Science a GoGo's Home Page Forums General Discussion Physics Forum Modifying Electrons in Atoms

Hi,
I am fairly new to the study of science, and a topic of interest came up in a discussion I had with a friend. In that discussion he claimed that scientists were able to modify electrons in one location and thus alter the electron makeup of another atom instantaneously no matter how distant. Can anyone please explain this to me and possibly provide a url?

Hi Rallem,

I get the impression that what you mean is something we call quantum entanglement. The notion was first introduced by Einstein, Rosen and Podolsky in 1935. Although Einstein was one of the major players in the formulation of quantum mechanics, he was never really happy with it. This is true for many of the other founders of quantum theory. For example, Schrodinger and Dirac both expressed a degree of dissatisfaction.

Some resources to check would be the following.

On the EPR:

http://en.wikipedia.org/wiki/EPR_paradox

http://arxiv.org/abs/quant-ph/0211012

On quantum entanglement check out:

http://en.wikipedia.org/wiki/Quantum_entanglement

http://plato.stanford.edu/entries/qt-entangle/


Dr. R.

Thank you Dr. Rocket, your response has given me lots to absorb, but I have a very basic question about the data provided by wikipedia about the EPR Paradox. In the Reality and Completeness section of the page it says

We have seen that a quantum state cannot possess a definite value for both x-spin and z-spin. If quantum mechanics is a complete physical theory in the sense given above, x-spin and z-spin cannot be elements of reality at the same time.

My question is, where did we see this? At wikipedia they are usually pretty good at showing sources of their information through hyperlinks, but that isn't done here. I can't simply accept something to be true, just because it is written down. I will need to have a source for the information and I need to go to that source to study it.

Hi Rallem:

Wouldn't the reference be "written down"?

Hi again Rallem,

If I read you right you are asking about about two of the most important themes in quantum theory, namely duality and the indeterminacy principle.

The idea of duality was first brought to light by the French physicist De Broglie. (Pronounced as d-broy-ee.) You see, Einstein had argued that light is not only wavelike, but also has particle characteristics. this would explain the peculiar aspects of the photoelectric effect. In de Broglie's thinking this seemed like a general idea. So electrons, normally thought of as particle, should have wave-like characteristics. Sounds pretty wild, but experiment confirms this. (Otto Stern's molecular beam experiments 1929 et seq. and several others.)

Along with Planck's quantum of action duality led Heisenberg to the indeterminacy principle.

Here are some links for starters.

On duality:
http://en.wikipedia.org/wiki/Physics

On the indeterminacy principle:

http://en.wikipedia.org/wiki/Uncertainty_principle

This one has a further reference to EPR. Try google-ing for some of the terms I have thrown out. There is lots of info out there.

Of course you should not accept written authority! The best approach to any branch to science is to have an open but skeptical mind. In addition to this you need to read and to study and, above all else, you must think. You may be a beginner, but at one time we all were. Just keep at it and it will come to you.

Dr. R.

Thank you Dr. Rocket, and jjw. I have gone through the links at wikipedia and the concepts for duality and the uncertainty principle, but I feel that they fail to explain away my question. Earlier on I stated that I was new to science, and that statement was not entirely true. In college in the mid 1990's I studied physics with calculus as an electronics engineering major. Since that time I have pretty much stayed away from the fields, but have recently decided to brush up and study the two fields.

In my earlier post I questioned their statement that if quantum physics was a complete field of study then an atom cannot have x spin and z spin at the same time. I think they must have meant that an x spin and z spin cannot be measured at the same time, but it would seem obvious that they can contain both observables. I think this may be the area in which I am getting confused and I think they are either treating all observables as equal or I am.

Hi Rallem,

Your confusion is completely understandable. It took me years to sort it all out and I still have a way to go. Let's see if I can put this in a few words that are easy to comprehend.

From a classical point of view an observation can be made with negligible interference on the part of the observer. If you want to measure the motion of, say, a pendulum, you see it and the measuring apparatus by means of reflected light. The pressure of this light is real but completely negligible and has no tangible effect on the motion.

Now Planck comes along and finds that there is a "quantum of action." This is just the fact that the smallest interaction between things is one quantum of energy. The quantum is small by ordinary standards, but significant at the atomic level.

After Planck comes Neils Bohr with a quantum theory of atoms. This is recognized as a preliminary theory by one and all, even Bohr. In this theory Bohr pictures the atomic electrons as orbiting a small nucleus. Now Heisenberg had a problem with this view. He maintained that electron orbits are not observable - not even in principle. He illustrated this with his gamma-ray microscope thought experiment. In this a quantum of gamma radiation is scatterred by an electron into a microscope. There is a certain level of indeterminacy in this operation that cannot be got rid of. (Actually Heisenberg got some of these ideas from his Doctoral advisor Arnold Sommerfeld, but that's a long story.) The point is that you cannot "see" what the atomic electron is doing - tracing an orbit or something entirely different.

