Warped Passages (70 page)

Facts so bizarre cry out for a deeper explanation. One of the most important lessons of the perplexing discoveries of the last decade is likely to be that space and time have more fundamental descriptions. Ed Witten succinctly summarized the problem when he said that “space and time may be doomed.” Many leading string theorists agree: Nathan Seiberg asserted, “I am almost certain that space and time are illusions” whereas David Gross imagines that, “Very likely, space and perhaps even time have constituents; space and time could turn out to be emergent properties of a very different-looking theory.”
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Unfortunately, no one yet has any idea what the nature of this more fundamental description of spacetime will turn out to be. But a deeper understanding of the fundamental nature of space and time clearly remains one of the biggest and most intriguing challenges for physicists in the coming years.

25

(In)Conclusion

It’s the end of the world as we know it (and I feel fine).

REM

Icarus Rushmore XLII used his time machine to visit the past and warn Icarus III of the disaster that awaited him should he continue driving his Porsche. Ike III was so astounded by his visitor from the future that he heeded Ike XLII’s warning. He traded in his Porsche for a Fiat and subsequently led a full, contented, and slower-paced life.

Athena was ecstatic to be reunited with her brother, and Dieter was happy to see his friend, though both of them were confused since it seemed as if Ike had never left. Athena and Dieter realized that the time travel that Ike reported to them was pure fiction. Even in dreams, the Cat never looped through time, the Rabbit never reached a stop with extra time dimensions, and the quantum detective refused to contemplate such odd behavior of time. But Athena and Dieter preferred happy endings. So they suspended disbelief and accepted Ike’s fantastic story all the same.

Despite the impressive physics developments of the last few years, we don’t yet know how to harness the force of gravity or teleport objects across space, and it’s probably too soon to invest in property in extra dimensions.
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And because we don’t know how to connect universes in which you could loop through time to the one in which we live, no one can create a time machine, and most likely no one will do so any time soon (or in the past).

But even if ideas like these remain in the realm of science fiction, we live in a wonderful and mysterious universe. Our goal is to learn how its pieces fit together and how they’ve evolved into their current state. What are the connections that we haven’t yet figured out? What are the answers to questions like those I asked in the previous chapter?

Even if we have yet to understand the ultimate origin of matter at the deepest level, I hope I’ve convinced you that we do understand many aspects of its fundamental nature on the distance scales we have experimentally studied. And even if we don’t know the most basic elements of spacetime, we do understand its properties for distances far away from the Planck scale length. In those regimes, we can apply physical principles we understand and deduce the sorts of consequences I’ve described. We’ve encountered many unexpected features of extra dimensions and branes, and those features might play a critical role in solving some of the puzzles of our universe. Extra dimensions have opened our eyes and our imaginations to amazing new possibilities. We now know that extra-dimensional setups can come in any number of shapes and sizes. They could have warped extra dimensions, or they could have large extra dimensions; they might contain one brane or two branes; they might contain particles in the bulk and other particles confined to branes. The cosmos could be larger, richer, and more varied than anything we imagined.

Which, if any, of these ideas describes the real world? We’ll have to wait for the real world to tell us. The fantastic thing is that it probably will. One of the most exciting properties of some of the extra-dimensional models I’ve described is that they have experimental consequences. I can’t overemphasize the significance of this remarkable fact. Extra-dimensional models—with new features that we might have thought were either impossible or invisible—could have consequences that we might see. And from these consequences, we might be able to deduce the existence of extra dimensions. If we do, our vision of the universe will be irrevocably altered.

There might be tests of extra-dimensional spacetime in astrophysics or cosmology. Physicists are now developing detailed theories of black holes in extra-dimensional worlds, and have found that although they are similar to their properties in four dimensions, there are subtle
differences. The properties of extra-dimensional black holes could turn out to be sufficiently distinctive that we will be able to discern recognizable differences.

Cosmological observations might also ultimately tell us more about the structure of spacetime. Observations today probe what the universe looked like billions of years ago. Many agree with predictions, but several important questions remain. If we live in a higher-dimensional universe, it must have been very different earlier on. And some of those differences might help to explain perplexing features of observations. Physicists are now studying the implications of extra dimensions for cosmology. We might learn about dark matter hidden on other branes, or cosmological energy stored by hidden higher-dimensional objects.

But one thing is certain. Within the next five years, the Large Hadron Collider particle accelerator at CERN will turn on and probe physical regions no one has ever observed before. My colleagues and I are eagerly awaiting that time. The LHC is a great bet—for scientists it doesn’t get much better. Experiments at the LHC will almost certainly discover particles whose properties will give us new insights into physics beyond the Standard Model. The exciting thing is that no one yet knows what those new particles will be.

During the time I’ve been doing physics, the only new particle discoveries have been particles that theoretical considerations told us we were pretty sure to find. Not to undermine those discoveries—they were impressive accomplishments—but finding something genuinely new and unknown will be far more thrilling. Until the LHC starts running, no one can be really certain where to best concentrate their efforts. Results from the LHC are likely to change the way we view the world.

