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Time Machines

Super-Science, NOT Fantasy!

Science Fact: Scientists say building a time machine may be extremely difficult. But time travel is not against the laws of physics!

For thousands of years, scientists and philosophers have talked of time as a river that flows steadily onward year after year. But what if there were a way to swim against the flow, or to run down the bank ahead of the river? Might we be able to journey back and forth in time just as we travel through space? The idea is not as far-fetched as it sounds, and the implications for the future are intriguing.

Ever since Einstein, scientists have considered three-dimensional space and time not as two different things, but as different aspects of four-dimensional "space-time." Quantum physicists, who study the world of subatomic particles, often find it easier to explain events by assuming time runs backward as well as forward, however much it defies common sense. At the other extreme, cosmologists looking at the universe on a grand scale have found that space and time can be warped by gravity and speed.


Back in the 1940s, German mathematician Kurt Goedel proved that if we could warp and twist space-time enough – creating what he called "closed, timelike curves" – then we could bore tunnels through time itself. But no one knew how to do the twisting – until black holes. The gravitational pull of a black hole is so enormous that it distorts the very fabric of space-time into what is called a singularity. When singularities were found to spin, it was proved that closed, timelike curves not only can occur – they must occur. The singularity forms a doughnut shape in space-time, while the hole in the middle is a perilous gateway to somewhere – or when.

On the following pages we’ll show you some ways it may be possible to travel in time without breaking the rules.




Time Machines in our Future!...
Interview with David Anderson, Ph.D.
August 3, 1999 in Frankfurt, Germany

Dr. David Anderson is the President and founder of a very unique company called the Time Travel Research Center based in the United States of America. His background is one of the world’s most experienced in the field of space-time study and includes more than twenty years of activity in the field of time control research.

GMD: Dr. Anderson, thank you for joining us.

Anderson: You are quite welcome. I appreciate and thank you for the opportunity to be here today.

GMD: Dr. Anderson, I have to say that your company is one of the most interesting I have ever encountered. Can you tell us a little about the Time Travel Research Center?

Anderson: Of course. The Time Travel Research Center is a privately-owned research laboratory based on Long Island, New York in the United States of America. The company was founded in 1995 and is exclusively dedicated to the advancement of the science, technology and research that will deliver practical time control and someday time travel capabilities. I believe we are a leader in the development of capabilities to pursue this goal and are the only company of its kind dedicated exclusively to pursuing the achievement of time control and time travel. We support private research and development and also pioneered and manage the development of the TRI-STAR Information System which is the worlds largest knowledge base of science, technology, and research applicable to the subject of time and time travel. The TRI-STAR system’s simulation programs also represent one of the most advanced space-time virtual laboratories in the world today, designed and optimized specifically for research in this field of study. The Center also founded and manages the Time Travel Research Association, the largest time travel interest group in the world.

GMD: I have heard a lot about the Time Travel Research Association. But before we discuss this could you tell us exactly how did you get started? How does someone become interested and involved in time control research and development?

Anderson: It initially began when I was very young. I suppose I had developed a strong interest and ability in mathematics and physics at an early age. After scoring very high on a government exam of some type the United States Air Force began repeatedly trying to recruit me to join the Air Force and participate in their advanced research and development programs. This continued for a couple years during high school and while I was finishing my undergraduate degree program at West Virginia University. At the time I didn’t know that I might be involved specifically in space-time research, but it was a strong interest for me at this time. I finally accepted their offer and then spent almost five years as a United States Air Force Officer, Flight Test Engineer, and Scientist, conducting advanced space-time research at the prestigious Air Force Flight Test Center in the Mojave desert.

GMD: What did you do during your tenure at the Air Force Flight Test Center?

Anderson: I did many things... many things that I still cannot discuss today. But I can say that the focus of my work was in the research, development, test and evaluation of space-time models and systems. It was here at the Air Force Flight Test Center that I began building a detailed understanding and passion for space-time physics. I moved from project to project developing new mathematical methods and models to help advance space-time study, test and evaluation. I remember that just as I was finishing my graduate program with California State University that I became almost completely obsessed in trying to solve a very specific and elusive problem. The problem was to explain an unpredicted and unexplainable variance in position that some of our space-based satellite systems experienced over longer periods of time. I was finally successful in solving the problem by creating a predictable and reliable mathematical model. However, even though my model worked, it took several more years for me to refine and really understand it. When I finally did I couldn’t believe what I was looking at.

