**by James F. Woodward**

*I first wrote about James Woodward’s work in my 2004 book Centauri Dreams: Imagining and Planning Interstellar Exploration, and have often been asked since to comment further on his research. But it’s best to leave that to the man himself, and I’m pleased to turn today’s post over to him. A bit of biography: Jim Woodward earned bachelor’s and master’s degrees in physics at Middlebury College and New York University (respectively) in the 1960s. From his undergraduate days, his chief interest was in gravitation, a field then not very popular. So, for his Ph.D., he changed to the history of science, writing a dissertation on the history of attempts to deal with the problem of “action-at-a-distance” in gravity theory from the 17th to the early 20th centuries (Ph.D., University of Denver, 1972).*

*On completion of his graduate studies, Jim took a teaching job in the history of science at California State University Fullerton (CSUF), where he has been ever since. Shortly after his arrival at CSUF, he established friendships with colleagues in the Physics Department who helped him set up a small-scale, table-top experimental research program doing offbeat experiments related to gravitation – experiments which continue to this day. In 1980, the faculty of the Physics Department elected Jim to an adjunct professorship in the department in recognition of his ongoing research.*

*In 1989, the detection of an algebraic error in a calculation done a decade earlier led Jim to realize that an effect he had been exploring proceeded from standard gravity theory (general relativity), as long as one were willing to admit the correctness of something called “Mach’s principle” – the proposition enunciated by Mach and Einstein that the inertial properties of matter should proceed from the gravitational interaction of local bodies with the (chiefly distant) bulk of the matter in the universe. Since that time, Jim’s research efforts have been devoted to exploring “Mach effects”, trying to manipulate them so that practical effects can be produced. He has secured several patents on the methods involved.*

*Jim retired from teaching in 2005. Shortly thereafter, he was diagnosed with some inconvenient medical problems, problems that have necessitated ongoing care. But, notwithstanding these medical issues, he passes along the good news that he remains “in pretty good health” and continues to be active in his chosen area of research. Herewith a look at the current thinking of this innovative researcher.*

Travel to even the nearest stars has long been known to require a propulsion system capable of accelerating a starship to a significant fraction of the speed of light if the trip is to be done in less than a human lifetime. And if such travel is to be seriously practical – that is, you want to get back before all of your stay-behind friends and family have passed on – faster than light transit speeds will be needed. That means “warp drives” are required. Better yet would be technology that would permit the formation of “absurdly benign wormholes” or “stargates”: short-cuts through “hyperspace” with dimensions on the order of at most a few tens of meters that leave the spacetime surrounding them flat. Like the wormholes in the movie and TV series “Stargate” (but not nearly so long and without “event horizons” as traversable wormholes don’t have event horizons). With stargates you can dispense with all of the claptrap attendant to starships and get where you want to go (and back) in literally no time at all. Indeed, you can get back before you left (if Stephen Hawking’s “chronology protection conjecture” is wrong) – but you can’t kill yourself before you leave.

Starships and stargates were the merest science fiction until 1988. In 1988 the issue of rapid spacetime transport became part of serious science when Kip Thorne and some of his graduate students posed the question: What restriction does general relativity theory (GRT) place on the activities of arbitrarily advanced aliens who putatively travel immense distances in essentially no time at all? The question was famously instigated by Carl Sagan’s request that Thorne vet his novel *Contact*, where travel to and from the center of the galaxy (more than 20,000 light years distant) is accomplished in almost no time at all. Thorne’s answer was wormholes – spacetime distortions that connect arbitrarily distant events through a tunnel-like structure in hyperspace – held open by “exotic” matter. Exotic matter is self-repulsive, and for the aforementioned “absurdly benign” wormholes, this stuff must have negative restmass. Not only does the restmass have to be negative, to make a wormhole large enough to be traversable, you need a Jupiter mass (2 X 10^{27} kg) of the stuff. This is almost exactly one one thousandth of the mass of the Sun and hundreds of times the mass of the Earth. In your livingroom, or on your patio. Warp drives, in this connection at least, are no better than wormholes. Miguel Alcubierre, in 1994, wrote out the “metric” for a warp drive; and it too places the same exotic matter requirement on would be builders.

