“In comparison with the distances light travels, all distances in the dimensions of space, even those involving airplane travel, are so very small that we essentially move only along the time axis, and we age continually. Only if we are able to move away from our frame of reference very quickly, like the traveling twin . . . would the elapsed time shrink to near zero, as it approached the speed of light. Light itself . . . covers its entire distance through space-time only in the three dimensions of space . . . nothing remains for the additional dimension . . . the dimension of time . . . because light particles do not move in time, but with time, it can be said that they do not age. For them “now” means the same thing as “forever.” They always “live” in the moment. Since for all practical purposes we do not move in the dimensions of space, but are at rest in space, we move only along the time axis. This is precisely the reason we feel the passage of time. Time virtually attaches to us.”
—Jürgen Neffe, Einstein: A Biography (1956)
Why is it called TARDIS? Sure, you probably know that the name TARDIS was invented by Susan (birth name Arkytior), who was an original companion and granddaughter of the First Doctor. And you no doubt know that TARDIS stands for Time And Relative Dimension In Space. Let’s put to one side for a moment all this stuff about relative space and time dimensions. After all, TARDISes are meant to move through time and space by disappearing here and reappearing there, which they do using a component called the “dematerialization circuit.”
What’s of more interest to us in this chapter is how time ticks on the TARDIS. You may recall earlier on page 76 when talking about the Blinovitch Limitation Effect, we mentioned that one of Albert Einstein’s great realizations was that time is not absolute. We also spoke of the famous adage of popular relativity theory, moving clocks run slow.
How does this help us answer the question as to how time ticks on the TARDIS? It’s associated with Einstein’s teaching that any measurement of time uses the idea of simultaneous events. All our judgments in which time plays a part are always reckonings of simultaneous events. For example, if the Doctor says, “The TARDIS arrived here at ten o’clock,” he or she means, approximately speaking, “The small hand on my analogue watch pointing at ten and the arrival of the TARDIS were simultaneous events.” In short, Einstein argued that simultaneous events in one frame of reference would not necessarily be simultaneous when viewed from another frame. Einstein called this “the relativity of simultaneity.”
Let’s do a thought experiment. Einstein was famous for them, so it seems fitting. Warning: this will be a weird thought experiment. After all, this is Doctor Who. In our experiment, the TARDIS is seated exactly in the middle of a passenger railway car with glass sides. (I told you it would be weird.) The TARDIS and the railway car are the moving frame of reference and we are seated on a railway station platform, which is the stationary frame of reference. The train that carries the TARDIS and the railway car is passing through the station on which we’re seated.
Now imagine the TARDIS sends out a pulse of light in the forward direction of the glass-sided car, and at the same time sends out a pulse of light in the backward direction. Let’s also imagine that the doors at each end of the railway car open automatically when the light pulses arrive. To the Doctor in the TARDIS, both doors of the railway car will open at the same time. That is, simultaneously. This makes sense. The TARDIS is seated equidistant from each door. And as the time taken to reach each door is given by distance divided by speed, where the distances are equal and the speed is the speed of light, the time of each door opening will be the same.
But to us, seated on that railway station platform, the back door will open before the front door. That is, not simultaneously. Don’t believe me? Think about it. For us, stationary as we are, we see the back door move forward to meet the light pulse from the TARDIS. And we see the front door move away from the light pulse. This also makes sense. As the front door moves away from the light pulse, the pulse now has to go a greater distance on its journey than the backward light pulse does. That’s because the back door is moving toward its light pulse, making a shorter distance for its journey. And, as the distances are not the same, the journey times to front and back doors will be different. In short, the doors will not open simultaneously.
But which observer is correct, I hear you scream? Do the doors open simultaneously, or not? At the risk of infuriating the reader even further, I am duty bound to say that both observers are correct. This “simple” example proves Einstein’s point. Simultaneous events in one frame of reference (the Doctor in the TARDIS on the train) would not necessarily be simultaneous when viewed from another frame (we observers seated on the platform). You may need some time, and perhaps a stiff drink, to get used to this.
Hopefully, you are beginning to see that space and time measurements are relative. They depend on the motion of the observer. But, once more, how does this help us answer the question as to how time ticks on the TARDIS? It’s in the understanding that the “clocks” being used, both on the train and TARDIS and on the railway platform, are both behaving normally. If the moving TARDIS clock changed when the Doctor was moving, then he or she could tell they were moving by noticing the clock change and that’s simply too weird.
No, the answer is all in the fact that the stationary observer, us, sees something completely different when they look at the moving clock. As the speed of light must be the same for all observers, we stationary observers must hear more time elapse between ticks of the moving TARDIS clock than on our stationary clock. In other words, moving clocks tick slower than stationary ones, stretching out the interval between ticks and effectively meaning that each moving second is longer than a stationary second. And that not only explains how time ticks on the TARDIS, but also helps explain time problems such as the Twin Paradox.