# Theories of the Universe: Quantum Mechanics vs. General Relativity

## Quantum Mechanics vs. General Relativity

##### Universal Constants

**Quantum foam** is the term used by physicists to describe the violent activity of the quantum world. When you combine all of the characteristics of quantum mechanics—such as wave functions, probability, and uncertainty—you get a pretty active interaction among the forces and particles. You can also think of it as a quantum soup, happily bubbling away.

##### Universal Constants

**Quantum electrodynamics**, or **QED**, is the theory that describes the way electrically charged particles interact with one another and with magnetic fields through the exchange of photons. Also known as **relativistic quantum field theory**, it's quantum because it includes all of the quantum characteristics like probability and uncertainty; it's a field theory because it includes Maxwell's electromagnetic field equations; and it's relativistic because it incorporates the concepts of space and time from the special theory of relativity.

Before we go on, I think it's a good idea to clearly present the problem of unification. What is the main obstacle in the way of uniting the four forces and all of the elementary particles? Well, remember that what physicists have been trying to accomplish is the uniting of the microcosm and the macrocosm. These two areas of cosmology are represented respectively by quantum mechanics and general relativity. In the case of quantum mechanics, we have a world that operates on uncertainty, probability, and complimentarity. If we could look through a microscope at this tiny universe, we would see random quantum undulations resembling something looking like a storm on the North Atlantic Ocean. And if we took this into outer space, into the smooth gravitational field of a planet, you would no longer have the smooth warp of space-time described by the spatial geometry of general relativity. At this microscopic level, the gravitational field would be warped by the frenetic energy of the *quantum foam*. So this fundamental incompatibility of quantum mechanics and general relativity occurs not on the level of everyday life, or even in the vastness of the universe, but at the most fundamental level where the building blocks of matter have their existence.

### The Gravity of the Situation

The inability to reconcile general relativity with quantum mechanics didn't just occur to physicists. It was actually after many other successful theories had already been developed that gravity was recognized as the elusive force. The first attempt at unifying relativity and quantum mechanics took place when special relativity was merged with electromagnetism. This created the theory of *quantum electrodynamics*, or *QED*. It is an example of what has come to be known as *relativistic quantum field theory*, or just *quantum field theory*. QED is considered by most physicists to be the most precise theory of natural phenomena ever developed.

In the 1960s and '70s, the success of QED prompted other physicists to try an analogous approach to unifying the weak, the strong, and the gravitational forces. Out of these discoveries came another set of theories that merged the strong and weak forces called *quantum chromodynamics*, or *QCD*, and quantum electroweak theory, or simply the electroweak theory, which you've already been introduced to.

If you examine the forces and particles that have been combined in the theories we just covered, you'll notice that the obvious force missing is that of gravity. But hope is around the corner! The search for the *primary theory* is still underway. And in order to understand the theories that hold the highest possibility of resolving the dilemma, we need to examine and define the small little corner of the universe where this unification could take place. Although we've spent a lot of time discussing forces and particles, theories and solutions, the one thing we haven't discussed very much are the units of measurement that will allow physicists and you to understand and communicate their discoveries. In other words, we need a clear picture of the scale at which unification takes place.

##### Universal Constants

**Quantum chromodynamics**, or **QCD**, is the theory that describes the way quarks interact with one another by the exchange of gluons (remember gluons are the messenger particles that carry the strong force). Quarks come in three different colors: red, blue, and green. The “chromo” part of the name comes from the way the “color” of the quarks changes when it interacts with gluons.

### The Planck Scale

In other sections we've talked about how important measurement is to comprehending the “relative” nature of the universe. We use common terms like inches, feet, meters, pounds, and kilograms to describe physical characteristics such as weight, length, or distance of objects. What kind of units of measurement would we use to describe our essential unifying theory? Well, the best place to start is with units that are common to all aspects of nature. Universal constants interestingly enough fit the bill. Let's see what they are.

- Planck's constant,
*h*: This constant defines the ratio between a particle's energy and its frequency. If used in conjunction with the mass and the charge of an electron, the size of all atoms, of all kinds, anywhere in the universe can be determined. - The speed of light,
*c*: This speed is constant under all conditions, and is one that you're already familiar with. - Newton's constant,
*G*: This constant measures the strength of the gravitational force. Einstein proved that energy and mass are convertible into one another and gravitation is a force proportional to the amount of energy (mass) a system has, so everything in the universe feels the gravitational force.

##### Universal Constants

Some physicists don't like the phrase, “the theory of everything,” to describe the ultimate theory of the natural world. It's somewhat misleading, because it's not really the theory about everything in nature. It doesn't include the weather, baseballs, psychology, or people. They feel that the idea of a “final theory” or **primary theory** sounds more appropriate.

##### Black Holes

Just when science thought it was safe to assume that universal constants, like the speed of light, were absolute, a discovery comes along to throw a wrench in the works. Recently a team of international researchers found something that could make the basic laws of nature questionable. While examining the light coming from a quasar, which is an extremely bright object that produces 10 trillion times the energy per second as our sun, scientists found discrepancies in the spectrum patterns they were comparing. The differences suggested that something, possibly the speed of light, had changed by the time it reached the earth, trillions of miles from the quasar. To quote one of the scientists, “We don't know what has changed, we don't know whether it's the speed of light, or the electron charge or Planck's constant,” and he added, “There are theoretical reasons for preferring the speed of light.” In two or three years they'll know whether they're right or wrong. If they're right, that will have a profound impact on physics.

With these three constants, we should be able to combine them into units of measurement that will reflect the scale at which the unification takes place. Each of these units is named after Max Planck and can be described as follows:

**Planck length**(*hG*÷ c^{3})^{1/2}This is the quantum of length, the smallest measurement of length that has meaning. It's equal to 10^{-35}meter and is about 10^{-20}times the size of a proton.**Planck time**(hG ÷ c^{5})^{1/2}This is the quantum of time, the smallest measurement of time that has any meaning. Within the framework of the laws of physics as understood today, we can say that the universe came into existence when it had an age of 10^{-43}seconds.**Planck mass**(hc ÷ G)^{1/2}This mass is equivalent to 10^{-5}grams. This is small by everyday standards, but 10^{19}times the mass of a proton, and would be contained in a volume roughly 10^{-60}times that of a proton. This represents an enormous density that has not occurred naturally since the big bang.

These three units, along with others such as Planck density and Planck temperature, define the Planck scale. And all of these units express the smallest possible measurement that can be made in trying to understand what happened in the infinitesimal moments after the big bang. When we discuss superstring theory in “It's All Held Together with Strings,” the units we just discussed will be used to describe the level at which unification is possible.

Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with **Alpha Books**, a member of Penguin Group (USA) Inc.

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