To Infinity and Beyond: A Journey of Cosmic Discovery

The understanding of time has also been impacted by modern science. Relativity not only changed how the fabric of space is viewed but also how time intersects with space: The fabric of spacetime is both weirder and vaster than originally thought possible. Even though the idea that the universe had a specific beginning was resisted by some scientists, it has always been embraced by various cultures throughout the world; creation stories are ubiquitous in myths and religions.

This discovery led to questions about the size and age of the universe. These measurements are made possible by calculating the luminosity of stars, thus determining their distance from Earth in light-years. After many decades of recalculations, the universe is estimated to be about 13.8 billion years old. The oldest signal detected in the universe—the cosmic microwave background (CMB)—also indicates the temperature of the universe, which again confirms that the cosmos had a definitive beginning. While the temperatures of stars and other matter fluctuate, the CMB remains constant, which confirms that at one moment in time, the universe “was all in the same place experiencing the same event at the same time” (246): the Big Bang.

If the universe is expanding from the specific point of the Big Bang, then it stands to reason that it has a finite edge—though the authors clarify that what is meant by this is the edge of the “observable universe.” Beyond a certain point, light may not be able to penetrate for the telescopes to see. Thus, the universe itself may be infinite, but this speculation will always be beyond the purview of human knowledge.

Space and time are inextricably linked, and when scientists study stars, they also see into the past. The observations of the cosmos are not in real-time but images that make their way to Earth after years, decades, or much, much longer. Worldlines map the coordinates of an object (person or particle) in the intersection of time and space, and while travel through space is possible, travel through time is conjecture.

Warped spacetime could provide clues as to the possibility of time travel. Gravitational waves could reveal how this “warped spacetime” works differently than normal matter. Time travel has captivated the imagination of scientists and Hollywood directors alike. Einstein’s general theory of relativity posits that the speed of light is constant, while his special theory of relativity proposes that time is relative. One of the problems with traveling to the future is that one must not only choose the date and time but also the location of where one wants to go; however, one cannot know if that location will exist in that future time due to the universe’s movement. In addition, “time itself varies” to accommodate the constancy of the speed of light, according to mathematical equations (262). In space, astronauts age fractionally less than their Earth counterparts; this is due to the velocity at which the astronaut travels. Muons—particles about 200 times the mass of electrons—reveal the relativity of time. The rapidity of their decay is a tiny fraction of a second, as demonstrated in particle accelerators, yet they reach the Earth’s surface from space. This is only possible due to time dilation—the relativity of time.

Black holes are another mysterious cosmic phenomenon. Black holes are giant stars that collapse under the weight of their own gravity, and they only consist of that gravity; they can only be detected by the matter that is being engulfed by them. At the event horizon, “gravity overwhelms the speed of light” (269). This monumental gravitational force creates time dilation: At the event horizon, a single second could represent millions of years on Earth.

Time travel to the past presents its own unique set of problems, yet, as the authors note, mathematical equations suggest that it is possible. They propose three potential ways, all involving FTL (faster than light) speeds, to travel backward in time: through a wormhole; through a warp drive; or by using tachyons (a theoretical particle that can move faster than the speed of light). A wormhole suggests one can bend space, bringing two distant points together like folding a piece of paper to have its two farthest edges touching. Wormholes, however, are inherently unstable and collapse immediately after formation. There is speculation that dark energy or dark matter—charged with a negative force—could keep wormholes open, though most scientists believe that wormholes are a practical impossibility.

A warp drive, as in the Star Trek franchise, creates a bubble around a spacecraft, allowing it to travel FTL within that localized space (relativity states that objects cannot move faster than the speed of light within spacetime). It also seems a practical impossibility due to the massive amount of energy required to create such a bubble. Tachyons are particles that can themselves travel faster than the speed of light, though they can never slow down or stop, or else they no longer exist. A tachyon text message, as the authors explain, would arrive before one sends it.

In addition, one encounters the problem of causality in time travel. The authors lay out a scenario: An astronaut receives a message that the Earth is destroyed and travels through a wormhole to save the planet before it happens, but this begs the question of who sent the original message. Called a paradox, this kind of event cannot occur in physics. A common thought experiment about time travel, the “grandfather paradox,” posits that if one interrupts the timeline to prevent one’s grandparents from meeting, then one would never have been born in the first place (think Back to the Future from 1985). One exception might be the causal loop: No matter how hard one tries, one cannot change the outcome of events, even when traveling back in time. Events have already happened, and one’s presence in the past only contributes to the same sequence of events. All the discussion about time travel also presupposes questions about the nature of free will. Either the future is predetermined, or humanity can change and shape it.

Finally, the many-worlds theory comes out of the field of quantum physics, where the smallest particles behave in ways that defy regular physics. For example, photons behave differently when they are observed than when they are not. The double-slit experiment—performed many times in many different permutations—reveals that when photons are fired through two slits against a wall while someone is watching, they form the pattern that one would expect: two straight lines. However, when nobody is observing, the photons form a random pattern. Quantum physics suggests that all possibilities are open until one path is chosen. Each time a decision is made, the universe splits into another universe where a different possibility is chosen; hence, there are many worlds. This is different from the multiverse, wherein infinite universes exist along the same spacetime continuum. The many-worlds explanation insists that no universe could interact with another; the other worlds simply do not exist within the confines of this cosmos.

The sidebars in this section include a discussion on how the age of the Earth was determined; how the Big Bang got its name (as a joke); how time travel would be exceptionally dangerous; how GPS and time dilation intersect; how dark matter functions in the expansion of the universe; how quantum foam operates in the microcosmic world; and how Stephen Hawking disproved that time travel was currently possible. They also tackle popular culture tropes such as how black holes are depicted in movies; how Hollywood (in this example, Star Wars) gets hyperspace travel wrong; and how the popular narrative about traveling back in time to kill Hitler is another example of a paradox—impossible in physics.