Few subjects in science evoke more emotion than time travel. Humankind's fascination with time travel dates back thousands of years. Although, there is no consensus recognizing which written work was the first to discuss time travel, many scholars argue that the Mahabharata, from Hindu mythology, is the first, dating between 700 BCE (Before the Common/Current/Christian Era) to 300 CE (Common/Current/Christian Era). The Mahabharata, which is one of the two major Sanskrit epics of ancient India, relates the story of King Revaita, who travels to heaven to meet the deity Brahma. When King Revaita returns to Earth, he is shocked to learn that many ages have passed. In today's science, we would say that King Revaita experienced "time dilation," which I will discuss in detail below.
Although many people believe that time travel is strictly science fiction, it is not. Actually, as odd as this may sound, we are all time travelers. We are traveling from the present to the future. Since all of us move at speeds far below the speed of light, we are essentially all moving in time at the same rate. We measure our movement in time in arbitrary units. The measurement may be in seconds, minutes, hours, etc., based on a conventional clock, or we may be measuring our movement in time on an atomic clock, which is the most accurate type of clock humankind has developed to measure the passage of time.
I realize viewing ourselves as time travelers is not what most people think about when they consider time travel. Most people envision traveling between points in time (e.g. from the present to the past or vice versa), without experiencing the intervening intervals. In this article, we will address forward time travel, since its science and experimental verification is widely accepted by the scientific community.
Einstein's special theory of relativity, published in 1905, was the first to give scientific meaning to forward time travel. This theory predicted a strange new phenomenon, time dilation. What is time dilation? It is a phenomenon where time moves slower for any mass accelerated near the speed of light. If that mass were a clock, for example, the hands of the clock would appear to be moving slower than a clock at rest. Time dilation is an experimentally verified fact. In 1977, Bailey et al. accelerated a muon, a negatively charged subatomic particle about 200 times heavier than an electron, close to the speed of light, using the CERN Muon storage ring. The accelerated muon's lifetime increased almost thirty times that of a muon at rest. This time dilation effect suggests that the muon is able to travel to the future, since it continues to exist at a future time. There is also a time dilation effect caused by gravity, but due to its complexity, we will save discussing gravitational time dilation for a potential future article.
Einstein's special theory of relativity, including time dilation, is widely accepted by the scientific community. In fact, Einstein's special theory of relativity is the "gold standard" of theories, having withstood over 100 years of experimental verification. However, to date we have not been able to build a time machine to send larger objects, like humans, into the future.
To provide insight into the engineering difficulty of transporting a human into the future, let us consider what is termed the "twin paradox." Picture a pair of twins. One twin boards a spaceship that leaves Earth at a velocity close to the speed of light (e.g. 95 percent of the speed of light), and returns to Earth a year later relative to the clock on the spaceship. When the space bound twin returns, he or she is one Earth year older. However, for the twin that remained on Earth, approximately three Earth years will have passed. The twin in the spaceship has time traveled to the Earth's future. The scientific community views the twin paradox a scientific fact, having proven its validity experimentally using particle accelerators.
What is the engineering difficulty regarding the above scenario? It is impossible using today's science to build a spaceship capable of traveling close to the speed of light. According to Einstein's special theory of relativity, both the energy required, and the mass of the spaceship approach infinity as the spaceship approaches the speed of light. Today's science is only able to accelerate subatomic particles close to the speed of light. The energy to accelerate a spaceship close to the speed of light requires we learn to harness extremely large energy sources. For example, we may need to learn to harness the energy equivalence of a star, similar to our Sun.
How close are we to building a practical time machine capable of sending humans to the future? This is a judgment call. If you examine the real science of time travel, there is widespread agreement on the theoretical foundation suggesting forward time travel is possible. Unfortunately, the engineering associated with building a machine capable of sending humans into the future has proved a formidable task. In my judgment, we are at least several decades, or more, away from solving the engineering problems associated with human time travel to the future.
This article discussed time travel to the future. Now you know it is real. Backward time travel, based on Einstein's general theory of relativity, is also a theoretical possibility, but presents even greater obstacles, including numerous time travel paradoxes. Time travel paradoxes are thought experiments illustrating violations of causality (cause and effect). I discuss forward and backward time travel in my book, Unraveling the Universe's Mysteries, available online at Amazon and Barnes & Noble, and on by blog, Science Questions & Answers (www.louisdelmonte.com).