The h Bar

12:00 AM

For most classical things, they all have well defined causations thus a unique causal network can be drawn to describe them. Using this analogy, their causality is like a crystalline solid with an ordering

Meanwhile, possibly in some quantum systems, you have indeterminate causality where things are in a superposition of A cause B and B case A. These are like some kind of quantum state that are ordered

But can we have states where the causal network that describe it is stochastic and depends on some order parameters like an amorphous solid

or better, "liquid causality" where the underlying causal network keeps changing and does not have a settle form, similar to a spin liquid?

Sir Cumference

3:12 AM

@Blue I'm pretty sure Abcd was joking

Or I hope (not trying to sound condescending)

vzn

4:15 AM

@Secret try looking into (QM) "probability current" or probability fluid

Secret

4:44 AM

Pretty sure a probability fluid is different from the notion where causal structure is dynamic

vzn

4:56 AM

He just graduated from college and wants to be an astrophysicist. Oh, and he's 11 / cnn

↑ seems likely very interesting AMA guest speaker o_O

Jake Rose

5:20 AM

Hey guys a question about SR. one of the assumptions of this is that you can't tell two inertial reference frames apart and that speed is just relative. But why couldn't you trace back to when one of the frames was accelerated? Then could you not deifniteively say that this frame was definitely the moving frame?

John Rennie

@JakeRose not necessarily. Both frames could have been moving and one frame decelerated to a stop.

Then the frame that experienced the acceleration would be the one that was stationary.

Jake Rose

So you couldn't tell if it was deacceleration or acceleration?

John Rennie

No

Jake Rose

Yeah I agree

So overall would it not be possible if you could trace back to all the forces which one was accelerating?

*moving not accelerating

John Rennie

Accelerating just means your speed has changed. You can tell how much the acceleration changed your speed, but you can't tell what your speed was before or after the acceleration.

Jake Rose

5:32 AM

Oh I see

thanks for the help!

btw @JohnRennie experienced my first May ball

what a bloody party

John Rennie

@JakeRose :-) I never paid to go to a May Ball but I worked as security at a few of them

Jake Rose

Oh really how come? I had to leave town before I could work for any but hopefully gonna earn some quick money next year

Cows

finished v2 of code. It's in the cloud now

John Rennie

You just have to know the organisers. The deal is usually that you have to work for a few days setting everything up then again afterwards to clear everything away, then for half the night as security. Then you get to spend the other half of the night as a quest.

Jake Rose

@JohnRennie did you do pure physics second year?

Pretty good strategy

John Rennie

5:36 AM

I don't recall being paid though

Jake Rose

May balls are extortionate

nearly cried after seeing what was left of my student loan

John Rennie

@JakeRose yes I agree. I was pretty broke as a student and paying to go to a ball was way outside my reach.

Just paying the rent on my digs was a stretch :-)

Jake Rose

The May balls employ everybody now adays. Unless you're on the committee you're strong paid (even if it's pretty poorly)

John Rennie

I guess that's a lot more professional, but it seems a shame

Jake Rose

Tell me about it. Magdalene accommodation is certainly not the best.

It's because all of the food and drink staff (not including the private vendors) are students. So it's probably a few too many people to let in.

John Rennie

5:38 AM

No student accomodation is great. My brother went to Imperial College and some of the places he stayed were so bad the rats had left.

Cambridge is actually not bad compared to a lot of places.

Jake Rose

Imperial seduce you with the really fabulous first year accommodation then throw you out for the rest

Oh no trust me I count my lucky stars with my accommodation even if it is a tad expensive

Did you do pure physics in your second year? Or a bit of chemistry too?

John Rennie

I did advanced chemistry and ordinary physics in my second year. By then I had discovered quantum chemistry and decided that was what I wanted to do.

Jake Rose

Ahhhhh. Was it split into physics I /II and similarly for Chem back then?

John Rennie

Yes, for both physics and chemistry you had an advanced course for people who planned to specialise in the final year and an ordinary course for people who just wanted some knowledge but weren't going to specialise.

So I did advanced chemistry then all chemistry courses in my final year. The ordinary physics was very useful though and I'm glad I did it.

Jake Rose

Ah gotcha.

John Rennie

5:44 AM

I would have been tempted to do both advanced chem and advanced physics but the timetable didn't allow it.

Jake Rose

I'm always interested to see what people who do a physics a chemistry and then do other random one

Could yo you part II physics without part I?

