Who is Sabine Hossenfelder and what are her research interests?

[marcus]

Nice idea, but we each need to earn enough to eat.

kea, if I could think of a research idea that was right for you then you could have it free. it would be yours.
I don't understand the "but, we each"
people who suggest things don't need to share the tempie caviar.

what I think is that "foundational questions" sounds intriguingly wacky.
let's try to think here of some really "foundational" questions that one or some of us could research

(unless you think this is bad taste, or rather not)

 

[Kea]

kea, if I could think of a research idea that was right for you then you could have it free.

Why, thank you, Marcus, but I'm not sure that my digestive system would find it particularly nutritious.

let's try to think here of some really "foundational" questions that one or some of us could research...

Like the what does the removal of an ontological framework for quantum physics have to say about the existence of omniprescient beings? Yes, that sounds fairly foundational to me.

 

[marcus]

Like the what does the removal of an ontological framework ...

but it never HAD an ontological, did it? not since maybe Prince de Broglio I mean?

=========

I hope you try again. OK sure we won't offer any suggestions if that's better for you. But it just seems right, almost a duty, for you to enjoy a Templeton grant.

 

[Kea]

...but it never HAD an ontological, did it?

Quite true, I think. But somehow, after all this time, this remains a topical and foundational question.



P.S. Sorry, Hossi, that we have turned a PI chat into a fqxi one. Pies do taste better than fqxies, don't they?

 

[arivero]

Hey, I am serious about my anti-gravitation stuff. The cosmological implications are quite interesting. There is nothing spiritual about it, it's just a GR-extension. I don't think that is weirder than http://en.wikipedia.org/wiki/Quintessence_(physics)" .

I was sleep-less yesterday, so I missed the opportunity of a pun: "quintessence" was the same object (substance, element) that Marcus refers as the "Empyrean substance", wasn't it?

 

[vanesch]

Hey, I am serious about my anti-gravitation stuff. The cosmological implications are quite interesting.

In my extreme naivity, there's something about these anti-gravity things that I don't get, if you want to keep some form of the equivalence principle. Back to simple "Einstein" experiments. The basic idea of the equivalence principle is that locally, there's no distinction between gravity and an accelerated reference frame (up to tidal effects, which are "second-order local"). So there should be no distinction between an experiment at the surface of the earth, and an experiment in a rocket in space, pulling 1 g.
But from the moment you introduce these anti-gravity things, that idea doesn't run anymore. Now, I'm not convinced by extending the allowable transformations to save part of the equivalence principle. It is, as far as I understand, simply dead, if you have anti-gravity stuff. I know of course about spin-1/2 things, and spinor tranformations which are different from tensor tranformations. But all spinors are theoretical constructs which SERVE to construct observable tensor quantities. I don't know of any directly observable spinor quantity, honestly. Usually, you need two spinors to construct an observable.
But with antigravity things, they simply *go the other way* in the two experiments (in the rocket, and on the surface of the earth).
You're probably thinking of "squaring the "going the other way"" so that twice going the other way goes the same way or something, similar to the spin-1/2 spinors, but that doesn't fly as long as this antigravity thing is observable, no ?
It's not the same as saying that, because of spinors, suddenly a full rotation in space is over 4 pi, and not over 2 pi. As far as I know, a rotation over 2 pi only flips the sign of *theoretical constructions* and not of a real observable quantity (in that, when I turn around my apparatus over 2 pi, that suddenly I see different outcomes).
But - unless I completely misunderstood you - antigravity things behave observationally different in a rocket at 1 g and on the Earth's surface, no ?
So that we CAN make the difference between both, and I thought that *THAT* was the essence of the equivalence principle ?

 

[marcus]

I was sleep-less yesterday, so I missed the opportunity of a pun: "quintessence" was the same object (substance, element) that Marcus refers as the "Empyrean substance", wasn't it?

For shame! It is a black day when you miss a pun. but these lapses are rare.

Personally I don't know. there may be different physical models used by the alchemists or by the medieval world theorists.

surely there are Five essences: earth, water, fire, and air, plus the fifth (the quintessence)

and one would suppose that the quintessence is the highest and best, so that the outermost sphere ought to be made of it.

but the word "empyrean" suggests that the outermost sphere is made of the Fourth essence, fire. so I am confused. when you learn the correct answer I hope you will be kind enough to share it with us.

 

[hossi]

In my extreme naivity, there's something about these anti-gravity things that I don't get, if you want to keep some form of the equivalence principle.

So that we CAN make the difference between both, and I thought that *THAT* was the essence of the equivalence principle ?

