The Physics of

The Show
      Doctor Who is a long-running British show about the Doctor, a Time Lord able to travel through both time and space. Using his ship disguised as a police box, the Doctor and his everchanging companion traverse the universe and, more often than not, saving whatever world they've landed on.
     On air since 1963, the show is now on its Eleventh Doctor (Time Lords regenerate rather than die), and the show is as popular as ever. Our project aims to investigate several elements of the show as they relate to physics. As with many sci-fi shows, Doctor Who relies on its viewers to accept its science as fact. Within this site, we will examine weapons used on the show; the Doctor's favorite tool, the sonic screwdriver; and even his ship, the TARDIS (Time and Relative Dimension in Space).

The Physics
On the Show...
     The TARDIS (Time And Relative Dimensions In Space), is the Doctor's vehicle (for lack of a better word). It is disguised as a 1950's London Police Box and it is bigger on the inside than it is on the outside. Much, much bigger on the inside. This is the case because the TARDIS is "dimensionally transcendental", which the doctor explains as being similar to a cube that is really far away, but large, which is able to hide behind a cube that is close but smaller. Except that in the case of the TARDIS the cubes are brought together and separated instantly and constantly.
     For another view of the outside of the TARDIS, click here. A fraction of the inside is pictured at the right. Even the image of the inside is deceptive; it shows only the control room. The TARDIS is said to contain at least one library, a large wardrobe, and even a swimming pool, amongst other rooms.
     Most of the time the way that the TARDIS travels is to disappear and then reappear or "dematerialize" then "re-materialize", however occasionally it can fly. To fly it maintains its same size and shape and just launches into the air.
     One of the things that changes often is the TARDIS' locking mechanism. At some points it uses a simple lock and key, at other times it responds to the doctor snapping his fingers, and in recent seasons it has had remote locking capabilities not unlike modern car locks. You can see the physical key's evolution here.
In Reality...
     Though physically impossible, the "dimensionally transcendental" nature of the TARDIS is its best known attribute. Physics can disprove the ability of the aformentioned two cube theory by stating the fact that nothing moves faster than the speed of light, which would make it impossible for the necessary movement to take place without it being seen. Another way to consider it is to view physical space as a conserved quantity. Space (as in physical space), does seem to follow rules similar to that of conserved quantities such as momentum or energy. If we consider describing physical space in the physics term of a conserved quantity, the extreme size difference bewtween the interior and exterior of the TARDIS is shown to be impossible. If the exterior takes up approximately 250 cubic feet (estimate) of the limited amount of space within the hypothetical "system", than the inside can only take up 250 cubic feet, unless it takes space from somewhere else, which in turn would show on the outside.
     The TARDIS' travel is at best unrealistic. There is no explaination for how an object with living people can dematerialize and then re-materialize. There are many things that can be transmitted from one location to another without being seen, some examples include; radio broadcasts, television shows and heat. The first two are examples of electromagnetic waves, while heat travels by conduction, convection or radiation. However, none of these start out as solid matter and then become electromagnetic waves, only to re-materialize at another point. A flying TARDIS is much easier to believe in, as we have many vehicles that can fly. The TARDIS' shape, however un-aerodynamic, would not prevent it from taking flight... if it had some method of propulsion. We can look to the properties of conserved momentum to disprove that the TARDIS could fly in the way that it does on the show. A rocket (or a TARDIS) cannot take-off without expelling something (gas, an object, air) in the oposite direction that it wishes to go. This is because of conserved momentum, which is a quantity that must stay the same within a defined system. In the case of a rocket, the heated fuel projects "down" (negative amount) while the rocket wants to go "up" (positive amount). The amounts of the two forces must cancel each other out. The same is true of steering, the rockets that we have in the real world have miniature rockets on their sides to create torque to steer the rocket while is travels. The TARDIS has none, there are no steering mechanisms to speak of on the TARDIS.
     The locking mechanism on the TARDIS is perhaps the system that is most based in reality. The simple lock and key has no obvious flaws, and the remote-entry key sounds very similar to the remote-entry fobs that come with many cars now-a-days. How does a remote-entry key work? The basic set up, (which is explained in great detail and with excellent pictures here) is that the fob that is attached to your key chain is basically a really small radio station, and your car lock is a really tiny radio receiver. The receiver is set to only respond to a very specific frequency, specifically the same one that is being transmitted. For security purposes the transmitter and the reciever are made so that they cycle through a bunch of different frequencies at the same rate, so they always stay in synch, but so that no one can capture the frequency and use it to break into your car. This is completely feasible in the TARDIS! The snapping activated lock is also not out of the question, but the security would be much weaker. If the lock on the TARDIS was "snap" activated it would be sound activated, and basically anyone who could snap at the same frequency as the doctor could gain access to the TARDIS. This seems a highly unlikely overthought and it is much more likely that the snapping was only a part of the whole procedure of unlocking the TARDIS, the other parts may have been unseen

Weapons from All Around the Galaxy
Electromagnetism Pulse as a Weapon
On the Show...