Cows

@JohnRennie what inspired you the most to get into Quantum chemistry

John Rennie

@JakeRose no

Well technically you could go to the lectures and sit the exam but I suspect you'd find it far too demanding

@Cows I've always been interested in applied science i.e. relating it to the real world, and quantum chemistry seemed like it did exactly that.

@JakeRose bear in mind my knowledge of the Cambridge courses is 35 years out of date :-)

Cows

The thing that makes me wish I really liked chemistry a lot is the fact that one could use it to create medical drugs. This to me is super magical.

I mean I like to see chem in action but I am horrible at it lol

Jake Rose

@Cows organic chemistry is da bomb. Just fun hexagons.

I found chemistry an overall pretty easy subject. At least first year it's a bunch of hand wavy-mess

Cows

5:58 AM

@JakeRose I tried studying Ochem on my own some time ago, failed like a boss lol. But yeah Ochem and BioChem are essential to human survival

Captain Bohemian

chemistry in university level can only be easy for people with amazing memory power

Jake Rose

I found biochhem one of the more boring bits. Synthesis is what I found most interesting.

John Rennie

@JakeRose I agree. But quantum chemistry is very different. You're calculating molecular wavefunctions and from them the properties of the molecules.

2

Jake Rose

You can get through a surprising amount of organic with just a good conceptual understanding. Most of the basic mechanisms are just common sense then rather than memory.

Cows

what makes me marvel the most about life is drug action pathways

Jake Rose

6:01 AM

Now geology. Theees a subject which was fun.

Cows

I can't even imagine how people think about these things

Captain Bohemian

my experience with chemical course is there are too many terms to remember in time so that I often don't know what the teacher is talking about when they intersperse their lecture with multiple chemical terms.

Cows

@JohnRennie not that I will understand :P , but what is planck data saying?

John Rennie

@Cows there's a good summary here

Jake Rose

I think our lectures went through them pretty well. If we talk about something we tend to talk about the idea more so than the name so you can still get by with knowing minimal terms. Sadly being a good (insert science branch here) does usually involve a bit of name learning.

Novalium Company

6:14 AM

Hello everybody!

I've been working on my game recently and haven't got much time, but when I finish it, I'll be back regularly :-)

Howdy all

Helloooooo :-D

What's up?

just chillaxing

2

user2236

Chillaxing

:-)

user157860

7:28 AM

@JohnRennie, hi, could you confirm your comment and give a judgement on the answers I got? I also found your formula $ v = \frac{at}{\sqrt{1 + (at/c)^2}} $ here, which one applies to my example? Thanks

P.S. When you can spare some time, could you give a short answer specifying the difference among those formulae?

John Rennie

@user157860 The two situations are very different.

user157860

which one applies to my case?

John Rennie

Suppose you have a spaceship with a steady acceleration i.e. it's firing its rocket motor with a steady thrust as measured by the people on board. That's when you get the equation:

$$ v = \frac{at}{\sqrt{1 + (at/c)^2}} $$

This happens because (a) time is dilated on the spaceship and (b) length is contracted on the spaceship.

user157860

So, I need the schwartzild radius? could I find the final velocity just finding the PE ?

John Rennie

So the observer on Earth and the observers on the spaceship disagree about the length of a second and a metre.

For the black hole you need to calculate the trajectory of a freely falling object.

user157860

7:34 AM

No, I do not mean a black hole, just a massive planet or a neutron star

Cows

@user2236 :)

John Rennie

This is a fairly standard problem. In general you have to solve an equation called the geodesic equation, but for the Schwarzschild metric the symmetry allows you to take a number of shortcuts and you get the equation:

$$ v = \left(1 - \frac{r_s}{r}\right)\sqrt{\frac{r_s}{r}}c \tag{1} $$

Where $v$ is the velocity and $r$ is the radial distance as measured by an observer far from the black hole.

user157860

Consider the case of a neutron star, there is no Sr, a is just 3 Km/s^2, can I just add the PE and from the total, through SR work out the new velocity?

John Rennie

No

I assume you're asking for the velocity as measured by an observer far from the black hole, and that will be given by the same equation as for a black hole.

user157860

R is 10^8 cm,

John Rennie

7:40 AM

Beta Decay has considered the case of a constant acceleration like the spaceship I described, though I don't recognise the final equation they got. It might be correct and just be in a more complicated form than the one I'm used to.