Dear vanesch,

thanks for the smart question Indeed, the extension of GR I have proposed is a relaxation of the equivalence principle.

I understand the equivalent principle this way: locally physics is as in special relativity. This is still valid. However, I found that there are two possible ways to go from the local tangential spaces to the total curved space. The one is the usual one, which leads to the notion of tensors and the tensor calculus that comes along with it. The other way leads to a similar structure, with quantities whose transformation behaviour under general diffeomorphism is modified. From their transformation behaviour one can construct an associated covariant derivative in the usual way, such that it respects the transformation behaviour. This is essentially the reason why these fields (particles) do not move on geodesics.

You might say, the equivalence principle is valid up to a two fold degeneracy. When you introduce a field in this theory you have to specify which kind of transformation behaviour it belongs to. This doubles the particle content of the standard model. Each particle comes with it's anti-gravitating partner.

You can interpret this in the Newtonian limit: a particle has an inertial mass and a gravitational mass. Either both are identical, or the one is the negative of the other.



B.

 

[vanesch]

I understand the equivalent principle this way: locally physics is as in special relativity.

This means, if I understand correctly, that locally, (apart from tidal effects), you cannot observe any gravitational field to distinguish it from a uniformly accelerating frame, right ? So it is sufficient to "accelerate in the other direction", in other words, to "fall along", and everything should happen AS IF THERE WAS NO GRAVITY, right ?
Now, consider a local inertial frame initially "falling along" with your anti-gravity particle, towards the sun, say. It will have, at a certain point, a certain position and momentum, so we can define a "tangent" inertial frame. In this frame, initially, your anti-gravity particle is at rest, right ? But it won't stay that way ! It will start accelerating in your local inertial frame! On the other hand, a normal particle, initially at rest in your inertial frame, will stay at rest - by definition of it being an inertial frame.
So your inertial frame is only an inertial frame for "normal" particles, and not for "antigravity" particles ? But then locally, physics is NOT as in special relativity, no ? Where particles, free of interactions, should follow a uniform motion (and in particular, when initially at rest, should stay at rest). Some do, and others don't. And from this difference, we can then find out, locally, that the "inertial frame" is falling in a gravity field.

Let's now go to outer space, far away from our galaxy, and put ourselves in a rocket, floating freely. We now put our anti-gravity particle at rest in our rocket: it stays at rest. And so does the normal particle.

So we succeeded in making a difference between a free falling frame in a gravity field, and a "true" inertial frame in outer space. Exactly what was forbidden, no ? The entire idea of the equivalence principle was that this was impossible, I thought. I don't see how you can *partially* relax this. Something is impossible in principle, or not. For instance, in special relativity, it is in principle impossible to distinguish one inertial frame from another, by just doing local experiments. That's the entire contents of the relativity principle (which is already present in Galilean relativity). From the moment that there is ONE single way to do so, the entire structure of special relativity falls apart ; or even, galilean relativity falls apart (that was exactly what happened when the Maxwell equations defined a single velocity c which could be locally measured: the effect of having a way of locally establishing an absolute velocity killed off Galilean relativity, and hence the group of galilean transformations and introducing the ether ; only to be replaced by special relativity and the Lorentz group). There was no way to partially relax galilean relativity: after having an absolute c, it was dead.

What's supposed to be impossible, by the equivalence principle, is to make a distinction between a free-falling frame in a uniform gravity field, and an inertial frame "in outer space". At least, that's how I understand it.
With my normal particle / anti-gravity particle set, I can make the difference ; I can even find out the absolute acceleration of the gravitational field that way, and hence the "background" inertial frame in which this gravitational field is present. And if that's the case, then the entire geometrical picture of gravity as a curved 4-dim spacetime manifold falls apart, no ? Because we now have a NON-CURVED background spacetime on which we have gravity as a field, like any other.
And once we have that, to me, the equivalence principle and from it, the requirement of general covariance, are dead. In the same way as galilean relativity, and its related group, were dead after having a fixed c.

 

[ccdantas]

Hello to all!

Very nice discussion forum.

Dear Sabine (hossi):

I wish you the best of luck in your professional career! You know? You are (or will be) in a place I very much would like to be (PI), and working on things I very much would like to be working on. I mean, full time. Also, you are so young... Have you realized how lucky you are?

I don't mean to envy you... Argh... Yes, I do envy you!

We cannot have all we wish in life. I have a permanent (job) position (*not* exactly in physics), my hunsband also has a permanent job, I have a wonderful kid and own a beautiful house. My life is now absolutely stable. But at the same time I will never have the opportunities and mobility you have.

