@JohnRennie Just saw you 'drop in'! Hi

@OneMug hi

Yes, I've just got up. It's 5 a.m. here in the UK.

@JohnRennie Kinda figured that was your time zone

@JohnRennie Haven't made it through that rocket equation page yet, have been sidetracked with Dirac's equations (again).

@JohnRennie Question for you: Would you agree with the statement that "Bound systems oscillate"?

@OneMug I'm busy for a few minutes checking servers. Checking the servers is the reason I have to be up at this ungodly hour :-)

@JohnRennie Understand! Leave you to it then.

@OneMug hi, my servers are all checked! :-) Yes, I'd agree that as a general rule all bound systems oscillate.

@JohnRennie Agree. Now if time is what is measured by a clock, and a clock counts repetitive cycles of some kind, that means the clock 'oscillates' in some fashion, agreed?

I'm not sure I agree with the statement that time is what is measured by a clock.

Time is a coordinate just like the spatial coordinates. It's a label we attach to spacetime points to distinguish them from each other.

I wouldn't say that time is what is measured by a clock. Rather I would say a clock is a device that behaves periodically with time.

psyarxiv.com/e5gy3

@JohnRennie Einstein is credited with the statement that "Time is what is measured by a clock". I read somewhere (can't find it at the moment) that that is not really an accurate translation of what he if fact stated (in German). Do you perchance, know what the proper translation of his definition of time should be?

Impossibility can not only be seen, but also felt

but can we take that one step further, by fooling the laws of physics that impossible is possible

@OneMug I don't know the quote, but bear in mind that we now understand relativity a lot better than Einstein did. Any complicated theory takes time to be fully understood. That means you need to be careful about taking Einstein's quotes as gospel. That can be a path to madness :-)

I don't mean any disrespect to Einstein. No doubt physicists 100 years from now will laugh at our naive understanding of quantum field theory.

@JohnRennie Very true. Best I keep my madness to myself. lol.

@JohnRennie Skipping over that definition of time for now and continue with the clock question. Is a clock a device that counts the cycles of some repetitive process? And if so, does that mean that a clock must be some kind of closed system (because it 'oscillates')?

I don't think a clock has to be closed. Energy can flow in and out of a clock e.g. with a mechanical clock energy flows out due to friction and you gave to put energy in by winding it.

I feel as though you're trying to get at some concept by being prescriptive about clocks, and I'm not sure what you are working towards.

I suspect it isn't helpful to worry too much about what exactly a clock is.

morning

@JohnRennie I'm trying to reconcile a mental picture I had once of time being akin to the real number line, but with the + and - infinities connected, which doesn't quite fit with my current picture of time, but that old picture keeps coming back.

@NovaliumCompany morning :-)

@OneMug in relativity time is just a coordinate, and indeed there is no unique definition of the time coordinate because it depends on the observer.

That is, there are always three space and one time coordinates, but different observers will disagree about the directions of the time and space axes.

@JohnRennie Exactly. But still, it seems somewhat different than the other 3 coordinates, because we get all caught up with ideas like causality and the 'direction' of time and entropy and all that.

@OneMug have a look at:

@JohnRennie I have, last night. Very good question and answer. That's what got me going tonight!

@OneMug the thing that distinguishes the time dimension is a property called the signature.

@skullpetrol Exact same principles describes nazis and other crazy groups

You'll know that in 3D space Pythagoras' theorem tells us if we travel $dx$ in the $x$ direction, $dy$ in the $y$ direction and $dz$ in the $z$ direction then the total distance travelled is given by: $ds^2 = dx^2 + dy^2 + dz^2$. Yes?

Yes

morning fellows

@JohnRennie 5am? Jesus, what time do you go to work?

@OneMug in 4D spacetime you can include the distance travelled in time. But it goes into the equation with a minus sign:

$$ ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2 $$

It's that minus sign that distinguishes the time dimension from the spatial dimensions, and it's the minus sign that is responsible for all the weird stuff in relativity like time dilation and Lorentz contraction.