Long before Thorne and Alcubierre laid out the requirements of GRT for rapid spacetime transport, it was obvious that finding a way to manipulate gravity and inertia was prerequisite to any scheme that hoped to approach, much less vastly surpass the speed of light. Indeed, in the late 1950s and early 1960s the US Air Force sponsored research in gravitational physics at Wright Field in Ohio. As a purely academic exercise, the Air Force could have cared less about GRT. Evidently, they hoped that such research might lead to insights that would prove of practical value. It seems that such hopes were not realized.

If you read through the serious scientific literature of the 20th century, until Thorne’s work in the late ‘80s at any rate, you will find almost nothing ostensibly relating to rapid spacetime transport. The crackpot literature of this era, however, is replete with all sorts of wild claims and deeply dubious schemes, none of which are accompanied by anything resembling serious science. But the serious (peer reviewed) scientific literature is not devoid of anything of interest.

If you hope to manipulate gravity and inertia to the end of rapid spacetime transport, the “obvious conjecture” is that you need a way to get some purchase on gravity and inertia. Standard physics, embodied in the field equations of Einstein (GRT) and Maxwell (electrodynamics), seems to preclude such a possibility. So that “obvious conjecture” suggests that some “coupling” beyond that contained in the Einstein-Maxwell equations needs to be found. And if we are lucky, such a coupling, when found, will lead to a way to do the desired manipulations. As it turns out, there are at least two instances of such proposed couplings advanced by physicists of impeccable credentials. The first was made by Michael Faraday – arguably the pre-eminent experimental physicist of all time – in the 1840s. He wanted to kill the action-at-a-distance character of Newtonian gravity (that is, its purported instantaneous propagation) by inductively coupling it to electromagnetism (which he had successfully shown to not be an action-at-a-distance interaction by demonstrating the inductive coupling of electricity and magnetism). He did experiments intended to reveal such coupling. He failed.

The second proposal was first made by Arthur Schuster (President of the Royal Society in the 1890s) and later Patrick M.S. Blackett (1947 Nobel laureate for physics). They speculated that planetary and stellar magnetic fields might be generated by the rotational motion of the matter that makes them up. That is, electrically neutral matter in rotation might generate a magnetic field. Maxwell’s electrodynamics, of course, makes no such prediction. There were other proposals. In the 1930s and ‘40s Wolfgang Pauli and then Erwin Schrodinger constructed five-dimensional “unified” field theories of gravity and electromagnetism that predicted small coupling effects not present in the Einstein-Maxwell equations. But the Schuster-Blackett conjecture is more promising as the effects there are much larger – large enough for experimental investigation. And George Luchak, a Canadian graduate student (at the time), had written down a set of coupled field equations for Blackett’s proposal.

Some worthwhile experiments can be done with limited means in a short time but only a fool tries to do serious experiments without having a plausible theory as a guide. Plausible theory does not mean Joe Doak’s unified field theory. It means theory that only deviates from standard physics in explicit, precise ways that are transparent to inspection and evaluation. (The contra positive, by the way, is also true.) So, armed with Faraday’s conjecture and then the Schuster-Blackett conjecture and Luchak’s field equations, in the late 1960s I set out to investigate whether they might lead to some purchase on gravity and inertia. The better part of 25 years passed doing table-top experiments and poking around in pulsar astrophysics (with its rapidly rotating neutron stars with enormous magnetic fields, pulsars are the ultimate test bed for Blackett’s conjecture) to see whether anything was there. Suggestive, but not convincing, results kept turning up. In the end, nothing could be demonstrated beyond a reasonable doubt – the criterion of merit in this business. However, as this investigation was drawing to a close, about the time that Thorne and others got serious about traversable wormholes, detection of an algebraic error in a calculation led to serious re-examination of Luchak’s formalism for the Blackett effect.

Luchak, when he wrote down his coupled field equations, had been chiefly interested in getting the terms to be added to Maxwell’s electrodynamic equations that would account for Blackett’s conjecture. So, instead of invoking the full formal apparatus of GRT, he wrote down Maxwell’s equations using the usual four dimensions of spacetime, and included a Newtonian approximation for gravity using the variables made available by invoking a fifth dimension. He wanted a relativistically correct formalism, so his gravity field equations included some terms involving time. They were required because of the assumed speed of light propagation velocity of the gravity field – where Newton’s gravity theory has no time-dependent terms as gravity “propagates” instantaneously. You might think all of this not particularly interesting, because it is well-known that special relativity theory (SRT) hasn’t really got anything to do with gravity – notwithstanding that you can write down modified Newtonian gravity field equations that are relativistically correct (technospeak: “Lorentz invariant”).