What Einstein said was that even if you can't see (observe) the atomic electron, it must be doing something. Also, when it is doing, whatever that is, it must have properties, e.g., momentum, even if we don't know what these actually are. Now here is the "rub." Some physicist said that this is not the case, these things are created only in an experimental setting designed to show them, i.e., measurement = creation. This is a complex issue. If you are interested checkout the Stanford Encyclopedia of Philosophy on-line:

http://plato.stanford.edu/entries/qt-uncertainty/

This article is not too technical so you might find it readable.

Dr. R.

Nice introduction Dr. R. There is no doubt that owing to the quantum of action you will not be able to make perfect measurements; however, this is also the case classically (although as pointed out by you the percentage involved is smaller). My problem is the following: I cannot see how the interpretation of the wave function's intensity as a probability distribution has anything to do with experimental difficulties. By using this interpretation, it implies that even if it were possible to make perfect measurements one will still obtain a probability distribution; i.e. the probability aspect is inbuilt in Nature. Thus this interpretation of Born should be analysed in thought experiments as if it were possible to make perfect measurements. When doing this one can easily demonstrate that this interpretation violates the conservation of energy. By extrapolating from such a thought experiment to the case where one has to take the quantum of action into account, one then finds that this conclusion is also valid in the latter case.

JB,

One must carefully distinguish between indeterminacy and uncertainty. The former is a quantum concept while the later is classical. Heisenberg, himself, only used the term uncertainty with great reluctance.

A quantity is uncertain if it has a value that is unknown but otherwise completely determinate. Consider the phase space for a free one dimensional motion of a particle. Suppose that the position and momentum is uncertain by the amounts delta_x and delta_t. The actual position in phase space, classically, will be some point in the rectangle formed by the uncertainties. We just don't know which one it is. As the system evolves the area of the "uncertainty patch" will change due to a "phase shear." In other words the uncertainty will grow. (If you know about Liouville's theorem, you might pause to reflect on this.)

In quantum theory indeterminacy is more fundamental than any of the wave functions and is at the root of the probabilistic interpretation. Think about the gamma-ray microscope experiment. See, for example,

http://www.aip.org/history/heisenberg/p08b.htm

and the associated links.

The whole point of the uncertainty principle is that some things are just not observable. As for violating the conservation of energy, reconsider the indeterminacy between energy and time. I believe that you will find that the putative problem vanishes.

You suggestion that "this interpretation of Born should be analysed in thought experiments as if it were possible to make perfect measurements". This has been done by the late David Bohm and others following de Broglie's notion of a pilot wave. However, there are serious ontological problems with this approach. To date these have not been unravelled.

Dr. R.

Dr. R,
I know what you are saying, and I am well informed on Liouville's theorem. I also know about the attempts that have been made by Bohm using de Broglie's "pilot" wave. Whether you call it "uncertainty" or "indeterminacy" does not solve the ontological problems you have with all the approaches from Copenhagen to Bohm.

According to my insight the problem has all along been the interpretation that matter consist of particles, and that the (delta)p gives the uncertainty or indeterminacy of the momentum of the particle. It just cannot do that for a time-independent (stationary) wave, because for matter waves (having mass) momentum is determiined by relativity; i.e. by how fast the observer is moving relative to the matter or vice-versa. This is not a problem for light because light is always travelling at a speed c relative to any observer. Thus, if (delta)p gives the uncertainty in momentum, is this uncertainty the same relative to any inertial reference frame? Hardly likely.

These problems disappear when it is assumed that a free electron is a localised wave with a centre of charge. The ground-state energy of the wave is then the electron's mass and therefore the electron also has a centre of mass. It acts like a point particle when "viewed from outside" the localised wave (classical mechanics and electrodynamics apply as we know from numerous experiments), while quantum mechanics kick in when the wave functions start to overlap by a critical amount.

Thus instead of assuming there are electrons "whizzing around the nucleus" (and that they are doing this without radiating light in total violation of Maxwell's equations) the electron-charge distributed within the intensity of the electron-orbital and is thus stationary as it should be.

What the uncertainty relationship for "momentum" and position gives is just the size of the wave; and as for any other wave, this is determined by the boundary conditions. Thus a free electron is a localised entity as can be verified experimentally, while when encountering a double split it spreads to go through both and then superpose on the other side. When, however, making a measurement on the other side before superposition occurs the electron becomes localised again and no diffraction pattern develops.

What I find strange is that science has come to doubt experimental results and, since Bohr, Born and Heisenberg, resorted to "magic" (Voodo inerpretations): i.e. things happen when we dont look which we cannot fathom and even if we look we still cannot fathom what is happening except if we look many times. In fact, by discarding the concept of particles and postulating waves that can change their spatial extent when the boundary conditions change, leads to a total causal interpretation for quantum mechanics. There should not be any doubt that when an entity passes a double slit and contributes to the formation of a diffraction patter, then this entity MUST have gone through both slits. This is what we know from experiment and this is what we have to conclude as experimental philosophers.

JB,

You are confused.

Dr. R.

Do you want to bet that within 10 years you will have to admit that you are the one that has been confused?