Force fields are real

Force fields made frequent appearances in the games that I played and the sci fi movies I watched as a kid (OK, continue to watch).  These fields usually protect a ship or soldier from the various forms of harm raining down upon them.  For example, in the video game “Scorched Earth” – an old favorite of mine – players competed as tanks, lobbing various munitions at one another.  Here you can see 6 tanks with force field shields around them.

A screenshot from the DOS classic Scorched Earth.

At some point growing up, force fields went the way of warp speed and the lightsaber – as far as I could tell, they existed only in the realm of science fiction.  So I distinctly remember sitting in my undergraduate E&M (Electricity and Magnetism) class and realizing “whoa, force fields are real.”  Now, as a research scientist, one of my primary focuses is to measure and even “see” force fields.  

So what do I mean that force fields are real?  What is a field even?  It’s admittedly tricky to describe what a field is.  It’s not really made of any thing.  It’s easier to instead describe what a field does.  In that sense, a field is simply the mechanism by which two or more distant objects interact.  They don’t touch.  But they can exert forces on each other through invisible fields.  It's easier to understand through examples.

You’re probably pretty comfortable with the idea of force fields, maybe without even knowing it.  Gravity pulls an apple to the ground.  This is because the apple experiences Earth’s unseen gravitational force field.  That same force field keeps the moon in orbit around the Earth.  

What other force fields exist?  Well, nearly everyone knows the expression “opposites attract.”  Although often referring to romance and dating, here I have in mind the electric fields created by charged objects.   A positive charge is attracted to a negative charge.  Conversely, two negative charges repel one another.  Like the Earth and the apple, there is no “stuff” connecting the charges.  Instead, a charge produces a field that extends out into space away from it.  This communicates to everyone nearby: “hey, I’m negative.  If you’re positive, come hang out.  If you’re negative, stay away!”

LEFT: A negative charge creates an electric field that points toward it.  This field gets weaker as you move farther from the negative charge. CENTER: The electric field produced by the negative charge exerts an attractive force on positive charges.  RIGHT: Other negative charges are repelled.

We rarely see the impact of electric force fields because most day-to-day objects are uncharged.  You and I are uncharged.  Sure, we’re made up of negative electrons and positive protons – but because there are an equal number of them, we have no net charge.  Your desk has no charge.  The air you breathe… you got it, neutral.  None of these objects can be pushed or pulled by electric fields.  However, there is a simple experiment that you can do to experience electric force fields: rub a balloon on your head.  Doing so transfers charge from your head to the balloon.  The balloon has now gained some charge – let’s say negative charges.  It grabbed these charges from your hair.  Because your hair has lost those negative charges, it is now positively charged.  Negative balloon.  Positive hair.  What happens? 

Now repeat with two balloons, rubbing both on your hair.  Both balloons will become negatively charged.  If you then gently rest one balloon in each hand and slowly bring your hands together… the balloons begin to interact, repelling one another.  They are experiencing one another’s force fields!

Similar behavior is found with magnets.  We’ve all played with refrigerator magnets: holding one in each hand and bringing them together slowly, we find one of two outcomes.  When they get close enough, they might snap together.  Or you might find that they want to wiggle away from one another.  Either way, they are interacting from afar.  We can’t see the force fields they are producing, but we can “feel” their effects as we bring the magnets together. 

Just like the charges, we can use the phrase “opposites attract” as the rule describing the behavior of magnets.  Unlike charges, which can be positive or negative, a magnet always has a pair of “charges” at its ends. We typically call these the magnetic poles: the north pole and the south pole.  The north pole of one magnet attracts the south pole of another.

In addition to being able to hold photos to your fridge, magnets are useful for navigation – at least they used to be.  Before GPS, the low-tech solution for wandering the woods or sailing the high seas was the compass needle.  Itself a magnet, a compass needle has both a north and a south pole.  And because Earth has a magnetic field (it too is just a giant magnet!), the magnetic compass needle feels a force that causes it to “point north."  A bit of trivia: since the north pole of your compass needle points to the Earth’s geographic Northern-most point, that point is actually a magnetic south pole.

