Who is Sabine Hossenfelder and what are her research interests?

[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.

:rofl:

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? :tongue2:

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

 

[marcus]

Hamburg!?

What about Karlsruhe or Stuttgart? :tongue2:

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?