It was suggested, as I had written a novel with alternate time lines, and time paradox, that I should blog about time travel. Well, it just so happens I did write something, about wormholes and time travel, about ten years ago. So I pulled it out, dusted it off, and here it is. Take into account this was written, as I say, ten years ago. This is based on a documentary, and internet research, that I had done at the time. I, personally, find it fascinating, but it's not everyone's cup of tea, and may even be used for those that suffer from insomnia.
A paper discussing wormholes—wormholes first appeared in Einstein's theory of gravity, in 1913—states that: To be stable, wormholes need lots of negative energy—a rare commodity in the universe. Such energy has not yet been found, but quantum mechanics suggests that it does exist. The problem is that we don't know whether the laws of quantum mechanics allow enough negative energy to be concentrated in such a way as to allow wormholes to exist.
Although wormholes could exist throughout the universe—one possible location is at the center of black holes—traveling through one of these might prove extremely difficult since the wormhole would be so unstable that it would collapse as soon as a spaceship, or even a ray of light, entered it. This is because there would not be enough negative energy to hold it open.
In a paper on "Wormholes and Time Machines" by Berndtson, Gunnarsson, and Johansson it states: The problem is that a macroscopic wormhole is not static in structure, it's rather a shape that expands from a singularity with zero throat radius to maximum radius and then shrinks back to a singularity again. This expansion-reduction of the radius would be very quick. Even light would not have a chance to pass through the wormhole before it shrinks back to zero radius again. In fact, any now known matter that would fall into the wormhole would pull it together through gravity. If constructing a mathematical model of an open wormhole that allows passage, the equations of general relativity says that matter with an enormous negative pressure is needed to uphold the wormhole gravitationally. The magnitude of the tension of the matter must be greater than the energy density of the matter itself. This would leave us with a material that will have a negative energy density relative to a light beam traveling through it.
We are talking of exotic matter, so-called because there is no such matter now known. There are some indications that exotic matter can exist. For example between two metal plates there can be field fluctuations that have a negative energy density relative to the field fluctuations in free vacuum. Evaporating black holes also imply that exotic matter can exist.
Another problem with wormholes is that fields can destroy them. Fields are in some solutions able to increase their strength for each passage through a wormhole. If the wormhole has a focusing effect, the total field strength becomes infinite and will therefore destroy the wormhole. If the hole is defocusing, it converges towards a finite value and the wormhole can survive. The only matter that can make the wormhole defocusing is exotic matter. If exotic matter exists and has the ability to uphold a wormhole without interacting with and harming the traveler, then there is a physical possibility of a traversable wormhole and for it to even work as a time machine.
A wormhole can be turned into a time machine by keeping one mouth of the wormhole fixed while moving the other. This can be made through gravitational attraction or by charging it electrically while moving it with electric fields. Traveling from the stationary mouth to the moving and back again could then send a traveler back in time.
A message sent through a wormhole would take a shortcut in space-rime, arriving almost at once if the wormhole is short.
The lifetime of wormholes, however, are argued to be too short lived for information to be communicated between the two external regions.
The real problem with traversing a wormhole is that none of the geodesics connecting these two external regions are time-like or light-like. For information to cross from one external region to the other, it would have to follow a space-like path during at least part of the journey. In other words, in order for information to cross from one side of a wormhole to the other without winding up hitting the physical singularities it would have to travel faster than light in a way not allowed even by general relativity. If information could do this, then the first problem of stability wouldn't be a problem at all as the information could travel through the wormhole arbitrarily fast and make it through before the wormhole connection was broken.
So when it comes down to it we are not even sure that exotic matter exists and if it exists it isn't very probable that we can collect enough of it to feed a wormhole. We also have to find a macroscopic wormhole to feed with this exotic matter. Current research indicates that it is impossible to open a wormhole bigger than 10^-35 meters. Therefore it will take more creative thinking before any scientist or novelist can fit a human into a wormhole, especially one that small. The problems are many and if it really is possible it will take many years before we overcome them.
In fact, physicists don't agree that time travel is possible. Stephen Hawking wrote in 1993: "...the best evidence we have that time travel is not possible, and will never be, is that we have not been invaded by hordes of tourists from the future." Time travels might be allowed theoretically but real ones are still science fiction at this point.
Then again, there is always the science of Ronald L. Mallett. I recently watched a documentary on his theories and ideas and found it fascinating.
According to Mallett, with the current technology it is possible to send subatomic particles, at the very least, back into the past.
We've know for a long time that time travel is possible, in theory. Mallett has a blueprint for a time machine, a means to achieve it.
Sending particles back in time would also allow information to take the same route, it could be used to send scientific understanding from the future...but then it would get a little strange.
Although the professor's underlying idea is new the science is not. Time travel, travel in the fourth dimension, allows a person to be in two places at once. And in fact has already been accomplished, although admittedly only to a minute degree, by the Russian space program in the later part of the 20th century. Traveling at 16,000 miles per hour has propelled them a fraction of a second into the future.
Experimental physicist, Chandra Roychoudhuri, specializing in laser technology, hopes to create a machine that will use the principle of flexible time to send particles into the past.
In 1905, 26-year-old Albert Einstein showed how space, time, and energy are linked. We know he got it right because his theory led directly to the atomic bomb.
This is the very same theory that should allow real practical time travel. It all has to do with the speed of light.
Einstein's big idea was that the amazing speed of light holds the key to everything from the untold power of the atom to the possibility of time travel.
The speed of light remains the same, no matter how fast you are moving towards it or away from it, it will appear to pass you at the speed of light.
