Traversable wormholes may enable time travel to the past. This post is based on material from my new book, How to Time Travel.

Let us begin our discussion by understanding the scientific meaning of a “wormhole.” There are valid solutions to Einstein’s equations of general relativity that suggest it is possible to have a “shortcut” through spacetime. To picture this, consider a piece of paper with a dot at opposite corners. In Euclidean geometry, normally taught in high school, we learn that the shortest distance between the two points is a straight line. However, valid solutions to Einstein’s general relativity equations suggest that the two points on the paper are connectable by an even shorter path, a wormhole. To visualize this, simply fold the opposite corners of the paper with the dots, such that the dots touch. You have created a representation of a wormhole. You have manipulated the space between the dots by folding the paper to allow them to touch.

Unfortunately, there is no scientific evidence that wormholes exist in reality. However, the strong theoretical foundation suggesting wormholes (i.e., valid solutions to Einstein’s equations of general relativity) makes their potential existence impossible to ignore.

The first type of wormhole solution to Einstein’s equations of general relativity was the Schwarzschild wormhole, developed by German physicist Karl Schwarzschild (1873–1916). Unfortunately, although the Schwarzschild mathematical solution was valid, it resulted in an unstable black hole. The unstable nature of the Schwarzschild wormhole suggested it would collapse on itself. It also suggested that the wormhole would only allow passage in one direction. This brought to light an important new concept. Faced with the unstable nature of Schwarzschild wormholes, American theoretical physicist Kip Thorne and his graduate student Mike Morris demonstrated a general relativity “traversable wormhole” in a 1988 paper. In this mathematical context, a traversable wormhole would be both stable and allow information, objects, and even humans to pass through in either direction and remain stable (i.e., would not collapse on itself). As is often the case in science, one discovery leads to another. Numerous other wormhole solutions to the equations of general relativity began to surface, including one in 1989 by mathematician Matt Visser that did not require negative energy to stabilize it.

As discussed above, traversable wormholes may require negative energy to sustain them. Several prominent physicists, including Kip Thorne and British theoretical physicist/cosmologist Stephen Hawking, believe the Casimir effect proves negative energy densities are possible in nature. Currently, physicists are using the Casimir effect in an effort to create negative energy. Obviously, if successful, the amounts of negative energy will likely be small. Because of the amount of negative energy that may result, I suspect the first wormholes developed will be at the quantum level (i.e., the level of atoms and subatomic particles).

We have merely scratched the surface regarding the science of wormholes, but we did accomplish one important objective. We have described how a traversable wormhole would allow spacetime travel via shortcuts in spacetime. This means we could connect two points in time or two points in space via a traversable wormhole. However, there is a hitch regarding time travel to the past. According to the theory of relativity, we cannot go back to a time before the wormhole existed. This means that if we discover how to make a traversable wormhole today, a year from now we can go back to today.

You may wonder why a wormhole constructed today would not allow us to go back to yesterday. To understand this conundrum, we need to understand just how a wormhole works as a time machine. Here is one scenario. Imagine you are able to accelerate one end of a wormhole to a significant fraction of the speed of light. Perhaps you could use a high-energy ring laser (i.e., a laser than rotates in a circle). As you twist the space, you create the “mouth” of the wormhole, something like a tunnel. After you enter the mouth of the wormhole, you are now somewhere in the wormhole’s “throat.” A “tunnel” is a good analogy to what is occurring. Now imagine you are able to take the other entrance of the tunnel, which is at rest and called the “fixed end,” and bring it back close to the origin. Time dilation causes the mouth to age less than the fixed end. A clock at the mouth of the wormhole, where spacetime accelerates near the speed of light, will move slower than a clock at the fixed end.

Given the above understanding of how a wormhole acts as a time machine, let us address why it is only possible to go back to the time of the wormhole’s construction. Imagine you have two synchronized clocks. If you place one clock at the mouth, and you place the other clock at the fixed end, they will initially read exactly the same time, for example, the year 2013. However, the clock at the mouth, influenced by the twisted space, is going to experience time dilation, and therefore move slower than the clock at the fixed end. Let us consider the case where the clock at mouth of the wormhole moves, based on the rate of twisting spacetime, one thousand times slower than the clock at the fixed end. In one hundred years, the clock at the fixed end, which experiences no time dilation, will read 2113. The clock at the mouth will still read 2013; only one tenth of one year will have passed due to time dilation at the mouth of the wormhole. From the fixed end, where no time dilation is occurring (i.e., the clock reads 2113), you can walk back to the mouth of the wormhole, where the clock still reads 2013. You will have walked one hundred years into the past. Notice, though, you cannot go back beyond the time of the traversable wormhole’s construction.

This post is based on material from my new book, How to Time Travel. Click How to Time Travel to browse the book free on Amazon.

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