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Name: Oscar
Status: student
Grade: 9-12
Country: United Kingdom
Date: Fall 2012


Question:
I am aware than quantum mechanics works using probability, through the unusual method of probability waves, which measure the probability of each way the particle could get there, and adds them all up to make the final probability. And loads of cool stuff comes about because of that. But I have two questions to do with quantum probabilities. 1) Is it that quantum particles are inherently random, or is it simply the case that it is impossible to isolate particles from the forces acting on it, and it is impossible to know enough information to predict how the forces will act upon particles? (like a more complex version of the system which makes weather random) 2) How can it be possible to use quantum mechanics in any kind of practical way, as it involves working with every possible way a particle (or multiple particles) to its destinations, which would surely would be infinitely many? Is there a special method of doing that which I am not aware of? Or are physicists able to select a handful to work with?

Replies:
Quantum particles are inherently random. The randomness of a quantum particles is described by a wave function, which is a technical term, and this wave function generates the probability waves. You may be surprised to know that the concept of "force" does not apply in the quantum world. Instead, one uses potential energy.

It is impossible to know enough information to completely predict the trajectory of a quantum particle. This idea is in contrast to the fact that we can know enough information to completely predict the trajectory of a big particles, such as an airplane.

The ideas of quantum mechanics are used everyday, but they are not apparent. For instance, the operation of the computer screen you are looking at right now can be explained by the "jumping" of electrons. The "jumps" that the electrons make can be predicted by quantum mechanics.

J. S. Grell


In answer to (1), the equations of quantum mechanics only yield probabilities. If they correctly describe particles, then the particles are inherently random. It is also true that, in practical situations, particles are almost never undisturbed by outside forces, and those forces can be so complicated (by large numbers of particles whose behaviors and locations are not known very well) that apparent randomness results.

In answer to (2), you do not have to know what each individual particle is going to do in order to know how a large collection of particles is going to behave. Even before quantum mechanics this notion was demonstrated in thermodynamics, fluid flow, etc. Quantum mechanics gives the pattern of behavior, rather than the exact behavior of individual particles, and this is often sufficient to predict how large collections of particles will behave.

Tim Mooney


Oscar,

Probability is a fundamental of quantum mechanics. The Uncertainty Principle tells us that we can not measure position and motion at the same time. That leaves us with looking at probable motion and position.

Quantum particles and weather: neither is random, they are chaotic. They are dynamic systems that are sensitive to initial conditions. Any smartphone screen or pair of polarizing sunglasses demonstrates practical quantum behavior that is totally invisible to most of our population.

Most physical scientists make assumptions based on those probabilities and progress through the reduction to practice.

All things probable! Peter E. Hughes Ph.D. Milford, NH


Oscar, In a given situation, only a few measured values are possible for each quantity of a particle. One of the major limitations is energy. Another is position: for example, an electron in a certain atom will probably stay in that atom. If an event ?measures? a quantity of the particle, only one of the possible values exists. BEFORE measuring, however, any probability-based combination of values can exist. In a simple example of value 1 and value 2, a particle could carry 70% value 1 and 30% value 2. When you measure the quantity, there is a 70% probability that the particle becomes only value 1, and a 30% probability that the particle becomes only value 2.

Each quantum particle has a sum of possible states. Until the state is in any way measured, all exist. As soon as something requires the state be measured, one of the states is ?chosen?. Only one state remains. Reality is as if only one of the possible measured values has ever existed for the particle. This is why quantum physics was so difficult to discover. You cannot measure the quantum properties directly.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


1. As far as the math and experiments show, quantum processes are truly random, not simply chaotic. It is not that we do not know enough precision to predict the exact outcome, it is that the particles themselves are inherently imprecise, due to their wave nature.

2. Of course, we cannot get a result from a calculation that takes infinitely long. It is necessary to have some way to truncate it. Basically it is a matter of mathematical tricks. The trick lies in determining when figuring additional terms will not substantially change the results. This is an area of ongoing research.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Oscar,

Two great questions, one of which is still debated. The first was posed early on in the development of Quantum Mechanics, and it is called the “hidden variable” question. Is the behavior of particles inherently random, or do we simple do not know enough about them? In other words, are there forces involved that we just do not know about causing the behavior of particles? If we knew enough information, could we predict to certainty the behavior (present and future location) of particles? Interestingly (and counter to our intuition), randomness does seem to be fundamental to Quantum Mechanics and the behavior of the universe. Randomness, however, does not mean complete chaos. As a poor example, consider a carnival attraction in which you had to throw a baseball through a small hole in a faraway wall to win a prize. You throw many balls and some go in and some do not. If you threw many balls, you might have a certain percentage of balls make it through the hole. Depending on the skill of the person throwing, this percentage might be higher or lower. In fact, you might be able to plot the skill level of the person versus the number of successful throws. Now take that plot and consider any person throwing many balls at the hole. You could then “grade” the skill level of that person as the percentage of successful throws by using your plot. If you put up a curtain between you and the thrower, you have the question posed by you. All you are given is the situation (person throwing the ball and your skill plot), and the result (number of successful throws). From this information you determine a skill level. Are there hidden variables here? Of course there are. Theoretically, if you could determine how the ball is thrown, you could determine whether or not it would, in advance, make it through the hole. The curtain prevents this knowledge, but then do you throw up your hands and say, “I cannot determine the skill level of this person because the situation is complete chaos!” Of course not. You have a series of many throws for each person, you pick out the percentage of successful throws, and you determine a skill level from your plot. Quantum mechanics does something similar to this, only all indications are that there is nothing behind the curtain (“Pay no attention to the man behind the curtain,” as the Wizard of Oz said). No hidden variables exist, and the trajectory of each ball is not unknown, but unknowable. I stated that this analogy is poor to begin with, and the reason is that once we state that the information is “unknowable” our reaction is, “but just because we do not know it does not mean it does not exist! Pull down the curtain and look!” Our analogies break down because we are familiar with the classical world where we can plot the force and trajectory of a baseball to great accuracy using Newtonian physics. In the atomic world, the behavior is different, and we must use Quantum Mechanics. We have no real-world experience with the way this physics behaves, and so we think it is counter-intuitive. It is not, just different.

Kyle Bunch, PhD, PE


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