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6.4: Section 3: What are the Implications of Some of Modern Physics?

Difficulty Level: At Grade Created by: CK-12
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Question 14: What are some of the implications of quantum mechanics and relativity? In the news there is mention of string theory, black holes, parallel universes, and other bizarre things.

One of the theories that is being explored as a possible unification theory (a theory that is more general and works to bring together quantum theory and general relativity) is string theory. The idea is that instead of the universe being composed of small point-particles, it is composed of infinitely-thin, rubber-band-like strings that vibrate. Recall how earlier we said that protons are made of three quarks, but the electron is an elementary particle, and it has no building blocks. In saying this about the electron we say that it exists only at a point, and does not have any radius (it’s not the sphere that you may be picturing). If an electron took up any space at all, there would be some sort of building-block material that is smaller than the electron. However, string theory says that these tiny, vibrating strings are the basic building blocks of all matter (including electrons), and what’s more, the theory seems to smooth out the problems that exist between general relativity and quantum mechanics. String theorists are attempting to rectify inconsistencies that have been observed by finding a more general theory that encompasses all of the laws of physics.

The length of a string in string theory would be about a Planck length (Planck was a scientist who made great leaps in quantum mechanics), which is about one hundred billion billion times smaller than the nucleus of an atom (that’s way too many zeros after the decimal to type here). They are so tiny that scientists cannot even begin to find experimental evidence of them. Presently string theory lies in the realm of mathematical theory.

How would we detect them? Well, it would help to know how scientists currently detect the particles inside an atom. It may seem archaic, but essentially physicists shoot tiny particles at other particles and then measure what happens. It’s kind of like closing your eyes and trying to find the shape and size of an object by throwing marbles at it and watching what the marbles do after they bounce off the object. What happens if you use big marbles as opposed to small marbles? You might gather by intuition that the smaller the marble, the better and more refined your understanding of the mystery object is. Have you ever played with that toy that’s made of hundreds of pins in a frame? If you push your hand into the pins and then take your hand away, you can see a “picture” or relief of your hand, like a mold. If the toy only had a few pins, the picture of your hand would not be very clear, but because there are so many pins that map out the landscape of your hand, you can see a very clear mold of your hand. The same is true for the “atom smashers” (the fond name we give the machines that speed up particles and shoot them at other particles). The smaller the particles are that we shoot, the better picture we get of the thing at which we are shooting.

The problems with general relativity and quantum mechanics occur at lengths a bit shorter than a Planck length, which is about the size of a string, or so it is theorized. This is extremely tiny. If physicists can shoot strings at particles, perhaps they can see inside the atom to its very tiniest of structures. However, we have a problem, assuming strings do exist. When you give a string a lot of energy (higher frequency), after a certain point, it starts to grow in size (again, theoretically), which does not help the cause of trying to peer into the very tiny world of the subatomic (Greene \begin{align*}155\end{align*}). This “growing” effect is not expected until you try to pump the string with enough energy to probe scales that are smaller than that of a Planck length, so anything larger than a Planck length should still be accessible. Scientists are at a bit of an impasse here, but string theory has a ways to go if it’s going to be supported by experimental evidence. We are not even close to experimenting at this energy level.

There is some hope for string theorists at the new Large Hadron Collider at CERN in Europe. The energy that this atom smasher can give the accelerated particles (the “bullets” being shot) is much less than it needs to be to see strings (it’s not even in the ballpark). However, physicists might be able to see the effects of string theory. For example, you may not be able to see around a corner, but you may be able to detect that somebody is standing behind the corner by seeing his or her shadow. You are not directly seeing them, but you see the effects of their existence. One of the results of string theory is that gravity is not just a field, as you may have learned earlier in the year in your physics class, and physicists may be able to detect this by using the collider at CERN.