GMD: What was it?

Anderson: It turned out to be an absolutely complete space-time model. It included every aspect of relativistic physics even including consideration for details like frame-dragging that is caused by the gravity and spin of the Earth and the Moon. This resolved the discrepancy in the satellite position almost perfectly. What was most exciting wasn’t the existence of the mathematical model itself, but the relationships that fell out of it.


At this point in time I began to develop what I labelled "Time-warped Field Theory" to describe these relationships and how they could be applied for practical time control applications. I left the United States Air Force to continue my work and spent almost all of my time since then working to fund my research, advance my theories, build our TRI-STAR virtual laboratory, and plan the launch of the Time Travel Research Center.

GMD: So is the goal of your research is to produce a time machine?

Anderson: It does seem that anytime someone mentions space-time physics this question arises almost within the same breath. Can we build a time machine that we can step into that can teleport us anywhere-anytime? Sometimes it seems that this is always considered to be the "holy grail" of space-time physics. I won’t deny that some of our research focuses in this direction. This will be realized, I am sure of that. But I believe most of us will see the application of time-warped field theory and time control in more practical everyday applications first.

GMD: What kind of applications? I have heard something about your ties to something called "Project Darkstar." Is this a military weapons program and the application you are referring too?

Anderson: Well "Project Darkstar" is one of our projects... and I suppose the name does sound a little sinister and brings to mind weapons of mass destruction. However, Project Darkstar’s first application will probably be in medical applications, but its not limited to this field. Where did you hear about the project?

GMD: From two different places, one in Australia and another in the United States. But that isn’t important. Can you tell us more about the results you have produced and anything you can about project Darkstar?

Anderson: Well Project Darkstar is one of our first projects to apply time-warped field technology for practical application. So far we have been successful in creating and demonstrating small self-contained time-warped fields. The current field size we are moving to now is about 10 to 12 centimeters in diameter. A considerable amount of work will be required to increase the field size from here but it is definitely achievable with more time and funding. But the field size of 10 to 12 centimeters we are creating now is more than enough for many short-term applications we are already researching.

GMD: Wait, lets slow down. I don’t understand exactly what this field is. What is it and how it can be used?

Anderson: It is a self-contained spherical time-warped field. Within its boundaries we can actually accelerate or decelerate to a certain degree the rate at which time passes relative to the rate of time outside of the field. Perhaps the best way to describe it is to discuss some of the applications of this technology. Lets go back to the medical application.

We are currently researching several applications in the medical field. One would be transplant organ preservation. The time-warped field will be used to preserve organs or tissues awaiting transplant. In this case the organ would be stored in a special container within the time-warped field.


Here it would be exposed to a significant retardation in the rate of time passage that would keep the organ healthy and fresh for a longer period of time. This will greatly increase the success rate of transplant operations and will also provide a solution for organs to be stored and made available for longer periods of time... so they can be available when they are really needed.

Another area of great interest and application of this technology is for scientific test acceleration or retardation. Not only in the medical field but in many others. In many disciplines the speed at which research can be accomplished, or results can be produced, is gated by the length of time required by certain natural processes or chemical reactions.


Utilizing the time-warped field technology we will be able to actually accelerate this testing and research, hopefully without compromising the quality of the results. This will have tremendous advantages in many industries and research around the world.


The number of new avenues in research and development this could open up are significant.

GMD: So this is all theoretical, right?

Anderson: No, the technology is real.

GMD: Well, this sounds hard to believe, what actual results have you produced to verify this?

Anderson: Several. First, we have demonstrated time rate acceleration and retardation using both mechanical and electronic clocks. Placing one clock with the field and a reference clock outside we can show the time rate divergence as the field is "adjusted." We also recently demonstrated the effect on a living organism successfully accelerating and retarding the germination and growth of plant seedlings.


