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RESEARCH
QUANTUM GRAVITY
Quantum gravity is concerned with unifying Einstein’s
general theory of relativity with quantum theory to create a
single theoretical framework. At Perimeter, researchers are actively
pursuing a number of approaches to this problem including loop
quantum gravity, spin foam models, emergent gravity, and causal set theory.
CAN
GRAVITY
AND
QUANTUM
PHYSICS
COEXIST
?
Perimeter reseachers made strides in several directions over the past year, seeking to take our
understanding of space and time beyond Albert Einstein’s special and general theories of relativity.
They have also created new models that express the hypothesis that space and time have an
atomic structure.
Many Perimeter researchers have been particularly focused on approaches to quantum gravity that
are classed together as being “background independent.” Background independence embraces
one of the basic principles of Einstein’s general theory of relativity: that space and time are relational.
They have no fixed, absolute structure to serve as a static background for doing physics. Instead,
space and time exist in a dynamically evolving network of relationships.
Theorists at Perimeter make use of a multiplicity of approaches in order to winnow out the best
model of a given phenomenon. Several different background-independent approaches, though,
converge on a common, if surprising, hypothesis: space and time must have an atomic structure.
According to this hypothesis, at a miniscule level – 20 orders of magnitude smaller than the
atomic nuclei of matter – the smoothness of space breaks into discrete units. The phenomenon is
analogous to the way water appears to flow continuously even though it is actually made up of vast
numbers of individual molecules. It has led Perimeter researchers to some of the most enticing and
important questions facing theoretical physics today:
1. What are the atoms of space and time? What laws do they satisfy?
2. How does the apparent smoothness of space and time emerge from their atomic structure?
3. How do the known laws of physics emerge as approximations of the fundamental laws
obeyed by the atoms of spacetime?
4. Are there observations and experiments by which the atomic structure of space and time
could be confirmed and studied?
The answer to the fourth question turns out to be an emphatic yes. While the scale of the atomic
structure of space and time might seem inconceivably small, it appears possible that spacetime
atoms – or at least their effects – might be observed.
It is an ironic rule of doing physics that observation of smaller phenomena demands larger tools.
Searching for evidence of the atomic structure of spacetime requires the biggest observational
instrument in existence: the universe itself. When light and cosmic rays travel for hundreds of