumference In one of our labs we had to measure the spinning frequency of a painted top with a stroboscope using exactly this effect - the frequency of the stroboscope is a multiple of the spinning frequency when the top appears to stand still. It's an easy way to measure rotation frequency contact-free. (Coincidentally, that lab also taught me that looking at a stroboscope for hours while hungover is one of the less pleasant experiences... :P)

Sir Cumference

@ACuriousMind I mean with a strobe light we can effectively control the rate at which our eyes sample the position. But with or without the strobe light, our eyes should still have a sampling frequency

And by extension the spinning top would be able to go over the Nyquist frequency

ACuriousMind

@SirCumference Our eyes do not work like digital visual sensors - they don't work by taking a whole image at fixed intervals.

So while there is a limit to the frequency which we can resolve (i.e. at which something goes from individual frames to smooth motion), they don't really have a "sampling frequency"

Semiclassical

@ACuriousMind a case where easy = headache-inducing :P

Sir Cumference

@ACuriousMind Wait for real?

ACuriousMind

You just have a bunch of receptors in your retina that fire when they're hit by light they're receptive for - it's an asynchronous process

Sir Cumference

4:28 PM

Huh, I'd assumed a spinning tire on the road could in principle seem motionless

Welp that's a bummer, the Nyquist theorem isn't as obvious in everyday life as I'd assumed

Semiclassical

if our eyes did work like digital virtual sensors, then TV/computer monitors would look like they do when you take a video of them i.e. rastered

enumaris

5:06 PM

They are making neuromorphic cameras that do operate like our eyes though :)

there's no "shutter" or "shutter speed", just a continuous stream of spiking signals from receptors

You can then process the data using neuromorphic chips and spiking neural networks

:D

neuromorphic MNIST is slowly progressing to be almost as good as regular MNIST

Sir Cumference

Astronomers watch six galaxies suddenly fire up into quasars

@enumaris Huh, I see

enumaris

fun times

DO YOU SMELL WHAT THE ROCK IS COOKING?

White Prime

Possibly of interest to above discussion: youtu.be/yr3ngmRuGUc

Slereah

@enumaris are you cooking rock

Good for mathematics, I hear

enumaris

I might b

White Prime

5:21 PM

I wonder what Einstein could've come up with after a massive bag of phet.

Also, disclaimer: Slereah initiated the narcotics talk.

enumaris

did I miss a rock=crystal meth reference or something

Sir Cumference

@enumaris you misspelled math

but otherwise yes that's what slereah said

enumaris

sounds legit

Rudi_Birnbaum

5:51 PM

@EmilioPisanty I started something, but not sure if it makes any sense physics.stackexchange.com/questions/503822/… especially the S(r)-S(r') term gives my headaches ...

More Anonymous

@ACuriousMind I re-read the post (which was deleted) and I can see why you came to the conclusion you came to. I was far too tired and not in a state to judge if my post was up to quality. (I learned it's not a great idea to write up something immediately after completion of the idea due to possible exhaustion). My problem now is I have re-written the entire post (yes they have some common equations) however I feel if this one was given a chance then it would not be closed. If I am right then re-written post is sufficiently different to warrant a new question post?

Any relevant meta posts anyone?

White Prime

6:35 PM

Has anyone checked the recently discovered repeating FRB's for some kind of code or pattern? Maybe it's other intelligent life's way of signalling to us that that's where they are.

Long shot, I know.

I'll see myself out.

RE The idea that it's space itself that's stretching when it comes to the expansion of the universe: How can emptiness stretch?

enumaris

6:50 PM

distances between objects in free-fall stretch

White Prime

Are there any sensible suggestions as to where the primordial atom came from? I have contempt for Hawkings awful No Boundary theory.

I really shouldn't say these things here, in the Cathedral of the Clerics of Relativity etc (meant half endearingly, half not, but good natured).

I'll shut up now.

bolbteppa

7:28 PM