We set up a control group outside of the field and repeated several tests where a seedling was allowed to germinate and grow within the field itself. Here we repeatedly and consistently demonstrated control and actual time rate divergences between the two test groups.

GMD: Why plants? Why wouldn’t you... or have you tested this on people or animals?

Anderson: No, testing this on animals or people is much too dangerous at this time. First the field size is much too small. But more importantly this would be very dangerous for the following reason.

The field boundaries have some very "unique" characteristics that would be very dangerous if mis-applied at this time. This is why we decided to focus on organ preservation and scientific test acceleration first. But we don’t exclude the possibility and we even anticipate that after much more development that we will be able to create stasis fields and then eventually certain types of disease regression capabilities in the future.

But given the dangers it is much better that we walk before we run here.

GMD: It sounds like you have a lot of passion and interest for the application of your T.w.F. Technology (Time-warped Field technology) for medical and health care use?

Anderson: Yes, I do. The impact this technology could have on accelerating research and finding cures for diseases like heart disease, cancer, diabetes and aids is profound. We do believe that our TwF Technology will eventually permit certain types of actual disease regression as our development continues and we find safer ways to use it on a living person.


But in the short-term the benefits in accelerating and opening up new avenues of research will have such a large positive impact that we feel it may entirely change the way the world looks at and performs research and may help us move more quickly to new cures or treatments for these diseases.

GMD: It sounds like the benefit of this new technology could have a tremendous influence on medical cure and treatment research.

Anderson: Yes, absolutely.

GMD: You mentioned earlier that Project Darkstar wasn’t limited to just medical applications. What other applications do you see for this technology?

Anderson: Well the applications we just discussed as I mentioned are short-term applications. As we continue to study and develop our Time-warped Field technology we see other possible uses of the technology emerging. One of the first is the development of "containment fields."

GMD: Containment fields... What is this and how would it be used?

Anderson: As I mentioned earlier the boundaries of a Time-warped Field have some very unique characteristics. Some of which are very dangerous to living tissue. But some of these same characteristics that make them dangerous may allow them to be used to contain materials or energy that if exposed could cause damage if released. I suppose I’m not explaining this well. Its usually easier to discuss the technology and application with my presentation materials.

But for example, we see T.w.F. technology possibly being used to create a containment field that can be used with nuclear reactor cores to prevent the escape of dangerous radiation which can be very harmful or deadly to people, or any living creature nearby. Another application would be to protect against the escape of hazardous materials during handling, storage, or transport. There are many materials and organisms that are very deadly if exposed to living organisms and this could be another new application where this technology could have a tremendous impact.

I have to stress that analysis of the Time-warped Field boundaries is difficult and we still have much to learn. Another of our projects, "Project Prime-Zero", is dedicated totally to the analysis of the boundary characteristics of the field. This is where our Tri-Star simulator and virtual laboratory is key. So far the results of simulations on the system have been very promising and have not excluded that these types of containment fields may be possible with further research and development. In fact, the results suggest strongly that they are completely possible within the realm of the mathematics and physics of T.w.F. theory.

GMD: It sounds like your computer simulators are critical to projects like Prime-Zero and some of the others. Can you tell us a little about this?

Anderson: Well, you are absolutely correct, our virtual laboratory is key to analyzing and learning more about Time-warped field technology and its application. Our simulation programs may represent the most advanced space-time virtual laboratory in the world. It is a system that has been designed and optimized specifically and successfully for space-time research and development. Hmm... I probably should control myself here because this is a topic I like to talk too much about and we do not have that much time. But we are very proud of our programs and they have been invaluable to our research. I have personally invested now more than fifteen years in the development of the Tri-Star system and with that type of "investment" its easy to get carried away.

GMD: That’s okay, tell us more about it.

Anderson: Okay. As I mentioned the system can simulate a complete and accurate space-time model for almost all of our research needs. The Tri-Star system’s simulation programs were designed with one goal in mind -- to advance our efforts to achieve time time control and time-travel technology. It is a very unique scientific research platform.

The system is probably most unique in that it is very flexible and quickly adaptable to the various types of T.w.F. analysis and testing we need to do. By bridging the walls between physics, mathematics, and computation... and injecting our space-time model... the Tri-Star system now delivers a powerful research and development environment for us.

