Does Quantum Mechanics Apply to Large Objects?

If it is possible for electrons to be in multiple places at the same time, why is this not the case for larger object like atoms, molecules, or people?As far as I can see, electrons are practically immeasurable masses moving around at practically immeasurable speeds. This "allows" them to essentially break the rules put forth by much of physics. If larger masses were able to move at a similar or proportional speed, would they be able to be in multiple places at once as well?
- Pavan Iyengar (age 13)
Orlando, Florida, United States of America

The question is, does quantum mechanics (QM) apply to large objects as well as to small ones?  If it does, then why don’t we see the same weird QM effects with baseballs as with electrons and other microscopic objects ?  In particular, if an electron can be in two places at the same time, why don’t baseballs exhibit the same behavior?

This is a very interesting question.  I will give you an answer, but beware: The right answer is, we don’t know.  The reason is that, although most physicists believe that large objects (including you, me, and the Sun) all behave quantum mechanically, the predicted effects are much too subtle to have been observed with current technology. For this reason, we can’t be sure that the answer I’m about to give is correct.

To begin with, I will refine your question a bit.  What does it mean to say, “The electron can be in two places at the same time”?  A physicist will insist that the only way to answer this question is to compare the measurements predicted by this assertion with those made by its alternative, “The electron can only be in one place at a time.”  Physicists are very hard-nosed about this, because measurement is our only way to learn how things behave.

With this preamble, I will develop the answer in parts. Here is the sequence:

1. QM talks about probabilities, not certainty.
2. A short discussion of waves and interference.
3. QM probabilities are determined by the wave properties of the objects.
4. In many situations, what we observe depends on the interference between waves that come from different places.

I will describe a measurement that has been made many times that demonstrates that an electron must sometimes be in more than one place at a time.  I will also discuss why this measurement is not feasible with baseballs.

1:
QM describes every measurable quantity in terms of probabilities.  For example, if one asks, “Where is the electron?”, QM tells us the probability of obtaining each of the possible results.  If there are several possibilities (the usual situation), then there is no way to know with certainty which result will be obtained.  It is important to keep in mind that not only do we not know what result will be obtained, but also the electron is not at any one particular place before we make the measurement!  This is what is meant when we say, “The electron can be in two places at the same time.”

2:
For position measurement in particular, the probabilities are described by waves that move around in much the same way as water waves.  For our discussion here, one important property of the waves is their wavelength.
Look at the picture; it’s a photo of a water wave that is passing through a barrier that has two holes in it.  The important features are:

• The wave has a wavelength.  The bright arcs are the crests, and the dark arcs are the troughs of the wave.  The distance between crests is the wavelength.
• Each hole acts like a source of the wave.  That’s why the crests are circular - a wave spreads out from its source.
• There are regions where the waves add up (labeled by “strong waves here”) and other regions where they cancel (“weak waves here”).  This is called constructive and destructive interference.
• The interference pattern (where the strong and weak regions are) is a consequence of the fact that the wave passes through both holes.

If there were only one hole, the wave pattern would be significantly different.  Look at the second picture.  It is a photo of a water wave passing through a barrier that has only one hole.This means that we can tell whether the wave passed through one or two holes by looking at the wave behavior far away – we don’t need to look at the holes themselves.

3:
Let’s put 1 and 2 together.  We need to understand something about QM waves.  Water waves are the motion of the water molecules.  What are QM waves?  To the best of our knowledge, they do not correspond to the motion of any physical object.  In particular, a QM wave does not correspond to any “wavy” motion of the electron it is describing.  Its only physical significance is to describe the probability of finding the electron at a particular place.  This is an unsatisfying state of affairs – how can there be something that does not have any physical existence except to describe the behavior of something else?  I’m not going to answer that question.  It has bothered physicists for the last 90 years, and there is not (yet) a satisfying answer.   So, as they say, “Live with it.”

4:
Why can’t we see this effect with large objects (baseballs)?  Look at the first photo again.  The distance between the locations of constructive and destructive interference is approximately equal to the wavelength.  Therefore, we need to know the wavelength of a moving baseball.  I’m not going to go through the details here.  Because a baseball is much, much more massive than an electron, its wavelength is much, much smaller.  In fact, the wavelength of a pitched baseball is about 10^-34 meters.  That’s an incredibly small distance, much smaller (by a factor of a quadrillion, or so) than any distance that we have ever measured.  That’s the reason that we don’t measure the weird QM effects that are much more obvious for tiny particles at the atomic scale.

Jon Thaler

(published on 06/05/2016)