 So how do your recent papers deal with these huge discrepancies?
Our new ideas say nature has only one mass scale—the scale of the weak interactions. From this flows the idea that gravity is weak because the particle mediating the gravitational force, the graviton, lives far away from us in new extra spatial dimensions. The relative strength of the forces was formerly understood in terms of separation in energy space, but what we are now saying is the extreme weakness of gravity follows from its separation in new positional space having real extra dimensions.
Our new picture is that the 3-D world is embedded in extra dimensions. Gauge forces and their particles are confined to this 3-D plane, but the graviton lives mostly outside this plane in the extra dimensions. That means the graviton exists mainly away from us, and hence its interaction with us, the force, is weak.
What’s the size of this space? Note that gravity is very weak, which tells us the graviton spreads into extra space for interaction. To account for the observed weakness of gravity, the scale length of two new dimensions has to be about a millimeter, which is gigantic! For three or more dimensions the size becomes smaller than 1 mm.
The basic notion underlying our work is to explain the observed separation in energy levels by separation in real space. This gives us a totally new perspective for addressing theoretical and experimental problems.
High-Impact Papers by Savas Dimopoulos,
Published since 1996
(Ranked by average citations per year)
| Rank |
Paper |
Total
Citations |
Average
cites
per
year |
| 1 |
I. Antoniadis, et al. "New dimensions at a millimeter to a fermi and superstrings at a
TeV," Phys. Lett. B, 436(3,4):257-63, 1998. |
360
|
160
|
| 2 |
N.
Arkani-Hamed, S. Dimopoulos, G. Dvali, "The hierarchy problem and new dimensions at a millimeter," Phys.
Lett. B, 429(3,4):263-72, 1998. |
467
|
144
|
| 3 |
N. Arkani-Hamed, et al. "Phenomenology, astrophysics, and cosmology of theories with submillimeter dimensions and TeV scale quantum gravity," Phys. Rev. D,
59(8):6004, 1999. |
242 |
108 |
| 4 |
S. Dimopoulos, et al. "Experimental signatures of low-energy gauge-mediated supersymmetry breaking," Phys. Rev. Lett.,
76(19):3493-7, 1996. |
155 |
31 |
|
|
So are the ideas testable?
Perhaps. Let me give an example of an experimental signature for extra dimensions. The Large Hadron Collider (LHC), under construction at CERN, Geneva, will have an energy up to 14 TeV. Now think about collisions between two TeV particles: in a collision at such high energies the gravitational force becomes large because the distance of closest approach is very small. If these particles are radiating energy into extra dimensions it might be detectable.
Here’s an analogy to understand this: imagine that our universe is a two-dimensional pool table, which you look down on from the third spatial dimension. When the billiard balls collide on the table, they scatter into new trajectories across the surface. But we also hear the click of sound as they impact: that’s collision energy being radiated into a third dimension above and beyond the surface. In this picture, the billiard balls are like protons and neutrons, and the sound wave behaves like the graviton.
Quantitative studies of future experiments to be carried out by LHC show that any signatures of missing energy can be used to probe the nature of gravity at small distances. The predicted effects could be accessible to the Tevatron Collider at
Fermilab, but the higher energy LHC has the better chance.
These colliders are still under construction, but results also have consequences for "table-top" experiments, being carried out here at Stanford, as well as the University of Washington and the University of Colorado. Here’s the basic idea: imagine there are two extra dimensions on a scale of a millimeter. Next, take two massive particles separated by a meter, at which distance they obviously behave according to the well-known rules of 3-D space. But if you bring them very close, say closer than one millimeter, they become sensitive to the amount of extra space around. At close encounter the particles can exchange gravitons via the two extra dimensions, which changes the force law at very short distances. Instead of the Newtonian inverse square law you’ll have an inverse fourth power law. This signature is being looked for in the ongoing experiments.
Your recent papers address whether what we already know in physics and astrophysics is compatible with large extra dimensions.
Well we haven’t found any showstoppers! An important constraint comes from the supernova of 1987 because hot stars can cool by emitting gravitons into the extra space. Gravitons are weakly interacting particles, so if they are produced anywhere inside an exploding supernova they can escape immediately which would be a new way of releasing energy from the supernova. Supernova 1987A is the hottest astrophysical object for which we have detailed understanding. Matter in the supernova was subjected to nuclear temperatures, and the implied constraint to the extra dimensions was in the
submillimeter-to-micron range.
Unfortunately, the submillimeter range is below the spatial resolution of the table-top experiments, so they probably won’t find anything, except in the special case where just one of the extra dimensions is large, in which case gravity will follow an inverse cube law.
How has the physics world reacted?
At first we faced denial. We had deliberately used the word "sub-millimeter" in our first paper. Physicists were surprised, to say the least, that such a thing was not already excluded experimentally. I remember a stage in 1998 when colleagues wondered if we had not forgotten some crucial experiment. We were not discouraged. No! We gave talks on the ideas, and by July 1998 had analyzed the laboratory and cosmological constraints. That paper marked a sea-change in opinion: physicists began to think this was an interesting idea. By the fall of 1998 we were showing how to do real physics. Now several study groups are taking us very seriously: the high citation rates speak for themselves.
Personally I am not surprised by the reaction. Revolutionary ideas go through a cycle: denial, followed by "okay it is consistent but can you do anything with it?" and finally, once you show how to do real physics, you may get the third phase where many physicists become interested in the field. The same thing happened to me and Giorgi back in 1981 when we first proposed the supersymmetric extension of the standard model of particle physics. Initially there were the usual skeptics but now it is completely accepted.
Oddly, for me, the major competitor to these proposals for extra dimensions is the supersymmetry extension. But let’s recall some of the disadvantages of the standard model. First, it shuts out gravity. Second, it has 18 free parameters, many of them very small. Third, the vacuum energy is 120 orders of magnitude larger than what you would naively guess from the standard model.
Proposing extra dimensions to space is a drastic step. But once you have the extra space you can attribute the smallness of some quantities to the statement that their origin is somewhere far away inside space, just as an astronomer might attribute the faintness of a galaxy to its large distance. For example, maybe the smallness of the electron mass arises because its origin is far away inside the extra dimensions.
My view is that both of the big ideas I have worked on are testable in the next decade by
LHC. The two frameworks have complementary features. I’m greatly looking forward to the
outcome! 
Science
Watch®, May/June 2001, Vol. 12, No. 3
Citing URL: http://www.sciencewatch.com/may-june2001/sw_may-june2001_page4.htm |
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