Earth has its own magnetic field, which exerts a force on compass needles.  The north (red) pole of the compass needle is attracted by Earth's south magnetic pole, which actually is located at the geographic north pole.  Confusing, I know.

This cartoon starts to give you a sense of what to envision when you think of Earth’s magnetic field.  It extends from the Earth’s core out into space (not just outer space, but everywhere in space – where you’re sitting, and where the satellites orbit).  Just like the electric field of the negative charge shouted "hey, I'm negative," the Earth's magnetic field communicates "I'm magnetic" and exerts forces on other magnetic objects.  The large swooping arcs that connect the magnetic north pole to the south pole show what the magnetic field “looks like.”  We can’t see it of course.  But with another magnet – like a compass – we can detect it.  Earth has a force field!  It’s very real, and very measurable!

A simpler and more dramatic way to “see” the force field from a magnet is to sprinkle iron filings onto a tabletop, and then set the magnet down atop the filings.  The iron filings act like many small compass needles (iron is magnetic), each aligning itself to the line of magnetic force at that point in space.

Any old magnet produces a magnetic field similar to Earth's.  You can see this using either a compass needle, or by sprinkling iron filings around the magnet.

Permanent magnets aren’t the only way to produce magnetic fields.  Remember the coil of wire I was working on in “Seriously though, everything is a voltage”?  That coil is used to produce magnetic field.  Here’s how: when a current runs through a wire, a magnetic field appears, circulating around the wire.

You could test this by holding a compass nearby.  Depending where the compass is placed around the wire, the needle will point in a different direction.  Now imagine wrapping that wire around on itself to form a circular loop.  The magnetic field will continue to circulate around the wire.  But in the middle of the loop, all of those circulating fields point in the same direction – straight out of the loop.

A loop or coil of wire with an electric current running through it produces a magnetic field that points straight out of the loop.  This is how an MRI scanner produces a magnetic field.

An MRI scanner generates a magnetic field in just this way.  Your body is inserted into a huge coil of wire.  The current is flipped on, and now you find yourself sitting in a very strong magnetic field.  That’s the “M” in MRI – Magnetic Resonance Imaging.

So there you are sitting in a strong magnetic field, why don’t you feel any forces pushing or pulling on your body, trying to reorient you like a compass needle?  Well, just as humans aren’t electrically charged, we also aren’t very magnetic.  (We are a little bit magnetic, otherwise MRI wouldn’t work… let’s get into that another time). 

This all goes to show why we haven’t seen the force fields of science fiction.  While we certainly know how to produce force fields (charges, magnets, and current-carrying wires), the objects on which these fields exert forces are limited.  Magnetic fields exert forces only on magnetic objects (in other words, don’t carry a heavy wrench in your pocket during your MRI scan).  Electric fields can exert forces on electrically charged objects.  But since humans are neither magnetic nor charged, you can’t use E&M fields to put up a shield that keeps other people out.  The same is true for incoming enemy fire – a magnetic or electric force field won’t protect you.

But that doesn’t mean force fields aren’t very real, and very useful.  We use magnetic fields all of the time.  Any time you listen to music, watch TV, or listen to a call on your cell phone, magnetic fields push and pull on speaker cones to generate sound.  Electric motors use magnetic force fields to cause rotation: whether to spin the blades of your smoothie blender, roll up a car window, or swing the windshield wiper blades.  Earth’s magnetic field exerts forces on incoming electrons and protons – an effect which leads to the northern lights.  And remember how “Everything is a voltage”?  We learned that electrons can be pushed from high voltage to ground (like a ball rolling downhill).  That difference between high voltage and ground – that’s just an electric force field, pushing on the electrons.  Every time we use electricity, we are using electric force fields.  So much for science fiction.  Force fields are very much real.