The h Bar

4:30 AM

@JohnRennie I have been doing testing using light. And my results are very worrying. Light doesnt have mass, but my findings are either false or aren't. And if they aren't false this isn't good.

@JohnRennie Here's the rundown. My hypothesis is L.A.S.E.R has density. The density of the L.A.S.E.R gets greater as thr power via Watts, mW, or Mega Watts goes up.

@JohnRennie Proof: Polyitic Graphite at 6mm thickness is diamagentic when an external magnetic field is close enough and is applied. Using a North to South locking configuration using magnets makes the Graphite stay still and locked in. Pointing the L.A.S.E.R at the Graphite causes it to move in the toward direction. Turning the photon energy into kinetic energy.

@JohnRennie Governing Theory: Any object that has mass has the potential for kinetic energy. Written: 2/21/17 @ 7:48 P.M. Written By: Scott ***** Section: L.A.S.E.R Potential Energy... Section Type: Experimental.

@JohnRennie How? Why? What? I didn't realize what my experiment was saying until now. I'm hoping something is wrong in my experiment, because it has to have an explaination. But I'm also hoping there isn't. Becaue it could open many good doors. Could you please help me out on this one?

John Rennie

@ScientistSmithYT light doesn't have mass but it does have momentum so it does exert a force on the objects it hits.

Scientist Smith YT

@JohnRennie So then how does it not have mass? The governing theory is that any object with mass has potential for kinetic energy. I'm so confused.

John Rennie

162

Q: If photons have no mass, how can they have momentum?

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the product of mass and velocity, so, by this defini...

Scientist Smith YT

I'll take a look at it and come right back.

@JohnRennie I'm back, I see it's kind of a hot buttion topic. One answer says it has relativistic mass and then the other answer goes aginst that. So what should I think about it? Two answers but they battle aginst each other. And a comment went aginst it as well, but some comments go for it.

I'm looking at the basics of laws but they seem to contradict a little. One says you cant have mass without velocity. And another says you can.

John Rennie

4:48 AM

Mass in modern relativity means rest mass and that is undeniably zero for a photon. If you define the "mass" as momentum divided by velocity than you get a "mass" for the photon, but this is not a useful definition of mass. The answers differ simply because they mean different things by the word "mass".

Scientist Smith YT

So then how do we as scientists determine what it is then? They seem to contradict in a way that isn't good.

Or even if light has mass or not?

John Rennie

In special relativity there are special variables called scalar invariants. These are special because of the way they behave under transformations between different observers.

In particular a scalar invariant has the same value for all observers so they are kind of universal variables.

Anyhow, rest mass is a scalar invariant, and that makes it special. That's why all relativists define the word "mass" to mean "rest mass".

Invariant mass

The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object or system of objects that is independent of the overall motion of the system. More precisely, it is a characteristic of the system's total energy and momentum that is the same in all frames of reference related by Lorentz transformations. If a center-of-momentum frame exists for the system, then the invariant mass of a system is equal to its total mass in that "rest frame". In other reference frames, where the system's momentum is nonzero, the total...

Scientist Smith YT

Say the value at all observers was 20. How would that number make sense at the observers point of view from the graphite in perspective to the light?

Because the light seems to be moving away from the graphite. But the graphite is moving away from the light. In the light perspective.

Emilio Pisanty

6:28 AM

@ACuriousMind you were saying? =P

Dunno if it'll be enough to make the rep cap today, but it's still good anyways

Scientist Smith YT

@JohnRennie Did I say something wrong?

John Rennie

@ScientistSmithYT I'm working I'm afraid ...

Scientist Smith YT

@JohnRennie Job? Or with someone else?

Emilio Pisanty

@ScientistSmithYT that argument doesn't actually work. Saying "from the perspective of X" means "from an inertial frame that co-moves with X", and there are no inertial frames that move at the speed of light

John Rennie

@ScientistSmithYT bit of both

Emilio Pisanty

6:35 AM

Light doesn't have a "perspective"

Scientist Smith YT

@JohnRennie Ok, I'll chat later, thanks. :)

ACuriousMind

@EmilioPisanty the top-scored answer is accurate, imo. The accepted one isn't really wrong but doesn't answer the question, I'd say.

Scientist Smith YT

@EmilioPisanty I'm thinking of the frame of reference at the front of the light traveling to the object.

@ACuriousMind I think it answers the question to directly. Making it seem that it isn't answering the question.

@ACuriousMind The person asking the question seems to ask about energy states, and the accepted answer tells us the correct answer so directly that it seems that it isn't answering it. At First I thought it didn't answer the question. But when I took a closer look and examined the basics of each word used, the wording used and how it's used as well as what each statement and word means. It answers the question.

