Two different questions that both matter
When someone asks "will my signal reach Andromeda?" they're usually asking two distinct things:
1. Will the photons physically arrive?
2. Will anything be able to detect them?
These have very different answers.
To the first: yes. In the vacuum of space, radio waves are electromagnetic radiation traveling at the speed of light. Nothing stops them. They don't decay, scatter into nothing, or run out of energy. The photons from a transmission today will reach the Andromeda Galaxy in approximately 2.537 million years — barring an interaction with matter along the way, which is very unlikely in the near-vacuum of intergalactic space.
To the second: no, almost certainly not — at least not with any technology comparable to what we have now. The signal is real and physically present, but spread so thin over the vastness of space that detecting it would require an impossibly large receiver.
Both things are true. Neither cancels the other out.
1. Will the photons physically arrive?
2. Will anything be able to detect them?
These have very different answers.
To the first: yes. In the vacuum of space, radio waves are electromagnetic radiation traveling at the speed of light. Nothing stops them. They don't decay, scatter into nothing, or run out of energy. The photons from a transmission today will reach the Andromeda Galaxy in approximately 2.537 million years — barring an interaction with matter along the way, which is very unlikely in the near-vacuum of intergalactic space.
To the second: no, almost certainly not — at least not with any technology comparable to what we have now. The signal is real and physically present, but spread so thin over the vastness of space that detecting it would require an impossibly large receiver.
Both things are true. Neither cancels the other out.
The inverse square law: the math of spreading energy
Here is the core of the issue. When a radio signal is transmitted from a point source (or a directional antenna), it spreads outward as an expanding sphere. The energy is distributed over the surface of that sphere.
The surface area of a sphere scales with the square of its radius: A = 4πr². So when the signal travels twice as far, it's spread over four times the area. When it travels ten times as far, it's spread over one hundred times the area.
This is the inverse square law: signal power (per unit area of a receiver) is proportional to 1/r².
Let's run some numbers. Say a Cosmic Echo transmission leaves Earth with an effective radiated power (ERP) of 1 megawatt — a realistic figure for a focused parabolic dish transmission.
• At 1 light-year (~9.46 trillion km): the power density at a 1-meter² receiver is approximately 2.8 × 10⁻²⁸ watts. Extraordinarily faint, but current radio telescopes operate at sensitivities around 10⁻²⁶ watts — so still above the noise floor at this distance with a large dish.
• At 1,000 light-years: power density drops by a factor of 10⁶. Now at ~2.8 × 10⁻³⁴ watts — well below what any existing radio telescope can detect.
• At 2.537 million light-years (Andromeda): approximately 10⁻⁴⁸ watts. Utterly undetectable by anything conceivable with present physics.
But the photons are still there. Every single one. Traveling at the speed of light.
The surface area of a sphere scales with the square of its radius: A = 4πr². So when the signal travels twice as far, it's spread over four times the area. When it travels ten times as far, it's spread over one hundred times the area.
This is the inverse square law: signal power (per unit area of a receiver) is proportional to 1/r².
Let's run some numbers. Say a Cosmic Echo transmission leaves Earth with an effective radiated power (ERP) of 1 megawatt — a realistic figure for a focused parabolic dish transmission.
• At 1 light-year (~9.46 trillion km): the power density at a 1-meter² receiver is approximately 2.8 × 10⁻²⁸ watts. Extraordinarily faint, but current radio telescopes operate at sensitivities around 10⁻²⁶ watts — so still above the noise floor at this distance with a large dish.
• At 1,000 light-years: power density drops by a factor of 10⁶. Now at ~2.8 × 10⁻³⁴ watts — well below what any existing radio telescope can detect.
• At 2.537 million light-years (Andromeda): approximately 10⁻⁴⁸ watts. Utterly undetectable by anything conceivable with present physics.
But the photons are still there. Every single one. Traveling at the speed of light.
Voyager: the benchmark for long-range detection
Voyager 1 is the best real-world data point we have for detecting signals at extreme distances. Launched in 1977, it now sits more than 24 billion kilometers from Earth — in interstellar space.
Voyager 1's transmitter has a power output of just 22.4 watts — roughly equivalent to a refrigerator light bulb. NASA's Deep Space Network (DSN) can still detect its signal using 70-meter parabolic dish antennas.
But 24 billion kilometers is only about 0.0000026 light-years. Alpha Centauri, our nearest star, is 4.37 light-years away — nearly 2 million times further. To detect Voyager's signal from Alpha Centauri would require a receiving dish with an area roughly 4 trillion times larger than the DSN's 70-meter dishes.
A Cosmic Echo transmission at 1 MHz with a focused parabolic dish transmits more power than Voyager — but the physics scales the same way. The signal is detectable within the range of human technology at current sensitivity thresholds. Beyond that, the signal continues but falls below our ability to receive it.
At what distance does the signal become undetectable? With current human technology and a 1-megawatt ERP, a few hundred light-years at best. Beyond that, you'd need a receiver the size of a large moon.