The system is used extensively before and after our hard experiments to compare actual results v.s. computer predictions. More and more though the greatest value has been in exploring and documenting the nature and possible applications of time-warped field boundaries. Overall I can’t express how valuable this program has been to our success and progress across the board.

GMD: I would like to see the system someday. You mentioned that the system addressed "almost all" of your research needs. Why "almost all?" Can you explain?

Anderson: I did not realize I was that revealing. Yes, there are some areas where we need to improve our simulation efficiency. The nature and characteristics of the time-warped field boundaries as I mentioned are very complex. I was probably understating this point. The mathematical model of the field in this area is extremely complex. Our Tri-Star simulation programs model the time-warped field accurately but due to the complexity our analysis and research in this area is progressing slower than we like, but it is progressing.

GMD: Perhaps you should put your simulator inside the time-warped field to accelerate your work.

Anderson: Actually, we have seriously thought about it and are looking at this as a possible long-term application of time-warped field technology. We believe that when we increase the field-size that we may be able to in fact do this.

There is much we still need to learn but it is quite possible that T.w.F. technology will create new possibilities and avenues to increase computer power and performance. Today we have parallel processing... who knows, tomorrow we may have multi-dimensional processing in an accelerated time domain. The technology may also have some separate advantages in the computer manufacturing process and it is very possible that T.w.F. technology will open some new paths in super computer development and applications.

GMD: It seems like there are some exciting applications for use of this technology today. And it seems that there are even more you are trying to explore for the future. Are there any other applications out there you have considered?

Anderson: Certainly, but we will have to wrap-up after this so I can be sure to make my flight. If not... I will have a very long walk to Bucuresti.

GMD: Okay.

Anderson: Well, researching this particular application its not a high priority effort, but as our Prime-Zero Project reveals more about the nature and characteristics of the time-warped field boundary we may find that the technology might provide a new source of power. This power source efficiency could be extremely high efficiency and would be a 100% environmentally clean technology. Again, there are many unknowns and much research to do, but this is a real possibility.

GMD: Our understanding of time has certainly changed in recent years, hasn’t it?

Anderson: The quest to understand time has been going on for thousands of years. But yes, you are correct. The last hundred years have revealed a lot. Time dilation and time control are achievable. This is science-fact not science-fiction. Its easy in each of our daily lives not to see this. But time control and even time travel are no longer considered to be a pseudo-science... they are accepted scientific, mathematical and physical fact. In fact, they are a essential basic element and part of the fabric of the universe we are living in right now. I suppose for many its just another example of how science-fact can be stranger than science-fiction.

GMD: If someone is interested how can someone learn more about time and time travel?

Anderson: I would first recommend visiting our web-site at The web-site present many views on the study of time and time travel including views on physics, mathematics, philosophy, metaphysics and even its implications and ties to spirituality. We also publish a quarterly journal called The Space-Time Journal that is packed with a lot of good and exciting information on many subjects surrounding the study of time.

There are also many good books available on the subject. One of the best I always recommend for someone just getting started is Paul J. Nahin’s "Time Machines." Also, "Space-time Physics" by Edwin Taylor and John Wheeler is a great introduction to a more mathematical view of space-time physics. We also maintain a comprehensive index and guide to hundreds of books and videos on the subject on our web-site if someone needs more sources.

We also manage an association called the "Time Travel Research Association." This is an association that networks time travel information and interests from around the world. We currently have more than 8,000 members from more than 78 countries. We sponsor the association in an effort to try to help advance the study and development of time control and time travel for anyone who is interested. The Time Travel Research Association is a great way to keep informed on news an updates and we even offer a free membership option. Its a great way to study and learn. Another organization I think many would find interesting is the "International Society for the Study of Time." I believe they have a web-site at

GMD: One last question. What should we look for coming from the Time Travel Research Center in the future?

Anderson: Well if our research continues to resolve itself as we’ve planned... and we predict it will... we will all be seeing some fascinating changes and the opening of an entire new industry based on this technology. Time-warped Field (TwF) technology, as we continue to refine and enhance its application, will have a profound impact on our world. The future will definitely be an exciting "time."

