Part III: Time Travel and Relativity – in other words, not ignoring the Laws of Physics

Part III: Time Travel and Relativity – in other words, not ignoring the Laws of Physics


Ed Note:  The original have the necessary formulas but could not be uploaded…

When we think of Time Travel we invariably think of one man; Albert Einstein. He gave us the modern mathematics of time travel and advanced the questions to a level which allowed us to test the hypotheses. He did not however give us Relativity, he did give the world  General Relativity and Special Relativity, both of which we shall detail later. First though, Relativity. To understand the concept of relativity we need to go back in time to 1632 and the Book Dialogue Concerning the Two Chief World systems (Dialogo sopra i due massimi sistemi del mondo). This was the book that eventually had Galileo found “vehemently suspect of heresy”. That however, is another story.

Galileo, by any account, was one of the most significant scientists of his time and probably to his detriment, one of the greatest self-publicists of his time. The book was originally published with the permission of the Roman Inquisition, but his mistake was to alienate former allies in the Jesuits and the Pope himself.   

The basic principle upon which Relativity stands is that if an action occurs and two people observe it from different points, the challenge is for them to agree when and where the activity occurred. His principle of relativity states that there is no physical way to tell between a body moving at a constant velocity and a stationary body. We can determine that one body is moving relatively to the others, but it is impossible to determine which of  the objects is moving and which is immobile (without an external reference point).This concept works for all bodies from ships to planets and stars. Galileo explained his principle by the example of two ships.

“Shut yourself up with some friend in the main cabin below decks on some large ship and have with you there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide [Note David Eckstein/Samuel Edelstein: This should rather read 'narrow' instead of 'wide'. Think of a bottle with a narrow neck on its top! ] vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all these things carefully (though there is no doubt that when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. In jumping, you will pass on the floor the same spaces as before, nor will you make larger jumps toward the stern than toward the prow even though the ship is moving quite rapidly, despite the fact that during the time that you are in the air the floor under you will be going in a direction opposite to your jump. In throwing something to your companion, you will need no more force to get it to him whether he is in the direction of the bow or the stern, with yourself situated opposite. The droplets will fall as before into the vessel beneath without dropping toward the stern, although while the drops are in the air the ship runs many spans. The fish in their water will swim toward the front of their bowl with no more effort than toward the back, and will go with equal ease to bait placed anywhere around the edges of the bowl. Finally the butterflies and flies will continue their flights indifferently toward every side, nor will it ever happen that they are concentrated toward the stern, as if tired out from keeping up with the course of the ship, from which they will have been separated during long intervals by keeping themselves in the air.  ...“  [05-187f]”
Looking first to the Salvatius Ship, called after the book’s narrator. In short Galileo described how the effects of constant motion (constant velocity)are the same as the effects of stillness, we cannot tell what is motion and what is not. Note, this is constant motion not acceleration which is anoter force to act on a body (ship). With a point of reference (outside) it is impossible to gage which body is moving. We have seen this, for example on a train. While parked at a station with another train sitting on a line next  to you, as one starts to move you cannot tell which one is moving until you have an external reference point to calibrate against what is being observed. Locked in the ship with no view out  and a calm see it will not be possible to determine if the ship is moving  or at rest (acceleration will as mentioned cause additional forces on objects so movement can be determined)

What  we learn from this is that the relative (uniform) motion of the two ships does not impact the laws of motion on the other/either ship. A cannon ball dropped from the top of the main mast on the Salvatius Ship will appear from the other ship (Segredus) to fall in a parabolic course while looking to fall straight down from the Salvatius’ mast.  The same hold true should Segredus be the one dropping cannon ball from the mast of his ship. Now let’s take this to the modern age; in space above (or below, or to the side) Earth are two ships and a joint beach ball. Free from gravity or other forces, the ball does not accelerate in any manner as no force (in any x, y, or z) direction is applied. The motion of the ball is uniform in respect to both ships.

This principle leads on from the principle of inertia, bodies operating on a horizontal plane will not change their velocity as long as no force is applied to them. “The laws of nature  do not permit you to determine if you are stationary, and thus your speed (and that of anything else) always has to be stated as relative to another object”

We are all familiar with the idea of Einstein’s two space ships but is was Galileo who showed us that within a system (ship) the elements, (all having the same velocity) will move relative to each other; not indicating a moving of stationary object. If the ship is totally closed to the world and moving on a calm sea, out sailor, Salvatius will throw the ball up and down catching it each time no differently whether the ship is moving or not.