     Electromagnetic pulses notably appear as weapons in two Doctor Who episodes. In the episode "Voyage of the Damned," the EMP takes the form of a cylindrical device taken from the cyborg body of a character named Bannakaffalatta. The EMP unit proves to be the only effective weapon against the Host-robots who look like angels and are programmed to kill all remaining survivors of the crash of the luxury interstellar cruiser The Titanic. The device produces an electromagnetic pulse which neutralizes the robots. One downside of the EMP unit in the episode is that it has to be periodically recharged.
     The episode "The Age of Steel" also contains an electromagnetic device built as a grenade. It neutralizes a Cyberman. Cybermen are a race of cyborgs who used to be humanoid but began to implant and increasing amount of artificial parts in their bodies for self-protection. Over time, the race became cold and calculating-more machine than man. Cybermen are central enemies to the Doctor throughout the series. In this episode, the EMP has the unintentional effect of disabling the Cyberman's emotional inhibitor, allowing it to remember how it was prior to conversion into an artificial creature. This leads the Doctor to deactivate the robot body and then kill him with the sonic screwdriver.
In Reality...
     Electromagnetic pulse weapons are intended to overload electronic circuitry by producing an intense electromagnetic field. They are based on the principle that electric current creates a magnetic field and that changing magnetic fields can in turn produce an electric current. However, with an electromagnetic pulse it is on a much larger and intense scale. It is either a high-intensity burst of electromagnetic energy over a short period of time or a deluge of electromagnetic radiation resulting from an explosion or a suddenly shifting magnetic field. The result is electric and magnetic fields that can damage other electrical systems through voltage and current surges.
     In terms of a national security threat, the military was originally most concerned with the possibility of a nuclear EMP which would cripple the U.S. by wiping out all or most of electronic devices. It has been well known and documented as early as the 1940s and 1950s that nuclear explosions create an EMP. The electrical disturbances created by a nuclear blast are caused by the Compton effect. The Compton effect is that photons of electromagnetic energy (in this case intense gamma radiation) knock loose electrons from atoms with low atomic numbers (in this case the oxygen and nitrogen atoms in the atmosphere). The flood of electrons interact with the Earth's magnetic field, creating a fluctuating electric current, which rapidly induces a powerful electromagnetic field to emerge. The EMP leads to strong electric currents in conductive materials over a large area.
     For electronic equipment to be affected, the weapon has to be above the visual horizon. Generally, the higher the amplitude at which the weapon is detonated, the greater the area it will affect. EMPs are not directly lethal to humans, but can bring a country to a standstill by disabling all electronic devices at once. Low-level EMPs would temporarily mess up electronic systems, more intense ones would cause computer data to be lost, and very powerful ones would permanently destroy electronic equipment. Non-nuclear EMPs can be produced, but they do not have near the same power as those produced by a nuclear explosion. Finally, in an ironic twist, older devices using vacumn-tube technology would stand a better chance of surviving an EMP attach than newer technology. Newer technology often centers around consumer-grade semiconductor devices, which is more sensitive topower surges.
     Electromagnetic pulses can definitely be created and weaponized. However, the depiction on the show is not entirely plausible. An EMP would immediately disable any electronic device, theoretically including evil robots as well. However, it would also disable any nearby electronic including any lights, etc. Additionally, the show is extremely fuzzy about how the pulse is produced and how it would be aimed at only one object. Overall, an electromagnetic pulse could be plausibly be used to effectively stop a murderous robot, but only by wiping out all the other electronics in the vicinity.
     You can see a video of the effects of electromagetic pulses here.

Delta Waves... Killer Brain Waves?
On the Show...

     In the episode "The Parting of the Ways," the Doctor constructs a delta wave generator to combat the Daleks. The Daleks are a pitiless robot race set on destruction. The Daleks are the main enemies of the Doctor. Unfortunately, the delta wave generator will not only defeat the robots, but fry the brains of any living being, including humans on nearby Earth. The Docto believes that death would be better than enslavement by the Daleks. However, the Doctor cannot bring himself to use the delta wave weapon and luckily finds other means of defeating the Daleks in the episode.

