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Return to E-mail Discussion page Next in threadOne of the great challenges for modern physics is to unify the theories of quantum physics and general relativity. Roughly speaking, quantum physics was developed to explain what happens on very small scales (atoms, etc.), while general relativity was developed to explain gravity on very big scales (stars, galaxies, etc.). While the 2 theories work remarkably well in the regimes where they each apply, physicists run into problems trying to make them work in domains where both theories are needed. Attempts to bring the 2 together are referred to as "quantum gravity" theories. This post is a start at providing some intuition about why it's difficult to make quantum theory and general relativity work together in a single theory of quantum gravity. 1) General relativity - Einstein's theory of gravity is based on the equivalence of gravity and acceleration. Locally, you can't tell that you're in a freely falling elevator in the Earth's gravity, rather than just drifting in space far from any gravitational influence. No experiment you do inside the elevator can tell the difference. But LOCAL is important here. If you release a ball from each hand, they will just float where you release them. But if you were in a really big elevator and released them thousands of miles apart, you would notice that they drift toward each other over time, as if pulled together by a mysterious force. Really they are just drifting together because they're both falling toward the center of Earth. But in this way you could tell the difference between floating in empty space, and free fall near a gravitating object. The point is that how small you must confine your region in order to be "local enough" to not notice depends on the setup - near Earth, a regular sized elevator is plenty local. Near a black hole, the region needs to be much smaller. 2) Although quantum theory deals with the very small, it is inherently non-local: you can't define things with arbitrary precision (one expression of this is the familiar Heisenberg uncertainty relation). So the difficulty in unifying GR and quantum comes about when "how local you have to be" for GR to apply is smaller than what you can define for quantum theory. A to be" for GR to apply is smaller than what you can define for quantum theory. A very large mass in a very small space creates this type of situation. Another way to look at it is to say that GR works with a backdrop of space and time that is continuous, while quantum theory reveals that nature is fundamentally "grainy" - like pixels on a computer screen. They work together fine as long as you're working on scales where the graininess doesn't become apparent (just like you don't normally notice the grains in your photos), but if you try to look at something where the size of a pixel is close to the size of what you want to look at, you realize that there is a fundamental conflict. Anyway this is just a start to get ideas flowing. A good book to look at if you'd like to explore this topic further is "Three Roads to Quantum Gravity," by Lee Smolin. More details can also be found on Wikipedia: http://en.wikipedia.org/wiki/Quantum_gravity http://en.wikipedia.org/wiki/Strong_equivalence_principle Todd |
