String Theory Quantum Mechanics and Gravity The start

Outline • The 20 th Century Crisis • Quantum Mechanics vs Relativity • String

Two Pillars of 20 th Century Physics • Special Relativity: the framework for describing

Special Relativity M. C. Escher ‘Relativity’ • The first completed revolution of 20 th

It’s the Law! • Absolutely nothing can move faster than light c = 300,

Relativity of Simultaneity • Observers can disagree whether A or B is earliest. String

Relativity of Simultaneity • Observers can disagree whether A or B is earliest. •

The Uncertainty Principle The more precisely the position is determined, the less precisely the

Apparent Inconsistency…. • If particles are known to be precisely at positions A and

Consistency Restored! • In Nature consistency is restored by ensuring that observers who disagree

Einstein on a Roll: General Relativity • In General Relativity, gravity is attributed to

General Relativity • General Relativity requires test observers whose positions and times are precisely

Quantum Gravity? • At small distances large ‘quantum fluctuations’ in the gravitational field make

String Theory: the idea • All matter consists of small one-dimensional objects (strings). •

String Theory: the idea • Enormous economy of description: • All particle types are

String Interactions • All interactions consist of the splitting and joining of these elementary

String Varieties, circa 1990 • The consistency of gravity with Quantum Mechanics makes String

Uniquely Predictive • String theory predicts space has 9 (or 10) spatial dimensions and

How Would We Know? • There may be 9 spatial dimensions, since we would

Experimental Tests? • Strings were thought to have to be very short in order

D-Branes • String theory is bigger than previously thought. • Normally, the ends of

Why Consider This? • Good Things Happen with both strings and D-branes: • Previously-hidden

The Brane World • Some states are trapped on the branes, and others are

Strings Might Be Big • String size is inferred from the (very weak) strength

New Perspectives • Perhaps some particles can move faster than others? • Perhaps not

Is Space-time Emergent? • In some circumstances physics at distances smaller than the string

Brane Cosmology • Cosmology describes the Universe given the distribution of matter within it.

String Summary • String theory is the only known theory where gravity and quantum

The Theory of Everything? • For the first time in the history of human

Perhaps a beautiful relationship…. String Theory

…. …. . and a classic ending. String Theory

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String Theory Quantum Mechanics and Gravity: The start of a beautiful relationship? Cliff Burgess String Theory

Outline • The 20 th Century Crisis • Quantum Mechanics vs Relativity • String Theory • A Theoretical Balancing Act • Possible Problems? • D-Branes and M Theory • Holography • The Brane World • Outlook String Theory

Two Pillars of 20 th Century Physics • Special Relativity: the framework for describing the physics of objects moving at speeds close to the speed of light. • General Relativity: the extension of this theory to include gravity. • Quantum Mechanics: the framework for studying the physics of very short distances. String Theory

Special Relativity M. C. Escher ‘Relativity’ • The first completed revolution of 20 th Century physics: • Relativity of Simultaneity • Time Dilation • Length Contraction • E = mc 2 • Speed of light as a maximum speed. String Theory

It’s the Law! • Absolutely nothing can move faster than light c = 300, 000 km/sec. time space String Theory

It’s the Law! • Absolutely nothing can move faster than light c = 300, 000 km/sec. • All observers agree on this speed, regardless of their own motion. time space String Theory

It’s the Law! • Absolutely nothing can move faster than light c = 300, 000 km/sec. • All observers agree on this speed, regardless of their own motion. • Causal influences cannot travel ‘outside the light cone’. time space String Theory

Relativity of Simultaneity • Observers can disagree whether A or B is earliest. String Theory

Relativity of Simultaneity • Observers can disagree whether A or B is earliest. • Causality is not violated because no information can get from A to B since it cannot travel faster than light. String Theory

…. Enter: Quantum Physics String Theory

The Uncertainty Principle The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa. Werner Heisenberg, 1927 String Theory

Apparent Inconsistency…. • If particles are known to be precisely at positions A and B, then they move with arbitrarily high uncertainty in their speed. • In particular there is some probability that they could be moving faster than light! String Theory

Consistency Restored! • In Nature consistency is restored by ensuring that observers who disagree on the order of events A and B also disagree on whether influences pass from A to B or in the opposite direction. • This can only work if every particle has an antiparticle having an identical mass and interaction strengths but opposite charge. Richard Feynman String Theory

Antiparticles Exist! String Theory

Einstein on a Roll: General Relativity • In General Relativity, gravity is attributed to the curvature of space and time. • Slowing of time in gravitational fields. • Bending of light by gravitational fields. • Predicts the existence of Black Holes. String Theory

General Relativity • General Relativity requires test observers whose positions and times are precisely known, but with negligible mass. • They can map out space-time without themselves gravitating. • Another, more difficult, conflict with the Uncertainty Principle! String Theory