Inside the ship the ball looks to be moving thus :         
Simply up and then down as expected, however if we take one side of the ship and  replace it with a one way mirror  (stay with me) So that Salvatius cannot see out and has no  external; reference We now see the same ball moving in a parabola, as it moves  alone at the steady velocity relative  to the observation point.

The external View: Moving with stead velocity relative to an observation point the same ball going through the same motions will appear to also have a horizontal element to its movement rather than the seemingly purely vertical movement  observed internally.

As we move to the Einsteinian logic we start to think in terms of time and space, but before we can do that we should firm-up on some key factors in relation to relativity, - namely time, space and gravity. For this we will now proceed  to England and Sir Isaac Newton.
Looking from inside the ship the ball goes up straight and down again straight at velocity V1,  over a distance of V2t (t = time).
From outside an observer will see the train move in a direction at velocity V2t. the stationary external obserser will see the ball moving a greater distance along a horizontal plane (the sum of the distance the ball moves within the ship plus the distance of the ship’s travel. The observer sees the ball at velocity V3. The velocity of the ball seen be the external observer can be found using good old fashioned Pythagoras 
In essence we are seeing two different views of the one occurrence, both relative a particular  viewing point. If a coordinate system (such as the ship or more modern space ship, essentially a self-contained unit) “A” moves at constant velocity in a straight line with respect to an inertial frame  “B” is also an inertial frame. With two inertial frames moving only uniform to each other and in a straight line, it cannot be determined whether one of the vessels is at rest in space (that is a area with only the two reference vessels and no other external reference point).
The principle is:  “any two observers moving at constant speed and direction with respect to one another will obtain the same results for all mechanical experiments”  In other words, "the mechanical laws of physics are the same for every observer moving uniformly with constant speed in a straight line.

Newton
Having briefly looked at one aspect of Galileo’s contribution in this area of physics and science, we will now look to see how his work influenced those who came after him. Perhaps one of the most significant of those people being Sir Isaac Newton. Like  Galileo, Newton was a man of many talents advancing knowledge of physics; gravity, motion and thermodynamics in particular, the understanding of our place in the stars/universe and even the minting of coins for which he was responsible  at the Royal Mint in the Tower of London. On a somewhat reassuring note we should also remember that despite being one of the greatest minds of his, or any other, generation, he like many others fell victim to the South Sea Bubble.  Having initially invested and made a small fortune, he reinvested and lost a huge fortune.  As with Galileo, we could look to many aspects of Newton’s life, career and relationships (famously bad tempered and difficult to get on with, heavy metal poisoning is suspected) however we will focus on his work in relation to Relativity. As with Galileo, the laws were first published in book form in “Principia Mathematica Philosophiae Naturalis” in 1686, about 50 years after Galileo’s Dialogue. The Principia is still in print and indeed it is easier to find a Latin version than it is to get one in English

Newton's laws
Newton's Laws of Motion help us to understand how objects behave when they are standing still, when they are moving, and when forces act upon them. There are three laws of motion.

First Law of Motion
“An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force”.  This law is often called  "the law of inertia".  Basically what Newton's First Law is saying is that objects behave predictably. If a ball is sitting on your floor, it isn't going to start rolling about unless a force acts upon it to cause it to do so. likewise objects in motion do not change their direction unless a force causes them to move from their path.

Second Law of Motion
“The force acting on an object is equal to the mass of that object times its acceleration.” This is written in mathematical form as: F = ma 
 F is force, m is mass and a is acceleration, Force is understood to be “The Change of momentum with change of time”. Stated also as when a force acts on an object, it will cause the object to accelerate. The larger the mass of the object, the greater the force will need to be to cause it to accelerate .  when a force acts on an object, the object accelerates in the direction of the force. If the mass of the object does not alter, acceleration will increase as the force applied increases; likewise if the force is decreasing/increasing the mass will increase/decrease acceleration respectively. Force is directly proportional to acceleration, while mass is indirectly proportional to acceleration. Using calculus (invented by Newton to support the laws) we find:
F = m(dv/dt)
is the differential change in momentum per unit time, it is a characteristic  of a moving body determined by the product of the body’s mass and velocity. Because acceleration is defined as the instantaneous change in velocity in an instant of time (dv/dt),   giving the equation we first saw above.
F = ma                                 
 So far so good, and now for the one everybody can quote:

Third Law of Motion
“For every action there is an equal and opposite reaction”. Applying a force on an object means that the object in question will push back with the exact same force, but in the opposite direction.

