A Mechanical Pulse

Can we synthesize a beating heart directly from stem cells?  Did Dorothy’s friend, the Tin Man, have a heart?  Does blood flow through the veins of computer engineers?  Not yet, yes, and not until proven otherwise.

The term pulse has taken on a biological meaning in popular culture.  It is used to describe the gush of blood sent streaming from the heart throughout the body.  You can feel your pulse rate by pressing against your neck.   Fictional characters always check the wrist to see if someone is dead, but the neck seems like a more reliable indicator (if I were a doctor using the wrist to be fancy, I would probably pronounce some living people dead).

In science, a pulse takes on a more general meaning.  It is a single disturbance that propagates through a medium or material.  The source of the pulse may be a beating heart, but it can just as easily be a loud horn.  The loud horn creates a sound of a certain intensity, which travels in all directions through the medium of air.  Sound waves may appear to be mystical, but it is just a game of broken telephone between neighbouring air molecules; information, in this case, longitudinal vibrations on a molecular scale, is being transported. 

The propagation of sound in air is an example of a mechanical pulse, but simpler examples exist.  If a string is constrained at one end and held in tension, a mechanical pulse may be sent along it by imposing an external lateral impulse at the free end.  The resulting phenomenon is actually quite beautiful. 

The string wishes to return to equilibrium; it wishes the balance it had a moment ago to be restored.  So, the laterally displaced fraction of string revolts – it hurries back to where it came from.  In so doing, it excites the piece of string next to it laterally.

The chain reaction that manifests in the string is known as a mechanical wave, or pulse.  It is like a line of dominoes of lateral excitation.  The string is essentially communicating information of a disturbance across its length.  The pulse appears to be a single wave travelling horizontally along the string, though as we will soon see, it is incorrect to think of it as such. 

The speed with which a pulse appears to propagate along a string is determined by the amount of tension in it and its mass per unit length.  Waves travel faster in strings with greater tension and with a lower density.  Specifically, the wave speed in m/s, is given by v = (T/µ)^0.5, where T is the tension in Newtons, and µ is the linear density in kg/m.  As it turns out, the severity of the external excitation, or source, has no influence on how quickly the pulse travels.  The source determines the shape (amplitude) of the propagating disturbance, but has no say on how quickly it is conveyed.  It is kind of like when your computer freezes – the time the operating system takes to restore itself is independent of how loudly you yell at it. 

In reality, the string itself is not moving left to right or right to left.  The motion of a horizontal propagating wave is an illusion: in reality, the only motion occurring is in the vertical direction.  Locally, particles of string move up and down in sequence, giving the impression that horizontal motion is occurring.  It is like the wave you see at sporting events in stadiums.  Members of the audience are not actually moving from side to side, merely up and down in sequence.  The resulting phenomenon gives the impression that horizontal motion is occurring.  Imagine that each sport fan participating in the wave is a particle of string, and you begin to properly visualize what a mechanical pulse is.

Have you witnessed a mechanical pulse in other facets of life?  Sure you have.  Have you ever mowed the lawn with an electric cord?  In an effort to displace a distant section of the cord out of the path of the mower, you do not walk towards it and do it manually.  Instead, you give a sharp tug to your end of the cord, sending a mechanical pulse along its length.  You then give yourself a nod of approval as the cord adjusts its position to your will.

You may have also seen mechanical pulses in action when unplugging electronic equipment.  If you are a guy, you are probably lazy (you call it efficient).  Instead of unplugging something manually, you send a pulse along its length.  When the disturbance reaches the outlet, the plug snaps out of it, and you develop a smug grin on your face.  This action appeals to you for two reasons: (1) you did not need to walk a few feet to unplug the thing, and (2) you are doing something you were told not to do as a child (you rebel you).

Hopefully, this introduction to mechanical waves has been informative.  Perhaps the next time you see a green line representing a heart beat pulsate on Grey’s Anatomy, you will think about mechanical pulses travelling along a string.  You will then pick up the remote control and press a button.  This action will catalyze yet another pulse, and your television will turn off.  Your neurons will fire in an effort to stand up, and your muscles will contract. 

In life, information is transmitted by a variety of sources through a vast array of media.  However, the information is not always communicated via speech.  Whether it is a beating heart or an oscillating piston, biological and mechanical systems speak to us every day.  It is a scientist’s job to listen carefully to what they are saying, and through experimentation, research and development, manage to interpret their language.