Let’s take a moment to discuss general relativity. Einstein helped us view gravity in a new way that is described in general relativity. We discussed special relativity earlier when we were exploring time dilation and Lorentz length contraction (and the constant speed of light), and we mentioned general relativity, but did not go into a conceptual description. General relativity addresses time and space as a fabric, and Einstein helps us visualize by telling us to picture the space-time fabric as a giant rubber sheet (although in reality space-time is not flat like a piece of paper). On this rubber sheet you should picture all the celestial bodies (Sun, planets, stars, etc.) resting. The larger the object, the more it presses down on the rubber sheet. (Please suspend the fact that there is no gravitational force to pull the planets and stars down on the rubber sheet. This isn’t quite a perfect metaphor.) This view of space-time helps us to better picture how gravity is communicated from object to object and helps us answer the question of how the Moon knows or feels the presence of the Earth, and thereby causes it to have its present motion. The problem with Newtonian physics is that there is no mention of how planets “feel” gravity. In Newtonian physics, gravity just “is.” Newton was aware of this problem as you can see in the quote below.

“Tis unconceivable [sic] that inanimate brute matter should (without mediation of something else which is not material) operate on and affect other matter without mutual contact…. That gravity should be innate, inherent and essential to matter so that one body may act upon another at a distance through a vacuum, without the mediation of anything else by and through which their action or force may be conveyed from one to another is to me so great an absurdity that I believe no man who has in philosophical matters any competent faculty of thinking can ever fall into it….” (Newton, “Letter”)

Picture yourself and an elephant standing on a trampoline. Even with your eyes closed you could sense the presence of the elephant (although you may not know that it’s an elephant) by the way it causes you to slide and lean in a little toward it on the trampoline. And, the larger the elephant, the greater it would affect your position next to it. Einstein managed to help us resolve Newton’s problem by helping us see that the celestial bodies affect one another through the distortion of the fabric of space-time in which they exist.

String theory seems to suggest that this isn’t quite the end of the story, rather just a blurry view of the real universe. String theory suggests that there exists a small particle that physicists call the graviton that communicates the force, just like the strong force has the gluon to communicate between quarks (called a force carrier). String theorists believe that gravity is not a very weak force, as is now the general thought, but that its strength is lessened because it is spread over more than just our dimension, and that parallel universes exist. These gravitons are thought to travel between these folds of parallel universes, and they are expected to travel at the speed of light and to be massless (only massless particles can travel at the speed of light, a consequence of relativity).

As a side note, you may wonder why presently the force of gravity is considered a weak force. It governs the motion of the planets and stars, so at first thought it seems like it should be a very strong force. But consider how a balloon that you rub on your hair is able to lift your hair against the gravitational force of the earth that is pulling down on your hair. When you rub a balloon on your head, some of the electrons are “rubbed off” on your hair and transferred to a localized region (balloons are insulators, so any charge you transfer sticks right where you put it) on the balloon, and it’s a relatively few number of electrons. Just a few electrons can attract the now positively-charged hair on your head (by rubbing electrons off your hair you have taken away negative charge, which leaves an unbalanced positive charge), and lift it very easily, despite the pull of Earth’s gravity. Gravity, therefore, must be a very weak force as compared to the strong force of electromagnetism. But perhaps string theorists are on to something if the gravitational force’s force carriers, gravitons, are spread out over more than one universe (parallel universe); then it would appear weak.


Black Holes

You have probably heard the term black hole and wondered exactly what it is. First, a black hole is not really a hole, a term first coined by John Wheeler in 1969. He called it a hole because it appears as a black, featureless area in space.