The laws of motion form a basis from which we can now look to relativity and how it progressed. Up until the time of Newton and the age of Enlightenment (the ancient Greek view of the world largely still governed.  Aristotle was the reference point for all learning of mechanics (physics). Whereas Aristotle held that the Natural state of matter was stationary (at rest) with respect to the Earth, Newton (and Galileo before him) held the constant state of matter to be one of constant velocity. The Newtonian perspective was that space, motion and time were all absolute and were a uniform inertial frame (just as the boat was for Galileo’s Salvatius)  and that time passed evenly throughout at an eternally fixed and inexorable rate without relation to anything external

It is this linking of time, motion and space that is critical to our understanding and advancement of space-time.  The laws of physics are equal for any observer moving at constant velocity. The constant velocity allows us to have an inertial frame of reference, (when an observer is moving at a constant velocity), providing an environment where the Laws of inertia hold true.

The key to Newton’s laws of Motion  is the Second Law: F = ma , or    a=F/m, the velocity is not the key, but rather the  change of velocity. Due to relativity, every observer moving at the same constant velocity will measure/observe the same forces on  an object as they are from the perspective of acceleration of the object. Following on from the Galilean example, until Salvalius has a deceleration (or acceleration) force applied he will not notice any movement, however as a result of Newton’s first law, as the ship decelerates he will find himself slipping forward  as the forces are applied.  If there is a second ship moving at the same velocity, both will seem not to be moving relative to each other without an external frame of reference. If we were to hang a pendulum in the galley and apply some very powerful sails to alter the velocity by a noticeable acceleration we wound see that the pendulum no longer swings as if it were stable (at constant velocity, even if v=0)    the pendulum will hang at an angle because the  pivot point is being pulled away from the pendulum. 

Okay, Newton’s law of motion support the theories of relativity so far, however. One of the important aspects of modern relativity and time travel is light and electromagnetism. Newton of course did much to advance our understanding of the laws of physics and particularly motion ( as well as thermodynamics, but that’s another story) and also light itself with the publication of Opticks in 1704. The finer understanding of the nature and speed of light would come some years later.

Before we jump to everybody’s favourite  Swiss physicist, we should  take a slight diversion or two for some final background. Firstly to the Dutch physicist Hendrik Lorentz.  From what we have seen so far Newtonian mechanics (physics) does not vary in relation to Galilean transformations concerning two inertial frames (at same speed and direction). However this presumes time is a universal concept, with it being the same everywhere from all observers, in other words in line with Special Relativity. The Lorentz transformations show measurements of space/time by two observers are connected, the observers moving at different velocities may measure different distances, elapsed times etc. The Galilean transformations function at relative speeds much smaller than the speed of light.  The Lorentz transformations can be summarised as follows:The frame moves at velocity v, in direction x with respect to the fixed reference frame. The coincide at t=t*=0 (which like the bad soccer game was tied 0-0 at the start) . X* is moving. The Galilean transformation shows coordinates from the fixed frame in terms of its location  in the moving reference frame, which is in line with human intuitive thinking of what we would think it should be like.

This is really theoretical material but it helps on the trip to the final answer.  Looking briefly at the Lorentz transformations we see as follows. The moving primed frame moves with velocity v in the  direction x with respect to the fixed reference frame. The reference frames coincide at t=t'=0. The point x' is moving with the primed frame.  All as normal so far. The Lorentzian equations for this are: ( the original version has the equations but Blogger would not take them)

This might seem theoretical but in  Minkowski (the mathematical space in which Einstein’s theory  of Special relativity is most conveniently formulated, the ordinary dimensions of  space are joined by a single dimension of time-space, the Lorentz transformations preserve the space-time interval between any two events. They show  the transformations in which the space-time event at the origin is left fixed.  These are important to this essay not just to explain the history of relativity but also to show that as our understanding of it grows so does the complexity of what needs to be considered.

