MadSci Network: Physics |
Hello, Phillip.
You’re checking what you’re learning against what you’ve already
learned: great job! Develop this skill and you will find it easier and
easier to assimilate new ideas.
In this case, you’ve taken your teacher’s comments that “they can’t
measure how fast” an electron transfers from one level to the next and
extrapolated to “I think there has to be something faster than light.” A
faster-than-light conclusion seems dubious, so I suspect that’s when you
wrote to MadSci. You were right to do so – I hope I can clear up some of
the confusion.
The short answer is that a quantum leap does, in fact, take some time,
so there’s no violation of the speed of light. Furthermore, there’s not
an immediate change in special coordinate, so you’re not looking at
a “teleportation” through space either. Your teacher is still largely
right about our inability to measure how fast the transition occurs, but
I’ll get to that near the end.
Based on your question, I suspect your picture of an electron is like a
little ball whirling around a nucleus, like a planet around a sun. If the
allowed states can be compared to the orbits of our actual planets, you’re
wondering how the ‘earth’ can make a transition to the ‘mars’ orbit
without traveling anywhere in between. Furthermore, even if it could
tunnel to the mars orbit or something, how could it do it instantaneously
without traveling faster than light?
If this is the root of the problem, I’ve got the answer for you: that’s
the wrong picture for how an electron orbits a nucleus. The truth of the
matter is that the electron doesn’t whirl around the nucleus; it just IS
around the nucleus. The electron can be thought of like a thick cloud
enveloping the nucleus up, down, and all around.
The shape of this cloud is important. Different states corresponding to
different electron energies are represented by differently-shaped clouds.
For example, some clouds are spherically-symmetric like balloons while
others are dumbbell shaped. Generally, the more structure to the cloud
the higher the electron energy associated with it. The bigger the cloud
is the higher the energy, as well.
So, when an electron jumps from one state to another, it “morphs” the
shape of its cloud. There’s always some overlap between the two clouds
(or else it wouldn’t transition to the new state) so there’s never an
issue of the earth-to-mars problem. And the process always takes some
time, too, so you don’t need to worry about instantaneous motion.
When your teacher told you that this transition time couldn’t be
measured, he or she was largely right. You certainly cannot measure the
transition time for any one atom. You cannot make that kind of
measurement without screwing up the very electron cloud you’re trying to
measure. And when you try to measure the behavior of lots of atoms you
find that you get a whole bunch of different transition times. The
_average_ transition time of a collection of atoms, however, will be the
same each time you measure it.
OTHER LINKS:
Here is the answer to an almost-identical MadSci question!
Re: How can Quantum Jumps happen instantaneously?
Two more relevant MadSci posts:
Re: How long lasts the jump of electron from one energy level to another?
Re: How long does it take for an electron to jump orbits.
If you’re interested in the Bohr model of an atom (from which most people
get their view of an atom with quantized ‘orbits’, and which served as the
starting point of quantum jumps for most people) you can go to this very
good site:
Bohr's
Atom
You’ll see the solar system picture, at first, but further
reading will lead you to more-realistic models of an atom.
Try the links in the MadSci Library for more information on Physics.