Yes, decisions are often so hard to take. They always involve losing things or some kind of compromise.

I enjoy your blog and have superficially read your last paper (I hope to find time to read it more carefully). BTW, I must apologize that somewhere in my blog, I said you were a "he"... Of course, WHO quickly corrected me. I also watched your PI presentation, very well done! Congratulations.

Just to not have born in an underdeveloped country (like me) is already a *big* gift! I am sure that, with such a great potential and excellent opportunities (and lots of hard work), you will succeed professionally.

Best wishes,
Christine

 

[hossi]

So your inertial frame is only an inertial frame for "normal" particles, and not for "antigravity" particles ?

The freely falling frames that fall along with both types of particles are not identical. Right.

So we succeeded in making a difference between a free falling frame in a gravity field, and a "true" inertial frame in outer space. Exactly what was forbidden, no ? The entire idea of the equivalence principle was that this was impossible, I thought. I don't see how you can *partially* relax this. Something is impossible in principle, or not.

You can relax that partially by saying it is impossible to distinguish between the accelerated particle and it's reaction to gravity. And it is impossible to distinguish between the accelerated anti-grav particle and it's reaction to gravity.

You have implied that I have both at once and that I can't distinguish between acceleration of them and their reaction to gravity. Which you found to fail, with which I agree.

From the moment that there is ONE single way to do so, the entire structure of special relativity falls apart

Well, no, why? I still have usual diffinvariance, tensor calculus etc for the usual fields. I have additional fields with different transformation behaviour. Their properties are, when transformed the right way, also invariant under coordinate trafos etc.

I can even find out the absolute acceleration of the gravitational field that way, and hence the "background" inertial frame in which this gravitational field is present. And if that's the case, then the entire geometrical picture of gravity as a curved 4-dim spacetime manifold falls apart, no ? Because we now have a NON-CURVED background spacetime on which we have gravity as a field, like any other.
And once we have that, to me, the equivalence principle and from it, the requirement of general covariance, are dead.

I am really sorry, but I can not quite follow your arguments. You can measure the gravitational field, yes. Whether the presence of a field is detected or not does not depend on the particle's nature. Either both notice it, or both don't. They both notice it in a different way though. I don't see how that messes with the geometrical picture of gravity. When space is flat, they both move on the same line, when space is curved they don't *.



B.

* Restrictions apply, but that is a lengthy story.

 

[hossi]

Dear Christine,

I really like your blog, it's very well organized and maintained. Thanks for the nice wishes. It probably helps neither you nor me, but to me it seems, you have everything I would like to have a permanent job, a husband with a permanent job, children and a house. Add to that a couple of cats and I quit PI. I really don't look forward to yet another move.

BTW, I must apologize that somewhere in my blog, I said you were a "he"...

No problem. I don't always use my first name as author cause it seems there is no one else in physics with my family name - which is long enough by itself. I admit I was more offended by being called a novice. Like, I have been in education since 12 years. How old do I have to get until I can call myself a professional physicist? I take it now, it was not meant to be insulting, I was just having a bad day.

Take Care

B.

 

[ccdantas]

Hi Sabine, (btw how would you like to be called?)

Thanks for replying and the kind words about my blog...

Now, would you be happy with a permanent job in... err... software engineering? Or would you stick with physics even though you'd never know where you would be the next year? (and this, for an unknown period of time...)

I just didn't have even the second possibility (well, perhaps it is an exaggeration, but that was how I saw it at that time)... Although my boss is nice enough to let me dedicate to research a small fraction of the time, that is not enough for me, and I am really *depressed*. Reading Epictetus and Marcus Aurelius (the stoics) has been somewhat helpful. But, you see, I cannot complain...

You asked: "How old do I have to get until I can call myself a professional physicist?"

I don't know. Just do not care about this. Your *are* young and somewhat starting your research career anyway. And that is *good*! Just concentrate on your work and do the best you can. The result of your work will speak for itself as time goes by, and in fact, going to PI is already an excellent indication that people have a professional respect for your work.

All the best of luck, Sabine!

Christine

 

[vanesch]

I am really sorry, but I can not quite follow your arguments. You can measure the gravitational field, yes. Whether the presence of a field is detected or not does not depend on the particle's nature. Either both notice it, or both don't. They both notice it in a different way though. I don't see how that messes with the geometrical picture of gravity. When space is flat, they both move on the same line, when space is curved they don't *.

