There has been much debate about what is often called “free” energy—energy that can supposedly, with the right technology, be drawn straight out of the atmosphere, and in very abundant supply. The debates are about whether the stuff actually exists or not, what it would actually cost were it to be harnessed, and if it does exist is it truly as abundant and efficient as it's being made out to be by proponents of research and development into this potential alternative energy source.
When one hears the phrase “free energy device”, one might be hearing about one of several different concepts. This might mean a device for collecting and transmitting energy from some source that orthodox science does not recognize; a device which collects energy at absolutely no cost; or an example of the legendary perpetual motion machine. Needless to say, a perpetual motion machine—a machine which drives itself, forever, once turned on, therefore needing no energy input ever again and never running out of energy—is impossible. However, it is not so simple to say that a new technology for harnessing the energy “floating” in the atmosphere is impossible. New technologies replace old ones all the time with abilities that had just been “impossible”. Harnessing the power of the atom for providing huge amounts of energy was “impossible” until the 1940s. Flying human beings were an “impossible” thing until the turn of the 20th century and the Wright Brothers' flight.  
The biggest claim of the proponents of “free” energy is that enormous amounts of energy can be drawn from the Zero Point Field. This is a quantum mechanical state of matter for a defined system which is attained when the system is at the lowest possible energy state that it can be in. This is called the “ground state” of the system. Zero Point Energy (ZPE) is sometimes referred to as “residual” energy and it was first proposed to be usable as an alternative form of energy way back in 1913 by Otto Stern and Albert Einstein. It is also referred to as “vacuum energy” in studies of quantum mechanics, and it is supposed to represent the energy of totally empty space. This energy field within the vacuum has been likened to the froth at the base of a waterfall by one of the principal researchers into and proponents of Hal Puthof. Puthof also explains, the term 'zero-point' simply means that if the universe were cooled down to absolute zero where all thermal agitation effects would be frozen out, this energy would still remain. What is not as well known, however, even among practicing physicists, are all the implications that derive from this known aspect o quantum physics. However, there are a group of physicists—myself and colleagues at several research labs and universities—who are examining the details, we ask such questions as whether it might be possible to 'mine' this reservoir of energy for use as an alternative energy source, or whether this background energy field might be responsible for inertia and gravity. These questions are of interest because it is known that this energy can be manipulated, and therefore there is the possibility that the control of this energy, and possibly inertia and gravity, might yield to engineering solutions. Some progress has been made in a subcategory of this field (cavity quantum electrodynamics) with regard to controlling the emission rates of excited atoms and molecules, of interest in laser research and elsewhere.