# DJ's Reviews > Three Roads To Quantum Gravity

Three Roads To Quantum Gravity

by Lee Smolin

by Lee Smolin

DJ's review

bookshelves: popular-physics

Feb 16, 2009

bookshelves: popular-physics

Recommended for:
any young scientist interested in today's fundamental questions in physics

Read in January, 2009

Lee Smolin stormed his way onto my fantasy grandfather list the fateful summer of 2008 when I realized physics and I were more than just a fling. His The Trouble with Physics was a fatherly introduction to the current state of the edges of theoretical physics and I was hooked. Needless to see, I was ecstatic to find "Three Roads to Quantum Gravity" under my Christmas tree this year and devoured it on plane ride to Thailand soon after.

"Three Roads to Quantum Gravity" is Smolin's briefing to the public on several current approaches to the quest for the unification of quantum theory and gravity. It weaves a tale that passes through quantum information theory, cosmology, and many other delicious details but, importantly, begins by focusing on the fundamental qualities that he believes any current theory should possess. The book is openly biased against the somewhat publicly alluring string theory, and I recommend that any reader complement this dish with a side of Brian Greene's The Elegant Universe.

Smolin's criteria run something like this.

This last problem has an interesting parallel in theories of the brain. Sensory information propagates to each of us at very different rates light faster than sound, sound faster than smells (particle diffusion), etc. Signals in the brain also travel at finite speeds (far slower than the speed of light). These finite speeds apply to both sensory information and "decisional" information other areas in the brain.

Continuing our discussion of information in the universe, Jakob Bekenstein's studies of black hole entropy have led to the recently popularized holographic principle - a surface bounding a region of space contains all the information contained within. Information is "projected" from the 3D interior onto the 2D boundary. Whether to view the 2D projection as fundamental and the 3D interior as "derived" or vice versa is still (as far as I know) an open question for theory. Bekenstein also placed a boundary of the possible amount of information represented, suggesting a quantization of space. In this picture, geometry can be viewed as defining the information channels of the universe.

One final fascinating theoretical nugget that Smolin presents is that accelerating observers are granted a window into the quantum fluctuations of space in the form of experiencing a hot photon gas whose temperature is proportional to their acceleration. The basic idea is that quantum fluctuations normally go undetected since they have no energy but when you accelerate "fast enough" your relative motion allows you to experience them as having an energy. You experience this random motion as heat.

"Three Roads to Quantum Gravity" is Smolin's briefing to the public on several current approaches to the quest for the unification of quantum theory and gravity. It weaves a tale that passes through quantum information theory, cosmology, and many other delicious details but, importantly, begins by focusing on the fundamental qualities that he believes any current theory should possess. The book is openly biased against the somewhat publicly alluring string theory, and I recommend that any reader complement this dish with a side of Brian Greene's The Elegant Universe.

Smolin's criteria run something like this.

**The theory must be background-independent**. It must be a physics based on relations, not on absolutes.**Processes, not objects, must be fundamental**. This gels with the idea of a human being as a pattern of information and not a specific set of matter (guess what percentage of your atoms are recycled every two weeks).**The theory must accommodate many different observes**. The finite age of the universe and special relativity's upper bound on speed mean that no one in the universe has access to all information available and each observer indeed has a*different*picture based on their location. Black holes place additional information boundaries throughout the universe. A single observer's "information space" might look somewhat like a sphere of Swiss cheese, the holes representing black holes, the cheese representing areas which the observer may receive information from, and "beyond the cheese" as the space from which information hasn't had time to reach the observer just yet.This last problem has an interesting parallel in theories of the brain. Sensory information propagates to each of us at very different rates light faster than sound, sound faster than smells (particle diffusion), etc. Signals in the brain also travel at finite speeds (far slower than the speed of light). These finite speeds apply to both sensory information and "decisional" information other areas in the brain.

*This means that different parts of the brain have access to different information at any given time*. Future theories of consciousness may very well have something powerful to say about about observer-dependent physics (and vice versa).Continuing our discussion of information in the universe, Jakob Bekenstein's studies of black hole entropy have led to the recently popularized holographic principle - a surface bounding a region of space contains all the information contained within. Information is "projected" from the 3D interior onto the 2D boundary. Whether to view the 2D projection as fundamental and the 3D interior as "derived" or vice versa is still (as far as I know) an open question for theory. Bekenstein also placed a boundary of the possible amount of information represented, suggesting a quantization of space. In this picture, geometry can be viewed as defining the information channels of the universe.

One final fascinating theoretical nugget that Smolin presents is that accelerating observers are granted a window into the quantum fluctuations of space in the form of experiencing a hot photon gas whose temperature is proportional to their acceleration. The basic idea is that quantum fluctuations normally go undetected since they have no energy but when you accelerate "fast enough" your relative motion allows you to experience them as having an energy. You experience this random motion as heat.

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*Three Roads To Quantum Gravity*.## Reading Progress

04/13 | marked as: | read |

message 1:by Jimmy (new)Then we will understand travel is not dictated by space or time but is simply a change of view, instantly.

The above is possible at a highly energetic dimension wherein the 'lesser' dimensions and the slower worlds appear to an observer who slows down his own receptivity

Conversely, a kind of omniscient view and relationship is produced when the observer accelerates his / her own natural apparatus.

All this needs training of the apparatus or organs or bodies, whatever you may term it. Some human beings have succeeded in the adventure but their names have become famous not as scientists or explorers but eagerly latched on by vested interests as heads of religions.