Ages 8–12 · about 6 minutes
The Field Hiding Behind the Fields
How can an electron feel a magnet it never comes near?
The big idea
Something invisible can change you even where nothing is pushing you.
- potential
- A hidden readiness in space. It can change things without pushing them.
- phase
- Where a wave has got to in its up-and-down cycle, like a step in a dance.
- longitudinal
- A wave that squeezes along the way it is going, the way sound does.
How can an electron feel a magnet it never comes near?
Picture two cyclists on a ring road around a windmill. One rides clockwise, the other the opposite way. A gentle swirl of air circles the windmill. Nothing at all is blowing on the road itself.
Yet when the two riders meet again, they are out of step. One had the swirl behind them the whole way. This actually happens, and it happens to electrons.
What we know for sure
Start with a bit of clever old math. In 1903 a mathematician named E. T. Whittaker showed something neat. The whole electric and magnetic field can be rebuilt from two simpler things called potentials.
Potentials feel less real than fields. A field pushes on you. A potential just sits there, ready. So for years people treated them as bookkeeping.
Then in 1959 two physicists proposed a beautiful test. Their names were Yakir Aharonov and David Bohm. Wrap a magnet up so that outside it the magnetic field is exactly zero. Now send electrons around the outside, along two different paths.
Nothing pushes on those electrons. They should not notice a thing. But they do. They come back with their waves shifted, exactly as predicted.
In 1986 a team led by Akira Tonomura measured it beautifully. This is settled, textbook physics now. A potential can change matter where no force acts at all.
What scientists are testing right now
Now the frontier. Light waves wave from side to side, across the way they travel. Sound squeezes along the way it travels instead. Sound is longitudinal.
Can an electric wave be longitudinal? In three places, absolutely yes. Right beside an antenna. Inside a plasma. And running along the surface of the ground or the sea. Engineers work with all three.
The open question is empty space. Could a longitudinal electric wave cross a vacuum and carry energy? Some inventors say yes, and there are patents describing machines.
The test is cheap and clear. Put a meter on every wire going into the machine. Then measure everything coming out. If more comes out than went in, that is enormous news.
Nobody has published that measurement yet. Scientists are testing this idea, and this website will report the answer when it arrives.
Why it matters
Think about what a potential gives you. It is a way of changing something without shoving it.
That is a completely different kind of control. Superconductors already use it, and the next lesson puts it to work.
Your turn
If nothing pushed the electrons, what actually changed them? And how would you tell a real new effect from a wire you forgot about?
Nobody has closed this question yet. The bench test is small enough for a school lab. Maybe you will run it.
Try this at home
Two kinds of wave, one afternoon
You need: A long skipping rope or shoelace, a wooden table, and a friend
- 1.Lay the rope along the floor. Have your friend hold one end still.
- 2.Flick your end sharply left and right, and watch the wiggle run down the rope.
- 3.Now press your ear flat against the table and ask your friend to tap the far end.
Notice: The rope wave waves across the way it travels. That is how light works. The tap you hear travels by squeezing the wood along the way it travels. That is longitudinal, and it is how sound works. Both are real, and they behave completely differently.
For grown-ups and older readers
This lesson comes from Chapter 10: Longitudinal Electrodynamics and Scalar Waves and the map unit Scalar waves and the field behind the fields.