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.

Visualization: thescientificdrop.com | Data: NASA, HIPPARCOS Star Catalogue

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

The decoded Arecibo Message 1974.
Each pixel encodes information about humanity. | Public Domain

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.

Are our signals actually detectable? The sobering physics

Here is where we have to temper the romanticism with real physics. Most of our signals follow the Inverse Square Law the intensity of a signal drops proportionally to the square of the distance from the source. Double the distance and the signal is four times weaker. At stellar distances, this effect is catastrophic for detectability.

By the time a standard 1980s television broadcast reaches a star 50 light-years away, its power density is so low that it is effectively indistinguishable from the natural radio noise of the universe the cosmic microwave background, stellar radio emission, and interstellar plasma turbulence. To a distant observer, Earth does not sound like a planet full of people broadcasting entertainment. It sounds like a very slightly "warm" radio source a marginal anomaly that would require extraordinary instrumentation even to notice.

A 2010 study estimated that detecting Earth's leakage radiation from even 1 light-year away would require a receiver roughly 900 kilometres in diameter far beyond anything humanity has ever built, and presumably far beyond casual alien eavesdropping capability. Only deliberate, high-power, narrow-beam transmissions like the Arecibo Message have any realistic chance of being decoded at stellar distances.

We are not broadcasting. We are whispering, accidentally, into an ocean of noise. The bubble exists but it is a bubble of extraordinary faintness.

What this means for the Fermi Paradox and the Great Silence

This visualization offers a sobering perspective on the Fermi Paradox the question of why, in a universe of hundreds of billions of galaxies each containing hundreds of billions of stars, we have heard nothing.

If we have only been detectable for 120 years, we have covered approximately 0.000002% of the Milky Way. If an advanced civilisation exists 1,000 light-years away, they will not hear us for another 880 years. If they went extinct 500 years ago, we missed them entirely. If they are actively listening but using different frequencies or detection methods, our leakage radiation is far too faint for them to notice without extraordinary effort.

The Great Silence might not be evidence of absence. It might simply be evidence of geometry and timing the brutal arithmetic of a galaxy so large that even signals travelling at the speed of light take tens of thousands of years to cross it, and civilisations so rare that the probability of two of them being close enough to hear each other at the same moment in cosmic time is vanishingly small.

This also connects directly to our Fermi Bubbles post the enormous structures above and below the galactic centre that remind us how much energy the universe moves around without our involvement, and how small our contribution to the cosmic conversation really is.

And for a sense of just how far our physical messengers have travelled compared to our radio signals, the Voyager probes at 170 AU are still well inside the innermost shell of the radio bubble. Our fastest machines have barely covered a light-day. Our radio waves have covered 125 light-years. The difference in scale between those two numbers tells you everything about why interstellar travel remains a dream while interstellar communication remains a theoretical possibility.

Frequently Asked Questions

How far have human radio signals traveled?

As of 2026, the earliest escapable signals have travelled roughly 125 light-years from Earth, creating a total bubble diameter of about 250 light-years. Most signals at that distance are incredibly weak and effectively undetectable.

Can aliens actually detect our TV signals?

Almost certainly not. Standard broadcasts follow the Inverse Square Law power drops with the square of distance. A receiver roughly 900 kilometres in diameter would be needed to detect Earth's leakage from just 1 light-year away. Only deliberate narrow-beam transmissions like the Arecibo Message have any realistic chance of being decoded at stellar distances.

What was the Arecibo Message?

Transmitted on 16 November 1974, it was humanity's first deliberate high-powered signal aimed at deep space. Designed by Frank Drake and Carl Sagan, it encoded information about human DNA, our solar system, and the transmitting telescope as a 1,679-bit binary pictorial message aimed at the M13 cluster, 25,000 light-years away. It has currently travelled about 52 light-years.

Is Voyager inside or outside the radio bubble?

Very much inside. Voyager 1 at 170.5 AU is only about 23 light-hours from Earth a tiny fraction of the 125 light-years our radio signals have covered. Our electromagnetic signals travel enormously faster than any physical spacecraft.

What is the radio sphere or radio bubble?

The volume of space containing all electromagnetic radiation ever leaked from Earth powerful enough to escape our ionosphere. It expands at the speed of light and currently spans approximately 240 light-years in diameter.

How long until our signals reach the nearest star?

They already did. Alpha Centauri at 4.37 light-years received our first signals around 1904. Any new signal sent today would arrive there in about 4.2 years.

Further Reading

On The Scientific Drop

• Where Is Voyager 1 Now? Our farthest physical messenger, still inside the innermost shell of the radio bubble.
• The Zoo Hypothesis One possible explanation for why the radio bubble has received no reply.
• Fermi Bubbles The enormous galactic structures that dwarf everything in our radio sphere.
• Apophis 2029 Flyby More original data visualization using NASA JPL tracking.

Official & Academic Sources

• ESA HIPPARCOS Catalogue The stellar position data used to generate the visualization in this post.
• Arecibo Observatory The Arecibo Message Official documentation of the 1974 transmission.
• SETI Institute The organisation actively listening for signals from within our radio bubble's range.
• Loeb & Zaldarriaga (2010) Eavesdropping on Radio Broadcasts The academic paper estimating detectability of Earth's leakage radiation.