Voyager 1's transmitter has a power output of just 22.4 watts — roughly equivalent to a refrigerator light bulb. NASA's Deep Space Network (DSN) can still detect its signal using 70-meter parabolic dish antennas.
But 24 billion kilometers is only about 0.0000026 light-years. Alpha Centauri, our nearest star, is 4.37 light-years away — nearly 2 million times further. To detect Voyager's signal from Alpha Centauri would require a receiving dish with an area roughly 4 trillion times larger than the DSN's 70-meter dishes.
A Cosmic Echo transmission at 1 MHz with a focused parabolic dish transmits more power than Voyager — but the physics scales the same way. The signal is detectable within the range of human technology at current sensitivity thresholds. Beyond that, the signal continues but falls below our ability to receive it.
At what distance does the signal become undetectable? With current human technology and a 1-megawatt ERP, a few hundred light-years at best. Beyond that, you'd need a receiver the size of a large moon.
What about advanced civilizations?
Here is where it gets philosophically interesting.
The SETI community has long discussed the concept of a "Dyson sphere" — a hypothetical megastructure that encloses a star and captures most of its energy output. A civilization with this capability would have receiver arrays of almost unimaginable size. At that scale, could they detect a signal from Earth?
The physicist Phillip Morrison (co-author of the 1959 paper that launched SETI) estimated that a civilization with a Kardashev Type II technology level (able to harness the full energy output of their star) could potentially detect Earth's most powerful transmissions from across the galaxy.
We don't know if such civilizations exist. But the signal doesn't care about our technology limitations. It travels regardless. If something exists at Andromeda with the technology to detect a 1-megawatt radio pulse from 2.537 million light-years away, the photons will arrive in 2.537 million years. The signal will be there.
The point is not that anyone will necessarily receive your message. The point is that your message exists in space. The photons are real. They're out there. That's different from a symbol, a certificate, or a database entry.
The SETI community has long discussed the concept of a "Dyson sphere" — a hypothetical megastructure that encloses a star and captures most of its energy output. A civilization with this capability would have receiver arrays of almost unimaginable size. At that scale, could they detect a signal from Earth?
The physicist Phillip Morrison (co-author of the 1959 paper that launched SETI) estimated that a civilization with a Kardashev Type II technology level (able to harness the full energy output of their star) could potentially detect Earth's most powerful transmissions from across the galaxy.
We don't know if such civilizations exist. But the signal doesn't care about our technology limitations. It travels regardless. If something exists at Andromeda with the technology to detect a 1-megawatt radio pulse from 2.537 million light-years away, the photons will arrive in 2.537 million years. The signal will be there.
The point is not that anyone will necessarily receive your message. The point is that your message exists in space. The photons are real. They're out there. That's different from a symbol, a certificate, or a database entry.
What this means for your Cosmic Echo message
Cosmic Echo is honest about what a radio transmission is and isn't.
What it is: Real electromagnetic energy, physically propagating through space at the speed of light. Encoded in binary at 1420 MHz — the hydrogen line, the most scientifically significant radio frequency in the universe. Verifiable by a cryptographic hash. Tracked by a live Signal Tracker from the moment of transmission.
What it isn't: A guaranteed message to intelligent life. An unambiguously detectable signal at interstellar distances with current technology. A scientific SETI transmission (those require far more power).
What remains true regardless: the photons travel forever. The energy in your message is in space from the moment of transmission and will continue propagating outward at the speed of light for as long as the universe exists. It is the most permanent thing a human being can create.
The signal becomes undetectable to current technology somewhere between a few hundred and a few thousand light-years. The nearest stars in our catalog — Alpha Centauri at 4.37 light-years, Vega at 25 — are well within the range where a sufficiently sensitive receiver could detect a focused 1-MHz transmission.
Beyond that, the photons keep going. Andromeda. The Virgo Supercluster. The edge of the observable universe. Outward, forever, at the speed of light.
That's what makes it meaningful. Not the detection probability. The physics.
What it is: Real electromagnetic energy, physically propagating through space at the speed of light. Encoded in binary at 1420 MHz — the hydrogen line, the most scientifically significant radio frequency in the universe. Verifiable by a cryptographic hash. Tracked by a live Signal Tracker from the moment of transmission.
What it isn't: A guaranteed message to intelligent life. An unambiguously detectable signal at interstellar distances with current technology. A scientific SETI transmission (those require far more power).
What remains true regardless: the photons travel forever. The energy in your message is in space from the moment of transmission and will continue propagating outward at the speed of light for as long as the universe exists. It is the most permanent thing a human being can create.
The signal becomes undetectable to current technology somewhere between a few hundred and a few thousand light-years. The nearest stars in our catalog — Alpha Centauri at 4.37 light-years, Vega at 25 — are well within the range where a sufficiently sensitive receiver could detect a focused 1-MHz transmission.
Beyond that, the photons keep going. Andromeda. The Virgo Supercluster. The edge of the observable universe. Outward, forever, at the speed of light.
That's what makes it meaningful. Not the detection probability. The physics.
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Hydrogen line frequency
299,792 km/s
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