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Flying circus of physics

Using your head as a remote car opener

Monday, February 01, 2016


Video series

The seventh video in my Flying Circus of Physics video series with Cleveland State University has now been posted:

Using your head as a remote car opener 
Jearl Walker
February 2016  This effect has been rumored for years but these days we have some video evidence. A car fob (remote opening device) has a maximum range. If you are even farther from the car than that maximum, the radio frequency signal that you send toward the car is too weak to trigger the detector, and so the car does not lock or unlock. However, strangely enough, if you hold the fob next to your body (the head is usually used), you increase the maximum range of the fob.

Here is one example, from the BBC show Top Gear: Top Gear

We can easily say that the presence of the body enhances or amplifies the signal (that amounts to simply saying the maximum range is increased), but what exactly happens? Here is one explanation:

According to the explanation, your open mouth somehow sets up a resonance that builds up the signals, such as resonance can build up in a woodwind instrument. I don’t think so. The fob puts out a radio signal at a wavelength of about a meter, much longer than the dimensions of your mouth. Besides, unless you are a Cyberman from Doctor Who, your mouth is formed from biological materials, not metal from which radio signals will bounce around.

Here is another explanation:

This explanation is much better. The radio signal causes oscillations of the charged particles in the head or the water in the container, and those oscillations send out additional radio signal. But how?

Here is my explanation, and below I’ll tell you how you can check it. The fob signal is emitted in a wide range of angles because when you search for your car in a parking lot, trying to aim the signal directly at the car’s detector would be difficult. The signal is not a laser beam. So, only some of the signal is actually intercepted by the detector and the rest is wasted.

When you are very close to the detector, the detector occupies a fairly wide angle and intercepts much of the signal. As you walk away from the detector, it occupies an increasingly smaller angle and the signal is more spread out and thus weaker. Eventually, the intercepted signal is too weak to trigger the detector.

If you then place the fob near your head (or any other portion of your body), your head intercepts some of wasted signal. That signal is an electromagnetic wave (a radio wave but otherwise like visible light). The oscillating electric fields of that wave cause any free or semi-free charged particle in its path to oscillate back and forth (electric forces from the electric fields act on the charged particles). When charged particles oscillate, they radiate electromagnetic waves. That is how a simple AM radio antenna works. Electrons in the antenna are forced to oscillate up and down the antenna, and they radiate the radio waves off to the sides.

You have lots of charged particles in your head (and elsewhere). The wasted fob signal causes some of them to oscillate at the frequency of the wave, and so they emit radio waves off to the sides at that frequency. Some of that fresh radiation happens to be toward the detector, and thus the detector is receiving added signal in addition to the signal coming directly from the fob. That can be enough to trigger the detector.

Oh, but don’t worry about the radio waves. Unless you live in an extremely remote region, radio waves go through you all the time from all the communication systems (cell phone, television, radio). And when I say that the particles oscillate, their motion is on the atomic scale, not something that would make you tingle.

Here is how you can test my explanation. Find the maximum range of a fob when held at arm’s length, and then move somewhat beyond it. (That may be tricky if your car is like mine and refuses to acknowledge any signal when the fob is pressed several times in a short time.) Hold the fob next to your chest, neck, head, or even thigh. Can you trigger the detector? If so, then hold a container of water away from your body with the fob next to it, as we see in the video. However, don’t use tap water as that demonstrator did. Use distilled water a pharmacy or food store. Distilled water does not contain the salts and other impurities found in tap water and thus has no charged particles. So, in my explanation, the fob should not be able to trigger the detector when you use distilled water.

Next, gradually add table salt (NaCl) to the container. The molecules split into positively charged sodium ions and negatively charged chloride ions. I believe that when enough salt has been added, the fob will trigger the detector.


Are you looking for the stories from the last several months? They are in the archives:

Human slingshot (intoxicated SHM), 1.276
Marshawn Lynch running through defenders, 1.275
Pen ink is non-Newtonian, otherwise your shirt would be soaked, 2.58
Pub trick --- fluorescence of a gin and tonic, 5.19

Football players downed by lightning (really, upward streamers), 5.2
Black egg becomes glimmering, 6.169
Spiral arms of water, 2.200 
Pub trick -- pouring a black and tan, 2.187

Pointy ice drops, 4.20
Japanese samurai horo: silk cloth versus arrows, 1.130
Juggling hammers, hitting a nail on the head, 1.25
Pub trick --- optically reversing arrows, 6.168

Sloshing, 2.199
Coffee heated in hot sand, 4.115
Pinhole sites and selfies, 7.33
Pub trick -- balancing pins on glass rim, 1.239

Loop-the-loop with cars, bikes, skateboards, and simply running, 1.270
Pub  trick --- spoon into mug, 1.271
Transparent when wet, 6.86
Hula-hoop, 1.84

Lethal upward streamers in an electric storm, 5.1
Floaters in your eye, 7.5
Pub trick --- musical wineglass, 3.43
Standing and walking in a strong wind, 1.227

Pub trick --- beer bottle tapping prank, 2.76
Bull riding, 1.92
Mianus Bridge collapse, 1.71
Sliding stick across outstretched fingers, 1.172

Café wall illusion, 7.58
Tire explosions, 4.114
Pub trick --- using thermal stress to open a wine bottle, 4.113
Giant Leg towers, 1.272

Pouring tea in an airplane during a barrel roll  2.197
Pub trick --- water from nowhere  6.166
Survival strategies of emperor penguin huddles   1.197
Racing over a rail crossing  7.59

V1 flying bombs tipped by Spitfire airplanes  1.273
Smallest electric train  5.61
Whispering bench in New York City's Central park  3.63
Pub trick --- separating salt and pepper  5.60

Tesla coils  5.62
Royle's self-pouring teapot  2.198
Self-righting of overturned turtles and the toy known as gomboc  1.274
Pub trick --- the self-righting bottle  1.106

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