In a "uniform gravity field", spacetime is NOT curved. It is in fact identical to "free" spacetime, which is flat. THIS is (as I understand it) the entire idea of the equivalence principle, no ? In other words, according to the equivalence principle *there is no such thing as a uniform gravity field*. Nevertheless, you can make a difference between flat space (uniform gravity field) and flat space ("out there half way towards Andromeda") using "antigravity" matter. This very fact, to me, kills the entire idea of gravity being a result of a curved spacetime.
We CAN already make a difference between flat spacetime and curved spacetime: it's called "tidal effects". THIS is the only thing, according to the curved-spacetime-picture of gravity, which is "gravity": the curvature of spacetime. A flat spacetime is a flat spacetime. But "antigravity" matter will act differently in 'certain kinds of flat spacetime' than in others, which means that there is an EXTRA QUALITY now assigned to certain flat spacetimes (which is called "gravity"). But that implies that gravity, after all, is NOT the curvature of spacetime, no ? Because we can now make the distinction between two IDENTICAL spacetime manifolds...

 

[hossi]

But "antigravity" matter will act differently in 'certain kinds of flat spacetime' than in others, which means that there is an EXTRA QUALITY now assigned to certain flat spacetimes (which is called "gravity"). But that implies that gravity, after all, is NOT the curvature of spacetime, no ? Because we can now make the distinction between two IDENTICAL spacetime manifolds...

Hi vanesh,

Spacetime is either (globally) flat or it isn't. There are no 'certain flat spacetimes'. If spacetime is globally flat, both particles do the same thing, no matter what. If it is curved, they can move on different trajectories.

It seems to me you are using the 'uniform gravity field' as a synonym for acceleration. To use the equivalence principle, you say the effect of gravity (locally) is the same as acceleration in flat space. For the anti-g particle, its just the opposite acceleration. I.e. the ratio of intertial to graviational mass in inverted. It might be helpful to forget for a moment that there are two types of particles and only to consider the second possibility.

Yes, exactly, tidal effects allow to measure the curvature. I.e. the (infinitesimal) transport of the local frame differs for both particles. You start at one point where both were equal, then they will deviate from each other after some \delta x.



B.

 

[hossi]

Now, would you be happy with a permanent job in... err... software engineering? Or would you stick with physics even though you'd never know where you would be the next year? (and this, for an unknown period of time...)

Hi Christine,

I thought about your post last evening and had about 1 million things to say, most of which I'd definately not post in an online forum. Feel free to email me anytime, you find my email on my homepage.

I know that I am very very lucky. I still remember Lee's call with the job offer from PI, which came only some days after my talk at PI (US Airways had not even found my baggage then.) I almost dropped the stupid phone. It IS a great opportunity, but it's also kind of scary...

Indeed, about 80% of the time I wish I had a job like yours! (So do most of my friends). What is wrong with software engineering - my younger brother is in the same field. Unlike me, he has a reasonable income, a safe job, social security, a retirement plan, an appartment worth being called so, lives with his girlfriend, in a country where they speak his language, and - most importantly - he has a life. I know, the grass is always greener on the other side, blahblah, but a friend of mine (she is also a postdoc in physics) said recently 'What other job is there were you get treated like **** every day'.

But I am German, and I just like to complain

More seriously: if I compare my status with friends in 'real' jobs that are about my age, I have no status - after all that time. I might be young for a postdoc, but that is really sick at age 30. It's a disease the whole field suffers from: it takes an eternity to be taken serious.

Maybe I have been in the US for too long, but I genuinly believe that you can do what you really want to, if you only try hard enough. Sounds probably naive, but why don't you write some papers and get yourself some invitations for seminars. Have a look at the green grass.

Take Care,

B. (prefer B, even my new Japanese officemate can pronounce that)

PS: Answer from the German Science Foundation, they say no.

 

[vanesch]

Spacetime is either (globally) flat or it isn't.

And locally, to a good approximation ?

Can we say that some domain of a spacetime manifold is, to a good approximation, flat ? (eg, the Riemann curvature tensor vanishes over the domain, or is, at least, extremely small? )

Consider two such domains. One domain is relatively close to the world line of a huge black hole (so there will be SOME curvature, but you can make it as small as you want, by making the black hole as massive as you want and increasing the distance). Call it domain A.

Another domain is very far from any matter or energy. Call it domain B.

In a Newtonian limit, one would say that domain A is in a relatively homogeneous gravitational field, while domain B is free of gravity.
In both domains, there is so good as no tidal effect observable (very tiny Riemann tensor).
But in general relativity, both are rather equivalent, flat lumps of spacetime. So locally, they are equivalent concerning their metric structure. Now, let us choose coordinates over both of these domains which make this flat metrical structure explicit (in other words, take local coordinates which correspond to inertial observers). In domain A and in domain B, in such coordinates, normal particles undergo a uniform motion on a straight line.
But anti-gravity particles accelerate (strongly) in domain A, while they undergo uniform motion in domain B. (at least, I assume, that these anti-gravity particles accelerate AWAY from the black hole).