GMD: Dr. Anderson, thank you for your... "time" today.

Anderson: Your quite welcome. Thank you.

Dr. David Anderson can be contacted by e-mail at  or by regular mail by writing to the Time Travel Research Center, P.O. Box 1047, Smithtown, NY 11787-8547, UNITED STATES OF AMERICA.





Time Machines
by John Gribbin

TIME TRAVEL has become, if not respectable, then certainly fashionable in some quarters of the physics world over the past decade or so. Much of the blame can be laid at the door of the astronomer Carl Sagan, who was writing a science fiction novel in the summer of 1985, and asked the relativist Kip Thorne, of CalTech, to come up with some plausible sounding scientific mumbo-jumbo to "explain" the literary device of a wormhole through space which could enable his characters to travel between the stars.


Encouraged to look at the equations of the general theory of relativity in a new light, Thorne and his colleagues first found that there is nothing in those equations to prevent the existence of such wormholes, and then realized that any tunnel through space is also, potentially, a tunnel through time. The laws of physics do not forbid time travel.

This realization had two consequences. When Sagan’s novel, Contact, appeared in 1986 it contained a passage that read like pure Sf hokum, but which was (although few readers realized it at the time) a serious science factual description of a spacetime wormhole. And as Thorne and his colleagues began to publish scientific papers about time machines and time travel, the spreading ripples have stimulated a cottage industry of similar studies.

Curiously, this anecdote does not feature in Paul Nahin’s otherwise remarkably comprehensive account of the fact and fiction of time travel. Nahin is a professor of electrical engineering at the University of New Hampshire, and the author of several published science fiction stories, some dealing with the puzzles and paradoxes of time travel. He tells us how he discovered, and "devoured" science fiction stories at the age of ten, and this book is clearly a labour of love. The approach is scholarly, with 36 pages of footnotes, nine technical (but not overly mathematical) appendices, and a no-holds-barred bibliography. Nahin’s style is distinctly more sober than the material he deals with, but what he lacks in sparkle he certainly makes up for in comprehensiveness.

The approach, in line with the author’s background, is from the fiction and towards the fact. Old favourites, such as H. G. Wells and Frank Tipler, make their expected appearances, as do less familiar time travel fictions from the nineteenth century (comfortably predating Albert Einstein’s theories) and more obscure scientists and philosophers. And, of course, the familiar time travel paradoxes get a thorough airing.

There are, though, two major weaknesses in Nahin’s treatment of the science. The lesser is his discussion of black holes, which is weak and sometimes a little confused. Much more importantly, though, he fails to appreciate how the "many worlds" interpretation of quantum mechanics allows a time traveller to go back in time and alter the past without producing problems such as the notorious grandfather paradox. In the conventional version of the paradox, a traveller goes back and murders his grandfather as a young boy, so the traveller could never have been born, so grandfather never died -- and so on.


But in the many worlds version (championed today by David Deutsch, of the University of Oxford), the act of killing grandad creates a new reality, so that when the traveller then goes forward in time he is no longer in his own world, but in the universe "next door". This explains, for example, some of the more subtle touches in the "Back to the Future" trilogy of movies, which Nahin comments on while missing their point entirely. But although the book is flawed, it is still welcome.


It does not lend itself to being read from front to back like a novel, but is ideal to dip in to and hop around in, like a time traveller dipping in to history. It is also a first class reference book for anyone interested in the Sf side of time travel, and one that will be welcomed by the fans -- at least, they will welcome it when and if it becomes available in paperback at a sensible price.

Go Back


Worm Holes

Since the 1930’s, physicists have speculated about the existence of "wormholes" in the fabric of space. Wormholes are essentially gateways between different parts of the universe and are made by linking a pair of black holes. This effectively creates a tunnel through time and space: A traveler entering at one end would exit the other at a different time as well as a different place.


The difficulty lies in keeping the wormhole open while the traveler makes his journey: If the opening snaps shut, he will never survive to emerge at the other end.

