What Are Tachyons?
Understanding faster-than-light particles
Introduction
Tachyons are hypothetical particles that always travel faster than the speed of light in vacuum. The name comes from the Greek word "tachys," meaning swift. First proposed by physicist Gerald Feinberg in 1967, tachyons represent one of the most fascinating and controversial concepts in theoretical physics.
While photons (light particles) travel at exactly the speed of light, and ordinary matter travels slower than light, tachyons would exist in a realm where the speed of light is not a barrier but a lower limit. This fundamental difference leads to extraordinary and counterintuitive properties.
It's important to note that tachyons remain purely theoretical - no experimental evidence for their existence has ever been found. However, they continue to be studied because they offer insights into the nature of space, time, and causality.
Key Properties of Tachyons
1. Faster-Than-Light Speed
Tachyons always travel faster than light. Unlike ordinary particles that approach the speed of light asymptotically, tachyons cannot slow down to the speed of light - it represents an impassable barrier from below, just as it does from above for ordinary matter.
2. Imaginary Mass
According to special relativity, tachyons would have imaginary rest mass. This doesn't mean they lack mass entirely, but rather that their mass is described by an imaginary number (a multiple of the square root of -1). This mathematical peculiarity is necessary to maintain consistency with Einstein's equations.
3. Inverse Energy-Speed Relationship
Counterintuitively, as a tachyon loses energy, it speeds up. Conversely, adding energy to a tachyon would slow it down. At infinite speed, a tachyon would have zero energy. This is the opposite of how ordinary particles behave.
4. Time-Like Properties
From certain reference frames, tachyons would appear to travel backward in time. This raises profound questions about causality and the nature of time itself, potentially allowing for effects to precede their causes.
Comparison: Tachyons vs Ordinary Particles
| Property | Ordinary Matter | Light (Photons) | Tachyons |
|---|---|---|---|
| Speed | Less than c | Exactly c | Greater than c |
| Mass | Real, positive | Zero | Imaginary |
| Energy adds | Speed increases | Frequency increases | Speed decreases |
| Lower speed limit | Zero (at rest) | c | c |
| Observed? | Yes | Yes | No |
Theoretical Basis
Tachyons emerge as mathematical possibilities when extending Einstein's special theory of relativity. The famous mass-energy-momentum relationship allows for three classes of particles:
- Tardyons (Bradyons): Ordinary particles with real mass that travel slower than light
- Luxons: Massless particles like photons that travel at the speed of light
- Tachyons: Hypothetical particles with imaginary mass that travel faster than light
The mathematics of special relativity doesn't forbid tachyons - it merely indicates they would have unusual properties. However, their existence would challenge our understanding of causality and may require new physical principles to prevent paradoxes.
The Causality Problem
One of the most significant issues with tachyons is their potential violation of causality - the principle that cause must precede effect. If tachyons could carry information, it might be possible to send messages to the past, creating paradoxes.
The classic example is the "tachyonic antitelephone," a thought experiment where tachyon signals could theoretically be used to send information backward in time. This would allow for situations where you could receive a message before it was sent, or even prevent yourself from sending the message in the first place.
Many physicists argue that if tachyons exist, there must be some principle that prevents them from carrying information or being controlled in ways that would violate causality. This remains an active area of theoretical investigation.
Why Study Tachyons?
Despite their hypothetical nature, studying tachyons serves several important purposes in physics:
- Testing Physical Theories: Tachyons help us understand the limits and implications of special relativity
- Exploring Causality: They provide thought experiments for investigating the nature of time and causality
- Quantum Field Theory: Tachyonic fields appear in some quantum theories and may explain phenomena like symmetry breaking
- Cosmology: Tachyon-like behavior has been proposed in some models of cosmic inflation
- Mathematical Physics: They offer interesting mathematical structures to explore