Time dilation is basically the same effect as a pencil appearing shorter if you turn it

I am thinking about spacetimes with null signatures in one of its dimensions

say (-,0,+,+)

I wonder if that means travelling along the direction where the signature is 0 does not do anything

I don't think it would end up being very well behaved

You have a whole plane where any movement will have a distance $\leq 0$

Not the best for causality

hmm fair

@SirCumference I work from home, so I get up at 04:59 to start work at 05:00 :-)

5:00 is still very early

@JohnRennie Oh. I was gonna say I was suddenly terrified of how early I may need to wake up once I eventually begin working lol

I will probably be still asleep until 9:00

My quality of life significantly improved after I stopped waking up at 7am for high school

@SirCumference my job is to check all our servers early so I can identify problems in time for the engineers to fix them before the working day starts.

But the early start is fine by me because I finish early and have the rest of the day free.

It just means going to bed early.

Ah then you have enough sleep then

@Secret yes, I generally get to bed around 9 p.m.

@JohnRennie Thanks for your time tonight, but it's well past my bedtime here, and I think my mind has already gone to bed! I'd better let the old body follow it! Continue another time?

@JohnRennie how could one fall asleep so early? I have never gone to bed that early. Even when I was in elementary school, the earliest time I could go to bed is 11 pm.

@OneMug yes, I'l be around all this week.

OK. nite all

@CaptainBohemian your internal cycle gets shorter as you get older. When I was 18 I was just like you. In the holidays when I didn't have to get up at a set time I'd stay up later and later and get up later and later. As I've got old and decrepit I gradually went to bed earlier and got up earlier.

@JohnRennie Huh, that's interesting. I've kind of been hoping to someday be a "morning person", but I find myself staying up later and later

Welp good to know it may one day happen

@SirCumference there are disadvantages to getting old ... :-)

Fair enough lol

but going to bed too early can't watch stars.

@JohnRennie the wind blows in the robe of the Grim Reaper, thunder booming behind it

The Grim Reaper needs to eat less cabbage

Oh snap

Shouldn't make fun of the grim reaper at your age

You're on the list

I heard meteor showers generally appear in deep night so if you go to bed so early, you can't watch them.

@CaptainBohemian I'm just hoping I'll someday see the Milky Way disk

I've always lived with light pollution :/

@Slereah we all are, I'm just nearer the top than you :-)

@JohnRennie You don't know that

I just might get crushed by a piano today

I'm learning to juggle with pianos today

@SirCumference me too. I have never seen milky way and a meteor shower. I really hope to see them.

It's not only light polluted but also cloudy often. That's partly why I said I like places with abundant sunshine.

also, I found I can get up early when the sun rises high enough to shine into my eyes and feel energetic in a sunshine-abundant place no matter when I go to bed last night.

It is a lovely morning in Chester. The sunlight is streaming through my window (making it hard to read my screen!). I could hear the birds singing in the trees if I wasn't playing Motorhead at high volume.

Lovely day for a bit of scrumpy?

Hello one and all! Is anyone here familiar with the Damour, Soffel, Xu post-Newtonian formalism?

'fraid not

Sugar!

Mind your language

it's eclipse season again! A total solar eclipse occurred on July 2 in southern American. earthsky.org/tonight/south-american-solar-eclipse-on-july-2 I wonder if a lunar eclipse will occur soon and my area can see it.

last night probably over 7 o'clock, I saw the crescent moon briefly.

I think on July 2 it's downpouring here.

@JohnRennie I still don't understand what time is. I mean, I understand parts of your answer (the relative motiom cunfuses me), but I'm not feeling satisfied. Maybe we still haven't grasped the idea of time?

I was thinking, a lizard doesn't have and understand what consciousness is, so what if we don't have a crucial part of the brain that helps us understand what time is? What if we are simply not evolved enough to understand time, just like a cat isn't evolved enough to do complicated maths?

Einstein is credited with the statement that "Time is what is measured by a clock"

Length is what is measured with a ruler :P

So this confirms my theory that humans are stupid?

what I mean is we need a starting point to build a theory upon and those definitions will suffice for now

@skullpetrol Yes but a clock cannot measure time, since it has infinite decimals, just like a ruler cannot exactly measure length?

do we need "infinite decimals" to make real measurements?

It's the human brain that measures the clock. Does this mean time is a consequence of the complicated inner workings of this 3 pound jelly tissue?

@skullpetrol Well, I've read the most accurate clock can measure to a 1 second errornin 30 mil years

yup

would you be satisfied if all I required from a measurement is that it refers to a value?

a measurement is what you get from measuring

time is a measurement

In my opinion, the "time is defined from it's measurement" thing is just a simple excuse humans have come up with for the fact that they still don't know what time is.

