The h Bar

4:00 PM

@JohnDuffield Btw, it would serve surely as a signpost for the future.

@JohnDuffield I think as understand grows in a topic, things considered previously as mathematical constructions, can later become real. Black holes, antiparticles began all as theoretical things only. I think if you want to make an antimatter drive, it is not important, how real are the virtual particles, and from the engineering viewpoint, they are perfectly real.

@JohnDuffield Math serves physics, physics serves engineering. On the end of the chain, there is the Human, creating wonderful things with his hands, and with his mind. If I would have 500 rep to give away, I would ask about, what are the possibilities of the particle acceleration by focused laser beams.

Itachí Uchiha

@JohnDuffield,http://www.amazon.in/John-Duffield/e/B003VMY4VK/ref=dp_byline_cont_book_1

Is this your book?

John Duffield

@peterh : see this SLAC article or Wikipedia: "The Texas Petawatt laser facility at the University of Texas at Austin accelerated electrons to 2 GeV over about 2 cm (1.6x10^21gn).[5] This record was broken (by more than 2x) in 2014 by the scientists at the BELLA (laser) Center at the Lawrence Berkeley National Laboratory..."

@ItachíUchiha : yep. Only self-published I'm afraid.

peterh

@JohnDuffield But I think the site requires brave initiatives, and although I prefer engineering to phylosophy between the borders of physics, I think your question is interesting from a phylosophical viewpoint. So, I voted it up and I will vote it to leave open. :-)

John Duffield

@peterh : it isn't a philosophical question.

peterh

@JohnDuffield Wow! There is also a new, highenergy laser facility in my birth city! Now I can already at least imagine, what could they do.

@JohnDuffield 1GeV/cm means 100TeV in a km!

John Duffield

4:18 PM

@ItachíUchiha : we know how a spring works. When you compress a spring you add energy to it. This energy is stored in the electromagnetic field, in the space between the atoms. It's a little like pushing two magnets together when their north poles are facing one another. One way of expressing the dimensionality of energy is pressure x volume. Note that we talk of stress-energy, and stress is directional pressure.

@peterh : accelerators won't tell you what a photon is.

peterh

@JohnDuffield Yes, but they tell if SUSY exists.

John Duffield

@peterh : what's the point in proposing a selectron if you don't know how gamma-gamma pair production works?

peterh

@JohnDuffield I think, the theoretical upper limit to the laser-induced acceleration happens if the photons accelerating the particles start pair production. I've done once a little bit of calculation about it, and I've found that TOE scale could be reached in around 10000km.

@JohnDuffield I think I know: the energy of the photons are transformed to the rest mass + kinetic energy of the electrons. To my child I would explain it with some classical mechanical analogy, for example some water wave thing.

John Duffield

@peterh : take a look at Supersymmetry: "For example, there would be a 'selectron' (superpartner electron), a bosonic version of the electron with the same mass as the electron, that would be easy to find in a laboratory." I don't need an accelerator to know that this is nonsense.

@peterh : I would use a Falaco soliton. See arxiv.org/abs/gr-qc/0101098 .

peterh

@JohnDuffield My main problem with the SUSY that it looks sweaty. As if they've picked up a quantum number randomly and they had started to play with it. But, I think, string theorists could probably explain very well, why exactly the spin quantum number is so interesting to mirror it, and I trust them.

@JohnDuffield What if the LSP exists as a well-known, stable particle?

@JohnDuffield What if photino == neutrino?

John Duffield

4:34 PM

@peterh : it doesn't. The only stable particles with unequivocal mass are the electron and the proton plus their antiparticles.

@peterh : But it isn't. I have to go I'm afraid. Bye.

peterh

@JohnDuffield Yes, but as I know, a symmetry breaking is suspected what makes the LSP rest mass too high.

@JohnDuffield Good bye :-)

dmckee

4:52 PM

@peterh Descriving hw springs work by invoking the typical intermolecular forces of a solid is neither too broad nor particularly complicated. The precise origin of the intermolecular forces and why they take on the dual character that they do is rather more difficult.

See for instance

83

Q: What does it mean for two objects to "touch"?

If you've ever been annoyingly poked by a geek, you might be familiar with the semi-nerdy obnoxious response of "I'm not actually touching you! The electrons in the atoms of my skin are just getting really close to yours!" Expanding on this a little bit, it seems the obnoxious geek is ri...

120

Q: Why doesn't matter pass through other matter if atoms are 99.999% empty space?

