If the Flash were instead running faster than the speed of sound, then he would arrive at the five-mile point before the sound he emitted at the ten-mile mark. He would then say “Rules” and continue on toward you. Since the “Rules” has less distance to travel, it would reach you before the “Flash,” so you would hear the words in the reverse order that they were spoken—that is, to you it would sound like he’d said, “Rules Flash.” (Actually it would be more like “Selur Hsalf”) This backward speech would not reach you until after he had passed you. Running faster than the speed of sound, he can cover the distance from five miles away to you in less time than the sound waves.
If he were running at exactly the speed of sound, then when he yelled “Flash” at the ten-mile point it would reach the five mile mark at the exact same instant as the Tornado Titan himself did. When he then says “Rules,” it takes off from the five mile point toward you at the same moment that the “Flash” does, so that they both reach your eardrums at the same moment, twenty-five seconds later. You don’t hear “Flash Rules” or “Rules Flash” or even “Suler Hsalf” but the two words superimposed at the same instant. Sound is a pressure wave, so the waves from the two words add up and create a larger vibration than if heard separately. The Flash would not even have to be talking or making noise as he advanced toward you—the disturbance created as he pushed the air out of his way would form a pressure wave that you would hear as a thunderous roar (or a “sonic boom”) at the exact instant the Sultan of Speed cruised by. If the Flash ran faster than the speed of sound, this disturbance would still be created. In this case he would race past you in relative silence, and then later on the sonic boom, traveling at the speed of sound, would eventually reach you, with explosive consequences. (The “Rules Flash” he spoke in the previous paragraph would be lost in the sonic crash.) The “crack” of a gunshot, or of Catwoman’s whip, are mini sonic booms created by the bullet or the tip of the whip moving faster than the speed of sound in air.
The danger posed by the indiscriminate creation of sonic shock waves by the Flash is recognized by current comic-book writers. In DC: The New Frontier, a 2004 revisiting of DC Comics’ Silver Age heroes, set in the late 1950s when they historically made their first appearances, the writer Darwyn Cooke describes a scene where the Flash races from Central City (hazily located in the American Midwest) to Las Vegas, Nevada. In caption boxes that describe the Flash’s thoughts as he runs cross-country, he tells us, “I wait until I’m clear of the city limits before I hit the sound barrier. That’s one I figured out the hard way a few times. Flying glass and pedestrians don’t mix.” Indeed they don’t, as graphically demonstrated in Flash # 202 (Vol. 2). In this story, our hero has lost his memory and, in civilian clothes, does not realize that he possesses superspeed. Acting instinctually when mugged by a street gang, his high-velocity movements blow out every window on the block and cause massive structural damage to the surrounding buildings.33
Regardless of the order in which you hear what the Flash says, whether it’s “Flash Rules” or “Rules Flash,” if you have keen eyesight and can read lips you can be sure of the order in which the Flash actually said these words. This agreement is due to the fact that the light reflecting from the Flash travels much faster than sound (186,000 miles in one second, compared with one fifth of a mile per second). This is how we are able to determine the distance a thunderstorm is from us by comparing the timing between the lightning and the sound of thunder.34
But what if the Flash were running close to the speed of light? All sorts of strange things happen concerning length, time, and mass for objects moving near light speed, as described by Albert Einstein in 1905 in his Special Theory of Relativity (it’s called “special” because it ignores gravity, which is accounted for in the “General” Theory of Relativity, developed in 1915). This is neither the time nor place to go into a full discussion of relativity. A fair treatment of the topic would overwhelm the present book. But I do want to mention a basic point about traveling near the speed of light that won’t take too long, and that we’ll build on when we consider the connection between electricity and magnetism in Chapter 19.