At least that's what I get from the answer and question asked.

I'm curious what everyone here thinks about me embarking on a journey to build a 15 stage 13 million volt Cockcroft-Walton Voltage Multiplier. Everyone knows that 10 stages is pushing boundaries. But 15 is another thing. I want to see if I can find a way to do it. And I was hoping on getting feedback from the lovely users here. Feel free to give professional opinions or personal opinions.

Hints, tricks or insight are also welcome.

Emilio Pisanty

7:01 AM

@ScientistSmithYT there is no such frame

@ACuriousMind OK, thx

I was primarily asking about the one that's now accepted

Slereah

7:26 AM

Hello

Do you think I can send a letter to a physics journal and it's just entitled DIG UP EINSTEIN'S GRAVE

and it's nothing but a plea to digitize every unpublished paper ever referenced

because I'm pretty sick of it

PM 2Ring

7:56 AM

54

Q: Why is there a controversy on whether mass increases with speed?

Some people say that mass increases with speed, some people say that the mass of an object is independent of its speed. I understand how some (though not many) things in physics are a matter of interpretation based on one's definitions. But I can't get my head around how both can be 'true' is an...

special-relativity mass speed mass-energy

Slereah

9:15 AM

Having read the Marzke paper (though not yet the thesis), I feel I really should write up my own version of synchronization for GR

This seems like a thorny topic

Slereah

11:52 AM

@RyanUnger CONSIDER THE FOLLOWING

Take a set of three points, $p_1, p_2, q$

The spacetime is globally hyperbolic

and $p_1$ and $p_2$ are on a timelike curve

Wait, no, not quite

Lemme start over

danielunderwood

@Slereah are you an angry old professor yelling at your students?

Slereah

@danielunderwood Only in my soul

Consider a point $p_1$ on a timelike curve, with a point $q$

There are (possibly several) null curves connecting $p_1$ and $q$

And there are (possibly several) null curves connecting $q$ back to $\gamma$, the original timelike curve

Is there such a pair of curves such that $\ell_1$ connects $p_1$ and $q$, $\ell_2$ connects $q$ and $\gamma$ (intersecting it at $p_2$), and this pair of curve is such that it is the shortest possible time interval between $p_1$ and $p_2$?

oh and both $\ell_1$ and $\ell_2$ are on the horismos

I ask because light rings make all my ideas a nightmare to check

Emilio Pisanty

608

Q: Firing mods and forced relicensing: is Stack Exchange still interested in cooperating with the community?

The last weeks and days have seen some erratic behaviour by Stack Exchange Inc., such as likely illegal changes to the content license and the firing of an upstanding community moderator with no explanation except copy-pasted responses (leaving many to believe it was for no good reason). It would...

discussion support moderators community

@mods Are there any upcoming changes to this site's moderators?

Slereah

Basically my issue is trying to define points $q$ with respect to an observer uniquely by a pair of null geodesics

But those may not be unique due to Issues

So I need to be able to at least figure out which null geodesics is on the horismos

Slereah

12:39 PM

citeseerx.ist.psu.edu/viewdoc/…

could b good

Bloody hell

now I have to consider $N$, space of null geodesics!

PM 2Ring

1:18 PM

Space of Null Geodesics would be a good title for an album of ambient music.

Slereah

1:35 PM

"The situation is simplest in Minkowski space, where N is projective null twistor space"

Whew

Good thing it's simplest

rob

@EmilioPisanty In the private space where the physics mods chat amongst ourselves, none of us has expressed any plans to leave the community over this issue. A couple of us have participated somewhat in the ongoing discussion in the network-wide mod-only chat room. I don't anticipate any changes to our team, but I don't speak for everyone. We'll keep you guys posted if that starts to change.

Ryan Unger

General F-theory Models with SU(3) x SU(2) x U(1) / Z_6 Symmetry - Andrew Turner

►

lol that "overview"

Slereah

@RyanUnger What's a sky

That paper keeps talking about the skies of points on a manifold

Oh wait nvm

It's defined in the paper

Ryan Unger

no idea

Slereah

1:50 PM

Apparently the set of all null geodesics isn't necessarily Hausdorff, but it is if the manifold is globally hyperbolic

