Quantum Theory Vs. General Relativity

20th century physics was a battle of the titans. In one corner: Quantum Theory, in the other corner: Relativity.

While it may seem on the surface that these two theories are competitive and mutually exclusive in nature, they are really complimentary views of the universe. In fact, Einstein was inspired by Max Plank's suggestion that radiation is composed of discrete packets (or quanta) of particles. This led Einstein to develop his theory of the photoelectric effect (which won him the Nobel prize in physics in 1921), which in turn provided the basis of Quantum Theory.

The difference between Quantum Theory and General Relativity in a nutshell is that Relativity deals with the universe that we can see, while Quantum Theory covers the sub-atomic universe.

General Relativity Explained.

Einstein's theory of General Relativity deals with the universe in the grand scale. It can be seen as a more accurate view of the universe than the laws of Newtonian Physics, which preceded Relativity. General Relativity elevated Albert Einstein to rock star status, not just in the scientific community, but pop culture too. The basis of General Relativity is perhaps the single most famous scientific equation ever: E = mc².

Most people on the street have seen or heard that formula, but few could tell you what it means.

In the equation: E stands for energy, m for mass, and c is the speed of light. But what this means is that mass IS energy, and literally the energy contained in any given object is equal to its mass multiplied by the speed of light. To put it another way, energy has mass and mass can be converted into energy. Atomic Bomb anyone?

But relativity goes beyond the atomic bomb. Another fundamental principle of General Relativity is that space and time are not two distinct concepts but are really two aspects of a single concept: spacetime. We only perceived these aspects as being distinct.

It is this perception of space and time that are relative to the observer, hence the name: Theory of Relativity.

This is often a point of much confusion for people. How can space and time be relative? After all, isn't a meter always a meter and an hour 60 minutes long?

Not so in Einstein's world. According to Einstein's view of the universe, how we view time and space depends upon our position or speed relative to the event we are measuring (or observing). This is Special Relativity. General Relativity holds that our perception of space and time depend upon the effects of gravity.

The two most prominent aspects of this are seen in the Lorentz Contraction, and Time Dilation effects.

Lorentz contraction.

When an object is moving toward or away from the observer, and at a speed of at least 1/10 the speed of light, the observer will perceive the object as shorter than it would seem when at rest.

That's at a constant velocity. Things get even weirder when we throw acceleration into the mix.

When an observer is at rest, or traveling at a constant velocity, he will see "flat" space, which would obey the rules of Euclidean geometry we all know and love. But when the observer is accelerating, he will see a warped spacetime geometry in which Euclid's laws break down.

Besides accelerated motion, another factor in how we perceive spacetime is gravity.

In fact, to an observer, a gravitational field is indistinguishable from a uniform acceleration. This means that an artificial sense of gravity can be created through uniform acceleration. If you suddenly awoke to find yourself on the floor in a closed room you would be unable to tell if you were held on the floor by Earth's gravity, or by the constant acceleration of 9.8 m/s² created by a spacecraft traveling through deep space.

An extension of this concept is that gravity warps spacetime. This is often presented with the image of a flat mattress, and a heavy object (like a bowling ball) placed in the center. If the mattress is of a sufficiently discount brand (i.e.: Cheap and flimsy), then the bowling ball will bend the mattress, creating a depression in the mattress where it lies. You then take a smaller and lighter ball, like a ping pong ball, and roll it toward the edge of the bowling ball. If you get it right, the ping pong ball will travel in a straight line toward the bowling ball, but bend and change direction to go around the bowling ball as it contacts the edge of the depression - much like water going down a drain.

This is in effect one of the outcomes of General Relativity - that gravity is nothing more than our perception of warped spacetime, caused by the mass of objects.

The implications of this in universal terms is that the greater the mass of an object, the greater its effect on spacetime and the greater its gravitational field. This paved the way for concepts like black holes, which are so massive some say they might lead to an actual tear in the fabric of spacetime. Think of it as a super massive bowling ball that tears through the mattress when you place it down.

Time dilation.

The second major aspect of Relativity is time dilation. Time dilation is essentially our perception of time being stretched out.