The Special Theory of Relativity can be boiled down to two statements that appear simple, but contain a wealth of physical insight. They are (1) nothing can travel faster than the speed of light (sorry, Superman and Flash), which is the same speed for everyone, no matter how fast they are moving, and (2) the laws of physics are the same for everyone, regardless of whether you are moving or not. The first point is the really weird one. If the Flash is running as fast as a speeding bullet, to us the bullet is traveling at 1,000 miles per hour, while to the Flash, running in the same direction at the same speed, the bullet appears stationary (which is why he is able to, “with a sweeping motion,” pluck it from the air so easily). But the speed of light is 186,000 miles per second to both you, standing still, and to the Flash, regardless of how fast he runs. Even if he is racing at half the speed of light—at 93,000 miles per second—the speed of light relative to him is not 93,000 miles per second, but still 186,000 miles per second, the same as for you standing on the street corner. How can this be?
When the Flash runs toward you, from the Flash’s point of view it is as if he is stationary but you are racing toward him. The Special Theory of Relativity states that for both you and the Flash, you must agree that the speed of light is 186,000 miles per second. In order for this to be true, Einstein argued that from your point of view, the Flash will appear thinner (that is, his length in the direction he is running will appear compressed) and time will seem to pass slower for him than for you. From the Flash’s point of view, a yardstick he holds would still be one yard long, and his watch keeps time just as it always has, but to him it is you who are moving rather than him, and he will make similar determinations about you (your length will be shortened and time will move slower for you, as it appears to the Flash). This is because in order to measure the length of a yardstick that the running Flash is holding, for example, you have to consider the front and back ends of the stick, and record the times when they pass a given point. For two people moving relative to each other (say, a running Flash and a stationary observer) it becomes impossible for them to agree as to whether or not two things happen at the same time when the events are separated in space and time. The shortening of lengths as a function of velocity will play a key role in helping us understand how an electrical current generates a magnetic field in Chapter 18.
The concept that two observers can disagree as to the ordering of events was certainly not invented by Einstein, nor is it unique to the Special Theory of Relativity. As pointed out by Lev Landau and G. B. Romer in their excellent short book, What is Relativity?, even when both observers are stationary they can disagree depending on where they’re situated relative to an object (Go to the house on the left. Which house? Which left? My left or your left?) Just such an ambiguity played a central role in the climax of “The Race to the End of the Universe,” in the December 1967 issue of Flash # 175. Space aliens force Superman and the Flash to run a race across the Milky Way galaxy, ostensibly as a competition on which they intend to gamble. However, the aliens are actually disguised supervillain foes of the Flash, and their true intent is to destroy the Scarlet Speedster and the Man of Steel. At the end of the race, the Justice League of America watches on monitors, yet no clear winner is declared. Those members of the Justice League watching the finish line from monitors placed on the left-hand side of the line claimed Superman had won the race, while those watching monitors on the right-hand side of the finish line are sure that the Flash is the victor. While this comic provided a graphic illustration of one of the fundamental principles of the Special Theory of Relativity, namely that differing observers could disagree about the simultaneity of events when objects are moving at high velocities, at least one reader in 1967 found the lack of a clear winner at the conclusion of this story to be a titanic rip-off.