Before we finally arrive with Einstein, we have one last stop and that is with the 19th century Scottish physicist James Clerk Maxwell. Maxwell gave us the theory of electromagnetic radiation, and in so doing linked together electricity, magnetism and light as the same thing essentially. He showed that electrical and magnetic fields travel through space as waves moving at the speed of light. Maxwell postulated that light is undulations in the same medium, that is the cause of electrical and magnetic phenomena.  Doing this, he showed the way to the spectrum of electromagnetic radiation. Defining fields as a tension in the medium, he stated his belief in a new concept - that energies resides in fields as well as bodies. This pointed the way to the application of electromagnetic radiation for such present-day uses as radio, television, radar, microwaves and thermal imaging.

Not forgetting the actual calculations for the speed of light, we will jump back to the 1670’s and Denmark, specifically to the astronomer Ole Rømer. While observing the transit of Jupiter and its moons he calculated the time light takes to travel to earth. His original calculation of 220, 000km/second is about 25% out but gave a method of calculation. It caused some controversy at the time, with opinion being divided, (Huygens and Newton both supporting). James Bradley developed this and confirmed the methodology in 1729 and was only 1000miles per second out. 

Even through Einstein’s experiments, the world had still not settled on the speed and nature of light. Maxwell’s waveforms were accepted in time and it was not until the 1980’s that the 17th General Conference of Weights and Measures declared that the speed of light is 299,792,458 meters per second. This may be refined in the future, but the criticality of this measure can be shown in the modern definitions of a meter and even a kilo; both are now determined in relation to light, further reflecting its nature as a constant. It is worth remembering that when Einstein developed the Theory of Special relativity in 1905, he ASSUMED that the speed of light was a constant.

Special Relativity
Having reviewed the history and a little of the nature of relativity  it is about time we looked at the theories of Special and General Relativity. The theory of Special Relativity is based on two postulations, both of which we have covered above to some degree;

•    Relativity: The laws of nature are the same in all inertial reference frames
•    The speed of light in a vacuum is the same in all inertial frames.

These are the two principles which give us such great science-fiction. Special relativity is restricted to objects that are moving at constant speed in a straight line; inertial motion. Einstein’s theory showed that the speed of light is a limit that can be approached but never reached or exceeded  by material objects. It is this idea that gave way to:

E= mc2

 In 1905 Einstein published a series of papers covering; photoelectric effect, the proof of atoms, Special Relativity and finally the paper that gave us  The Formula. Originally written as:
m= E/c2 later to become E= mc2 . It shows that mass and energy are the same physical entity, both being able to change into the other, but more on that a little later.

Rather than consider absolute space and time, Einstein used definitions which related to the state of motion of the observer; using the example of an observer on a track and another on the now famous train. Using the existing formula of x* = X – vt (see above) we note the speed of one observer relative to the other. T is the time at which the observed event occurs. The issue here is to consider light in relation to speed.

A ball thrown on a moving train will have a greater speed than a ball thrown with the same force by someone standing on the platform. This is because the speed of the train is added to that of the ball to give its total speed. But this isn't the case with light. If you measure the speed of the light produced by torches on a moving train and a stationary platform, you will get the same speed - the speed of the train doesn't matter. When you measure the speed of light it doesn't matter if you are moving or stationary, or if the source of the light is moving - the speed is always the same: 300,000,000 metres per second.  (Approx).

Here is comes. For the speed of light to  always be the same, something else must change. The measurements of speed and time in this principle depend on how fast you are travelling. As velocity accelerates to the speed of light, relativistic effects on time dilation (clocks running slow) and length contraction become more noticeable.
Using Lorentz  transforms Einstein  developed :

t′ is time as measured by the moving observer
 c is the speed of light.
From these, came new  equations the addition of velocities
u and u′ are the speed of any moving object as seen by each observer
 v is again the speed of one observer relative to the other.

From this; if a beam of light is projected from the front of the train, moving at the speed of light, an observer on the train will record the speed of the beam as c as per above. However, just as the first observation is in line with the equation, the second observer on a platform will also see the light with speed c, not 2c as might be expected. As we mentioned earlier for light to be constant, the other physical properties must change, the object travelling to light speed becomes shorted along its direction of travel, while time intervals become longer, with time running more slowly for the system as it travels close to the speed of light. The person on the train will measure time and distance (length) as normal, being as they were from his perspective when he set off (the traveller being in the train is part of the system moving at that speed, so the length of the train remains the same for him, as does the rate of passage of time. The observer on the platform, on the other hand will see the train get  smaller. While , eventually when the train stops it will be seen that time has passed more slowly for the train based observer. Comparing clocks, it will seem as the train based observer has jumped from an earlier time to the current platform time. An extended example of this is if observer 1 went on a lap of the solar system in a near light speed vessel and a clock, while observer 2. Waited on Earth with a second clock, both clocks having been synchronised prior to the vessel leaving. After about 5 years passing (from an earth frame of perspective) the vessel returns to earth, observer 2 will see that for him only four years have passed, the times are no longer synchronised.