So then, what is a black hole? Physicists think that a black hole is formed when a large star (a few times larger than our Sun) runs out of the fuel that maintains it, and because it’s so large, its own gravitational force pulls it into a dense area of matter that is small, but very massive. Recall from our discussion of general relativity that celestial bodies, such as stars and planets, distort and stretch the fabric of space-time, like giant bowling balls on a rubber sheet. This distortion of space-time affects the path of light, whether it’s light that may be traveling by, or light emitted by the star actually causing the distortion. Note that the gravitational force only “pulls” on objects with mass, like planets and stars and particles. Light has no mass, so gravity does not “pull” on light. However, the actual path on which it is traveling is affected, so the gravitation force does affect the path of light, just not by directly pulling on it. Scientists believe that when a large star collapses its mass becomes distributed over a very small volume (it’s very dense).This collapse greatly distorts the fabric of space-time, so much so that light cannot escape its distortion. For example, for a space shuttle to escape orbiting our Earth, it has to go a certain speed. This speed is called the escape velocity. The larger the gravitational pull, the faster the object must go to escape its pull (or the distortion of space-time). Think about it this way: When you go around a bend in your car, if you go slowly enough, it’s easy to maintain circular motion. However, if you speed up, there is a certain speed that will cause you to break free from the frictional force keeping you circling and you will slide off tangentially. The difference with orbits is that the force causing the circular motion (or centripetal motion) is not friction (like it is with your car). Instead it’s the gravitational force (or the warping of space-time by Earth’s large mass). A collapsed star is very massive and creates such a gravitational force (or distorts space-time so much) that the path of light turns right back in toward the center. Its path can’t overcome the warping of space-time. Because light is the universe’s speed limit, nothing else can even come close to escaping the space-time warping from the collapsed star. Thus, a collapsed star is called a black hole, as nothing can escape it, not even light (Whitlock, “Gravity”). (Recall that to see something you need to detect light bouncing off of it.)

Dark Matter

Physicists have taken pictures of distant interacting bodies (like a grouping of stars), and after some calculations, have surprisingly discovered that there isn’t enough matter there to cause such an interaction. In this case interaction means gravitational pull or orbiting. How can these stars be grouped when the mathematics doesn’t seem to add up? Scientists are now conjecturing that there is matter that exists that does not reflect light, or perhaps reflects just a very small amount of light, so that it cannot be detected. They have called it dark matter. The term dark matter does not infer that it is dangerous or bad, or that it’s like black holes. Instead the term dark matter means that very little light reflects, if any, so it appears dark, or undetectable, or it would be if it were not for the fact that the gravitational forces are not adding up correctly (another example of learning about something’s existence without really being able to “see” it).


The term antimatter may sound mysterious, so let’s shine a little light on it. Antimatter was predicted before it was experimentally discovered by Paul Dirac, a theoretical physicist who was developing quantum mechanics. In his theory he predicted the existence of a particle that is the same mass as an electron, but has an opposite charge (positive). Later, this particle was called the positron. You might wonder what the difference is between the proton and the positron, because you already know that the proton has a positive charge. Protons are very large as compared to electrons. Positrons and electrons are the same mass, just opposite in charge. When charged particles move through a magnetic field, they spiral, and the direction of their spiral depends on their charge. Physicists saw evidence of particles spiraling in two different directions, implying opposite charges. However, the particles were the mass of an electron, thus showing the first evidence of antimatter. Now physicists have been able to produce antiparticles in particle accelerators (like at CERN in Switzerland), however, they never last very long as they are almost immediately annihilated by their corresponding particle (matter and antimatter annihilate each other). For example, if an anti electron (positron) is produced, it will be annihilated by an electron in very little time. Scientists at CERN have been able to produce antimatter (an atom made of antiparticles); they created an anti hydrogen atom by causing a positron to orbit an antiproton. Again, it was short-lived, as it was annihilated by the prevalent electrons and protons we have. It is predicted by quantum mechanics that the creation and annihilation of matter and antimatter happens frequently, but is so fast we cannot detect it, and because the particles annihilate each other, conservation laws are not violated (they end up canceling each other out). This is an application of the Heisenberg uncertainty principle, which we have not discussed.

We could continue to discuss and delve deeper into what all this means, all this unintuitive physics, and the implications for how we view our universe, and you should continue thinking and reading about our universe, but for now let’s leave with a summary quote from a notable thinker.

“I know that this defies the law of gravity, but, you see, I never studied law.” (Bugs Bunny)

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