The document discusses various approaches to developing a theory of quantum gravity, including string theory, loop quantum gravity, and ideas about emergent or noncommutative gravity. It notes open problems like developing a coherent state of gravitons, achieving background independence in loop quantum gravity, and solving the problem of time in quantum mechanics and general relativity. Later sections discuss quantum spacetime and noncommutative quantum field theory as a potential approach, noting it will require reworking calculations from scratch while checking for issues like causality, symmetries, particles, interactions and spin-statistics.

1. Quantum
G rav ity
Tapio Salminen

2. Chapter 1.
Motivation

3. Theoretical
physics

4. You should do
things that make
you happy!

6. Or could it
possibly be?

8. Have to dig a
little deeper...

9. “Curiosity is a powerful
human impulse – some
distance below sex and greed,
I grant you, but far ahead
of altruism – and I was
simply curious.”
-- Robert Harris, Ghost

10. Chapter 2.
Quantum gravity

11. Horava Quantum
gravity graphity
Loops NC QFT
Quantum
G rav ity
Strings

12. The heart of the problem
- Einstein's equations

14. Geometry = Energy

15. Ideas of emergent gravity

16. Ideas of emergent gravity
String theory
A coherent state of gravitons

17. Ideas of emergent gravity
Still background dependent!
String theory
A coherent state of gravitons

18. Ideas of emergent gravity
Loop quantum gravity
Background independent
Problem of time

19. The problem of time

20. The problem of time
Quantum mechanics

21. The problem of time
General relativity

22. “Motion and change are primary. Nothing
is, except in a very approximate and
temporary sense. ... if we want to think
fundamentally we must not lose sight
of the essential fact that 'is' is an
illusion.”
-- Lee Smolin

23. Horava Quantum
gravity graphity
Loops NC QFT
Quantum
G rav ity
Strings

24. Chapter 3.
Quantum spacetime

25. Quantum theory

26. Quantum mechanics
Quantum phase space
Quantum oscillator

27. Quantum field theory
Quantum felds
and their excitations

28. Quantum spacetime
Quantum
coordinates

29. Looks like it
will be very
complicated...

30. Noncommutative quantum
field theory
Start with a usual, commutative, quantum field theory:

31. Noncommutative quantum
field theory
...and insert modified products:

32. You will still have
to check everything
starting from
scratch!

33. CPT
causality symmetries
Start checking!
Interactions spin-
particles statistics

34. So this is all just
familiar calculations
with a bit more
math?