The LHC will have enough energy to produce the new types of particle that promise to be so revealing. These particles could turn out to be superpartners or other particles that four-dimensional models predict. But they might also be Kaluza-Klein particles—particles that traverse extra dimensions. If and when we see those KK particles will depend entirely on the size and shape of the cosmos in which we live. Do we live in a multidimensional universe? And will the size or shape of that universe make KK particles visible?

All of the models that address the hierarchy problem have visible weak-scale consequences. The signatures of the warped geometry that addresses the hierarchy problem are particularly amazing. If this theory is right, we will detect KK particles and measure their properties from the clues they leave behind. If, instead, other extra-dimensional models describe the universe, energy will disappear into extra dimensions and we’ll ultimately detect those dimensions through the resulting unbalanced energy accounting.

We certainly don’t yet know all the answers. But the universe is about to be pried open. Astrophysical observations will explore the cosmos earlier, further away, and in more detail than ever before. Discoveries at the LHC will tell us about the nature of matter at distances smaller than any physical process ever observed. At high energies, truths about the universe should start to explode.

Secrets of the cosmos will begin to unravel. I, for one, can’t wait.

Glossary

action at a distance
The hypothetical instantaneous effect of objects on other, distant objects.

aether
A hypothesized invisible substance (now debunked) whose vibrations were supposed to be electromagnetic waves.

alpha particle
A helium nucleus (consisting of two protons and two neutrons).

anarchic principle
The statement that all interactions that are not forbidden by symmetries will occur.

anomaly
Symmetry violation that arises from quantum contributions to a physical interaction, but which is not present in the corresponding classical theory (in which quantum contributions are not taken into account).

anomaly-free theory
A theory for which the symmetries of the classical theory are also symmetries of the theory with quantum contributions included.

anomaly mediation
Communication of supersymmetry breaking by quantum effects.

anthropic principle
The reasoning that says, out of many possible universes, we could live only in a place where
structure
could have formed.

anti de Sitter space
Spacetime with constant negative curvature.
antiparticle
A particle with the same mass as another particle, but opposite charge.

atom
A building block of matter consisting of electrons orbiting a positively charged nucleus.

beta decay
Radioactive decay in which a neutron splits into a proton, an electron, and a neutrino.

black hole
A compact object that is so dense that nothing can escape from its surrounding gravitational field.

blackbody
An idealized object that absorbs all heat and energy and radiates it back in a manner determined solely by its temperature.

blackbody radiation
Radiation emitted by a blackbody.

boson
A particle with integer spin—1, 2, etc. (one of two categories of particle established by quantum mechanics, the other being the
fermion
); photons and the Higgs particle are examples of bosons.

bottom quark
A short-lived, heavier version of the down and strange quarks.

brane
A membrane-like object in higher-dimensional space that can carry energy and confine particles and forces.

braneworld
A physical setup in which matter and forces are confined to branes.

bulk
Full higher-dimensional space.

Calabi-Yau manifold
A six-dimensional compact space, defined by its particular mathematical properties, that plays an important role in string theory.

CERN
The Conseil Européen pour la Recherche Nucléaire (now called the Organisation Européenne pour la Recherche Nucléaire or, in English, the European Organization for Nuclear Research), a high-energy accelerator facility in Switzerland; future home of the Large Hadron Collider (LHC).

charm quark
A short-lived, heavier version of the up quark.

chirality
The
handedness
of a particle with spin.

classical physics
Physical laws that takes neither quantum mechanics nor relativity into account.

closed string
A
string
that loops around and has no ends.

collapse of the wavefunction
Reduction of the quantum state after a precise measurement fixes the value of the measured quantity.

compact space
A finite space.

compactified
A compactified space is one that is rolled-up into a finite size.

Compton scattering
The scattering of a photon off an electron.

cosmological constant
The value of a constant background energy density that isn’t carried by matter.

cosmology
The science of the evolution of the universe.

coupling constant
The number that determines interaction strength.

curvature
A quantity that describes bending or curving of an object, space, or spacetime.

D-brane
A brane in string theory on which open strings end.

dark energy
The measured vacuum energy in the universe that constitutes about 70% of the universe’s energy but is not carried by any form of matter.

dark matter
The nonluminous matter that carries about 25% of the energy in the universe.

de Sitter space
Spacetime with constant positive curvature.

deep inelastic scattering
The experiment that discovered quarks by scattering electrons off protons and neutrons.

desert hypothesis
The assumption that there are no particles, aside from those included in the Standard Model, that can be produced at energies below the unification energy.

dimension
An independent direction in space or time.

dimensionality
The number of quantities required to uniquely pin down a point.

dimensionality of a brane
The number of dimensions in which brane-bound particles are permitted to travel.