The short answer: indefinitely
Radio waves are a form of electromagnetic radiation — the same category as visible light, X-rays, and microwaves, just at a different frequency (lower frequency, longer wavelength). In a vacuum, electromagnetic radiation travels at exactly the speed of light: 299,792,458 meters per second.
In a vacuum, there is no medium for the wave to interact with — no air molecules, no friction, no resistance. The wave propagates outward indefinitely. There is no maximum range. The signal does not stop; it only gets weaker.
This is not a metaphor or an approximation. It is the literal physics of Maxwell's equations and the empirically verified behavior of electromagnetic radiation in space.
In a vacuum, there is no medium for the wave to interact with — no air molecules, no friction, no resistance. The wave propagates outward indefinitely. There is no maximum range. The signal does not stop; it only gets weaker.
This is not a metaphor or an approximation. It is the literal physics of Maxwell's equations and the empirically verified behavior of electromagnetic radiation in space.
The inverse-square law: how signals weaken with distance
While a radio signal never stops, it does get weaker. This weakening follows a precise mathematical law: the inverse-square law.
A radio signal radiates outward in all directions (unless focused by a directional antenna). As the wavefront expands, its energy is spread over an increasingly large sphere. The area of a sphere is proportional to the square of its radius — so when the radius doubles (the signal travels twice as far), the energy is spread over four times the area. The signal power drops to one-quarter.
In mathematical terms: Power ∝ 1/r² (where r is distance from the source).
This is why a signal from Voyager 1 — now over 24 billion kilometers from Earth — is detected by Earth's Deep Space Network antennas using a 23-watt transmitter. The signal is extraordinarily faint, but the equation still works. The energy is there; you just need a sensitive enough receiver to detect it.
A radio signal radiates outward in all directions (unless focused by a directional antenna). As the wavefront expands, its energy is spread over an increasingly large sphere. The area of a sphere is proportional to the square of its radius — so when the radius doubles (the signal travels twice as far), the energy is spread over four times the area. The signal power drops to one-quarter.
In mathematical terms: Power ∝ 1/r² (where r is distance from the source).
This is why a signal from Voyager 1 — now over 24 billion kilometers from Earth — is detected by Earth's Deep Space Network antennas using a 23-watt transmitter. The signal is extraordinarily faint, but the equation still works. The energy is there; you just need a sensitive enough receiver to detect it.
Real-world milestones: how far has Earth's radio reached?
Humanity has been transmitting radio signals since the late 19th century. The first intentional radio transmissions date to the 1890s. Here's what that means today:
~130 light-years: The approximate radius of the "radio bubble" — the sphere of space that has been reached by Earth's earliest radio transmissions. Any civilization within this bubble that has sensitive radio receivers could theoretically detect our transmissions.
~100 light-years: Earliest detectable TV broadcasts (1930s-40s era) are now approaching the 100-light-year mark. Our most powerful military radar signals have the same reach.
~50 light-years: Early cold war-era radar signals — including powerful early warning systems — have reached this distance.
In other words, if any civilization within 130 light-years has a sufficiently sensitive radio telescope, they already know about us. We've been broadcasting our existence for a century.
~130 light-years: The approximate radius of the "radio bubble" — the sphere of space that has been reached by Earth's earliest radio transmissions. Any civilization within this bubble that has sensitive radio receivers could theoretically detect our transmissions.
~100 light-years: Earliest detectable TV broadcasts (1930s-40s era) are now approaching the 100-light-year mark. Our most powerful military radar signals have the same reach.
~50 light-years: Early cold war-era radar signals — including powerful early warning systems — have reached this distance.
In other words, if any civilization within 130 light-years has a sufficiently sensitive radio telescope, they already know about us. We've been broadcasting our existence for a century.
Voyager 1: the ultimate demonstration of radio range
NASA's Voyager 1, launched in September 1977, is the most distant human-made object ever built. As of 2026, it is approximately 24 billion kilometers from Earth — well beyond the heliopause, in interstellar space.
