How Do Tachyons Challenge Our Understanding of Physics?
The universe is made of different types of particles interacting with each other and giving off energies of different types and spectrums. The universe, when analysed, is a very minute fashion revealing a plethora of particles that make up the atoms, called subatomic particles like neutrinos, bosons, quarks, leptons, etc.
Tachyon is one such hypothetical subatomic particle that always travels faster than the speed of light. Tachyons are still not physically detected in any experiment, but their existence is mathematically hypothesized.
What is Tachyon?
The word “Tachyon” is derived from the Greek word “tachy”, meaning swift. A tachyon is a quasiparticle that travels at speed faster than light. Tachyons exhibit unusual property wherewith the increase of speed of the particle, the energy decreases. In the case of other subatomic particles, the energy increases with an increase in speed. Thus, it would require an infinite amount of energy to slow down a tachyon to the speed of light.
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Discovery of Tachyons : ECG Sudarshan Tachyon
The first hypothesis pertaining to these faster-than-light-particles was given by German physicist Arnold Sommerfeld, in 1904, who called them meta-particles. Gerald Feinberg first coined the term tachyon in 1967 in a paper related to faster-than-light particles and their kinetics with respect to special relativity.
Now, when we talk about tachyon Sudarshan’s name deserves an obvious mention. Indian-American physicist, E.C.G Sudarshan proposed the quantum theory for Tachyons, which challenged Einstein’s theory that nothing can move faster than light.
Properties of Tachyons
In special relativity, tachyon-like particles have space-like four-momentum, as opposed to time-like four-momentum in normal particles. Currently, the mass of tachyons is considered imaginary. As per the Lorentz Invariant theory, if we implement the same formula for Braydon’s (slower than light particles), it shows that the total energy of the particle is dependent on its resting mass and its kinetic energy. When the velocity of the particle is greater than the speed of light, the denominator becomes an imaginary number, as the value under the square root becomes negative. As the energy of the tachyon has to be real, then the numerator, mc2 must be imaginary, thus indicating that the mass of the tachyon must be imaginary.
We have already established that the speed of tachyon increases with a decrease in energy. Energy starts approaching zero as the velocity of tachyon approaches infinity. Thus, as Braydon’s can’t break the light speed barrier, tachyons are unable to slow down to speed slower than light speed.
Observation of Tachyons
As tachyons move at speeds greater than the speed of light, it is not possible to observe them in real-time. After tachyons have passed through a point in space, the observes would see two images of it. The observer would observe an appearing and a departing image in the opposite direction. This is called the double image effect, which is normally observed in the light field of a superluminal object.
Cherenkov’s Radiation
It is theorized that a tachyon with an electrical charge would lose energy in the form of Cherenkov’s radiation, like every other subatomic particle. When they exceed the speed of light in that medium (speed of light varies in different optical mediums, the commonly referred speed of light is measured in hard vacuum). As in Tachyons, the speed increases with energy decreases; loss of energy causes it to accelerate and form a hyperbola in space-time. But, the single hyperbola is formed of 2 oppositely charged tachyons with opposite momentum, which annihilate when they reach an infinite speed at the same place in space.
Even neutral tachyons lose energy through gravitational Cherenkov’s radiation due to their gravitational mass. Interaction of tachyons with other particles causes irradiation of Cherenkov’s radiation into those particles.
Causality of Tachyons
If tachyons were able to move faster than light, then they would violate causality according to relativity and can give rise to situations like the Grandfather’s paradox. The situation can be understood by applying the relativity of simultaneity in special relativity, which states that there would be disagreement on the simultaneity of two events in two points in space, which cannot take place in different inertial frames of reference.
If we send a signal from one location to another, if the signal is moving at speeds below the speed of light, the transmission event is before the reception event (linear causality of events). However, signals faster than the speed of light, supposedly transmitted using a tachyon beam, would always have some points in time where the signal would be received before it is sent, almost like the signal has travelled back in time.
Did You Know?
The idea for faster-than-light-particles by Gerald Feinberg was inspired by a science fiction story called the “Beep”.
Tachyons have been used to explain technologies in science fiction movies like Star Trek, which has concepts like tachyon beam, anti-tachyon, etc. Who doesn’t remember deep space tachyons from the 3rd season of Star Trek: Deep Space Nine (Explorer episode)!
MMORPG Eve Online also featured six different types of "Tachyon Lasers" or tachyon light.