Phew

Dodged a bullet there

plz halp, I don't want to have to learn about twistors

I just wanted to do some experimental physics for a bit

Sir Cumference

whoa

isn't it weird how we're all using computers

which were made using physics

...i don't think i'm making much sense

Slereah

I don't use a computer

I'm just a voice in your head

Ryan Unger

I've said repeatedly that I'm against using computers

Sir Cumference

did you say it on your computer though

bolbteppa

2:07 PM

F theory is pretty scary

5

Q: SL(2,R) to SL(2,Z) in Type IIB String Theory

I heard from Prof. Katrin Becker (in her "SUSY for Strings and Branes - Part 1" lecture) that the classical $SL(2,\mathbb{R})$ symmetry in type IIB String theory becomes $SL(2,\mathbb{Z})$ in Quantum because of charge quantization. However, I cannot see how does it work. Is there any rigorous Mat...

symmetry symmetry-breaking string-theory

"One may describe type IIB string theory as a formally 12-dimensional theory called F-theory (due to Cumrun Vafa) whose 2 dimensions must be immediately compactified on a two-torus or, using the F-theory algebraic geometry jargon, an elliptic curve."

Slereah

2:21 PM

"Let $M$ be a strongly causal spacetime and $N$ its space of null geodesics. Then if $N$ is non-Hausdorff, $M$ must be nakedly singular"

Hm, lemme thing

What would be the space of null geodesics in 2D Minkowski space

$\mathbb{R} \times \mathbb{Z}_2$?

And I guess more generally $\mathbb{R}^{(n-1)} \times SO(n-1)$ for $n$-dimensional Minkowski space?

"One straightforward consequence of the quaternionic nature of the hat-conjugation acting on 2-spinors is that there is no non-trivial combination $\kappa^\alpha \omega^{\dot\alpha}$ which is preserved under the hat-operation. This is simply the statement that there are no real null vectors in Euclidean space!"

Slereah

2:46 PM

"Let $\alpha \in T^*M$, and define $f : \mathbb{R} \to T^*M$ by $f(t) = t \alpha$"

Wait what?

How do you define that

Do they mean scaling fiber-wise?

Ryan Unger

yes

Slereah

Hm

They talk of the geodesic vector field

$$X_G = n^i \frac{\partial}{\partial q_i} - {\Gamma^i}_{jk} n^j n_i \frac{\partial}{\partial n^k}$$

That's a section of $TTM$ right?

Perhaps even

dare I say

Part of the horizontal bundle

Akash. B

Hi

Slereah

3:05 PM

"So we discover that lines in twistor space intersect if and only if their corresponding points in $M_\mathbb{C}$ are null separated."

Augh

Akash. B

3:40 PM

Hmm I wonder why a digital weighing machine doesn't show reading of its own weight?

PM 2Ring

@Akash.B Try turning it upside-down. ;)

Akash. B

@PM2Ring but still how can I take note of it's reading ?

PM 2Ring

On a more serious note, scales generally have a way to adjust them to read zero when nothing is on the weighing platform / container.

Akash. B

the display will face towards the floor

PM 2Ring

@Akash.B Put it on a glass table.

Akash. B

3:45 PM

@PM2Ring yeah I saw it in google

Zero corrector

But did'nt make much sense to me

@PM2Ring great idea

PM 2Ring

It's very handy. Say you're a chemist, and you want to weigh some dangerous chemical for a reaction you want to do. So you put your empty beaker on the scales, adjust the scales to read zero, then add your chemical to the beaker.

Akash. B

Oh I see

PM 2Ring

@Akash.B The scales may not report their own weight accurately, since they probably aren't designed to operate upside-down. But even if they do work, the answer won't include the weight of the weighing platform, since that's subtracted by the zero adjustment.

Akash. B

@PM2Ring oh okay

PM 2Ring

@Akash.B Real chemists never contaminate the pans of their scales. And they don't generally weigh stuff directly in beakers, they prefer to use something much lighter (like a piece of paper), to get as much precision as possible. If you're weighing 0.1 g of stuff in a 100 g beaker, you're throwing away 3 decimal places.

JMac

4:17 PM

@PM2Ring Or using really expensive scales for no good reason.

Semiclassical

4:51 PM

@ACuriousMind "non-verbal media", to use the highfalutin word from our lab manual

Novalium Company

5:07 PM

How factories get their machines? I mean, along the line there a specific machines and robots that work only for that specific product. Do companies order custom made machines?

Semiclassical

From other factories, presumably

Novalium Company

There are factories that manufacture specific machines for other factories that will produce a product?

Semiclassical

I think so. I imagine some stuff is created in-house, but at some point you probably buy certain tools and go from there. (and of course the people you buy that from probably had to follow the same tack)

PM 2Ring

5:30 PM

The mathematician / physicist John von Neumann, one of the fathers of computer science, contemplated the problem of the minimal factory that can replicate itself, i.e., it can build copies of all of its machines from raw ingredients. (I guess it needs to be able to replicate the building too).