Perhaps. But where else can you start from?

true, but it's just a start, not a finish

We can modify the definitions latter with evidence from experience.

@skullpetrol :thums up:

:-)

:---)

(0:

+'_'+

Could you install one of these inside a humanoid robot? large.stanford.edu/courses/2013/ph241/jiang1

Just curious

You can install it in a human

RTGs used to be standard as batteries for pacemakers

Back in the... 70's?

as you don't need to recharge them

@skullpetrol quoting Einstein is not allowed

what about the evidence

Why do the most active editors here propose so many trivial edits, and edits that completely screw up sentences?

Because most edits are trivial edits

just fixing typos and notation and spelling

indeed, a partial lunar eclipse will occur on July 16.

Total eclipse of the heart

@Slereah A high percentage of the most active editors are just changing notations to what the proposed editors prefer, or changing spellings to American spellings. A lot of them don't fix typos or spelling, and often just introduce subjective changes. Often enough they actually misinterpret words and change the sentence so it doesn't make sense.

I also do that :p

I'll be damned if someone uses $< \psi, \psi >$ instead of $\langle \psi, \psi \rangle$

IDK, every time i open the queue it seems like a few aggressive editors doing edits that are trying to make the site conform to their specific style; which is not what edits are for if it's subjective style elements

shrug

FYI you're not someone I'm talking about, the people I'm thinking of seem to primarily participate in Physics SE through these edits

If you think that's an issue you should probably bring it up to physics meta

maybe bring up a few specific examples

I probably could; but I don't know if I'm the only one it pisses off

I don't see that it does any harm if people want to make fairly trivial edits to posts. It's not as if they are benefiting in any material sense.

Internet points are their own rewards

Changing spelling from UK to American or the reverse is specifically discouraged in a meta post that I now can't find. But otherwise stylistic changes don't do any harm.

Can I change everything to Australian instead

I've recently noticed a lot of edits changing $dx$ to $\mathrm d x$. Whatever.

I also do $SO(n)$ to $\operatorname{SO}(n)$

It's not them benefiting I'm worried about, it's the staggering amount of mistakes they seem to make relative to how prolific they seem to be editing. I get the feeling the community catches a lot; but I feel like they are adding more moderation work than they add value. It seems like over a third of the edits introduce more grammatical errors, or completely change sentence structure.

@JohnRennie physics.stackexchange.com/review/suggested-edits/266832 This example in particular really bugged me (I don't know if the system would have informed you about this, or if it got swept up before you saw it)

@JMac I didn't see the notification so I guess it got rejected while I was off the site.

I'll try to find the meta post on regional spellings.

@JohnRennie Probably, yeah. The Canadian in me was seething; knowing that both spellings were totally fine for most words (and that changing "though" to "through" turned your sentence into partial nonsense).

@NovaliumCompany time can be annoying to define and understand. It depends on the context. Relativity does away with the notion of absolute time and just defines relative intervals for example. Understanding what you cannot do, being defining time in a more stringent way, is just a s good as understanding it

The argument is often put forward against QM with regards to the measurement paradox. It is indeed strange that without measuring the quantity it exists in a probability space only. Just because it’s weird, and we can’t deifne it doesn’t mean it’s wrong. It just means that maybe the world works a little weirder than you thought.

@NovaliumCompany I apologise that’s not a stringent answer on ‘what time is’ but it may help to think about this when you don’t understand why the world is the way it is sometimes.

Dew point is the temperature at which the air is saturated. Why does dew point increase if more water vapor is added to a parcel of air? The way I understand it is through the “bucket” analogy. A parcel of air at a given temperature can hold a certain amount of water vapor. Greater temperature, greater bucket. And when it’s full, that is the dew point temperature. How could dew point increase when I add more water vapor into a given bucket/parcel of air? Any insights are appreciated!

@JakeRose I agree. The best glimpse of time I've got is that it's not absolute. The fact that time is relative and different for each observer (different movement speeds and different gravitational pulls) just makes the world even more interesting.

I don't understand why an object that is in a gravitational field with strong intensity (e.g close to a black hole) has it's time slowed relative to an observer outside that gravitational field.

