These are some notes on the difficulties with Time Travel for my book “Out of Time”
Trying to understand Time Travel.
There are a number of difficulties in trying to write a consistent story about people moving from the past to the future or visa versa.
If we assume that each person has only one time line then to jump to a point in the future would suggest that the person has now altered their time line. But if they were always going to jump to some point in the future then their time line hasn’t changed.
The classic paradox is if a person goes back in time to prevent his own birth does he cease to exist in the future? Or is he still travelling on his own time line. He obviously still exists but if he then goes forward to his own future will he be known or vanished?
Like wise what happens if he goes to the future or past to meet himself? Once you accept that time travel exists this is not a problem. It was always going to happen. If a person travels to the past to meet himself he will already know he has met himself in the past.
So what happens if that person who knows he has met himself in the past decides not to go into the past and meet himself? Then if he chooses not to go then does the past meeting not occur? But does he still have a memory of the past meeting? The paradox can only be resolved only by saying that if he decides not to go then he never has gone to the past to meet himself and therefore he has no memory of the meeting.
There is also the problem of spatial movement with time. The sun is moving at 220 km/s around the galaxy and the earth is orbiting the sun at about 18 m/s. Thus any travel through time to meet the past or future must also ensure that the location can be found. To put it simply: the spatial location of Hamgreen in 1873 is millions of miles from its location in 2015. This can only be feasable if time is some form of spatial dimension linked with the three dimensions of our normal world.
So is time all in the mind?
Firstly we have to decide what we mean by time. We are generally concious of time by the memory of movement. An object moves from A to B then we see A as the past and B as the present. With no memory, no movement, no sense of time. The knowledge of the past through memory also allows us to ‘see’ the future. Based on our memory of past events we can construct futures which in our minds seem as real as the past. These constructs also allow to plan or hope to change the future. So in our minds we are continually travelling backwards and forwards in time. It is therefore only a short step to imagining it is possible to travel through time. ….But this is all in the mind.
If we actually wish to visit the past or future it requires a physical translation which involves a different type of time; one that we were not generally aware of until James Maxwell produced his electro-magnetic field equations and Einstein his theories of relativity.
In our ‘normal’ world we live in a 3-dimensional space and we are only conscious of moving through it by the use of memory.
However, Einstein showed that when an object moves through space it actually moves through 4 dimensions, 3 spatial and a 4th “time” dimension which has some special properties which affect the other 3 dimensions. These properties are;
1) All objects in this 4 dimensional space travel at the speed of light. Never slower never faster. Even while you are sitting in your chair reading this you are travelling at the speed of light in 4D space.
2) An object at rest, i.e. an object not moving in the 3 space dimensions, will travel along the 4th dimension at the speed of light. The speed of light is ~300,000 kilometres/sec then in 1 second the object will travel a distance of 300,000 kilometres along this ‘time’ dimension. In our ‘normal’ world we have no sensation of this because everything is travelling at this speed in 4d space.
So what happens if an object moves through the three space dimensions with velocity v? It cannot go faster than light so the only thing that can happen is that time must slow down to compensate so that it still travels through the 4 dimensions at the speed of light. Or conversely, the distance it travels contracts to compensate.
What happens as we increase the velocity of the object? As the velocity of an object increases time slows down further until when the object is travelling at the speed of light the time reduces to zero. Thus it takes no time for an observer sitting on that object to go from point A to point B in space.
In other words, an object traveling at the speed of light only travels through space. It experiences no time in travelling from A to B.
This is not obvious to an observer. For example the nearest star is 4.5 light years away. If a photon leaves the star and travels to the earth as it travel at the speed of light it takes no time to reach the earth. However an observer sees it takes 4.5 years for the light photon to reach an observer on the earth. What is happening?
— the observer has moved 4.5 light years along the time dimension.
However, an object with mass can only travel slower than the speed of light, because an additional effect of the 4 dimensional space is that its mass increases with velocity and at the speed of light it will have infinite mass. Thus in accelerating from rest to the speed of light will require infinite force.