But this isn’t quite right. Special relativity has inertia implicitly built right into the foundations of the theory. Indeed, SRT is only valid in “inertial” frames of reference. ^{1} So, consider the most famous equation in all physics (that Einstein published as an afterthought to SRT): E=mc^{2}. But write it as Einstein first did: m=E/c^{2}. The mass of an object – that is, its inertia – is equal to its total energy divided by the square of the speed of light. [Frank Wilczek has written a very good book about this: *The Lightness of Being*.] If inertia and gravity are intimately connected, then since inertia is an integral part of SRT, gravity suffuses SRT, notwithstanding that it does not appear explicitly anywhere in the theory. ^{2} Are gravity and inertia intimately connected? Einstein thought they were. A well known part of this connection is the “Equivalence Principle” (that inertial and gravitational forces are the same thing) but there is an even deeper notion needing attention. He gave this notion its name: Mach’s principle, for Einstein attributed the idea to Ernst Mach (of Mach number fame). ^{3}

What is Mach’s principle? Well, lots of people have given lots of versions of this principle, and protracted debates have taken place about it. Its simplest expression is: Inertial reaction forces are produced by the gravitational action of everything that gravitates in the universe. But back in 1997 Herman Bondi and Joseph Samuel, answering an argument by Wolfgang Rindler, listed a dozen different formulations of the principle. Generally, they fall into one of two categories: “relationalist” or “physical”. In the relationalist view, the motion of things can only be related to other things, but not to spacetime itself. Nothing is said about the interaction (via fields that produce forces) of matter on other matter. The physical view is different and more robust as it asserts that the principle requires that inertial reaction forces be caused by the action of other matter, which depends on its quantity, distribution, and forces, in particular, gravity, as well as its relative motion. [Brian Greene not long ago wrote a very good book about Mach’s principle called *The Fabric of the Cosmos*. Alas, he settled for the “relationalist” version of the principle, which turns out to be useless as far as rapid spacetime transport is concerned.]

The simplest “physical” statement of the principle, endorsed by Einstein and some others, says that all inertial reaction forces are produced by the gravitational action of chiefly the distant matter in the universe. Note that this goes a good deal farther than Einstein’s Equivalence Principle which merely states that the inertial and gravitational masses of things are the same (and, as a result, that all objects “fall” with the same acceleration in a gravity field), but says nothing about why this might be the case. Mach’s principle provides the answer to: why?

Guided by Mach’s principle and Luchak’s Newtonian approximation for gravity – and a simple calculation done by Dennis Sciama in his doctoral work for Paul Dirac in the early 1950s – it is possible to show that when extended massive objects are accelerated, if their “internal” energies change during the accelerations, fluctuations in their masses should occur. That’s the purchase on gravity and inertia you need. (Ironically, though these effects are not obviously present in the field equations of GRT or electrodynamics, they do not depend on any novel coupling of those fields. So, no “new physics” is required.) But that alone is not enough. You need two more things. First, you need experimental results that show that this theorizing actually corresponds to reality. And second, you need to show how “Mach effects” can be used to make the Jupiter masses of exotic matter needed for stargates and warp drives. This can only be done with a theory of matter that includes gravity. The Standard Model of serious physics, alas, does not include gravity. A model for matter that includes gravity was constructed in 1960 by three physicists of impeccable credentials. They are Richard Arnowitt (Texas A and M), Stanley Deser (Brandeis), and Charles Misner (U. of Maryland). Their “ADM” model can be adapted to answer the question: Does some hideously large amount of exotic matter lie shrouded in the normal matter we deal with every day? Were the answer to this question “no”, you probably wouldn’t be reading this. Happily, the argument about the nature of matter and the ADM model that bears on the wormhole problem can be followed with little more than high school algebra. And it may be that shrouded in everyday stuff all around us, including us, is the Jupiter mass of exotic matter we want. Should it be possible to expose the exotic bare masses of the elementary particles that make up normal matter, then stargates may lie in our future – and if in our future, perhaps our present and past as well.

The physics that deals with the origin of inertia and its relation to gravitation is at least not widely appreciated, and may be incomplete. Therein lie opportunities to seek new propulsion physics. Mach’s principle and Mach effects is an active area of research into such possibilities. Whether these will lead to propulsion breakthroughs cannot be predicted, but we will certainly learn more about unfinished physics questions along the way.