This week Google described its ambitious plan to use magnetic nanoparticles circulating through the blood to detect and report back on signs of cancer or an impending heart attack. Some nanotechnology experts, however, have responded by asking whether Google’s project is more science fiction than medical reality.

“It’s very exciting that a company with Google’s financial firepower is taking on this big challenge,” says Chad Mirkin, who directs the International Institute for Nanotechnology at Northwestern University. But he says that what Google has described is “an intent to do something, not a discovery or a pathway to get there.” At this point, he says, the technology is speculative: it’s basically “a good Star Trek episode.”

Google’s basic idea is nothing new—researchers have been developing magnetic nanoparticle diagnostics and treatments for years (see “Nanomedicine”). In the announcement, Andrew Conrad, head of the Life Sciences team at the Google X research lab, said “essentially the idea is simple.”

The concept might be simple, but executing it isn’t. Employing nanoparticles in the body is very difficult, and it’s unlikely that Google’s vision will be realized anytime soon.

The main problem facing the search giant will be biology. Google intends to produce a nanoparticle pill that you can swallow. From there the nanoparticles would somehow get into the bloodstream, something Mirkin says requires “a big leap of faith.” Once in the bloodstream, they’re supposed to circulate, find their way to targets such as cancer cells, and then be collected for measurements. A magnet held near superficial blood vessels on the wrist, for example, could concentrate the nanoparticles in one place. Google did not say how it would measure a signal from the nanoparticles.

Each of these steps is challenging. For one thing, the body’s natural defenses are designed to eliminate foreign objects, Mirkin says, so Google will need to find a way around that.

In addition to challenges in delivering the nanoparticles and reading a signal from them, another key question is whether the system will be safe, says MIT professor Robert Langer. Indeed, says John McDonald, a professor at Georgia Tech, “one of the big hurdles we had with magnetic nanoparticles was their toxicity.” McDonald says that “Although anything is possible, I think there may be more effective ways to detect cancer and other diseases at an early stage than the approach envisioned by Google.”