I guess I'll have to learn the complicated SR maths to understand this, buut I'm not doing that for now.

@ZeroTheHero yay, money

@NovaliumCompany you’d have to understand GR to. It’s not trivial stuff and usually is taught in the third year of an undergraduate physics course

@NovaliumCompany you can get into SR with just algebra and trig

I can learn GR and SR, I just don't want to currently. I want to explore some other stuff, but GR and SR are definitely worth learning.

GR requires differential geometry to understand.

@NovaliumCompany time dilation is a result of a fundamental symmetry in relativity.

differential geometry is indeed a wonderful subject, but undergraduate curriculum doesn't teach it. don't know why.

@JohnRennie the upright d looks awful

@RyanUnger I generally don't use the upright d but I don't really care if people want to use it.

@JakeRose I told you as simple as I could. I know the formula of the power, It's I*V. But as I told you the current makes the magnetic field, Not the voltage, if you raise the current you'll raise magnetic field. The magnetic field causes the motor to move.

@dmckee I've got to do something with this MathMax!

I get it a bit, I think! You need high voltage to get high current. But why don't they use more radius for the coil to make motors? The resistance is equal to R = pL/A

@X4748-IR Like why don't they use really thick wires?

@JMac Yes, I mean opposite

The lower A gets The higher R gets

So really low radius means the wire can't supply as much current; but really thick wires are expensive and hard to manipulate

Anyways, Do you know how high voltage effects the magnetic field?

The first one is a jet motor, and the second one is a simple DC motor. I don't understand why the first one needs 15V

@X4748-IR How do you get 2 A x 50 V = 500 W?

@Loong I saw that too lol I think it's the output power

@X4748-IR 15 V, what do you mean, it says 50?

@JMac Yes, I read the datasheet, it says it needs 50 Volts

of course, I think it wasn't a yes-no question!

I guess I just don't understand the confusion, one is a 500 W motor and the other is 66 W

ibb.co/zf9LmZH

xdhmotor.en.alibaba.com/product/1921334555-218538928/…

Hmmm, I don't know why different websites say different things! I:

What sites saying different things about what? Those are completely different motors and would have different torques and stuff

@JMac About the motor. I know they are different. What I'm trying to say is some motors need much more voltage than the others instead of the current when they're supposed to be more powerful.

Just wanted to know how the voltage effects on RPM in a motor with a formula. I understand why high current does effect, but I don't understand why a motor needs 50V when it only needs 2 amps

@X4748-IR I'm not that experienced with the specifics of electrical engineering; but just based on the picture of that 500 W motor I can take a guess why it's high voltage low amps. They needed it to have a high power (it probably has much higher torque than the 66 W motor); but it also had to be fairly small. For high current they would have needed big wires; but lower current higher voltage allows for a smaller motor

Those that I sent were just some examples and are not important at all. you'll see similar motors by googleing

The relationship between volts, watts, RPM, stall torque, max load torque, etc; will all vary based on a lot of factors, like the type of motor, and how efficient the design is. You can use different amounts of energy at the same RPM for example, by having a different torque at the specific RPM

I don't know why I don't see anything about voltage in these formulas! /:

Or anything about power...

@JMac Hmm...

+ Phi = 10^8 lines => Current makes the lines..

I thought in motors everything is related to MMF = IN

Force wont necessarily linearly relate to energy. The law of energy conservation will hold for motors. So more the product of voltage and amperage is, the greater the power supplied will be, and thus the greater the torque x rpm that the motor can theoretically supply

@JMac Hmmm...

"The golden ratio really does show up everywhere"

@JakeRose Wait you get taught GR as an undergrad?

see amsmeteors.org/2019/07/… meteor showers therein all culminate after midnight.

but the hourly rates of these meteor showers are so slow that I doubt one can see one meteor after gazing the sky from 12 am to dawn.

low, not slow

I wonder why countries that were once controlled by the Soviet Union are called the "second world".

@JohnRennie What do you mean?

@SirCumference you have a world class GR guy at your school

I told you this years ago

Your fault for not capitalizing

@RyanUnger sorry

@ACuriousMind Yo, sorry for the out of the blue question, but as a physics MSc did you find it tough to find a job in software development?

I'm thinking about whether it's worth it to get a comp sci minor in case I decide to do programming. Not sure how much employers care about it tho