Nevertheless, BOTH domains correspond to locally FLAT pieces of spacetime.

There are no 'certain flat spacetimes'. If spacetime is globally flat, both particles do the same thing, no matter what. If it is curved, they can move on different trajectories.

And "locally" ? Do the anti-gravity particles do the same thing as normal particles in (to a good approximation) the locally flat spacetime not too far away from a huge black hole ?

It seems to me you are using the 'uniform gravity field' as a synonym for acceleration. To use the equivalence principle, you say the effect of gravity (locally) is the same as acceleration in flat space. For the anti-g particle, its just the opposite acceleration. I.e. the ratio of intertial to graviational mass in inverted. It might be helpful to forget for a moment that there are two types of particles and only to consider the second possibility.

What I don't understand, is this: from the structure of spacetime, it is always possible to find ONE frame (= coordinate set) which is "locally inertial" at point P (think it is called Riemann normal coordinate system). Normal matter is supposed, in such a frame, to behave like in free space (uniform motion on a straight line), at least, locally. This is true whether the spacetime is locally flat or curved.
Now, if some stuff starts *accelerating* in that frame, then, this is because of some *OTHER* interaction, but it can certainly NOT be due to the local structure of spacetime (= gravity), can it ? And what we have here, is that those anti-gravity particles start accelerating in the case of domain A. Of course, you can find now another frame, in which this anti-gravity particle is in uniform motion, but the point is that such a frame is not a Rieman-normal coordinate set. (= inertial frame).

So my point is that locally IDENTICAL lumps of spacetime (domains A and B) give rise to *different* behaviour for that anti-gravity particle ; which means, to me, that it cannot be the (local) structure of spacetime that describes this motion.

Of course, if ALL matter were "anti-gravity" there would not be a problem either (we just made a mistake in our assignment of the metric tensor, and it must be re-defined in order for the "co-moving frame" to be inertial now). But I don't think both can be coexisting and derive both from the metric structure of spacetime.
But of course, there can always be *another* interaction which makes these "anti-gravity" particles accelerate, for one reason or another.

 

[arivero]

Hi Sabine, (btw how would you like to be called?)

Thanks for replying and the kind words about my blog...

Now, would you be happy with a permanent job in... err... software engineering?

I can not tell for Sabine, but on my side, I have been two years in the private soft engineering, then moving to teach computer science for five or six years. My private consolation was more on the side of Lucretius (say, atomist once, atomist ever) and Archimedes, but even in this case one gets bored and the only thing I see as a solution is the one you both are afraid: to move.

(I think I will move to Cambridge for some months. BTW if anyone can offer a cheap room, I am hearing )

When I was younger I was even able to smell the wind of moving. Do you know, the last day of a meeting, when nobody left in the city because everyone has understood it is the hour to leave. The same thing can be sensed after months or years, at some moment the wind blows, and it depends of course of the roots you have grown and the debts you have not payed yet.

Or would you stick with physics even though you'd never know where you would be the next year? (and this, for an unknown period of time...)

I just didn't have even the second possibility (well, perhaps it is an exaggeration, but that was how I saw it at that time)...

I think I had the possibility but the real problem is when changing institutes does imply changing fields of research, and not being able to choose. If you "stick with physics" in this way -a very common mistake, most of your teachers did it-, you are just getting a work you are good to, "problem solver" (sometimes even very good), and then you can start to think that computer science is a good option too. Hossi, at the PI, is still able to choose his research and to choose it according to physics, not according "a nice problem to solve and publish about" (common mistake related to the previous one).

I have know experimentalists leaving *after getting tenure* when they become aware they were not able to choose experiment to do (it takes huge money and politics to control a single room in a lab).

 

[Kea]

Now, would you be happy with a permanent job in... err... software engineering? Or would you stick with physics even though you'd never know where you would be the next year?

...and I am really *depressed*. Reading Epictetus and Marcus Aurelius (the stoics) has been somewhat helpful...

Dear Christine

I am also very, very lucky - to have been born where I was, and to have had the opportunities I have had.

Since I was a teenager, when I first learned what physics was, I wanted to be a theoretical physicist. I'm still doing my PhD, over 20 years later. I have worked in software engineering (now that's not a bad job at all)...as well as managed a small lab in biomedical engineering for a year or so, as well as worked for an investment bank (not that I actually wanted to in the first place - long story), as well as...cooked, cleaned, waitressed, worked as an agricultural labourer, cashier, ski patrol, lift operator, ice cream seller, conservation worker and...I'll stop here, although that's not the full list.