For years, scientists believed that the transit was physically impossible. But recent research, especially by the U.S. physicist Kip Thorne, suggests that it could be done using exotic materials capable of withstanding the immense forces involved. Even then, the time machine would be of limited use – for example, you could not return to a time before the wormhole was created. Using wormhole technology would also require a society so technologically advanced that it could master and exploit the energy within black holes.


But the trip would not be impossible – just very, very difficult!

Go Back


Worm Holes Engineering

There is still one problem with wormholes for any hyperspace engineers to take careful account of. The simplest calculations suggest that whatever may be going on in the universe outside, the attempted passage of a spaceship through the hole ought to make the star gate slam shut. The problem is that an accelerating object, according to the general theory of relativity, generates those ripples in the fabric of spacetime itself known as gravitational waves.


Gravitational radiation itself, travelling ahead of the spaceship and into the black hole at the speed of light, could be amplified to infinite energy as it approaches the singularity inside the black hole, warping spacetime around itself and shutting the door on the advancing spaceship. Even if a natural traversable wormhole exists, it seems to be unstable to the slightest perturbation, including the disturbance caused by any attempt to pass through it.

But Thorne’s team found an answer to that for Sagan. After all, the wormholes in Contact are definitely not natural, they are engineered. One of his characters explains:

There is an interior tunnel in the exact Kerr solution of the Einstein Field Equations, but it’s unstable. The slightest perturbation would seal it off and convert the tunnel into a physical singularity through which nothing can pass. I have tried to imagine a superior civilization that would control the internal structure of a collapsing star to keep the interior tunnel stable. This is very difficult. The civilization would have to monitor and stabilize the tunnel forever.

But the point is that the trick, although it may be very difficult, is not impossible. It could operate by a process known as negative feedback, in which any disturbance in the spacetime structure of the wormhole creates another disturbance which cancels out the first disturbance.


This is the opposite of the familiar positive feedback effect, which leads to a howl from loudspeakers if a microphone that is plugged in to those speakers through an amplifier is placed in front of them. In that case, the noise from the speakers goes into the microphone, gets amplified, comes out of the speakers louder than it was before, gets amplified . . . and so on. Imagine, instead, that the noise coming out of the speakers and into the microphone is analyzed by a computer that then produces a sound wave with exactly the opposite characteristics from a second speaker.


The two waves would cancel out, producing total silence.

For simple sound waves, this trick can actually be carried out, here on Earth, in the 1990s. Canceling out more complex noise, like the roar of a football crowd, is not yet possible, but might very well be in a few years time. So it may not be completely farfetched to imagine Sagan’s "superior civilization" building a gravitational wave receiver/transmitter system that sits in the throat of a wormhole and can record the disturbances caused by the passage of the spaceship through the wormhole, "playing back" a set of gravitational waves that will exactly cancel out the disturbance, before it can destroy the tunnel.

But where do the wormholes come from in the first place? The way Morris, Yurtsever and Thorne set about the problem posed by Sagan was the opposite of the way everyone before them had thought about black holes. Instead of considering some sort of known object in the Universe, like a dead massive star, or a quasar, and trying to work out what would happen to it, they started out by constructing the mathematical description of a geometry that described a traversable wormhole, and then used the equations of the general theory of relativity to work out what kinds of matter and energy would be associated with such a spacetime. What they found is almost (with hindsight) common sense.


Gravity, an attractive force pulling matter together, tends to create singularities and to pinch off the throat of a wormhole. The equations said that in order for an artificial wormhole to be held open, its throat must be threaded by some form of matter, or some form of field, that exerts negative pressure, and has antigravity associated with it.

Now, you might think, remembering your school physics, that this completely rules out the possibility of constructing traversable wormholes. Negative pressure is not something we encounter in everyday life (imagine blowing negative pressure stuff in to a balloon and seeing the balloon deflate as a result). Surely exotic matter cannot exist in the real Universe?


But you may be wrong.


Making antigravity

The key to antigravity was found by a Dutch physicist, Hendrik Casimir, as long ago as 1948. Casimir, who was born in The Hague in 1909, worked from 1942 onwards in the research laboratories of the electrical giant Philips, and it was while working there that he suggested what became known as the Casimir effect.