Time dilation in Special Relativity is described by the Lorentz transformation (see above). Under Special Relativity, faster moving objects will experience slower moving passage of time relative to slower or stationary objects. The classic example of special relativistic time dilation is the Twin Paradox. The Twin Paradox is what Einstein called a Gedankenexperiment, or "thought experiment."

The basis of the Twin Paradox is to imagine that there are two identical twins. One twin leaves for a far away star on an interstellar spaceship that travels at very near the speed of light. The other twin stays home on Earth. If the twin on the spaceship travels for 50 years, when he returns to Earth he will see that his twin has aged 50 years while he appears not to have aged at all.

Time dilation in General Relativity pertains to the gravitational effect and is sometimes known as Gravitational Time Dilation. Under this theory, the greater the local gravity, the more distorted the passage of time appears. In areas of intense gravitational effect relative to areas of lesser gravitational effect, time appears to slow down. In the case of a black hole, where the gravitational effect approaches infinity, time is thought to essentially come to a standstill.

The Gravitational Time Dilation effect has been verified with the use of atomic clocks at differing altitudes, since the lower the altitude, the closer to the Earth and the greater the gravitational influence.

Another discrete difference between Special and General relativistic time dilation is that under Special Relativity, it will always appear that your clock is correct, but a clock in another frame of reference will appear dilated. In the example above, each twin will perceive the other twins clock as moving more slowly than his own.

Light Bending Effects.

The concept of gravity warping spacetime also carries some other aspects that changed the way astronomers perceive the universe. One such theory was something called Gravitational Lensing. This theory holds that if light traveling in a straight line were to pass near a massive object, that object would bend the region of spacetime around it and the beam of light would also bend to follow it - the light would change direction!

This led to some interesting work along the lines of determining whether light was a wave or composed of particles, extending Einstein's work on the photoelectric effect. The short answer is both, but that's a different discussion altogether.

Einstein's theory of General Relativity stands today as one of the turning points as well as one of the cornerstones of modern physics.

Quantum Theory.

As mentioned above, Quantum Theory deals with the sub-atomic universe, but it can also be thought of as the theory of physics of the extreme.

Where General Relativity and Newton's Laws give us a good understanding of the large scale universe, they are not all encompassing. Newtonian physics breaks down when speeds become too great or the mass of an object is sufficiently large. This is where General Relativity comes into play and provides a better understanding of the laws and mechanics of the Universe.

But Relativity also breaks down as things get extremely small. It turns out that Relativity is not such an accurate predictor of the state of the universe at a sub-atomic level.

Enter Quantum Theory.

Quantum, from the Latin quantus, literally means "the smallest discrete quantity of some physical property that a system can possess."

Quantum Theory deals in particles, and on a sub-atomic level there are two types of particles: bosons and fermions.

Along with the types of particles, Quantum Theory is concerned with the state of a particle. Here, state is defined by energy, momentum and location. The defining difference in these two types of particles is that two identical bosons can have the same physical state, but two identical fermions cannot. The universe, it seems, is a crazy mixed up world at the subatomic level.

In a particle system with sufficiently large quantity of particles, the classical laws of thermodynamics will be followed.

However, when there is not a sufficient quantity, there will also be insufficient energy in the system and most of the particles will be restricted in their ability to exchange energy. This kind of system is known as a degenerate system. In a degenerate system, the classical laws of thermodynamics are no longer in control, and so the laws of quantum mechanics take precedence.

Quantum uncertainty.

The famous (or perhaps infamous) Quantum Uncertainty Principle is centered on the quantum concept of state. Specifically, it says that the greater the accuracy of a measurement of the location of a particle, the more uncertain the measure of the energy of the particle. In short, you can know one, but not the other.

It was the inherent uncertainty of Quantum Theory that led Einstein to utter his famous quote, "I, at any rate, am convinced that He does not throw dice." This has been paraphrased and simplified through the years as "God does not play dice", but it belies Einstein's inherent distrust of the Quantum Theory.

Unified Theory.

So, 20th century physics left us with two seemingly disparate views of the universe. One provides a glimpse of the inner workings of matter, while the other gives us a view of the grandeur of the universe as a whole, but never the twain shall meet.

There is no theory that combines Quantum Mechanics and General Relativity. In fact, the search for such a theory is the quest of 21st century physics - the holy grail of physics.