I was quite sure in 1998 that I would never have the chance to return to physics - for the third time (another long story). But I did. Again. Far from having job security, I'm used to wondering where I'm going to sleep next month, and what I'm going to eat. When they discovered at the university that I can be a difficult person sometimes, they told me that I should - I quote - pretend I was 18 and put my head down.

Don't give up.
Cheers
Kea

 

[ccdantas]

Thanks to all who wrote about the questions I have raised, and put their own personal feelings and experience about them.

I agree with B. that there are many issues that not only she, but probably all of us, would not be willing to write openly about, that is, issues that go far beyond what is reasonable to post online. So I'll stop here.

Thanks a lot again and good luck to all of us!

Best wishes
Christine

 

[hossi]

What I don't understand, is this: from the structure of spacetime, it is always possible to find ONE frame (= coordinate set) which is "locally inertial" at point P (think it is called Riemann normal coordinate system). Normal matter is supposed, in such a frame, to behave like in free space (uniform motion on a straight line), at least, locally. This is true whether the spacetime is locally flat or curved.

Hi vanesh,

thanks for your thoughts

You mean local in a surrounding, including some infinitesimal region around this point (i.e. first derivatives included). In this infinitesimal region - as you point out - the gravitational pull will feel like an acceleration in flat space. The direction of which is inverted for the anti-gravitational particle.

From this naive pictorial point of view, I see no disagreement with the equivalence principle as I stated it above - namely that it holds for both types of particles on it's own. Both can be fooled by an angel pulling the elevator to believe that there is gravity. In the one case though, the elevator goes up, in the other case down, respecting that they feel the gravitational pull in the other direction. In the usual case, the ratio of inertial to graviational mass is 1, in the other case -1.

However, I take it, you have a very distinct problem with the mathematical formulation. I hope, I eventually get the point:

When you define the the Riemann normal coordinate system (which actually is not really a coordinate system on the whole manifold when I recall that correctly) you make a gauge requiring that the Christoffelsymbols vanish. This makes the geodesic motion look especially simple, i.e. as in flat space, 2nd derivative of x equals zero.

For this, you have used the form of the covariant derivative acting on the quantity to be parallel transported. The momentum of the anti-graviational particle behaves differently under such transport. The curve on which it remains 'parallel' therefore is a different one. Or, as you would have expected from the modified transformation behaviour of the new particle's momentum, it's covariant derivative is different.

Consequently, the Riemann normal coordinates for the anti-grav. particle are defined by a different gauge. I.e. by the requirement that the corresponding connection coefficients (which are related to the Christoffelsymbols but not identical) vanish. For details and indices, see paper.

So, you are right that the Riemann frame for the anti-g particle is not the usual Riemann frame. What is the problem with that?

Indeed, the anti-gravtational particle gets repelled from the black hole (unfortunately, the example with the motion in a Schwarzschild background dropped out of the paper because it was too long.)



B.

 

[hossi]

Thanks a lot again and good luck to all of us!

Dear Christine and all of you,

I also thank you for your comments. It is always good to hear other peoples experiences and stories, and to see that the personal side of the job-related issues affects many of us.

I absolutely can relate to what Christine says, I don't think I would be able to give up on physics. I think it is this addiction to physics which makes so many postdocs stay, despite the insecure future.

One might be tempted to call that a selection process, but imo the selection does not yield a wishful result. I.e. those who survive are not neccessarily those who can contribute best to the field. A non-neglibile fraction are those who primarily succeed in well selling themselves, those who know the right people or just come from the right places, and those who could be given a PhD for perfect mainstreaming.

I definately think though, it is possible to improve the situation.

@arivero : I can't smell the wind of moving. Or if, then it smells like U-Haul exhaust. But I can smell the wind of change, and I can smell some change coming.



B.

 

[vanesch]

However, I take it, you have a very distinct problem with the mathematical formulation. I hope, I eventually get the point:

When you define the the Riemann normal coordinate system (which actually is not really a coordinate system on the whole manifold when I recall that correctly) you make a gauge requiring that the Christoffelsymbols vanish. This makes the geodesic motion look especially simple, i.e. as in flat space, 2nd derivative of x equals zero.

Exactly. The Riemann normal coordinate system is a coordinate patch in an atlas, which is "Riemann normal" for a specific point, indeed, and can extend, or not, to the whole manifold. But as we're looking locally, this is good enough.
What we call "Riemann normal coordinate system" (from a math viewpoint) is what's usually called a local inertial frame from a physics viewpoint.
Ok, we're on the same line here.

