# Lisa Randall's Warped Passages

1. Jan 26, 2006

Staff Emeritus
I just went to hear Lisa Randall speak at Laurence University in Appleton Wisonsin, and very much enjoyed it.

I have her book, Warped Passages and in both the talk and the book she introduces an intriguing phenomenon. Assuming a (3+1) dimensional brane embedded in an extended higher dimensional spacelike bulk, she calculated the Einstein equations assuming the source was localized on the brane, and discovered that while the resulting gravity obeys the approximate inverse square law on the brane, it falls off as a sharp exponential orthogonal to the brane in the bulk space. Since in the Einstein theory gravity is identical to curved or warped geometry, this allows her to describe the bulk as warped. Hence her title.

This exponential fall-off recalls Yukawa's theory of a force carried by a massive boson, which was also characterized by an exponential decay. So the Randall theory is "as if" the graviton, massless on the brane, acquired mass when it left the brane.

She then imagines that the world we live in is one brane - she calls it the weakbrane after the weak energy scale, which characterizes our experimental physics - and there is a parallel brane, some distance away orthogonally in the bulk, which is the source of gravity. Since the strength of gravity has fallen exponentially by the time it encounters the weakbrane this accounts for the fact that gravity is so feeble at the weak energy scale.

A pretty model, and she claims it will be testable at the LHC. She even showed a slide which portrayed her, very fetching in a hard hat, examining the construction there.

2. Jan 26, 2006

### marcus

Some pictures of Lisa Randall, here's a relaxed one taken of her with John Brockman of edge.org.
http://blogs.law.harvard.edu/crimson1/pictures/viewer\$94

Here she is in a gallery of snaps of famous people, you have to scroll down about 7 rows:
http://www.edge.org/3rd_culture/trivers04/trivers04_photos.html

Here's her Harvard faculty webpage, with very handsome commercial studio portrait.
http://physics.harvard.edu/people/facpages/randall.html

Last edited: Jan 26, 2006
3. Jan 26, 2006

### FSC729

Wow

Super intelligent and HOT, I would definitely like to meet her during office hours, give me some tutoring on string theory .

John G.

4. Jan 27, 2006

### Chronos

marcus is always spotting the hot physicists . . .

5. Jan 27, 2006

### marcus

It was selfAdjoint who said she looked fetching in a hardhat. I admit nothing!

Last edited: Jan 27, 2006
6. Jan 27, 2006

### Sauron

Lisa Randall vs Fotini Kalamara (http://www1.greece.gr/GLOBAL_GREECE/SPOTLIGHT/thinkingatthespeedoflight.stm) [Broken], fine, lets give tweak to the controversy of strings vs LQG.

Jokes aside the theory of supernumerary dimensions is very interesting. It is ot original from Randal and Sundrum but it comes from a few years before.

And it is not strictly string theory. I have readed some of the articles of Lisa Randal and the actual calculations are not made using string theory but with general relativty and ordinary QFT.

String/brane theory acts like a motivation, of course, but the results don´t depend on it.

Last edited by a moderator: May 2, 2017
7. Jan 27, 2006

Staff Emeritus
Her latest version is not Randall-Sundrum but different. And yes, I thought she looked extremely nice in person; make of that what you will.

8. Jan 27, 2006

### Sauron

¿Different?

About her being nice or not i can´t say anithing, i don´t know her. And in facto it does ´t affect at all the apreciation about her scientific work, of course.

And i find these supernumerary theories the most interisting perspective of stringy posibly observable fenomenology.

I don´t know when her book will be traduced (if it is traduced at all) to spanish, that is, publied in Spain. But surelly if its traduced i´ll read it (even if i previously have readed the technical papers).

9. Jan 27, 2006

### josh1

Anyone who believes that this whole brane-world thing wont turn out to be a load of crap is a medical emergency that needs to be rushed to the hospital for an immediate physical intuition transplant.

10. Jan 27, 2006

### CarlB

Anyone can demonstrate that a square peg fits in a round hole if they allow a large enough diameter.

Carl

11. Jan 27, 2006

### duke_nemmerle

Darn!! That's sort of what I was trying to say with my only post here https://www.physicsforums.com/showthread.php?t=70695

Except I think I was saying that this effect might be that "dark matter" is the gravitational effect of matter on a nearby brane. That is, the mass of the gravitons leaving this nearby brane would affect us like dark matter does and us to them. Nearby branes would of course have similar Large Scale Structure due to this.