Information cannot travel fa
ster than light, so there will always be a discrepancy as to the order in which events occur. In order to balance out so that the one thing everyone agrees on is the value of the speed of light, it will appear that lengths are shortened in motion and that time passes more slowly. Comic books frequently get this wrong when dealing with characters who can travel at the speed of light (such as Negative Man of the Doom Patrol and Captain Marvel—not the “Shazam” guy, or the late 1960s-early 1970s officer of the Kree military, but an African-American heroine in the Avengers in the late 1980s who could transform her body into coherent photons of light). The other characters in the story should not be able to see these heroes, as there’s no way for light to be scattered from them if they are moving as fast as light. At best, they might appear as a spark of lightning from a great distance away, but would be invisible when nearby. The highest velocity the universe allows is the speed of light. As the Flash runs faster and faster, one would think that he should be able to pass this limit, but this can’t happen (in the real, physical universe—not the comic-book universe where it happens all the time). To explain this phenomenon from the point of view of a stationary observer, it must be that the faster he goes, the harder it becomes for him to further accelerate. From Newton’s second law (force is equal to mass times acceleration), we know that if the force that his running shoes apply stays constant but there is no corresponding acceleration, it must be because his mass has increased. So in addition to time seeming to be slower and lengths appearing to shrink, the mass of the running Flash will seem (to us stationary slow-pokes) to increase the faster he runs. This is occasionally referred to in the comic books. In Flash # 132, the Monarch of Motion is attempting to run at light-speed, but finds that he is in fact “slowing down. I’m going slower and slower, despite all my efforts . . .” As shown in fig. 17, the “grim truth dawns on the harried speedster,” namely that he can’t run any faster because he is getting heavier. Of course, those of us with an understanding of Einstein’s Special Theory of Relativity understand perfectly well what is happening to the Flash—an alien is shooting him with a “gravity increasing ray,” as shown in fig. 18! But taken out of context, fig. 17 provides an excellent illustration of Einstein’s Special Theory of Relativity.
In the more recent JLA # 89, the Flash has to move the entire population of 512,000 men, women, and children in Chongjin, North Korea, away from the imminent blast of an atomic bomb in a fraction of a second. In order to accomplish this feat, he must move at speeds very close to the speed of light. The relativistic consequences of his high speeds are alluded to once he has saved the town and collapses to his knees on a hilltop. As described in a caption box, “as his body sloughs off the screaming aftereffects of near light travel, eyes of almost infinite mass turn towards the blaze engulfing Chongjin.” Of course, to the Flash, he is stationary and it’s the rest of the world that moves and gains mass. By the way, it was the realization that an increase in the kinetic energy of an object is directly connected to a rise in its mass that led Einstein to derive E = mc2.
Fig. 17. The Flash, attempting to run at light-speed, discovers that he becomes heavier the faster he runs in Flash # 132. Is this a textbook illustration of Einstein’s Special Theory of Relativity?
Fig. 18. No! The Flash is the victim of an alien’s “gravity increasing ray!”
If nothing can travel faster than the speed of light, then how does the Flash’s Cosmic Treadmill allow him to move forward or backward through time? I’m sad to say that time travel doesn’t work in a reversible manner. The closer an object’s velocity comes to the speed of light, the slower time appears to pass, as regarded by a stationary observer. As will be discussed in a moment below, there is a barrier (I suppose one could call it a “time barrier”) associated with the change in mass with velocity that prevents any object from accelerating up to and beyond the speed of light. So time will appear to slow, but will never move backward. However, it is true that time travel into the future, in great leaps rather than a second at a time, is possible, if one can move fast enough.
In the Legion of Superheroes # 16 (Vol. 5), published in 2006, Supergirl from the twenty-first century suddenly flies a thousand years into the future. She had been pursuing an alien missile, sent at great speed by a race of would-be world conquerors to destroy the Earth. This missile was traveling at near light-speed, and the Maid of Steel had been chasing it for three solid days. After finally catching and destroying the missile, she meets a collection of teenage superheroes in the year 3006, the aforementioned Legion, and winds up joining them on a series of adventures. Supergirl and the Legion periodically express puzzlement over how she wound up a thousand years later from when she began chasing the missile. The comic eventually explained her time displacement as arising from her having crossed the path of a “zeta beam,” which apparently can move you dramatically forward in time. But Einstein’s Special Theory of Relativity provides a more natural explanation.