The simplest way to understand the Casimir effect is in terms of two parallel metal plates, placed very close together with nothing in between them. The quantum vacuum is not like the kind of "nothing" physicists imagined the vacuum to be before the quantum era. It seethes with activity, with particle-antiparticle pairs constantly being produced and annihilating one another.


Among the particles popping in and out of existence in the quantum vacuum there will be many photons, the particles which carry the electromagnetic force, some of which are the particles of light. Indeed, it is particularly easy for the vacuum to produce virtual photons, partly because a photon is its own antiparticle, and partly because photons have no "rest mass" to worry about, so all the energy that has to be borrowed from quantum uncertainty is the energy of the wave associated with the particular photon.


Photons with different energies are associated with electromagnetic waves of different wavelengths, with shorter wavelengths corresponding to greater energy; so another way to think of this electromagnetic aspect of the quantum vacuum is that empty space is filled with an ephemeral sea of electromagnetic waves, with all wavelengths represented.

This irreducible vacuum activity gives the vacuum an energy, but this energy is the same everywhere, and so it cannot be detected or used. Energy can only be used to do work, and thereby make its presence known, if there is a difference in energy from one place to another.

Between two electrically conducting plates, Casimir pointed out, electromagnetic waves would only be able to form certain stable patterns. Waves bouncing around between the two plates would behave like the waves on a plucked guitar string. Such a string can only vibrate in certain ways, to make certain notes -- ones for which the vibrations of the string fit the length of the string in such a way that there are no vibrations at the fixed ends of the string.


The allowed vibrations are the fundamental note for a particular length of string, and its harmonics, or overtones. In the same way, only certain wavelengths of radiation can fit into the gap between the two plates of a Casimir experiment. In particular, no photon corresponding to a wavelength greater than the separation between the plates can fit in to the gap. This means that some of the activity of the vacuum is suppressed in the gap between the plates, while the usual activity goes on outside.


The result is that in each cubic centimeter of space there are fewer virtual photons bouncing around between the plates than there are outside, and so the plates feel a force pushing them together. It may sound bizarre, but it is real. Several experiments have been carried out to measure the strength of the Casimir force between two plates, using both flat and curved plates made of various kinds of material. The force has been measured for a range of plate gaps from 1.4 nanometers to 15 nanometers (one nanometer is one billionth of a meter) and exactly matches Casimir’s prediction.

In a paper they published in 1987, Morris and Thorne drew attention to such possibilities, and also pointed out that even a straightforward electric or magnetic field threading the wormhole "is right on the borderline of being exotic; if its tension were infinitesimally larger . . . it would satisfy our wormhole-building needs." In the same paper, they concluded that "one should not blithely assume the impossibility of the exotic material that is required for the throat of a traversable wormhole."


The two CalTech researchers make the important point that most physicists suffer a failure of imagination when it comes to considering the equations that describe matter and energy under conditions far more extreme than those we encounter here on Earth. They highlight this by the example of a course for beginners in general relativity, taught at CalTech in the autumn of 1985, after the first phase of work stimulated by Sagan’s enquiry, but before any of this was common knowledge, even among relativists. The students involved were not taught anything specific about wormholes, but they were taught to explore the physical meaning of spacetime metrics.


In their exam, they were set a question which led them, step by step, through the mathematical description of the metric corresponding to a wormhole.

"It was startling," said Morris and Thorne, "to see how hidebound were the students’ imaginations. Most could decipher detailed properties of the metric, but very few actually recognized that it represents a traversable wormhole connecting two different universes."

For those with less hidebound imaginations, there are two remaining problems -- to find a way to make a wormhole large enough for people (and spaceships) to travel through, and to keep the exotic matter out of contact with any such spacefarers. Any prospect of building such a device is far beyond our present capabilities.


But, as Morris and Thorne stress, it is not impossible and "we correspondingly cannot now rule out traversable wormholes." It seems to me that there’s an analogy here that sets the work of such dreamers as Thorne and Visser in a context that is both helpful and intriguing. Almost exactly 500 years ago, Leonardo da Vinci speculated about the possibility of flying machines. He designed both helicopters and aircraft with wings, and modern aeronautical engineers say that aircraft built to his designs probably could have flown if Leonardo had had modern engines with which to power them -- even though there was no way in which any engineer of his time could have constructed a powered flying machine capable of carrying a human up into the air.