This can happen by space and time changing relative to the speed of light which stays fixed. If observer 2 was able to watch the vessel as it toured the solar system it wound be noticed that as the vessel got near the speed of light its length  got shorter. And now for the counter intuitive piece – from the equation it is shown that as a body nears the speed of light its mass increases and so it takes more and more energy to increase its speed further, closer to the speed of light. As it nears the speed of light the vessel will become so massive that there will not be sufficient available energy to make it go faster and reach/break the speed of light. As a consequence we learn that nothing with a mass can exceed the speed of light.
This can be shown in equation by:
 L0  = proper length
T0,  =  proper time, as measured by an observer on the vessel, , L and T are those values  as measured by a fixed observer. 
Looking to  E = mc2 we know that; E = energy, M = mass, C = speed of light and already we can see the relationship between speed, mass and energy.

From this it is starting to look as direct travel in to the future (as viewed by  observers from both frames of reference is not too possible, but  we can see that it is possible to have two bodies move at sufficiently different a speed to facilitate time dilation and what seems like time travel, one observer, relative to the other. This does not even have to from the realms of science fiction

The Hafele and Keating experiments of 1971
Four caesium atomic clocks were flown on regular commercial jets (on scheduled routes) around the world. Two flew east and the other two, west. This would test the theory. Looking at the flight paths for the trips a prediction of the loss/gain of time was made.
Upon return the clocks were compared to the atomic clock at the US Naval Observatory. Those flying east lost 59 +/-10 nanoseconds, while those flying west gained 273 +/- 7 nanoseconds, thus resolving the clock paradox. We can see from looking at special relativity that our options are limited due to the nature of the relationship between energy, mass and the speed of light. What happens when we add gravity in to the mix.
 
General Relativity
Having given the world special relativity, Einstein went on to develop the theory of General Relativity, which took account of gravity and brought us the understanding that objects of large masscan distort local space and time. This impacts our understanding of the very nature of the space around us.

Our knowledge of gravity tells us that two objects exert a force of attraction on one another. Sir Isaac quantified gravity as part of his study of  Laws of motion. Einstein’s work were largely thought experiments subsequently verified by experiments and observations by persons following. An example of this is the lensing of light around a massive object. How we see gravity is  from the curvature of space time. At its simplest it explains the motion of the planets such as in our solar system, the geometry of space-time dictates how the Earth moves around the Sun.
The Theory has been studied and verified in a number of ways, however we are going to jump to the science of Black holes  and worm holes and how they impact time travel. We know the density of a black hole distorts local time and space but actual time generally?
It might be worth taking a few seconds to determine the difference between a black hole and a wormhole. A wormhole is essentially a tube in space connecting two points (possible black holes or lesser events that sustain a wormhole, in theory if the conditions are correct a vessel could enter  one end and come out the other in a different time and place. A black hole pinches to a point and destroys anything pulled in to it.
For wormholes to be feasible they need to be sufficiently less violent than a black hole and most likely not have an event horizon or point of no return. There are questions about predicting how, or where or when  a vessel might emerge from wormhole. While we have largely accepted and can point to examples of black holes, wormholes are still the subject of our dreams and fantasies.

The theoretical study of wormholes indicates that the first ones close to the big bang, were microscopic and would probably have  grown in time. There is also the question of their stability. It is thought that adding “exotic” matter to the wormholes would stabilse them, however Dr Steven Hawking has suggested this is not possible, it is also thought that adding sufficient “regular” matter might work. As I type this and as you read it, it becomes immediately obvious that however possible it is highly improbable that we could tame or use a wormhole, due to its like unpredictability, size and energy.

General relativity tells us that gravity and acceleration are two names for the same event. In short there is no “force” of gravity. When we see satellites in orbit over the Earth, it is untrue to say they are in a zero gravity environment. They are actually in free-fall within the Earth’s gravitational field. They exhibit the effects of being in their own local inertial frame and so do not register the weight of their own mass. Satellites fall to earth.