Einstein's prediction that the speed of light is the same for everyone is one the strangest in physics, but it's true. It's been shown by hundreds of experiments that the speed of light remains the same no matter how fast you are moving towards or away from it (something else is changing, and that something is time). If an object is moving fast enough through space it can alter its passage through time.
The faster you travel the slower time passes for you (so what might appear an hour to you may be one hundred years to everyone else). In effect, you would be traveling one hundred years into the future.
Then you have the grandfather paradox, which suggests things are fixed not allowing for free will. You cannot change the past as it has already happened.
And then you have Dr. David Deutsch, the world's leading proponent of Parallel Universe Theory, who claims, that in addition to the universe we see around us, there are vast numbers of other parallel universes. Some are very like our own, differing only in the position of one atom, and others are very different.
This idea comes from the study of subatomic particles. The more we find out about them the more their behaviour seems absurd; which could actually explain why the future is not fixed but fluid, in a weird sort of way.
If you look at the universe on a very small scale you begin to see things that are very alien to our everyday experience.
In everyday life we are used to objects retaining their identity, a pen is a pen. A subatomic particle typically might change into another particle, or into two other particles, or particles might merge their identity and become one.
Sometimes a subatomic particle can be traveling along and then change course for no reason that we can observe. Or if this was the subatomic world, we could put an object on the table and it could ooze right through and pour to the floor.
This is the strange world of quantum mechanics, and physicists still argue about what is going on.
What's really happening is that the universe we see is only part of the physical reality. There are parallel universes, and each particle in our universe has counterparts in many of the parallel universes, and under some conditions these counterparts affect the particles that we can see. The universes are interfering with each other all the time. So particles in our universe could be hitting other particles in another universe, and neither universe would really know about it, and in that parallel reality a very different scenario could be playing out.
In a multiple universe time travel allows travel to take place between two similar and yet different universes, and because you are not in the universe you came from there is no need to fulfill any particular destiny, so neither universe contains a paradox, some would postulate, however, that this is not true time travel but inter-dimensional travel.
The theory implies that there are an infinite number of these parallel universes and that time travel would simply involve skipping from universe to universe to universe. A working time machine would, in effect, test this extraordinary idea. If it's correct then not only does free will exist but the nature of reality itself is very strange indeed.
Mallett says that the paradoxes that people talk about are only going to be resolved after we build the first time machine. Then we will know whether or not free will enters into it, whether there's multiple universes, or whether the universe is determined. That's going to have to be understood experimentally.
Although Einstein correctly worked out that traveling very fast sends you into the future, a further development of the theory should allow travel into the past.
The crux of Mallett's theory starts with intersecting powerful rings of laser light that twist time into a loop, an effect that Einstein predicted but was never able to prove.
The way it works is, a circulating beam of light creates a rotating region of space as though you were stirring a cup of coffee and in addition to twisting space in Einstein's theory, space and time are linked so you cause a twisting of time as well.
We think of a time-line as being a link from the past, to the present, to the future. If you can close that time-line into a loop you can go from the past, to the present, to the future, but we're on a loop so we can go from the future back into the past.
The basic form a time machine would take is a stack of lasers creating layers of circulating light and around them would be circulating loops in time. In building a time machine we would need to exceed the speed of light without breaking the laws of physics. We already have the answer. We know it exists, and we know where to find it. The problem is getting there.
What you need is a rotating black hole. A black hole that is spinning has a very interesting effect on the space and time around it. It drags them around with it and that means one can evade the rule about not exceeding the speed of light. As we already know a black hole's denseness creates a gravitational pull that is so strong not even light can escape it. This incredibly strong gravitational pull is able to grab the empty space around it and pull it towards its center like a whirlpool. Here physics is pushed to its absolute limit.
One can travel into the region where space and time are being dragged around the black hole, remain in that region traveling around with it and emerge at a time before we went in.
This natural time machine occurs only because the black hole has the power to move empty space. The phenomenonm, as you are most likely familiar with, is called frame dragging, one of Einstein's lesser known ideas.
Within that rotating region of space you can travel speeds up to the speed of light. To someone outside it would appear as though you are traveling faster than the speed of light and for them you will disappear from sight as you are traveling back into the past. However, black holes are not always readily available.
This is where Mallett had a stroke of inspiration. He realized that Einstein's theory was hiding the secret. Something that would be even better at twisting space than a black hole. It turns out that light is much more effective at twisting space and time. As stated before, his idea is to use a circulating light beam to twist space and close time into a loop. The key technical challenge is trying to get enough laser power to cause this twisting of space and time.
Modern laser technology is extremely advanced, some lasers can create conditions as hot as the center of the sun. An array of such devices could be rigged to fire at the same time and that could produce a cylinder of light with enough power to actually twist space.
An elementary particle fired on a corkscrew-shaped path down this tunnel would wind its way into the past. Seen from outside this experiment this would have some pretty bizarre consequences (this is where the strange part comes in).
Says Dr. Mallett: "I expect that particles might just simply appear out of nowhere, even though I didn't put anything into the machine. And those would be particles that would be from experiments that I performed next week, or a year from now, in which I am sending particles back to my time.
"This is a real time machine and that means that when you turn it on today, and you leave it on, it will only act during the time you have it on. So if I turn it on today and I leave it on for a hundred years, I can then travel from a hundred year from now up to today, but I cannot travel to a time before the machine was turned on.
"So, it's conceivable that the moment the machine is turned on that you might immediately start receiving messages from the future.
"If a machine can be created that would be stable enough to keep twisting time for a century or more it could become, in effect, a phone line through time. Anyone with future access to the machine could send messages back which begin to be recovered the moment it is turned on."