In Reality...
     Brain waves are generated by neurons in the brain. Neurons are cells that transmit information through electrochemical signaling. For more details about this process, presented simplistically, see here. It is possible to observe these electrical changes in the brain with an electroencephalogram (EEG). Brain waves are measured by frequency, in Hertz, and the lower the number of Hertz the less amount of brain activity that is going on. However, it is important to remember that we never just produce one specific brainwave at a time, but several different ones at the same time in different frequencies and strengths.
     Delta waves are high amplitude brain waves with a frequency of 1-4 Hz. (lowest brain wave frequency). The waves occur during state N3 slow-wave sleep and comprise 20% or more of the EEG record during this sleep state. EEGs of newborns show that delta waves predominate during their waking states and continue to appear in the waking states of five-year-olds. Healthy adults do not experience a lot of delta wave activity during waking states. Studies have shown that increased delta wave activity in adults in a state of intoxication or delirium or who have schizophrenia or dementia. Delta wave activity during slow-wave sleep declines during adolescence, with a drop of around 25% reported between the ages of 11 and 14 years of age.
     The idea that delta waves can be harnessed and used to fry the brains of living beings is purely in the realm of science fiction. Nor does it seem like it will ever be possible in the future. Delta waves only have negative implications when they are abnormally present, due to their link in the waking states of adults and mental illness.

 Sonic Screwdriver
On the Show...

     The Doctor's favourite tool is his sonic screwdriver. Despite its name, it looks nothing like the screwdriver you and I might use. When activated by the button on the side, the sonic screwdriver buzzes and emits sonic waves and the blue diode on the end glows. Does the sonic screwdriver live up to its name?
     The word "sonic" refers to anything having to do with sound and acoustics. We learned that sound travels in waves. The sonic screwdriver uses these sonic waves to perform many tasks, typically opening door locks and incapacitating electronic devices. We know that a sound wave of the right frequency can shatter a wineglass, but can it do all that Doctor Who says it can?

In Reality...
     In theory, yes. Just as a wineglass can shatter and the Tacoma Narrows Bridge collapsed at the right frequency, other objects can be affected by sound waves. It is not unreasonable to think that at the right frequency, a sonic wave can shift the gears of a lock to unlock it.
     In one episode, the Doctor's companion Rose uses the sonic screwdriver to fuse to broken ends of barbed wire back together. Can sound waves produces enough heat to melt metal? Again, in theory, yes. Scientists are now investigating why the Perseus galaxy cluster just can't seem to cool down. They believe it's due to extremely low frequency sound waves rippling through the cluster. They've yet to figure whether the waves' energy is producing the heat or somehow preventing the cluster from cooling. If they prove the former, then they make plausible the repair of the barbed wire by the sonic screwdriver.
     When held together, two sonic screwdrivers produce a terrible noise. In reality, when two sound waves collide, it is called wave interference. The peaks of one wave cancel out the troughs of the other, and two peaks or troughs will enhance each other. However, the resulting wave would not produce an earsplitting noise.

     On the show, the sonic screwdriver is a panacea to many of the obstacles in the Doctor's way. In reality, a sonic device could produce only a fraction of the results as it does on the show.


How Things Work textbook

On Electromagnetic Pulse:
Harris, Tom. "How E-Bombs Work." HowStuffWorks, A Discovery Company. Web. 20 April 2010. <>
"Electromagnetic Pulse." Wikipedia. Web. 20 April 2010. <>

On Delta Waves:
Shue, Dr. Karen. "Brain and Health: The Basics of Brain Waves." Web. 20 April 2010. <>
"Delta Wave." Wikipedia. 27 March 2010. Web. 20 April 2010. <>

On the Sonic Screwdriver

All photos are a copy right of BBC.

About Us

Marit Nelson - marit[@]
     - Marit is an elementary education major at UNC and she can't wait for the summer and swimming in the quarry!

Ann Marie Bartholomew - abart[@]
     - Ann Marie is currently a senior at UNC Chapel Hill and will be attending UNC next year to get her masters in speech pathology. She likes to bake, hang out with her friends, and play with her kitten Nibbler.

Alexandra Smith - prefaced[@]
     - Alex is an elementary education major at UNC who hopes to one day teach English in China.

This site was created for How Things Work
The course is Physics 100 and under the instruction of Dr. Duane Deardorff
University of North Carolina at Chapel Hill
Completed April 20th, 2010