FAQs on Tachyon: Concept, Properties, and Causality in Physics
1. What is the fundamental concept of a tachyon in physics?
A tachyon is a hypothetical particle that always travels faster than the speed of light (c). Unlike ordinary matter (called bradyons), which must always travel slower than c, tachyons are postulated to have a lower speed limit of c. The concept arises from mathematical solutions in special relativity and other areas of theoretical physics, like string theory, but their existence has not been experimentally proven.
2. What are the key properties that distinguish tachyons from other particles?
Tachyons possess several unique and counter-intuitive theoretical properties, including:
- Imaginary Rest Mass: To be consistent with the equations of special relativity, a tachyon would need to have an imaginary number as its rest mass.
- Speed and Energy Inversion: A tachyon would speed up as it loses energy, and slow down as it gains energy. It would require an infinite amount of energy to slow down to the speed of light.
- Faster-Than-Light (FTL) Travel: By definition, a tachyon can never travel at or below the speed of light.
3. How do tachyons differ from regular particles (bradyons) and light (luxons)?
Particles in physics can be classified based on their velocity relative to the speed of light:
- Bradyons: These are all ordinary particles (like electrons and protons) that have a real, positive rest mass and must always travel slower than the speed of light.
- Luxons: These are massless particles (like photons) that always travel exactly at the speed of light.
- Tachyons: These are hypothetical particles with an imaginary rest mass that must always travel faster than the speed of light.
4. Why would the existence of tachyons challenge the principle of causality?
The principle of causality states that an effect cannot occur before its cause. Because tachyons travel faster than light, they could theoretically be used to send signals into the past. According to special relativity, what one observer sees as a tachyon moving forward in time, another observer in a different reference frame could see as the same tachyon moving backward in time. This ability to communicate with the past could lead to logical paradoxes, such as preventing your own birth, which violates causality.
5. How can a tachyon theoretically gain speed by losing energy?
This paradox is a direct consequence of Einstein's energy-momentum relation (E² = (pc)² + (m₀c²)²). For tachyons, the rest mass (m₀) is an imaginary number. This mathematical property flips the relationship between energy and speed. For a normal particle, adding energy increases its momentum and speed. For a tachyon, decreasing its energy (moving it toward zero energy) would cause its speed to approach infinite velocity, while adding energy would cause it to slow down towards the speed of light as a lower limit.
6. If tachyons are consistent with some theories, why haven't they been experimentally detected?
There are several reasons for the lack of experimental evidence. Firstly, creating a tachyon would require a fundamentally different process than accelerating normal matter, as the light-speed barrier cannot be crossed. Secondly, a tachyon with very low energy would be moving almost infinitely fast, making its interaction time with any detector virtually zero. Most physicists believe that if tachyonic fields exist, they are likely unstable and would decay into ordinary particles almost instantly, a process known as tachyon condensation.
7. Could tachyons be used for time travel, and what is a tachyonic antitelephone?
The possibility of using tachyons for time travel is a famous thought experiment. A tachyonic antitelephone is a hypothetical device that uses faster-than-light tachyon signals to send a message to one's own past. The paradox arises when a person receives the message from the future and then acts to prevent the message from ever being sent. This creates a logical contradiction that makes such a device, and backward-in-time communication, problematic under the laws of physics as we understand them.
8. Is a tachyon simply a particle accelerated beyond the speed of light?
No, this is a common misconception. According to the theory of relativity, a particle that starts below the speed of light (a bradyon) can never reach or exceed it. As it approaches the speed of light, its mass increases, and it would require an infinite amount of energy to reach light speed. A tachyon is not an accelerated bradyon; it is a fundamentally different type of particle that is always and forever in a faster-than-light state, just as a bradyon is always slower than light.
9. What is the "reinterpretation principle" and how does it relate to tachyons?
The reinterpretation principle, proposed by O. M. P. Bilaniuk, V. K. Deshpande, and E. C. G. Sudarshan, is a way to resolve the causality paradox. It suggests that a tachyon observed to be travelling backward in time (and carrying negative energy) is indistinguishable from an anti-tachyon travelling forward in time (carrying positive energy). Therefore, any attempt to send a signal to the past would be reinterpreted by physics as creating a signal in the present that moves into the future, preserving causality.
10. Who were the key physicists behind the theoretical development of tachyons?
While the idea was explored by Arnold Sommerfeld, the modern concept and the name "tachyon" (from the Greek word 'tachys' for swift) were popularised by American physicist Gerald Feinberg in a 1967 paper. Independently, the concept of faster-than-light particles was also developed in 1962 by physicists O. M. P. Bilaniuk, V. K. Deshpande, and E. C. G. Sudarshan, who laid much of the theoretical groundwork.