So this means;
only massless objects such as photons travel at the speed of light.
objects with mass travel through time and space
So what does 4-dimensional space look like?
We see only in 3-dimensions. To try and understand how we might see in 4-D let’s look at how we see a person who lives in a 1-dimensional world.
Imagine an old chart recorded. The pen moves backwards and forwards along its track. This can be the 1-d person who can only move up and down this tract. We however can observe and record his path with time. As the chart moves the pen traces out as a graph the movement of the pen or 1-d person. We then have a view of the 1-d person’s life. On the chart the vertical trace is his movement in his 1-d movement through space and the horizontal trace is his movement through time. We can observe his path through time whereas he can’t. Hold that thought.
Now imagine a sheet of paper as a 2-d world with a black dot as a 2-d person. His world is always flat irrespective whether the paper is bent or twisted and he will think it has finite boundaries. However, If we twist the paper into a Möbius strip he will travel along it for ever and he will presume his world is infinitely long in one direction but bounded on either sideside.
Now imagine this 2-d creature of the surface of a Klien Bottle.
A Klien Bottle only exists in 4-d but every part of the Bottle is part of a 2-d surface with no edge or boundary and only one hole. It cannot be constructed in 3d space.
No matter which way he travels it will seem to him that there are no boundaries. His universe is infinite. However, we as 3-d creatures can see that his universe is bounded, i.e. the surface area of the Klien Bottle is finite.
As a 3-d observer we can trace the line of the 2-d person on the paper or klien bottle and construct a 3-d path of its journey. This is his time line which only we as a 3-d being can see. Hold that thought.
Now let’s take a long strip of paper. Mark the letter A at one end and the letter B at the other end. Now cut a square hole in it near one end just after the letter A.
Then make a single twist in the paper and bend it back under the strip so that it joins underneath the hole at the other end and the letter B can be seen in the square hole. The 2-d black dot person can either carry on along to the end of the twisted strip which brings him back to the origin or he can just walk into the end of the strip where it joins the square hole. The 2d observer then has a choice of two time lines to the future point B. Here we can see a way of travelling to a point in the future without having to travel along the normal time
Now let us look at a 3-d observer, us. We can only see in 3 dimensions. We are not able to observe the 4th time dimension. However what does an object look like to a super being who can see in 4 dimensional space where the 4th dimension is the time dimension?
Such a hypothetical being would have to live in a 5-d universe in order to see the 4-dimensional space.
If we take a sphere at rest in 3-d space, in 4-d space it would look like a rod with the same diameter as the sphere increasing in length by 300,000 kilometres per second.
To the superbeing this 4d rod would look real and it should be able to touch any part of this rod or any part of its existence in time.
Now what does a human being look like to this superbeing?
It would grow from a a single cell to a baby to an adult and then reduce to dust. This ‘life rod’ would be of finite length and follow the convoluted path the human would take from birth to death.
Now if this time or life rod is real can it be bent, moved, cut or changed by the super being? In fact could it be bent into a circle or moved backwards or forwards along its 4th time dimension.
If this is possible then it is a small step to move a person’s life rod or part of it back in time and space.
However in the absence of superbeing the top time line can still be distorted by the use of gravity.
Effect of Gravity
In the absence of matter or mass, space-time is described as ‘flat’. This means that each of the 4 dimensions are at right angles to each other and an object at rest will travel in a straight line along the time dimension at the speed of light.
Now in Newtonian physics all matter has an associated gravitational field .
In the Einstein universe the presence of mass or matter distorts space-time which we see as gravity.
With sufficient mass, space-time can be so distorted that space-time could be folded back on itself. It is possible that the time dimension could be bent back so that it nearly join itself. This point could in theory allow movement from a point in the past to a point in the future along the time line.
However, the important thing to remember for the moment, is: when you move through space you are compelled to move through time but, when you move through time (which of course you are always doing) you do not have to move through space.