“Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people.” Carl Sagan was the man who had once spoke this quote. If we take time to look at the big picture, the human race and the Earth we inhabit really is insignificant when it comes to the whole scheme of the universe. This is one of the reasons why so many people wish time travel and travelling to other universes were possible. We wish to figure out the universe in entirety, and we want to know if we’re alone or if there’s more than our version of “life.” Currently, time travel and travelling to other universes is nothing more than fiction that movies like Star Wars and books like The Time Machine by H.G. Wells have created. However, there are some real, as well as some theoretical, objects in space that could, theoretically, allow us to travel throughout time and space. These objects are: black holes, white holes, and wormholes. In this paper I will discuss what these objects are, how they work, and how they could, together, allow space travel.
A black hole is an invisible object in space that is so compact that, within a certain distance of it, even light isn’t fast enough to escape. Black holes are thought to be born from stars or other massive objects that collapse from their own gravity to form an object whose density is infinite. Once all the dying stars’ fuel for nuclear burning has run out, what’s life is the core; In a black hole, otherwise known as a singularity. The space surrounding the singularity, where the escape velocity must be equivalent to the speed of light is what’s called the event horizon; or “the point of no return.” (Seidel).
Speculation of black holes has dated back as early as 1783 when John Michell theorized that there might be an object massive enough to have an escape velocity greater than the speed of light. Simon Pierre LaPlace theorized not long afterwards that it is possible that the largest luminous bodies in the universe would be invisible (Is a Black Hole Really A Hole?). Black holes really came to light, though, after Albert Einstein developed and published his theory of relativity in 1915, in which he predicted space time curvature; going against Newtonian physics, which stated that all things in space travelled a straight line unless acted upon.
There are four different kinds of black holes that we know of: Static, Charged, Rotating, as well as a Supermassive. All four kinds are made up of the same three elements: The Photon Sphere, the Event Horizon, and the Singularity. There are always exceptions, though, which will be covered in greater detail later in this paper. The Static black hole, which is often referred to as Schwarzschild black holes after Karl Schwarzschild postulated them in 1916. The static black holes are the simplest of the bunch because they are simply defined as having a mass but no electric charge or angular momentum; when something doesn’t have angular momentum, that simply means that it is stationary and doesn’t spin (Research Paper on Black Holes). This type of black hole is said to have one photon sphere, one event horizon and one singularity. It’s also the only vacuum theorized that is spherically symmetric; meaning there is no observable difference between the gravitational field of this type of black hole and any other object with the same symmetry and mass.
The next kind of black hole, the charged black hole, is one which possesses an electric charge. It is said to have one photon sphere, one singularity, but two event horizons. Hans Reissner and Gunnar Nordstrom are responsible for theorizing this type of black hole and coming up with an equation to explain its possible existence. They discovered that if a small charge is added to a black hole, the event horizon would shrink and a second, inner horizon would form just above the singularity.
The more charge that this type of black hole had, the smaller the outer horizon becomes, while the inner horizon expands (Jillian). What happens, then, if the magnitude of the charge becomes as great as its mass? Reissner and Nordstrom predicted that both horizons would eventually vanish and leave a naked singularity. Having a black hole with no outer or inner horizon and simply a singularity would create a problem for those people who would attempt time travel. You would have absolutely no warning as to where the black hole began and you would instantly be sucked in and ripped apart by the hole’s tidal forces.
The third type is called the Rotating black hole and was proposed in 1963 by New Zealand mathematician Roy Kerr. His concept hinges on neutron stars, which are massive collapsed stars the size of Manhattan but with the mass greater than that of Earth’s sun (Kaku). Kerr postulated that if these dying stars collapsed into a rotating ring of neutron stars, their force would prevent them from combining into a singularity. Since this black hole wouldn’t have a singularity, Kerr believed it would be safe to enter without the fear of the infinite gravitational force pulling and stretching you until you were ripped to shreds. Because of this reason, this is the only type of black hole that is used when the theoretical discussion of time travel arises (Bonsor and Lamb).
The Supermassive black hole is the final type. Theoretically, it could be any of the three types: static, charged, or rotating. The only difference is that it is much, much larger. Supermassive black holes range anywhere from one million to billions of times the mass of our Sun (Research Paper on Black Holes).