Quantum Gravity? • At small distances large ‘quantum fluctuations’ in the gravitational field make gravity very strongly-interacting. • Cannot predict using General Relativity for distances shorter than the Planck length, ℓp. • Absolutely no candidate theory before the mid 1980’s. String Theory

String Theory: the idea • All matter consists of small one-dimensional objects (strings). • Strings just look like particles when they are not seen under sufficient magnification. • Originally thought to be only a single unique theory which makes mathematical sense. String Theory

String Theory: the idea • Enormous economy of description: • All particle types are simply different kinds of oscillations of the string. String Theory

String Interactions • All interactions consist of the splitting and joining of these elementary strings. • This is the only known sensible description of gravitational interactions at very short distances! • At long distances theory looks like General Relativity plus other interactions at low energies. • No parameters: the string length sets the units. String Theory

String Varieties, circa 1990 • The consistency of gravity with Quantum Mechanics makes String Theories almost mathematically inconsistent. • Nevertheless there appeared to be more than one kinds of consistent string theory. • Open strings, closed strings, heterotic strings, Type IIA strings, Type IIB strings… String Theory

Uniquely Predictive • String theory predicts space has 9 (or 10) spatial dimensions and 1 time dimension. • Experimental update: observed number of (large) dimensions is 3 space and 1 time. String Theory

How Would We Know? • There may be 9 spatial dimensions, since we would not know about the extra ones if they were extremely small. r < 10 -17 cm • Cannot yet predict the size of all dimensions. String Theory

Experimental Tests? • Strings were thought to have to be very short in order to properly reproduce the strength of Newton’s Law of Gravity. • Since strings are so short, it is extremely difficult to find decisive experimental tests for theory using only experiments on distances larger than 10 -17 cm. Theories without experiment guidance normally die through lack of progress… String Theory

D-Branes • String theory is bigger than previously thought. • Normally, the ends of open strings move freely at the speed of light. • Strings can also exist whose ends are anchored onto surfaces. • These surfaces are interpreted as large, massive objects, called D-branes, in spacetime String Theory

D-Branes String Theory

Why Consider This? • Good Things Happen with both strings and D-branes: • Previously-hidden ‘duality’ symmetries emerge, and all known string theories become related to one another by these symmetries! • Some weakly-interacting string theories are really the strongly-interacting limit of others! • Conjecture: all known string theories are different solutions to a more fundamental (11 -dimensional) theory (called M Theory). String Theory

The Brane World • Some states are trapped on the branes, and others are free to wander through all of the dimensions of spacetime. String Theory

The Brane World • Some states are trapped on the branes, and others are free to wander through all of the dimensions of spacetime. • It can happen that all of the known elementary particles and interactions (except for gravity) are brane bound in this way. String Theory

Strings Might Be Big • String size is inferred from the (very weak) strength of gravity compared with other interactions in 4 dimensions. • String theory predicts these strengths in 10 dimensions. • Gravity may be the weakest force because it ‘sees’ more dimensions than do the other interactions. • Strings can be as large as the present limits of detectability. String Theory

New Perspectives • Perhaps some particles can move faster than others? • Perhaps not all interactions ‘see’ the same number of dimensions of space? • Perhaps ‘space’ and ‘time’ only make approximate sense on very large distances. String Theory

Is Space-time Emergent? • In some circumstances physics at distances smaller than the string scale is identical with physics larger than the string scale. • Suggests that notions of distance may only be approximate, applying only for distances R » ℓs • Holography: Some string theories in 10 dimensions are equivalent with ordinary particle theories on the 4 dimensional boundaries. • In a proper formulation of theory space and time should not be assumed, but should emerge in the long-distance limit. String Theory

Brane Cosmology • Cosmology describes the Universe given the distribution of matter within it. • This can be dramatically different in the Brane World. P. Steinhardt WMAP String Theory

String Summary • String theory is the only known theory where gravity and quantum mechanics co-exist at high energies. • Major Lesson of the 20 th Century: Relativity and Quantum mechanics are almost inconsistent, and so together impose extremely strong self-consistency conditions. • The discovery of branes has radical implications for what string theory means. • • String theory (or M theory) seems unique after all. Size Matters: strings may be much bigger than thought. We may all be Brane bound. Experimental tests are much closer than had been thought. String Theory

The Theory of Everything? • For the first time in the history of human thought we have a candidate which could be a ‘Theory of Everything’. • It appears to consistently include gravity down to arbitrarily small distances. • It appears to be so tightly constrained by consistency as to be unique. • It must make contact with experiment, but there is not a time limit so long as progress is made. String Theory

Perhaps a beautiful relationship…. String Theory

…. …. . and a classic ending. String Theory