For this, you have used the form of the covariant derivative acting on the quantity to be parallel transported. The momentum of the anti-graviational particle behaves differently under such transport. The curve on which it remains 'parallel' therefore is a different one.

Exactly. That's my whole point, because LOCALLY there is no difference between this patch of manifold and a patch of manifold in deep space, concerning its metrical structure. BOTH are essentially flat, you see. So there is NO WAY in which to derive this OTHER curve, if the only thing that is given, is the metric.
The metric is THE SAME in the two cases, but the curves are DIFFERENT.

Or, as you would have expected from the modified transformation behaviour of the new particle's momentum, it's covariant derivative is different.

Consequently, the Riemann normal coordinates for the anti-grav. particle are defined by a different gauge. I.e. by the requirement that the corresponding connection coefficients (which are related to the Christoffelsymbols but not identical) vanish.

Ok, but a DIFFERENT set of connection coefficients corresponds to a DIFFERENT metric. So we now have TWO different metrics on our manifold. Is this what you are after ? But, it is a strange manifold who has two different metrics !

So, you are right that the Riemann frame for the anti-g particle is not the usual Riemann frame. What is the problem with that?

Well, it is a differential manifold with two different metrics. This screws up, if I'm not mistaking, the conformal structure of it: for each event, we now have TWO different sets of light cones. Well maybe you succeed in keeping the same conformal structure for both metrics, I don't know. But we now have, for each trajectory, two different eigentimes, and we have different length scales for the two metrics. What's space-like connected for one metric, is maybe timelike connected for the other. Ok, maybe one can make sense of this (I doubt it somehow, but then I didn't give it much thought and I'm not an expert on this), but it does give a serious blow to the entire idea of a spacetime manifold, no ?

However, we could see this differently: we could see one metric (the usual one) as the genuine metric (determining distances and times), and the second "metric" simply as an additional tensor which is NOT a metric, but which describes another interaction. A spin-2 field, if you want to. That's always possible. But then it is a coincidence that particles interacting with this field have their charge proportional to their mass, and it is certainly no "anti-gravity", but just another field (such as the EM field, but now a spin-2 field).

 

[ccdantas]

B. wrote:

One might be tempted to call that a selection process, but imo the selection does not yield a wishful result. I.e. those who survive are not neccessarily those who can contribute best to the field. A non-neglibile fraction are those who primarily succeed in well selling themselves, those who know the right people or just come from the right places, and those who could be given a PhD for perfect mainstreaming.

That is exceedingly lucid!

Best wishes
Christine

 

[vanesch]

B. wrote:

One might be tempted to call that a selection process, but imo the selection does not yield a wishful result. I.e. those who survive are not neccessarily those who can contribute best to the field. A non-neglibile fraction are those who primarily succeed in well selling themselves, those who know the right people or just come from the right places, and those who could be given a PhD for perfect mainstreaming.

That is exceedingly lucid!

Best wishes
Christine

What Sabine writes is indeed very lucid. However, it is true in many fields and domains of activity!

 

[josh1]

Will a moderator please either move this thread or lock it. It's enough already.

 

[reilly]

For wonderful German poetry, check out Wagner's Ring, Tristan and Isolde, and der Meistersinger (particulalry Hans Sach's aria,"Wann.. and, of course, the Prize Song), and Hofmannsthal's libretto for Strauss's glorious Der Rosenkavalier, just to promote a few. (Needless to say, I'm an opera nut)

Regards,
Reilly Atkinson

 

[marcus]

Will a moderator please either move this thread or lock it. It's enough already.

Quantum gravity is a puzzle isn't it?
this shows itself in the lives of those who pursue it---seekers, restless, even malcontents some of them, driven in various ways

it is not yet a cut-and-dried formulaized academic subject.

superstring approach does not seem to have worked out as expected and there are a number of people still looking
The non-string alternatives are something of a chimaera---they keep changing and reinventing themselves.

I think the human side of QG---the frustrations and the life stories, and the possible adventures---is part of it. So I am not unhappy with how the thread is going.

the same people as are persons in this thread (arivero, christine, bee, kea, vanesch...) can also take part in more impersonal intellectual threads as interesting ones appear.