@user157860 the Schwarzschild metric describes the spacetime gemoetry outside all non-rotating spherical objects, not just black holes. So it applies to a neutron star.

user157860

So, I cannot just add the PE to the KE of F?

John Rennie

No

Neither PE nor KE are well defined concepts once you have to take spacetime curvature into account.

Computing the motion in curved spacetime is a lot harder than that.

user157860

so, the best way is your formula $ v = \sqrt{\frac{r_s}{r}}c \tag{2} $ ?

John Rennie

Careful, that's the velocity as measured by a shell observer i.e. someone hovering at a distance $r$ from the black hole.

But shell observers have their time dilated so the velocity they measure is not the same as the velocity an observer far from the black hole would measure.

user157860

Yes, that's what I need I am standing at hf (10^11 cm) where I want to record the velocity

John Rennie

7:50 AM

@user157860 yes, in that case use $$ v = \sqrt{\frac{r_s}{r}}c $$

user157860

but does that formula take into account v at 10^12 cm? How can I fit that in?

John Rennie

It should be possible to show that this is approximately the same as the velocity you'd calculate using the change in PE as long as your far from the event horizon. I must admit I've never tried that.

user157860

Adding PE I found .9918 c, can you work out the value with your formula?

John Rennie

The equation I gave is for an object that falls freely from infinity starting with zero velocity i.e. at every point its speed is equal to the escape velocity. It's possible to do the calculation for an object that starts with a non-zero velocity i.e. it's speed is greater than the escape velocity but the resulting equations are more complicated. I don't remember them offhand though I have them somewhere.

@user157860 I'm not sure what you're asking. Your diagram is so cluttered I can't make out what you're trying to do. Can you explain it simply?

user157860

a comet F is at 10^12 cm from a massive star heading toward it at .99c it has 7.09 masses and 7.09-1 energy. it travels to 10^11 cm toward the star where it would reach c (+.01c), but considering relativity what is its actual speed when it impacts my ship at 10^11 cm from the star?

Secret

8:04 AM

0

Q: Can particals such as tachyons and anti matter be detected looking backward in time?

Figuring moving faster than light goes forward and backwards in time hypothetically, maybe thats why tachyons and or anti matter cant be seen or studied because we are conducting experiments to the here and now, present, when in our reality it wouldnt be seen in present for that fact alone that m...

quantum-mechanics

If tachyons are detectable, it will be basically indistinguishable from some correlations in the data, since we can only look in the present direction below the speed of light

John Rennie

@user157860 I'm busy for an hour or so, but that's not a hard calculation to do.

Secret

as well we will expect a lot of chrenkov radiation to be in the vicinity

user157860

@JohnRennie, thanks, take your time, ping me the answer.

or, ifyou feel like it, post a short answer just with the right formula and the result

Secret

scientificamerican.com/article/what-is-known-about-tachy

old scientific american: Seems there is nothing in our imagination that cannot be produced by quasiparticles

Physics Meta

0

Q: How to write mathematical expressions in questions using android devices?

I posted a question at Physics Stack Exchange site, and I uploaded a photograph of the calculation I had done on that. But, I was asked to write all my calculations in the question(and not in the photograph). So, How to write mathematical expressions in questions using android devices?

support feature-request mathjax tex formatting

Secret

8:16 AM

43

Q: Status of experimental searches for tachyons?

Now that the dust has settled on the 2011 superluminal neutrino debacle at OPERA, I'm interested in understanding the current status of experimental searches for neutrinos. Although the OPERA claim was a fun puzzle for theorists trying to find explanations, the claimed scenario wasn't anything li...

special-relativity particle-physics experimental-physics faster-than-light tachyon

Actually, what exactly do a tachyon signal look like (other than splitting into double images)?

Causality is overrrated, like everything else human

armaghplanet.com/blog/what-ever-happened-to-tachyons.html

> I have read some of the rationales and product descriptions on sites advertising this sort of junk and found only scare-mongering nonsense, aimed at extracting money from the unwell or anxious, proving that while space has a speed limit, there are no limits to folly and dishonesty.

There is only one thing that has the size of a proper class: Human Stupidity

Stupidity has no bounds

and bounds has no stupidity

and 2+2=5

Physics Meta

12:02 PM

0

Q: Question deleted without a trace and without notice?

I'm somewhat annoyed that one of my questions just vanished like it never existed. Shouldn't there at least be some kind of notification in my account left? And why not keep questions that don't have an answer, yet? The question was related to quantum scars - a relatively recent research topic, w...

Problem Solving Strategies

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