Right now, a sphere of electromagnetic radiation is expanding outward from Earth at the speed of light. It has been growing since the first powerful radio transmissions of the early 1900s. Today that bubble is roughly 240 light-years across and it contains every piece of music, every TV broadcast, every radar ping, and every deliberate message we have ever sent into the cosmos. To any civilisation with a sufficiently sensitive receiver sitting within that bubble, we have already announced ourselves.

The numbers are simultaneously humbling and staggering. Our radio bubble sounds enormous 240 light-years is about 2,270 trillion kilometres. Yet the Milky Way is roughly 100,000 light-years across. We have illuminated approximately 0.000002% of our own galaxy. In the cosmic ocean, we are a single drop of ink that has barely left the tip of the pen.

What is the human radio bubble?

The radio bubble is not a physical object it is a boundary of information. It marks the furthest point that any electromagnetic signal originating from human technology has yet reached, travelling outward in all directions at the universal speed limit: the speed of light, approximately 299,792 kilometres per second.

While Guglielmo Marconi was experimenting with transatlantic signals as early as 1901, most of those early waves were reflected back to Earth by the ionosphere the charged upper layer of our atmosphere that acts as a mirror for certain radio frequencies. It was not until the 1930s, with the adoption of higher-frequency equipment capable of piercing this barrier, that signals began reliably escaping into interstellar space.

Since these signals travel at the speed of light, the radius of our bubble in light-years is approximately equal to the number of years elapsed since the broadcast. A signal sent in 1936 has now travelled roughly 88 light-years from Earth. A signal sent yesterday has barely left the solar neighbourhood. Today, the radio bubble Earth has created spans approximately 240 light-years in diameter a ghostly, ever-expanding archive of everything we have ever said, broadcast, or accidentally leaked into the void.

How far have human radio signals traveled? The key milestones

Each major event in broadcast history corresponds to a different shell within the bubble a ring expanding outward like a ripple in a cosmic pond. Here are the most significant ones, measured by how far they have travelled as of 2026:

Each of these signals is still travelling. The 1936 Berlin Olympics broadcast is currently washing over star systems in the constellation Vela. The Apollo 11 Moon landing transmission is crossing through a region of space containing dozens of stellar systems. None of them have stopped, and none of them ever will they will propagate outward forever, getting weaker with every light-year, until they are indistinguishable from the background noise of the universe.

Visualizing the radio bubble with real stellar data

To create the visualization below, I used Python with stellar position data from the HIPPARCOS Catalogue the same precision astrometry dataset used by professional astronomers to plot nearby stars at their real distances from the Sun. The concentric rings represent the expanding shells of each major broadcast milestone. The result is both technically accurate and, frankly, a little unsettling.

What immediately stands out is how densely packed the inner rings are compared to the outer ones. The 1960s and 1970s represented an explosion in broadcast power military radar, civilian television, and space program transmissions all layered on top of each other. The outer edge of the bubble, representing our earliest escapable signals from the 1900s, is comparatively thin and faint both literally and metaphorically.

We often think of 120 years as a significant stretch of time. On a galactic scale, it is barely a heartbeat. Most of the 200 billion stars in the Milky Way have no idea we exist. We have essentially switched on a lamp in one small corner of a continent-sized forest and announced our presence to the nearest few trees.

Which stars have already heard us?

Several thousand star systems currently sit inside our radio sphere. Here is a breakdown of the most significant nearby stars and when they first entered the bubble:

Vega deserves a special mention. Carl Sagan chose it deliberately in his novel Contact as the source of the first alien signal because at 25 light-years away, Vega would have received our first powerful broadcasts in the 1920s and could theoretically be sending a reply that arrives around now. It is a beautiful piece of narrative physics, even if the probability of anyone actually being there to receive us remains unknown.

The Arecibo Message: Our one deliberate shout

Everything else in the radio bubble is accidental leakage from technology designed for terrestrial use. The Arecibo Message, transmitted on 16 November 1974, was different it was humanity's first intentional, high-powered, targeted broadcast into deep space.

Designed by Frank Drake and Carl Sagan, it was encoded as a 1,679-bit binary string 1,679 being the product of two prime numbers (23 × 73). Any mathematically sophisticated receiver would recognise this as intentional structure and arrange the bits into a 73-row, 23-column grid, revealing a pictorial message containing our number system, the atomic numbers of DNA elements, the structure of DNA itself, a human figure, our solar system, and a diagram of the Arecibo telescope that sent it.

It was aimed at the M13 globular cluster 25,000 light-years away. It will not arrive for another 24,974 years, and M13 will have moved by the time it gets there. As of 2026, it has only travelled about 52 light-years a tiny fraction of its intended journey.

The Arecibo Message was never really a practical attempt at communication. It was a proof of concept a demonstration that we could speak deliberately into the cosmos, not just leak accidentally into it. The rectangle that any mathematically aware civilisation would form from those 1,679 bits would show them exactly who sent it, and that we knew they would understand.

What makes the Arecibo Message remarkable is not that it will be received the odds of that are vanishingly small. What makes it remarkable is that 50 years later, it remains the clearest example of humanity choosing to speak rather than simply being overheard. Every other signal in the bubble was a byproduct of daily life. The Arecibo Message was a choice. A deliberate, calculated declaration that we are here, we are intelligent, and we wanted someone to know.