 

[marcus]

For me, the central post of this thread so far is Garrett's #26

New talk up on the Perimeter Seminar page:

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False

Am listening now...

to which I replied

good find! I didnt know she was giving a talk.
http://www.perimeterinstitute.ca/activities/scientific/seminarseries/alltalks.cfm?CurrentPage=1&SeminarID=627

Sabine Hossenfelder
Phenomenological quantum gravity: Pieces of the puzzle
Tuesday December 13, 2005, 4:00 PM

"The phenomenology of quantum gravity can be examined even though the underlying theory is not yet fully understood. Effective extensions of the standard model allow us to study specific features, such as the existence of extra dimensions or a minimal length scale. I will talk about some applications of this approach which can be used to make predictions for particle- and astrophysics, and fill in some blanks in the puzzle of quantum gravity. A central point of this investigations is the physics of black holes. I will comment on possible ways to proceed and on the missing pieces I find most important to look for."

I still didnt find the stream for this talk.
got it now. just needed to click on "seminar series" in the menu
I just listened to a few minutes

Then Bee says close on the heels of this talk, soon after she got back to SantaBarbara, she got a call from Lee Smolin offering (I guess) a postdoc gig at PI.

Now Bee will probably contradict me (she usually does) but one thing this means to me is that QG Phenomenology is a HOT TOPIC.
(maybe only relatively, maybe nothing of real interest is a conventional royal road to success, maybe everything involves spending time in the wilderness eating grubs and prophesying to the lizards and buzzards and Bee says she likes to complain) but to me at least comparatively a hot topic.

So if that is right, more people should be thinking about all the possible ways there might be to test different notions of QG.

Another straw in the wind: I think Bee said she proposed to the German Science foundation that they start a special research group at hamburg JUST FOR QG PHENOMENOLOGY and they actually were willing to do this and offered to put up some money.

maybe this is more a tribute to Bee's salesmanship and initiative than it is a credit to the German Science bureaucracy. But to me it also shows that QG phenom is beginning to have good possibilities.

Anyone want to contradict? I can only guess, maybe some other has firsthand experience

(there was also the Emmy Noether fellowship issue which you can read at Bee's blog)

 

[hossi]

But to me it also shows that QG phenom is beginning to have good possibilities.

Yes. Indeed. If nothing else, it's this optimistic message you should take out of this thread. (There will be an update on the situation on my blog any time soon).

Anyway, I am not really sure what to think about this thread, since it's titled with my name. And I guess josh is pissed off coz it's not his name. However, unless the discussion is about me or my cars, I appreciate any attempt to make science more human. A development you also find in recent popular physics books. You can't do science without the scientist. And (s)he comes with a story and a life, and hopefully a car better than mine.



B.

 

[nrqed]

Yes. Indeed. If nothing else, it's this optimistic message you should take out of this thread. (There will be an update on the situation on my blog any time soon).

Anyway, I am not really sure what to think about this thread, since it's titled with my name. And I guess josh is pissed off coz it's not his name.

B.

A bit mean.

In all fairness, if, say, in the quantum physics forums there were a few posters who are not moderators and with only a small number of posts would start discussing their taste in music or in cars, I am pretty certain that a moderator would quickly lock it up. (darn, I could have registered under a few names and have done that to prove my point).

If the moderators want to let the site turns into a chat forum or even a place to flirt but then the same rules should be applied to *everybody*.

Just my two cents.

 

[hossi]

A bit mean.

Was not intended. I meant to say, there is definately discussion in this thread that is interesting from a general point of view - as marcus points out, e.g. what is with the future of QG research, job options, etc. affects many of us, or the anti-g stuff that I have moved to another thread. Why that had to be done in a thread titled with my name is not clear to me. B.

 

[Astronuc]

Another straw in the wind: I think Bee said she proposed to the German Science foundation that they start a special research group at hamburg JUST FOR QG PHENOMENOLOGY and they actually were willing to do this and offered to put up some money.

Hamburg!?

What about Karlsruhe or Stuttgart?

QG I can readily accept. Anti-g, I'd have to think about that.

 

[marcus]

Hamburg!?

What about Karlsruhe or Stuttgart?

QG I can readily accept. Anti-g, I'd have to think about that.

watch out Astronuc, she is very feisty

even a nice Python beard will not protect you

 

[Astronuc]

I appreciate feisty and highly intelligent women.

B, before you leave UCSB, check out UC San Diego. After Waterloo, UCSD will look very nice.

I am partial to Suddeutschland, particularly around Karlsuhe and Stuttgart.


I just today stumbled across this thread, and it piqued my curiosity. I'll have to brush up on QG and some other topics before I ask anything intelligent, but one thing that comes to mind, is that if matter (rest mass) is quantized, then would not gravity be quantized? And why Anti-g? Is it because of the existence of matter/anti-matter?