Who is Sabine Hossenfelder and what are her research interests?

[hossi]

Padmanabhan makes very good (the best, sometimes) reviews on anything. The problem is that nobody quotes reviews, just say "As it is known...", so I guess he will need to ask people to quote him, even it he is not anymore in a publish or perish position.

Yeah, I actually like his reviews (those on cosmology).

Its not my experience though. It seems to me the ONLY thing people cite are the reviews (instead of all the single works).

And it wasn't about the reviews. The problem is, he always sends a whole list of papers that are - if at all - only very remotely connected to the topic. I certaintly don't mean to be impolite but I can't possibly cite everybody who has ever said anything on this and/or related subjects that might or might not have something to do with my work, or might eventually turn out to be related or whatever.

I could as well add a line at the end of every paper: "I am not going to discuss the following works [cite the whole arxiv]" - and "Don't trust me on the signs"

-B.

 

[josh1]

Well Hossi, that may be, but I think we can all agree that the really important question is why nobody has honored me with a thread under my own fake name. Seriously though when it happens (it's inevitable, like strings) write josh, not josh1. (I always use the same smiley)

 

[hossi]

look what I just found

http://whyfiles.org/siegfried/story17/"

 

[arivero]

look what I just found

http://whyfiles.org/siegfried/story17/"

Habitacorum Dei et Omnium Electorum. That goes for Templeton's prize :tongue2:

 

[hossi]

That goes for Templeton's prize :tongue2:

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)" .

*ohem* what does the latin mean? I learned latin at high school but that was about an eternity ago.

B.

 

[rtharbaugh1]

Hello Hossi

I just read the link and your blog which was linked to the link...and was wondering if antigravitating particles could be thought of as ordinary particles in time reversal, similar to the idea if I recall correctly in Feynman that positrons are consistant with time-reversed electrons?

Thanks,

R

 

[hossi]

...and was wondering if antigravitating particles could be thought of as ordinary particles in time reversal, similar to the idea if I recall correctly in Feynman that positrons are consistant with time-reversed electrons?

No. A particle of negative gravitational charge can not be replaced by a particle with opposite charge and positive energy, because in this case the energy is the charge. Actually, the anti-grav. has nothing to do with the time-reversal symmetry.

B.

 

[arivero]

*ohem* what does the latin mean? I learned latin at high school but that was about an eternity ago.

It is almost Spanish :-) It says something as "Celestial Empire(?), house of God and All The Choosen Ones".

In any case http://whyfiles.org/siegfried/story17/images/aristotle_earth.gif of the universe, it even includes the precession of the equinoxes

 

[marcus]

Habitacorum Dei et Omnium Electorum. That goes for Templeton's prize :tongue2:

I think the word is not Habitacorum but Habitaculum
(meaning habitation or dwelling-place)

the phrase I have seen is
Coelum Empireum Habitaculum Dei et Omnium Electorum

which means the Empyrean heavens, the dwelling place of God and of all the Elect.

the Elect are the chosen ones, the blessed, the saints etc.

Empyrean, oddly enough, is DIFFERENT from Empire even though sound the same, one Latin other Greek

Empire is IMPERIUM---Latin, a worldly earth-bound thing that obeys orders (imperatives) from the leader (imperator)

Empyrean comes from GREEK pyros FIRE, and means MADE OF FIRE,
the highest heavens were supposed to be made of a pure or sublimated crystallized fire, instead of from ordinary common stuff (clay, water, air)
Empyrean refers to the substance---sublime crystal fire, very hard, very smooth and perfect, very very whatever you can thing of very.

In the film ENGLISH PATIENT it says that "the heart is an organ of fire"
what they are talking about there is the old classical idea of empyrean.
I think.

 

[marcus]

Alejandro if we can prove by a thought experiment that an absolute qu. state does not exist and also that an absolute universal time does has no physical existence then this will show divine nonexistence

because if there were a divinity He would at least have an absolutely perfect clock
and His omniscient knowledge of the world would constitute an absolute quantum state.

and there ain't none

contradiction QED

============

I know, we all know this, it has been pointed out several times. But.
You mentioned the Templeton. Do they realize what they are doing by encouraging people to examine "FOUNDATIONAL" questions?

I fear that the Elect (omnium electorum) are going to shoot themselves in their feet of clay
and hoist themselves on their own theosophistical catapult.

 

[arivero]

Empyrean comes from GREEK pyros FIRE, and means MADE OF FIRE,
the highest heavens were supposed to be made of a pure or sublimated crystallized fire,

So the question mark in my translation, I was suspecting something was not right even for church latin... it was not a right-sound derivation from "impero" or "imperium" (and I can cope even with Ennio and Lucretius "induperator").

Thanks for the clarification, marcus! By the way, and off-topic, I remember there was some discussion about the origin of the root PYR- in greek language and its relationship with pyramid. Legend speaks of some ancient recipe of cakes of pyramidal form (obviously, baked in the fire) as the link between both words!

 

[arivero]

I know, we all know this, it has been pointed out several times. But.
You mentioned the Templeton. Do they realize what they are doing by encouraging people to examine "FOUNDATIONAL" questions?

I fear that the Elect (omnium electorum) are going to shoot themselves in their feet of clay
and hoist themselves on their own theosophistical catapult.

I do not blame Templeton, they do their work. They do not encourage to examine, they encourage to contemplate. It is a lot more problematic when our own journals (divulgation journals) do a passive presentation of science as something to be contemplated with awe. Against Faith the answer is action, reason-in-action.

 

[Kea]

I fear that the Elect (omnium electorum) are going to shoot themselves in their feet of clay and hoist themselves on their own theosophistical catapult.

Marcus,

Brilliant prose! Of course, my fqxi application (which I never intended accepting) was rejected. I regret deleting the letter I received, which contained some very flowery language and proudly informed me that I was now on their mailing list. Until then I wasn't quite sure whether to think of fqxi as another ordinary research institute, or not.

 

[marcus]

... very flowery language and proudly informed me that I was now on their mailing list. Until then I wasn't quite sure whether to think of fqxi as another ordinary research institute, or not.

I'd like to see you get a Tempie, Kea.
Maybe our slightly stochastic group brain here at PF can generate a winning idea for a Kea second-try proposal.

(I know individually I could not, but with collections of people you never can be sure what they are incapable of

 

[Kea]

Maybe our slightly stochastic group brain here at PF can generate a winning idea...

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

 

[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? :yuck: 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? :yuck: 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? :yuck:

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