According to Einstein’s Theory of Relativity, black holes (at least the static ones), do exist in space. Applying these black holes to time travel, though, seem to be impossible currently because of his Theory of Relativity, as well as the principles of the speed of light. The Theory of Relativity states that a particle (that has rest mass) with a subluminal velocity needs infinite energy to accelerate to the speed of light. However, to allow for time travel, one would have to be travelling faster than the speed of light, which is currently impossible (Bonsor and Lamb).
White holes are often called “anti-black holes.” This is because they’re black holes running backwards in time. Just as black holes swallow things irretrievably, white holes only spit things out. These objects would have a negative gravity and emit an extremely bright white light. However, it should be noted that white holes violate the second law of thermodynamics, and are only a thing of speculation (Hamilton).
Karl Schwarzschild not only proposed his own theory of a black hole with his Schwarzschild metric, his system also consists of a white hole and two universes connected by a wormhole (a topic which will be covered in just a few paragraphs).
In order for white holes to be of any use when it comes to time travel, parallel universes must exist. These parallel universes would exist alongside one another in hyperspace (Mendez). They would not be on the same plane of space; there would be no way of reaching them with simply a spaceship because they are not in the same dimension. Since they are parallel, they are in different dimensions, and in order to reach them one would have to travel, theoretically, through a black hole and exit in the other dimension through a white hole. These two holes would be connected by a wormhole, which will be discussed later.
One reason why white holes cannot exist in space is because they violate the second law of thermodynamics. The second law of thermodynamics says that any ordered system becomes more disorganized with time, and so a system which adds order, such as a white hole, is not at all possible (Wojcik).
A wormhole is a funnel in a region in space which connects two different places within the same space time or connects two asymptotically flat times (i.e. parallel universes). Wormholes, like white holes, are also purely hypothetical since it would require exotic matter, or matter with a negative energy density, to hold the tunnel open (Black Holes and Quasars). However, Einstein’s general theory of relativity allows for the existence of these since it states that any mass curves space time.
There are a few different types of theoretical wormholes. The Schwarzschild wormhole has an unstable throat and is connected to a black hole. This theoretical wormhole would be one where you could go past the black hole’s horizon without being torn apart by its tidal forces (Hamilton). As you travel further and further inside, you will eventually be able to see through the mouth on the other side, and eventually you would pass through the black hole, reach the white hole, and be spit out into a different universe.
Another type of wormhole is a Morris-Thorne wormhole. It differs in a few aspects from the Schwarzschild wormhole. First, this wormhole’s throat is more stable, and even though it still has an enormous tidal force at its mouth, it has no horizon. Because of this, it would only require planet size masses, which is much less than the other wormhole.
A Visser wormhole is formed by knitting together two ‘deep’ potential wells to form a throat. The inconvenience, though, is that it requires a lot of exotic matter to keep it open. Unlike a Morris-Throne or Schwarzschild wormhole, the Visser wormhole has significantly less tidal force due to this exotic matter.
The final type of wormhole is called an Ellis wormhole. There really aren’t many differences in this special case from the general case, other than that time flows smoother and the mouth is completely spherically symmetrical (Schimelpfenig).
Since wormholes would significantly shorten the amount of time it would take to travel from one point in space to another, many scientists have theoretically speculated that if any type of wormhole were connected to a black hole at one end and a white hole at another end, space travel would be possible (were these objects to exist).
As with every theoretical situation, there are problems that exist whilst considering the possibility. There are paradoxes, such as the Grandfather Paradox. Also, there are issues like an inconsistent casual loop and a consistent casual loop could occur and need to be considered.
“Imagine, you’re a time-travelling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to an 18-year old version of your father’s father. You raise your laser blaster, but what happens when you pull the trigger? You haven’t been bon yet. Neither has your father. If you kill your own grandfather in the past, he’ll never have a son. That son will never have you and you’ll never happen to take that job as a time-travelling assassin. You wouldn’t exist to pull the trigger, thus negating the entire string of events.” (Bonsor and Lamb).
This is what is known as the Grandfather Paradox, and it is probably the most famous paradox relating to time travel. This sting of events is also called an inconsistent casual loop.
While considering the possible issues with time travel, one must also consider the idea of a consistent casual loop. According to physicist Paul Davies, such a loop might play out like this: a math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with (Bonsor and Lamb). These issues and paradoxes make the thought of time travel an even more complicated possibility.
If time travel were to exist there would need to be black holes to act as an opening of a portal, a white hole, as an exit point of the portal, and a wormhole to act as a link between the two. While it is very interesting to theorize about how time travel could be possible, because of the different issues and paradoxes that would go along with it, time travel is a much more complex theoretical speculation than we had previously expected.