Leonardo could not even dream about the possibilities of jet engines and routine passenger flights at supersonic speeds. Yet Concorde and the jumbo jets operate on the same basic physical principles as the flying machines he designed. In just half a millennium, all his wildest dreams have not only come true, but been surpassed.


It might take even more than half a millennium for designs for a traversable wormhole to leave the drawing board; but the laws of physics say that it is possible -- and as Sagan speculates, something like it may already have been done by a civilization more advanced than our own.

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Cosmic Strings

As a variation on the rotating cylinder, some scientists have suggested using "cosmic strings" to construct a time machine. At the moment, these are purely theoretical objects that might possibly be left over from the creation of the universe in the Big Bang. A black hole contains a one-dimensional singularity – an infinitely small point in the space-time continuum.

A cosmic string, if such a thing existed, would be a two-dimensional singularity – an infinitely thin line that has even stranger effects on the fabric of space and time. Although no one has actually found a cosmic string, astronomers have suggested that they may explain strange effects seen in distant galaxies.

By maneuvering two cosmic strings close together – or possibly just one string plus a black hole – it is theoretically possible to create a whole array of "closed timelike curves." Your best bet is to fire two infinitely long cosmic strings past each other at very high speeds, then fly your ship around them in a carefully calculated figure eight.


In theory, you would be able to emerge anywhere, anytime!

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Tipler Cilinders

Civilizations with the technology to harness black holes might be better advised to leave wormholes alone and try the time-warp method suggested by U.S. astronomer Frank Tipler. He has a simple recipe for a time machine: First take a piece of material 10 time the mass of the Sun, squeeze it together and roll it into a long, thin, super-dense cylinder – a bit like a black hole that has passed through a spaghetti factory. Then spin the cylinder up to a few billion revolutions per minute and see what happens.

Tipler predicts that a ship following a carefully plotted spiral course around the cylinder would immediately find itself on a "closed, timelike curve." It would emerge thousands, even billions, of years from its starting point and possibly several galaxies away.


There are problems, though. For the mathematics to work properly, Tipler’s cylinder has to be infinitely long. Also, odd things happen near the ends and you need to steer well clear of them in your timeship.


However, if you make the device as long as you can, and stick to paths close to the middle of the cylinder, you should survive the trip!

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What If Tourists From The Future Could Visit Us? If time machines are possible, it is likely that someone in the future will already have constructed one. After all, in the future there is time to complete even the largest engineering project! Even if humans are not up to the task, creatures from other planets may try.

So why are we not overrun by visitors from the future? This is the argument used by the famous English physicist Stephen Hawking in what he called his "chronology protection conjecture." Like many other scientists, Hawking is troubled by the weird paradoxes of time travel. He argues that the universe simply couldn’t allow time travel to happen, because its evolution since the Big Bang cannot be reversed. If the universe were to contract instead of expanding, asks Hawking, would human beings "unevolve" in the same way they have evolved over millions of years?

A second explanation for the absence of visitors from the future is that none of the time machines envisaged so far lets the voyager go back before the moment the machine was first constructed. So relax. Since no one has built a time machine yet, out-of-time tourists are not a problem!

Technical limitations aside, the "many worlds" theory also solves most of the paradoxes of time travel. According to this theory, an infinity of universes is constantly being created. In quantum physics, when subatomic particles have a "choice" of options (such as going through one hole or another in a screen), they select one at random. The "many worlds" theory says that there is a universe for each possible choice made by the particle.

"Many worlds" solves another of the famous time travel paradoxes. Say you went back in time and shot your grandfather before he met your grandmother. Would you never have been born? If not, you could never have traveled back in time and shot your grandfather. Which means that you "were" born, so you "could" have gone back… According to "many worlds," when you go back in time you actually emerge in another universe that develops in parallel to our own.


But with an infinity of universes to choose from, how can time travelers ever hope to find their way back to the one they started out from?


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