To me, this would explain why gravity is a much weaker force(it leaks) and why there is so much more dark matter than matter(that it is this leakage from a nearby brane)

Are the two proporational at all(the factor by which gravity is weaker than the other forces and the factor by which there is more dark matter than baryonic matter)?

I dunno, just a thought

EDIT: Oh wow, I just realized I quoted her in my original article. I can't back it up with any maths or anything but it was still interesting to me

Last edited: Jan 28, 2006
12. Jan 28, 2006

Staff Emeritus
Josh1, Carlb, and Duke, you should learn at little about a subject before you diss it!

Randall claims her model (not theory!) produces detectable particles, with all the characteristics of existing particles like electrons, etc. but very massive. These are kind of the "shadows" of the actual particles (electrons, etc.) moving in the higher dimensional bulk, and the mass comes as our "view" of their momentum, which cannot be fully realized in our 4-D spacetime. My terms "shadow" and "view" are strictly my own understanding of her popularization in her book; her own definitions of these come from solving the equations; they are a strict consequence of her model (no tuning epicycles here!). She thinks these particles, or to be precise their signatures, will be observable at LHC when it starts doing experiments (maybe 2008 or so after a beam tuning period following its inauguration in 2007). So her model is falsifiable, and therefore good science by anybody's definition.

Last edited: Jan 28, 2006
13. Jan 28, 2006

### Tide

That's a trivial problem to the topologist.

14. Jan 28, 2006

### josh1

I understand brane-world physic quite well. But just because a theory makes verifiable predictions, that is, just because a theory meets the definition of what a scientific theory should be doesnt mean that it's a good theory. There are loads of theories that meet these requirements that are still baloney.

Randall-Sundrum proposed their idea to explain why gravity is so weak despite the fact that according to theory, the natural scale of gravitational phenomenon is the Planck scale. If theory was correct, gravitational phenomenon at this energy should be generic for all cosmologies. But if this is the case, where is this high energy gravitational phenomena? We certainly don't see it.

To solve this aspect of the hierarchy problem, Randall-Sundrum proposed the idea of a five-dimensional spacetime with the fourth spatial direction bounded on one end by a brane on which we live, and on the other by a "gravi-brane" on which gravity is of Planck strength. In other words, Planck strength gravitational phenomenon is generic in this cosmology, its just that its lives on the gravi-brane.

They then proposed that the geometry along the fourth spatial dimension is anti-de Sitter, but with a "warp factor" whose purpose is to scale the gravitational scale down as we move from the gravi-brane to our brane. Also, since our brane lives in a higher dimensional spacetime, the strength of gravity on our brane should in fact be much smaller than it is - zero if the transverse dimension is infinitely large. But Randall-sundrum showed that there choice of geometry effectively traps gravity on our brane giving the observed strength.

I just find this idea to be far too contrived and ugly to be correct. I think that most of the people who work on this idea are phenomenologists and astrophysicists, both of whom have a much more practical bent then "pure" theorists.

15. Jan 28, 2006

### duke_nemmerle

Actually I wasn't dissing it at all, it's something that fascinates me and I've thought about it before. I'm glad it was posted

16. Jan 29, 2006

### Mike2

Hi josh1,

Perhaps you can answer a question I have about branes inside higher dimensional bulk, etc. It seems to be that they are proposing the existence of a field restricted to some dimensions but not others. How it that possible? It seems like it would be a discontinuity for a field to only be on restricted dimensions and not on the other dimensions. As you travelled the bulk along these dimensions where the field was not, you would instantaneous have a change in the field from zeor to some values and instantly back to zero as you cross the dimension where the field was. Can reality sustain such discontinuities? But if there is no special field restricted to the lower dimensional brane, then it would seem that the brane is arbitrary, that there is nothing special about the brane. Does this sound like an inconsistency or something that cannot be physically real? Thanks.

17. Jan 30, 2006

### FSC729

Lisa Randall On Coast to Coast

Hello everyone, Lisa Randall will do a radio interview tonight (Jan 30, 2006) on Coast to Coast am with George Noory. See for yourselves:

Coast To Coast Am

The show begins at 10pm (Pacific Standard Time), if you can't listen to the show on the radio you can listen to a stream version:

WFLA (Click on listen live upper left hand corner)

Or

WJNO

John G.