Jonathan Corbett is a blogger who filed suit last week asking that the NYPD be banned from testing its new terahertz scanner, a bulky multimillion-dollar tripod-mounted device paid for by the Department of Defense, without "reasonable suspicion" or "probable cause."
His filing argues that "for thousands of years, humans have used clothing to protect their modesty and have quite reasonably held the expectation of privacy for anything inside of their clothing, since no human is able to see through them."
Police argue that the scanners would allow them to see concealed weapons, and indeed are an alternative to their controversial practice of "stop-and-frisk" searches. However, researchers have at times used terahertz (with varying success) to remotely detect and analyze substances such as explosives and narcotics, and the full capability of NYPD's new machine in the field has yet to be determined.
Despite the bulk and expense of the prototype, the market appears to be taking it seriously: Seeking Alpha reported that the US-based Advanced Photonix Inc. saw its stock soar 60% in a month as the trial was announced, though at present it appears some of that increase was short-lived. Terahertz scanners can be much smaller: in fact, researchers at Caltech and the University of Texas have designed CMOS terahertz chips to be used in cellphones, though these are designed for applications within four inches "for privacy reasons".
At some point the judicial system will need to decide whether terahertz is a new mode of vision that we have the right to use for its potential benefits, or whether it is an intrusive search and a violation of privacy.
Apart from the moral issues, there is the issue of safety. An MIT Technology Review article described "How Terahertz Waves Tear Apart DNA." This was based on a 2009 Rasmussen Group prediction that terahertz "tears apart" DNA lengthwise by separating the strands of the double helix in a short area.
However, this sort of DNA breathing does occur naturally, and a paper by Eric Swanson in 2011 disputed that terahertz could really do this to DNA anyway. But this month, Rasmussen and several collaborators followed up with a study finding that exposure to terahertz could cause consistent changes in gene expression by mouse mesenchymal stem cells - for instance, raising the amount of adiponectin RNA produced by 150 percent. This is based on the idea that the small separations of strands can open up specific regions of DNA, making them more active.
To put this in context, we should compare terahertz scanners with an alternative technology, the low dose X-ray scanners at airports which the TSA has recently withdrawn due to privacy concerns. X-rays are a form of light far beyond the ultraviolet, made up of high-energy photons that batter molecules apart (ionizing radiation).
Terahertz, by contrast, is at the other end of the spectrum, made up of low-energy photons intermediate between infrared light and microwaves. For this reason terahertz photons lack the energy to directly break DNA strands like X-rays, or even to cause chemical reactions as when pyrimidine dimers are formed by ultraviolet rays. What terahertz and X-rays share is that they are outside the range in which the sun radiates most of its energy. The natural environment is relatively dark in these frequencies, so our eyes have not evolved to detect them. Clothing is relatively transparent to them, but DNA, which under visible light is colorless in solution, is more likely to absorb these frequencies.
For the past 20 years biologists have tinkered with microwaves based on their ability to alter DNA's properties to make it easier to perform various analyses on samples, and still report new and remarkable results now and then. However, the effects reported for microwave treatment typically overlap substantially with the effects of ordinary heating from the microwaves absorbed, whereas the Scientific Reports article found no overall heat shock response in the cells exposed to terahertz, which remained at a controlled temperature.
But the paper's most provocative claim is that terahertz can affect genes very specifically, perhaps based on the frequency used. There are several papers published suggesting that the sequence of DNA affects its ability to absorb terahertz. In the most advanced application to date, published, as is becoming common nowadays, by a group in the People's Republic of China, researchers were able to distinguish bulk extracted DNA from two species of nematode, Bursaphelenchus xylophilus (a pine tree pest) and Bursaphelenchus mucronatus, by examining terahertz absorption. The study examined the entire terahertz frequency range using terahertz time domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy (FTIR), identifying a few specific frequencies at which the organisms differed
Though the group is nowhere even remotely near being able to take genetic fingerprints with their terahertz scans, results like this raise the question of whether specific organisms — and specific gene sequences — absorb particular frequencies of terahertz more intensely. If this is true, it is possible that a terahertz laser might activate different sets of genes depending on the exact frequency to which it is tuned.
As we begin to consider regulations for terahertz, the usefulness of scans on a broad range of frequencies will need to be considered. So must the potential for unique, as-yet undiscovered harmful effects of specific frequencies. The terahertz power used on the mouse mesenchymal stem cells was substantial — with the pulse laser, up to 30 megawatts, but in pulses of only 35 femtoseconds. With very high-power emissions, it may even be worth considering whether careful selection of particular frequencies could create a new kind of genetic weapon.
There are many decisions to be made in this area in coming years with far-reaching effects on our lives, and we will need to pay close attention to this new electromagnetic frontier if we are to have a voice in them.