How the universe sends messages: the physics of information

In today's interconnected world of computers and phones, everyone is talking about the information boom. We receive it through news, data streams, and endless scrolling, making it a vital part of our hectic lives. However, in physics, information extends far beyond human communication. It refers to how any change or influence moves from one body to another across space. Everything in the universe is in constant interaction. And every interaction, whether it's the Moon causing tides or sunlight warming our skin, conveys a message carried from one location to another. So what does information signify in the realm of physics?

Whether through rope tugs or phone calls, the universe communicates via different methods, all constrained by the universal speed limit of light.

In physics, information is not defined by meaning or content, but by change. If a physical system changes in a way that can, in principle, be detected elsewhere, then the information has been transmitted. A tug on a rope, a fluctuation in air pressure, a pulse of light, or a change in a gravitational field all qualify because each alters the state of another system. From this perspective, information can be classified by how that change propagates through space. Broadly speaking, we can identify three closely related modes of propagation.

• Through matter, as mechanical disturbances passed from particle to particle,

• Through fields, such as electromagnetic or gravitational fields that exist even in a vacuum,

• Through quantized excitations, like photons or phonons, which carry energy and change in discrete packets.

 These categories are not mutually exclusive. For example, sound waves can be described both as vibrations of matter and as phonons at the microscopic level. Still, they help us organize the different physical mechanisms by which information moves.

Each mode has its mechanism of propagation and characteristic speed, but all share one crucial requirement: the change must propagate through space and time. There can be no influence, signal, or message that appears somewhere else instantaneously, without first traveling through the intervening distance. Consequently, all modes of information must obey the fundamental principle of spacetime: no information can travel faster than the speed of light. Let’s now look at these modes one by one, starting with the most familiar.

Mechanical waves: information through matter

If you pull one end of a long rope, the other end doesn’t move instantly. The tug travels along the rope as a mechanical wave, a chain reaction of moving particles transferring energy and information about the pull from your end of the rope to the other. The speed of this signal depends on the rope’s tension and density, but is always finite, far below the speed of light. This simple example shows that even in everyday materials, information moves step by step, atom by atom.

Sound waves: information through air

When we speak, our vocal cords vibrate, creating tiny pressure variations that ripple through the air. These variations travel as waves, carrying the information encoded in words to another person's ears. The message moves at the speed of sound, about 340 m/s in air. Sound is also a mechanical wave, meaning it requires a material medium, a solid, liquid, or gas, to propagate. The speed of sound is highest in solids, reaching around 5,000–6,000 m/s in metals. Still, it remains much slower than the speed of light. In a vacuum, where there are no particles to vibrate, space is silent.

Light and radio waves: information at the speed limit

When you switch on a flashlight or send a message through your mobile phone, the information is transmitted as an electromagnetic wave. Depending on its frequency, this wave can appear as visible light or as invisible radio and microwaves. All electromagnetic waves travel at the speed of light, c, in a vacuum (or still very close to this value in the air). The speed c sets the limit on how fast information can travel, regardless of whether the source is stationary or in motion.

This means that whether you send your text from home or aboard the fastest rocket, the message would still travel at the speed of light. Your movement can't affect it; you can't speed it up or slow it down. Even if you could send a message from a car zipping at the speed of light toward your friend's home, the message would't be delivered any sooner. It would arrive together with the vehicle since both had travelled at the same speed limit.

Such cars, of course, don't exist in nature as massive bodies can't accelerate to the speed of a massless particle. Still, this idea nicely illustrates this principle documented in the postulates of special relativity. The constancy of the speed of light, c, establishes the universal limit for any motion, including the speed allowed by nature in communications.

Electric signals: information in circuits

In electronic devices like your computer, information is conveyed by variations in voltage and current. Although the electrons themselves drift through the circuit very slowly, typically at speeds of millimeters per second, the information is carried by the electromagnetic field generated throughout the circuit. When the voltage changes, the electric and magnetic fields surrounding the conductors rearrange, and this change propagates along the wire at a speed close to c, slightly reduced by the material's properties and the circuit's geometry.

This is why computers can operate at gigahertz speeds, even though their electrons barely crawl through the wires. However, despite remarkable engineering advances, no electronic device could ever transmit information faster than the speed of light.

Heat transfer: vibrations as messengers

Heat is transferred through atomic vibrations known as phonons. Consider a metal rod: when one end is heated, the atoms there vibrate more vigorously, disturbing their neighbors. This disturbance propagates along the rod as a collective vibration of the atomic lattice, a phonon. These vibrations convey information about temperature and thermal energy through the material at speeds comparable to that of sound in solids, which is vastly slower than the speed of light.

This explains why you can stir hot tea with a metal spoon without burning your fingers. The spoon transmits the tea’s temperature information through countless atomic interactions, and this process takes time. As with mechanical and sound waves, the speed of thermal information depends on the properties of the material, but it always remains well below nature’s ultimate speed limit, c.

Gravitational influence: the speed of gravity

Gravity might feel instantaneous in everyday life, but in physics, it behaves like every other carrier of information, taking time to propagate. If the Sun were somehow to disappear, we would learn about it only after about 8 minutes, the same time it takes light to travel from the Sun to Earth. During those 8 minutes, Earth would continue orbiting as if nothing had happened, responding to the last gravitational information it had received. The reason is that changes in gravity are not transmitted instantaneously. Instead, information about a change in a mass distribution is carried outward as disturbances in the gravitational field. These disturbances propagate through space at the speed of light, c, behaving in this respect much like electromagnetic signals.

It is important not to confuse gravitational waves with the gravitational field itself. In physics, information always involves change. A stable gravitational field produced by an unchanging mass does not transmit information. It simply exists as a static background. Only when the distribution or motion of the mass changes does the gravitational field change, and only those changes propagate outward as informative signals, known as gravitational waves.

Biological signals: information in living systems

When you send a text to your friend, your body sends messages of its own. Every decision you make, and every movement you perform, is brought into action by a vast network of electrochemical signals within your body. Before you reach for the phone, your brain initiates a command, sending a signal to your hand to act. This signal originates as an electrical pulse that travels through your body until it reaches the muscle in your arm. Upon contact, it triggers a chemical reaction that causes the muscle to contract.

These biological signals can propagate to about 120 meters per second, remarkably fast for such a complex process, yet still much slower than the speed of light. The information they carry is transmitted from one cell to another, triggering a sequence of responses, with each stage taking time. Even life itself must operate within the boundaries set by the universal speed limit.

Quantum entanglement: the tempting exception

Quantum physics introduces an intriguing twist that, at first glance, can challenge everything we have said so far. When two particles are entangled, their properties are so closely connected that measuring one instantly reveals information about the other. Since the very nature of the entanglement predetermines the outcome, the distance between them is irrelevant. This implies that even when we place them wide apart, we still should learn about the properties of the distant particle immediately after measuring the one at our location.

This outcome sparked suggestions that information can travel instantaneously across limitless distances. However, this impression is misleading. While entanglement creates strong correlations between particles, it does not convey information from one place to another.

Let's keep one particle on Earth and send its counterpart to the Moon. The researcher on Earth can learn about the particle on the Moon by studying its entangled twin. However, the information is obtained on Earth and remains there unless it is somehow passed to the researcher on the Moon. We could write a letter and send it via rocket, or we could use the quickest method and transmit the findings through electromagnetic signals. There are several options, but the result is the same. We cannot pass information from one place in space to another faster than the speed of light.