In 1949 Kurt Gödel showed that worldliness in closed space-time could curve back on themselves forming a space-time loop, called Closed Timelike Curves (CTCs). An object on a CTC worldline would eventually arrive back where it started,  at the same spacetime position. An older self would appear at one of its own earlier spacetime points.The Physicist Kip Thorne suggested that if one could trap one of the black holes that comprise the mouths of the wormhole it would be conceivable to transport it, preferably at speeds near the speed of light. The moving black hole would age more slowly than the stationary black hole at the other end of the wormhole because of time dilation. Eventually, the two black holes would become unsynchronized and exist in different external times. The natural time traveller could then enter the stationary black hole and emerge from the wormhole some years earlier than when he departed.Of course there are a few issues with black holes and wormholes (especially wormholes) that prevent us from popping back to various times and locations. We have not found a wormhole, even if we did we have no way of getting to them and even if we did we would need to make sure it was not a black hole that would remove us from existence. If we did find it was a wormhole, how do we know where it leads to, somewhere or time in our universe or another perhaps? There is a point known as the Cauchy horizon which is simply the middle of a wormhole or where two inverse or connected black holes meet – if this exists the issue is we know or can surmise nothing about it. Heaven only knows what is there or what could happen.

Of course harping back to the philosophical nature of time travel it is believed that if a CTC or bend in the timespace was used we could not travel back further than date CTC were perceived. The technology needed for such time travel is also far beyond our capabilities today, either to get to a singularity , to map or enter it and even go through it. We also mentioned “exotic” or regular, matter; how do we get such material there is sufficient  quantities and manage the subsequent energy reactions. These however are technicalities. Time travel into the past is allowed by General Relativity.
 
Conclusion
Can we travel back in time to prevent a zombie apocalypse? Yes and  no. The philosophy of time travel is like most others best answered by taking the economist’s approach “One the one hand…while on the other” looking purely in terms of thought processes, I would have to say that…sorry, the engineer in me is fighting this. It is theoretically and philosophically possible to go back in time. The new question is? What happens then? Do we remember why we went back, how far can we go back, as with the CTC discussion can we only travel back to a fixed boundary point in time. Then we need to consider our physical form when we go back and whether or not we are an adult (presuming we travel as adults) or a child separate to the one in the visited time. Then there is the discussion that we inhabit their physical being or ego and either go along for the ride as a separate entity r control events. So even if we define our physical and mental state on reaching the visit point, we then have to consider the external world, as it were. We need to consider if grandfather will allow us to kill him and then to determine the impact and causal relationship with other events.

If such events were successful, there would then be the consideration of the effects. If the initial reason for the time travel disappeared, what would have been the catalyst  for the time travel have been in the first place. Then there is the multiple universe answer, I have no issue with it, but let’s put t to one side for now. Looking at causality, I cannot help but feel there will be limitations, my concern would be on how those limitations managed to manifest themselves, the very manifestations would suggest links to causality that could be profound (and so possibly unlikely). In theory I am willing to say a form of time travel in to the past is possible but likely to be time and causally restricted.
However I refuse to consider that any kind of Wellsian or basic sci-fi time travel by simple stepping in to a contraption or pressing a button on your wrist-strap. All very entertaining just the same.  This is the point that the laws of physics kick in.  Wellsian time travel is just a non-started. The laws of physics tell us that in terms of the General theory of relativity using wormhole theory and CTCs, all things being equal (such as the wormhole being stable, sufficiently small/weak now to crush whatever enters, having fixed entry and exit points etc.), there does not seem to be anything in the laws of physics to prevent such events. Of course real world limitations make such event highly improbable.

So how does that help us, well however unlikely time travel into a fixed and predetermined point in space and past time may be (Jumping in to a point in the future is not even for discussion!), there is another option. Special relativity. If we can travel in a vessel at a speed sufficiently greater than that of the normal inertial frame  and return to earth at certain times in our chronology we will of course due to time dilation arrive at a time which to us is the future. Now if the world was sufficiently knocked back and depopulated while defeating a past zombie threat to have lost vital, say medical knowledge or abilities, we could consider such options load the necessary cargo on to a ship and travel off at speed  to a pre-determined time point and check if the cargo is needed. Considering that we had the technical ability to do such a thing, it is possible even now, to the mathematics necessary to predict when the ship would arrive back to earth given particular rates of acceleration and deceleration and a specific velocity. We might not be able to prevent a past one, but we might be able to reduce a risk of a new outbreak  in the future…

The next blog will be something a lot lighter and involving a lot less science, possibly…