In a reference frame moving along with Supergirl, three days elapse as she chases after the missile. But from her point of view, she and the missile are essentially stationary, and the rest of the universe is moving past her at great speed. If the universe’s velocity, relative to Supergirl, is close to the speed of light, she will observe a dramatic time-dilation effect. Using Einstein’s equations, if Supergirl’s velocity were 99.9999999963 percent of the speed of light, then a journey that she experiences as lasting three days would, for stationary observers on the Earth, take one thousand years. When she eventually slowed down and stopped, she would find herself a thousand years ahead of when she began. There is thus a physically valid way to travel into the future, with the caveats that (1) it would require extreme amounts of energy for an object of any real size to be accelerated to such a velocity and (2) it is a one-way trip, and flying backward won’t send you back in time. (Eventually the “zeta radiation” wore off, sending Supergirl back to the twenty-first century, enabling her to continue having adventures in her own comic book.)
But what about Superman, who frequently travels back in time by flying at great speed? The Warner Bros. 1978 film Superman: The Movie included a scene where, in order to undo the effects of a devastating earthquake (particularly Lois Lane’s death), the Man of Steel flies around the Earth so fast that he reverses the direction of time. Is this at all possible? Close examination of the scene in question, estimating the height above the Earth that Superman is flying, calculating the distance of an orbit and counting the number of circuits completed and the time he takes doing so, we conclude that Superman is indeed traveling at or faster than the speed of light! But that alone won’t save Lois Lane. Traveling that fast, he would jump forward into the far future, as his cousin did in the Legion of Super Heroes story, and might be in time to help some distant descendant of Ms. Lane, but not be able to save Lois herself.
While the Flash did not have to worry about length shortening (technically referred to as “Lorentz contraction”) too often, the slowing of time, or “time dilation” nearly cost him his secret identity. While the Scarlet Speedster is the fastest man alive, one of the quirks of his secret identity is that his alter ego, Barry Allen, is perennially late for his appointments. Often his fiancée, Iris West, would complain about Barry’s tardiness and wish he were more like the Flash, not realizing that they were one and the same. As fate would have it, Iris’s father would also notice Barry’s habitual lateness, and as described in “Slowdown in Time” in Flash # 141, was very intrigued by the fact that Barry’s watch always seemed to run slow. For Iris’s father was physics professor T. H. West, and he suspected that Barry’s watch was running slow because of time dilation effects arising from his superspeed crime-fighting exploits as the Flash. Spotting a robbery in progress, he called Barry, reported the crime underway, and asked Barry to synchronize his watch. Knowing that the Flash would have to run at great velocities to catch these criminals, he intended to meet up with Barry later on and compare the times on their watches. And it would
have worked, too—had Barry not reset his watch before meeting with Professor West. Tipped off by the strange request to synchronize watches, as the story concludes, Barry thinks: “As a scientist myself I’m also familiar with Einstein’s Theory of Relativity—and the effect of ultra-speed on clocks!” Sometimes even physics professors fall short when they try to outthink the Viceroy of Velocity!
In theoretical physics, there is one thing that can travel faster than the speed of light: particles termed “tachyons” that in fact can never travel slower than the speed of light. Under certain circumstances, they can appear to travel backward in time (which proved useful for Adrian Veidt in Watchmen, who employs tachyons to obscure the precognition abilities of Dr. Manhattan). Tachyons were proposed as a test of certain consequences of the Special Theory of Relativity. As far as we know, they do not exist, and more importantly, even if they were as common as crabgrass it doesn’t appear that they can interact with our physical world, in which no object can move faster than the speed of light. The Flash may travel backward and forward in time using his Cosmic Treadmill, but its only real value is in providing the Scarlet Speedster with a cardio workout.
The fact that Superman and the Flash would move through time thanks to their superspeed powers has not escaped the attention of physicists. Following the printing of the first edition of this book, a story was relayed to me concerning Superman’s meddling with the space-time continuum. Back in the 1950s a group of physics majors at M.I.T. wrote to Mort Weisinger, the editor of Superman comics at the time. They complained that a recent issue had shown Superman flying faster than the speed of light, in direct contradiction to Einstein’s theory. What did he have to say about that? To which Weisinger allegedly replied: Einstein’s is only a theory—Superman is fact!
The Physics of Superheroes: Spectacular Second Edition Page 15