The ghostly passage of one body through another is obviously out of the question if the continuum assumption were valid, but we know that at the micro, nano, pico levels (and beyond) this is not even remotely the case. My understanding is that the volume of the average atom actually occupied by m...

particle-physics electrons atomic-physics collision matter

77

Q: Why does matter exist in 3 states (liquids, solid, gas)?

Why does matter on the earth exist in three states? Why cannot all matter exist in only one state (i.e. solid/liquid/gas)?

thermodynamics material-science phase-transition states-of-matter

peterh

@dmckee I only joked :-) I know the PSE is not so bad. I also think the OP asks specifically from the springs.

@vzn "Insane" is not always pejorative in every language and in every context, for example "insanely fast" can also mean "very fast" in at least one I know. Similar nuances can be dangerous :-( Btw, also I prefer "insane" things to the "normal" ones. :-)

@vzn As I know, on English the "crazy" can have a similar secondary meaning.

Secret

https://www.youtube.com/watch?v=O7zewtuUM_0
The physics of archery

peterh

5:08 PM

Uhm, it was enough today. Good bye until tomorrow.

Alex

If we have a grounded conducting plane and a charge $q$ a distance $d$ above the plane, then the charge $q$ induces a negative charge on the conducting sphere in a region closest to the charge. Hence in this region I assume that $\rho \neq 0$ (since it is negative). Hence by Poisson's equation this implies that $\nabla^2 V = -\frac{\rho}{\epsilon_0}$ in this region. But if $V = 0$ since the plane is grounded, then we would get $\nabla^2 V = 0$. Which one is correct? Thanks.

Sir Cumference

5:28 PM

So, can anyone explain to me why the hubble sphere is expanding?

vzn

0

A: What exactly is a photon?

this is the big "zen" question of physics for centuries, thanks for asking it. other answers are good/ acceptable, this one (reputationally risky, but sincere/ detailed) takes in some ways radically different angle/ approach. other answers look toward the past, this one will attempt to do the nea...

Bass

6:48 PM

@peterh Once I had plans to develop an SR simulation, some $(2+1)D$ "strange world" arcade game where $c$ is comparable to everyday speeds. Unfortunately, as with all my game projects, there was not much of a usable result. But I think a good game could really make those theories more intuitive.

Mast

7:27 PM

@vzn You seem to have gotten a downvote on that, although I'm not sure why. Perhaps it's considered too much a wall of text?

A TL;DR section could alleviate that.

AKA summary.

DanielSank

7:40 PM

@Mast I doubt it.

The post probably got down-voted because:

1) Lack of punctuation.

2) No real focus.

3) Doesn't answer the question.

4) Saying things like ' "zen" question of physics'. What does that even mean?

I think readers want a more precise answer.

2

@vzn this isn't a criticism of you personally, just explaining my guess of why the post got down-voted.

Mast

I tried to fix the punctuation, but I can't help the rest. That's up to the author.

bolbteppa

@JohnDuffield what is the jist of the money section of your book

Some of them reviews are hilarious tbh

Why the aversion to just learning the math behind the theories, I can't understand it, there is this big question mark as to what it's all about when you ignore the math

DanielSank

7:58 PM

This site needs an authoritative list of examples of good and bad questions.

bolbteppa

Think of it as boarding a big wooden horse and going inside the gates undercover to get inside the belly of the beast

DanielSank

All this business about the homework policy, low quality questions, and essentially everything else we ever debate around here grows out of the fact that there are curious people in the world who do not understand how to ask a question well.

Mast

@DanielSank Make it a meta.

DanielSank

In the software community, we just point everyone to this.

@Mast Perhaps.

ACuriousMind

11:47 PM

@peterh This is precisely why I don't care to discuss with you: You have already decided that my goal is to "close and destruct newbie questions", while I have repeatedly stated that my view of the quality of a question is independent from its actual level. I know good high school questions about Newtonian physics as well as absolutely garbage questions about rather technical aspects of QFT. That you continue to assert I want to steer the site into a "MO-like direction" is ridiculous.

2

Physics Guy

Man, I've just read a 58 pages long article about dirac equation in curved spacetime.

ACuriousMind

I have absolutely no desire to debate either my voting patterns or my vision for the site with someone who just will not listen but instead continue to slander me. I've had enough ridiculous accusations just this month to last me the entire year, so I will not respond to you any further.

3

@PhysicsGuy Bad timing there :P

Physics Guy

;)

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