I could type this into a solver, which MIGHT help, but would also mean I have to get a lot of parentheses right...

3

Novalium Company

6:12 PM

We are von Neumann machines.

If we create a von Neumann machine, we will be a von Neumann machines which can create von Neumann machines.

If we create a von Neumann machine that can create a von Neumann machine, we have created a human baby.

i just blew ur minds #swagg

Emilio Pisanty

@rob thanks for the update =).

Novalium Company

How does this compress? It's basically centrifugal pumps in series?

JMac

@NovaliumCompany ... that logic is not good. Just because you could call humans von Neumann machines, it wouldn't make every von Neumann machine made by humans into a human, therefore creating a von Neumann machine that can create a von Neumann machine is not necessarily creating a human baby... unless you choose to very loosely define those terms

@NovaliumCompany Any more context on where that comes from or what we're looking at?

Novalium Company

6:28 PM

@JMac That was exactly what I was thinking while writing it.

@JMac Just a pic from google. I saw on Discovery Channel a place where they convert salt sea water into drinkable and they used such thing to create high pressure and somehow remove the salt.

It increases the pressure. I think it's centrifugal compressor or something like that, maybe a few in series.

JMac

@NovaliumCompany Seems like it's a fairly normal looking multistage centrifugal pump (which is, as you guessed, basically centrifugal pumps in series, just all in one package running on a shared shaft) googling multistage centrifugal pump shows a lot of pictures which seem to match. As far as salt removal goes, if it was vapour going through there it might be Vapor-compression desalination

Novalium Company

The propeller in each pump should be spinning faster than the previous one right? Cuz when solution goes through the first one, it accelerates (and increased pressure) and for the next one to accelerate it more, it must be spinning faster. Am I wrong?

JMac

6:43 PM

@NovaliumCompany The multistage centrifugal pump in series is used to increase pressure not flow rate compared to a single impeller at the same size and speed. Basically, if you want more flowrate at the same pressure, run the pumps in parallel, if you want the same flowrate and higher pressure, run them in series. Each stage of the pump just (approximately) adds on to the pressure added by the previous stage

Emilio Pisanty

@SanchayanDutta I'm going to do my best to avoid engaging further on that meta thread; there was a statement that absolutely needed to be called out and I did that, but I won't take the bait you're laying out there.

Novalium Company

@JMac I don't understand how the pressure is added but ok. I thought high pressure is high speed, therefore, if in series they are increasing the pressure, they should be increasing the speed as well?

JMac

@NovaliumCompany You could get high speed out of a high pressure fluid; but the speed it travels and it's pressure are also separate. Consider a pressurized vessel, the fluid obviously doesn't have a large bulk speed; but does have pressure. Now consider pressurized pipework, where the fluid has a pressure, and is also moving in bulk

If you put a hole in the pressurized vessel into a lower pressure environment, the fluid would gain velocity as it loses pressure

Analogously, pressure is like voltage, flow rate is like current. The pressure difference (voltage) drives the flow rate (current), and in doing so reduces the pressure of the fluid

Emilio Pisanty

That said, I personally find it deeply alarming to see some very tired old transphobic tropes getting used on Meta by SE moderators

(This shouldn't need saying, but just in case: content and people are different things. "what you said fits the pattern of a transphobic trope" is not the same as "you are transphobic".)

Novalium Company

@JMac I still don't understand in the multistage centrifugal pump, how can the solution have increasing pressure and travel at the same speed.

Anonymous

7:11 PM

@EmilioPisanty I detest the implication that I was laying out some bait. 'nyway, cya.

Ryan Unger

@EmilioPisanty what?

tpg2114

Please don't go down this route here

Keep it to physics.

Ryan Unger

Can you link to the drama?

JMac

@NovaliumCompany Why not? You're adding energy to the fluid, you get to choose how the energy is going to increase the flowrate or the pressure depending on how you design the pump that is getting it there.

tpg2114

Nope. Head over the mother Meta if you'd like to

Novalium Company

7:19 PM

@JMac So the pumps somehow increase the pressure and keep the flow speed the same?

JMac

@NovaliumCompany It could do that, yes. It's kinda hard to draw the line anywhere. Pumps increase pressure/flow rate so that the fluid as enough pressure and flow rate to do everything it needs to. Sometimes there are things that require high pressure, other times you might need to reduce pressure and increase flow; but the pump only cares about the fluid going through it

So basically a pump is designed to raise the pressure by a specific amount at a specific flow rate. This will vary based on the type of pump, how it is built, the speed the motor is running at, etc.

Novalium Company

@JMac Alright. Thanks for the help.

Emilio Pisanty

8:25 PM