Voyager 1 communicates with Earth using a 3.7-meter dish antenna and a 23-watt radio transmitter — roughly the power of a refrigerator light bulb. The signal takes about 22+ hours to reach Earth each way. Earth receives Voyager's signal using the Deep Space Network's 70-meter antennas.
The signal power when it arrives at Earth is approximately 10⁻¹⁶ watts — a tenth of a quadrillionth of a watt. So faint that it requires some of the most sensitive receivers ever built to detect it. But it is detected. Every day.
This is the most powerful empirical demonstration that radio signals in space have indefinite range: a 23-watt transmitter, 24 billion kilometers away, still reaches us across decades.
Voyager 1 communicates with Earth using a 3.7-meter dish antenna and a 23-watt radio transmitter — roughly the power of a refrigerator light bulb. The signal takes about 22+ hours to reach Earth each way. Earth receives Voyager's signal using the Deep Space Network's 70-meter antennas.
The signal power when it arrives at Earth is approximately 10⁻¹⁶ watts — a tenth of a quadrillionth of a watt. So faint that it requires some of the most sensitive receivers ever built to detect it. But it is detected. Every day.
This is the most powerful empirical demonstration that radio signals in space have indefinite range: a 23-watt transmitter, 24 billion kilometers away, still reaches us across decades.
What happens to a Cosmic Echo transmission over time
A Cosmic Echo transmission uses a directional parabolic dish antenna, which focuses the signal in a specific direction rather than broadcasting in all directions. This significantly increases the effective range compared to an omnidirectional antenna.
After transmission, your signal travels at 299,792 km/s and passes the following milestones:
1.3 seconds: Passes the Moon (384,400 km)
8.3 minutes: Passes the Sun (150 million km)
~4.5 hours: Passes Neptune, leaving our Solar System
~1.3 years: Passes the Oort Cloud, the outermost boundary of the Solar System
4.37 years: Reaches Alpha Centauri (if aimed there)
25 years: Reaches Vega
26,000 years: Reaches the galactic center
2.5 million years: Reaches the Andromeda Galaxy
It never stops. The universe has no edge (or if it does, it is expanding faster than light can reach it). Your signal propagates outward, forever.
After transmission, your signal travels at 299,792 km/s and passes the following milestones:
1.3 seconds: Passes the Moon (384,400 km)
8.3 minutes: Passes the Sun (150 million km)
~4.5 hours: Passes Neptune, leaving our Solar System
~1.3 years: Passes the Oort Cloud, the outermost boundary of the Solar System
4.37 years: Reaches Alpha Centauri (if aimed there)
25 years: Reaches Vega
26,000 years: Reaches the galactic center
2.5 million years: Reaches the Andromeda Galaxy
It never stops. The universe has no edge (or if it does, it is expanding faster than light can reach it). Your signal propagates outward, forever.
Can the signal be detected at interstellar distances?
This is the honest part of the answer: probably not by current human technology, at interstellar distances. The inverse-square law ensures that even a strong signal becomes extremely faint over light-years.
However, detection depends entirely on the receiver, not just the transmitter. A civilization with a radio telescope significantly more powerful than anything we've built could potentially detect focused radio transmissions from nearby stars. We don't know what technology exists beyond our solar system.
What we do know: the electromagnetic energy is real and it continues outward. The signal is there. Whether anything receives it is an open question — the same open question that drives SETI research.
For many people who use Cosmic Echo, the detectability question is secondary. The fact that the signal is real — that it actually exists in space, moving at the speed of light — is what matters. Not whether it's received, but that it was sent.
However, detection depends entirely on the receiver, not just the transmitter. A civilization with a radio telescope significantly more powerful than anything we've built could potentially detect focused radio transmissions from nearby stars. We don't know what technology exists beyond our solar system.
What we do know: the electromagnetic energy is real and it continues outward. The signal is there. Whether anything receives it is an open question — the same open question that drives SETI research.
For many people who use Cosmic Echo, the detectability question is secondary. The fact that the signal is real — that it actually exists in space, moving at the speed of light — is what matters. Not whether it's received, but that it was sent.
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