Are Electromagnetic Waves the Only Way to Transmit Information?

Electromagnetic waves—including radio, microwaves, visible light, and X-rays—are important ways to send information, but they are not the only ones. A message can be encoded in any physical change that a receiver can detect and interpret. Examples include variations in air pressure, the movement of an object, or changes in a stream of particles.

The key distinction is between what carries the message and how fast the message can travel. Special relativity places a limit on causal signals; it does not require every signal to be an electromagnetic wave.

What counts as transmitting information?

To transmit information, a sender must be able to choose between at least two possible states and make a change that a receiver can distinguish. For example, a sender might produce a loud or quiet sound to represent 1 or 0. The physical state carries the message; an agreed rule lets the receiver interpret it.

That physical state need not be light. Sound is a pressure disturbance moving through a material such as air or water. A moving object can carry a written note. A sequence of molecules can carry a chemical signal, and a beam of particles can be modulated to encode data. In each case, the receiver detects a physical change and reads it according to a code.

Examples of non-electromagnetic-wave signals

• Sound: Speech and underwater acoustic signals travel as mechanical waves through a medium. Sound does not propagate through empty space.
• Mechanical motion: A person can carry a message, or a device can transmit a pattern of taps along a solid object. The information travels with the moving matter or disturbance.
• Chemical signals: Cells and organisms can release molecules that travel to a receiver, where they trigger a response.
• Particle beams: A sender can encode a signal in the timing or number of particles reaching a detector. Neutrinos, for example, interact so weakly with matter that they can pass through large amounts of material, although producing and detecting useful neutrino messages is difficult.
• Gravitational waves: These are ripples in spacetime, not electromagnetic radiation, and in principle can carry information. Their extremely weak effects make them impractical for ordinary communication.

Some familiar channels combine several physical processes. In a wired circuit, for instance, a changing electrical signal is associated with electromagnetic fields in and around the conductors. Electrons in the wire do not travel from sender to receiver at the speed of the signal; the signal propagates through the electromagnetic interaction and the circuit’s structure.

A worked comparison: speech and light

Suppose you speak across a room to someone 10 metres away. Taking the speed of sound in air as about 343 metres per second, the sound takes approximately

$$\frac{10\ \text{m}}{343\ \text{m/s}} \approx 0.029\ \text{s}$$

to arrive: about 29 milliseconds. The information is carried by changing air pressure, not by a radio wave sent from your mouth to the listener.

A light signal over the same distance takes about

$$\frac{10\ \text{m}}{3.0\times 10^8\ \text{m/s}} \approx 3.3\times 10^{-8}\ \text{s},$$

or about 33 nanoseconds. Light is much faster in this example, but both channels transmit information. The comparison illustrates that a channel’s speed depends on the physical process and its medium.

The speed limit is a separate issue

In special relativity, no controllable signal can carry information faster than the speed of light in a vacuum, conventionally written as $c$. Signals in materials are generally slower. The limit applies to causal influence, not just to electromagnetic waves: a sound wave, particle, or moving object cannot be used to send a message into the past or beyond this limit.

Some wave quantities, such as phase velocity, can exceed $c$ in particular circumstances. That does not mean a receiver can use them to learn a newly chosen message faster than light. The speed relevant to communication is the speed at which information or a causal change can propagate.

Why entanglement is not a communication channel

Measurements on entangled quantum systems can show correlations that cannot be explained by ordinary local hidden-variable models. But a person measuring one part cannot choose the outcome so as to encode a message for someone measuring the other part. The distant observer’s local results, considered alone, do not reveal which measurement was made or which outcome occurred elsewhere.

To compare results and see the correlations, the observers must later exchange ordinary information through a classical channel, which is limited by the relativistic speed limit. Entanglement therefore does not provide faster-than-light messaging.

Common mistakes

• Equating “signal” with “light.” Light is one carrier among many; mechanical and chemical processes can carry information too.
• Confusing the carrier with the code. A binary message is an abstract pattern. It must be represented by detectable physical states, but those states need not be photons.
• Treating every faster-than-light wave speed as faster-than-light communication. A mathematical wave speed can exceed $c$ without allowing information to travel that fast.
• Assuming entanglement sends a controllable message. Correlations alone do not let one observer select a result that encodes a message for another.