Tachyons are theoretical particles believed to exceed the speed of light and move backward through time.
For tachyons, the possibility of exceeding light speed and traveling through time might be real. Science fiction excels at captivating us with scenarios that defy the universe’s physical laws. We are fascinated by the warp engines of the starship Enterprise propelling it past light speed, or when Barry or Wally—whichever incarnation of the Flash—accomplishes this feat in just a pair of yellow boots.
Similarly, we enjoy stories featuring adventurers like the Doctor or Doc Brown, who use peculiar, seemingly outdated technology to break the rules of causality. Imagine if there were a fundamental particle capable of all these feats—surpassing light speed like the Flash and journeying back in time without requiring a TARDIS, a DeLorean, or even yellow boots.
This is the concept of a tachyon. But make no mistake: these particles aren’t purely the musings of science fiction writers. Tachyons are grounded in the realm of “hard” science.
What is a tachyon?
Tachyons stand out as one of the intriguing concepts derived from Einstein’s theory of special relativity. This 1905 theory is founded on two key postulates: first, nothing with mass can surpass the speed of light (c), and second, the laws of physics remain consistent across all non-inertial reference frames. A significant implication of special relativity is that space and time merge into a singular framework known as spacetime. This means a particle’s movement through space is inherently connected to its movement through time.
The term “tachyon” made its debut in scientific literature in 1967, thanks to a paper titled “Possibility of Faster-Than-Light Particles” by Columbia University physicist Gerald Feinberg. Feinberg suggested that tachyonic particles might emerge from a quantum field characterized by “imaginary mass,” which explains why the first postulate of special relativity does not constrain their velocity.
This concept introduces the possibility of two types of particles in the universe: bradyons, which travel slower than light and constitute all visible matter; and tachyons, which travel faster than light, as noted by the University of Pittsburgh. A distinguishing feature between these particles is that while adding energy to bradyons increases their speed, reducing energy in tachyons causes their speed to increase.
Tachyons and time travel
One of the most significant outcomes of Einstein’s theory of special relativity is the establishment of a universal speed limit, represented by c—the speed of light in a vacuum.
Einstein proposed that as an object nears this speed limit, its mass approaches infinity, as does the energy required to accelerate it further. This implies that nothing can move faster than light. However, consider the possibility of a particle with negative mass, like a tachyon; its lowest energy state would make it travel at c. But why would this result in traveling backward in time?
The answer lies in the very notion of “relativity” integral to “special relativity.”
A commonly used tool to illustrate special relativity is the spacetime diagram.
Spacetime is populated with events ranging from the cosmic and violent, like a distant star’s supernova explosion, to the ordinary, such as an egg cracking on a kitchen floor. These events are plotted onto a spacetime diagram. As a particle moves through spacetime, it creates a worldline that tracks its trajectory.
Also occupying spacetime are observers, each with their own reference frame. These observers might perceive the sequence of events in spacetime differently. For instance, Observer 1 could witness event A, the supernova, occurring before event B, the egg cracking. Meanwhile, Observer 2 might see event B precede event A.
Events within an observer’s light cone can be connected by a signal traveling at sub-light speeds.
Every event has an associated light cone. If event B falls within the light cone of event A, the two could be causally connected. For example, a supernova might have knocked an egg off the kitchen counter—or perhaps the falling egg triggered the complete gravitational collapse of a dying star, somehow. This is possible because a signal traveling slower than light can link events within the light cone. The boundaries of the light cone represent the speed of light. To connect an event outside the light cone with one inside it would require a signal that travels faster than light.
If event A is inside the light cone and event B is outside, then the supernova and the egg-related incident cannot be causally linked. However, a tachyon moving faster than the speed of light could disrupt causality by connecting these events.
To understand why this poses a problem, consider it this way: imagine event A as the sending of a signal, and event B as the receiving of that signal. If the signal travels at or below the speed of light, all observers in different reference frames agree that A precedes B.
But, if the signal is carried by a tachyon and thus moves faster than light, some reference frames might show the signal being received before it’s sent. To an observer in such a frame, the tachyon seems to have traveled backward in time.
One of the core principles of special relativity is that the laws of physics should be consistent across all non-accelerating reference frames. This means that if tachyons can violate causality and move backward in time in one frame, they must do so in all frames.
A diagram illustrating how events are perceived at varying times across different reference frames.
Tachyons paradoxes
To understand how this leads to problems known as paradoxes, consider two observers: Stella, aboard a spacecraft orbiting Earth, and Terra, stationed on the planet’s surface. They are communicating by sending messages via tachyons.
If Stella sends a signal to Terra that travels faster than light in Stella’s frame but backward in time in Terra’s frame, and Terra then sends a reply traveling faster than light in her frame but backward in time in Stella’s frame, Stella could receive the reply before she sends the original signal.
What if Terra’s response says “do not send any signals”? In that case, Stella wouldn’t send the original message, meaning Terra would have nothing to respond to and therefore never sends the tachyon signal saying “don’t send any signals.”
Thus, not only do tachyons violate causality in every reference frame, but they also introduce severe logical paradoxes.
There are proposals for avoiding these paradoxes. The simplest solution is that tachyons may not exist. Another, less drastic suggestion is that observers in different reference frames might be unable to distinguish between the emission and absorption of tachyons.
This means a tachyon traveling backward in time could always be perceived as moving forward in time, because receiving a tachyon from the future inevitably creates the same tachyon and sends it forward in time.
Another proposition is that tachyons are unlike any known particles, in that they don’t interact and can neither be detected nor observed. This implies that the tachyon communication system used by Stella and Terra in the previous example cannot exist.
Similarly, some researchers suggest that tachyons cannot be controlled; their emission and reception occur randomly. Therefore, it’s impossible to send a tachyon carrying a message that violates causality.
Tachyons. Could we ever detect them?
Besides being likely incomprehensibly small like other particles, tachyons, which always move faster than light, cannot be detected on approach because they outpace any associated photons.
Once a tachyon passes, an observer would perceive its image as splitting into two distinct images, showing it arriving in one direction and disappearing in the opposite direction simultaneously.
If detecting tachyons using light during their approach is impossible, are there alternative methods to identify these faster-than-light particles?
Possibly. Although tachyons are theorized to have “anti-mass,” this still constitutes mass energy, suggesting they could exert a gravitational effect. Highly sensitive detectors might be able to discern this effect.
Another potential detection method could arise from their faster-than-light properties.
While the speed of light in a vacuum, c, is a universal speed limit, particles can travel faster than light in other mediums. When electrically charged particles are accelerated to and beyond the speed of light in certain mediums, such as water, they emit a type of radiation called Cherenkov radiation, as noted by the International Atomic Energy Agency.
Therefore, if tachyons are electrically charged, one way to detect them would be to measure Cherenkov radiation in the near-vacuum of space.
The power of imagination in science
Tachyons, whether they exist or not, highlight the crucial role of imagination in our ongoing quest to understand the universe. Even if these particles may never be measurable, our minds are free to explore their possibilities.
We can ponder the concept of a particle moving backward through time and what it reveals about the nature of time, the universe, and the events that fill them.
In a 1929 interview with George Sylvester Viereck published in “The Saturday Evening Post,” Albert Einstein famously remarked, “Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.”
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