The work of historians sure would be a lot easier with a time machine. Imagine how much we could learn about the past if we could just visit.
Unfortunately, we don't (yet) have the technology needed for time travel. But here are two real-world uses of wavelength-measuring sensors that can help us to get a better look at what happened in the distant past.
Much of what we know about ancient civilizations comes from the work of archeologists. Picture in your mind an archeological dig site. If you are anything like me, you probably envisioned something like a scene from an Indiana Jones movie: crowds of busy people scurrying around in a desert or jungle, digging holes, carrying away large amounts of dirt, and carefully cleaning and categorizing artifacts.
While you often have to dig to find ancient objects, it can be both destructive and impractical to disturb ground without knowing what's beneath it. The Society of American Archaeologists, in fact, calls archaeology a destructive science because excavating historical sites destroys them—meaning that they can never be recovered and must be approached with utmost care.
How, then, do you know where to start?
GPR in action at an archaeological site. Image courtesy of Archaeo-Physics LLC.
Enter ground-penetrating radar (GPR). Unlike a metal detector that is limited to metal artifacts, GPR can detect a variety of different materials and targets buried in the ground.
It works by emitting radio waves, usually somewhere between 16 and 2600 MHz depending on the composition of the ground. High-precision sensors then pick up the returning waves as they bounce off different objects and materials underground. GPR has even been used to find where holes have been dug and refilled by mapping the disturbances in the soil.
Image courtesy of Global GPR
These capabilities make it well-suited for use by archeologists, allowing them to look for potentially fruitful digsites in an easy, fast, non-invasive, and non-destructive way. GPR was notably used to explore the ruins of the Mayan city Holtun in Guatemala, despite the entire city being buried under several feet of earth and vegetation.
MIT's Terahertz Book Scanner
In the absence of a time machine, finding books or other writings from ancient times can be a huge help in understanding how people thought and what they did. Unfortunately, however, time often renders these books too fragile to open and study. What do you do then?
A group of researchers at MIT may have an answer. They've recently used a special device to read the ink on a piece of paper. This may not sound impressive in and of itself—unless you understand that, at the time, the paper is hidden below eight other pieces of paper also with writing on them.
Image courtesy of Nature Communications
So how does it work?
It's actually quite similar to GPR. The scanner is pointed at the book or stack of papers, which are placed at a very carefully measured distance.
The scanner emits a single bipolar pulse (that is, a single wavelength) then waits for the "echo". As the pulse travels through the various layers and pages of the book, it is partially reflected back by each one, but each reflection takes slightly longer to return. This means that the scanner will pick up a series of pulses of varying strength, depending on how well that particular layer reflects the pulse.
Layers with ink on them reflect differently than layers without, and this is recorded by the scanner. Before doing a full scan, researchers must first identify which frequency the ink is "visible" at for the scanner.
The book is then moved slightly and the process is repeated. This continues, scanning and moving, scanning and moving. Each of the reflected signals can be put together into a composite map with each pulse representing a layer. The time a pulse took to return indicates how deep in the book the layer is and the strength of the pulse gives some clues as to what the layer may have been made of.
One of the challenges of this technique is that the sensor picks up "shadows" of the ink on the surrounding pages. So work around this, the team at MIT developed an algorithm that allows the scanner to identify the shape of the letters on the page.
A visualization of the overlap of ink between pages. Image courtesy of MIT Media Lab.
Granted, most books have more than eight pages. But this is a huge step towards being able to study and preserve very old and valuable books without risk of damaging them.
Maybe someday we'll invent a time machine—or maybe not. But, either way, you can be sure that we will find more innovative and powerful ways to get a look at the past.