What Physicists Mean by "Information"

Why "Information" Sounds Deceptively Simple

In everyday life, "information" means facts, messages, or data in documents and chats. In physics, information is a measurable physical quantity tied to probability, entropy, and even heat. It shows up in black holes, quantum mechanics, thermodynamics, and computing, often with precise formulas attached.

That makes "information" a bridge concept: part physics, part computing, part philosophy of reality.

How Normal Thinking About Information Works

Everyday view:

Information is content: a text message, a photo, a song.
It's "abstract" and independent of the medium (paper, disk, brain).
Losing a file feels like losing information with no physical consequences except inconvenience.

This view misses that erasing or processing information has unavoidable physical effects (e.g., heat generation).

How Physics Thinks About Information

1. Shannon Information (Bits and Surprise)
In information theory, information is measured in bits, defined by how surprising an outcome is:

If a bit is always 1, reading it tells you nothing (no surprise → 0 bits of information).
If a bit is equally likely 0 or 1, reading it gives 1 bit of information (maximum surprise).

Formally, this is Shannon entropy: higher entropy = more uncertainty, more information needed to specify the state.

2. Thermodynamic Information (Landauer's Principle)
Landauer's Principle states:

Erasing 1 bit of information necessarily dissipates at least $$kT \ln 2$$ energy as heat at temperature T.

Erasing information ≠ free; it has a minimum energy cost. Information is thus not just abstract—it's physically tied to entropy and heat.

3. Quantum Information (Qubits and Entropy)
For qubits, information is measured via von Neumann entropy, a quantum generalization of Shannon entropy:

A pure quantum state has zero entropy (fully known).
A mixed state (probabilistic mixture) carries entropy (uncertainty).

Quantum information theory studies how information can be encoded, transmitted, and transformed under quantum laws (with constraints like no-cloning).

What This Notion of Information Is Good At

A. Unifying Computation and Thermodynamics

Landauer's principle ties bit erasure to heat generation.
This means computation has a fundamental thermodynamic cost, beyond engineering inefficiencies.
Future ultra-efficient computers must respect this lower bound; we can't do arbitrary reversible computation for free.

B. Explaining Physical Irreversibility
Microscopic physical laws are often time-symmetric, but macroscopic processes (eggs breaking, friction) are not.

Information perspective:

When macroscopic systems evolve, microstate details get scrambled into inaccessible degrees of freedom.
Losing track of that information corresponds to entropy increase.
"Arrow of time" is linked to information dispersal.

C. Black Holes and Holography
The black hole information paradox asks: what happens to information about matter that falls into a black hole? Early calculations suggested it's lost; quantum theory insists information must be preserved.

This tension led to holographic principle ideas where:

Information about volume may be encoded on boundary surfaces.
Black hole entropy is proportional to event horizon area, not volume.

Information becomes a central quantity in describing spacetime itself.

Real Problems This Concept Helps With

Designing low-power computing: Understanding the Landauer limit helps push chips closer to the fundamental energy floor.
Quantum error correction: Quantum information theory tells us how much redundancy is needed to protect qubits from noise.
Understanding black holes and quantum gravity: Information-centric theories (AdS/CFT, holography) try to reconcile gravity with quantum mechanics using information as a core ingredient.
Communications and coding: Shannon information directly drives compression limits, channel capacity, and error-correcting codes.

Common Myths

"Information is purely abstract; it doesn't affect physics."
False. Erasing information has a minimum energy cost and increases thermodynamic entropy.
"Information is always about meaning."
In physics, information is about distinguishable states and probabilities, not semantic meaning. Two random bitstrings with no human meaning can still have high information content.
"You can erase data without cost if you design hardware correctly."
Landauer's principle says no: any logically irreversible operation (like erasing a bit) has an energy cost proportional to temperature.
"Information and entropy are unrelated."
In information theory and statistical mechanics, entropy is literally a measure of missing information about microstates. More entropy = less knowledge, and vice versa.

Why Trending Now?

Quantum computing forces precise thinking about information storage, transmission, and loss in quantum systems.
Black hole information paradox remains a major driver of research in quantum gravity and holography.
Energy efficiency crises in data centers make Landauer-scale limits relevant for long-term chip design.

Physicists and computer scientists are converging on the view that information is as fundamental as energy or matter.

Are These Ideas a Threat?

To current computing models: Not immediately. We are far from hitting the Landauer limit in everyday processors; most heat is from engineering inefficiencies, not fundamental limits.
To intuitive notions of reality: Yes. Thinking of reality as "information + dynamics" is very different from everyday object-based intuition. It suggests that describing what is possible to know may be as fundamental as describing what exists.

Future Outlook

Information as a primitive of physics: Many approaches to quantum gravity and spacetime (e.g., holographic duality) treat information as the building block of geometry.
Reversible computing and beyond: Research into reversible logic and adiabatic computing tries to approach the Landauer limit, minimising energy per operation.
Quantum internet and secure communications: Quantum information (entanglement, teleportation) will shape future networks that move "information" in ways impossible classically.

Conclusion

When physicists say "information," they don't just mean data in your files—they mean a quantified measure of uncertainty, distinguishable states, and the physical cost of manipulating them. Through Shannon entropy, Landauer's principle, and quantum information theory, "information" becomes a physical entity tied to heat, irreversibility, and even spacetime itself. It is one of the deepest bridges between physics, computing, and the structure of reality.