Scientists from the SETI Institute have proposed a new explanation for the famous “Great Silence”—why, after 66 years of searching, we have not detected a single convincing artificial radio signal from space. According to their research, the problem may not only be the enormous distances or the rarity of technological civilizations, but also “space weather” around stars—flares, coronal mass ejections (CMEs), and stellar winds that strongly distort narrowband signals.

How stellar activity “smears” signals

According to Space.com, radio signals traveling through plasma and electrons in stellar wind or CMEs experience diffractive scintillation, an effect that spreads a very narrowband signal (only a few hertz wide) across a much wider frequency range. The signal’s power becomes distributed across that range, and its peak intensity falls below the detection threshold of modern telescopes.

“If the signal is smeared by the environment of its own star, it may drop below our detection thresholds—even if it’s actually there,” explained Vishal Gajjar of the SETI Institute.

SETI traditionally searches for extremely narrowband signals (only a few hertz wide) because:

  • nothing in nature produces such narrow radio lines;
  • they are easy to distinguish from natural background noise.

But if stellar activity spreads a signal across tens or hundreds of hertz, it becomes indistinguishable from noise.

What the calculations showed

The authors of the study—Vishal Gajjar and Grace Brown—first examined how the solar wind and CMEs affect narrowband signals from our own spacecraft, such as Voyager program and New Horizons.

They then extrapolated those results to two types of stars:

  • Sun-like stars
  • Red dwarfs (which make up about 75% of all stars in the Milky Way but are highly active)

Simulation results for signals around ~1 GHz—the frequency range where most SETI projects search—were striking:

  • around 70% of stars smear signals by more than 1 Hz;
  • about 30% smear them by more than 10 Hz (especially red dwarfs);
  • during a strong coronal mass ejection, the spreading can exceed 1000 Hz, making the signal completely invisible to narrowband searches.

Why this could explain the “Great Silence”

Researchers emphasize that even if technological civilizations exist and actively transmit signals, their messages might simply drown in interference from their own stars.

This is particularly relevant for red dwarfs, the most common stars in the galaxy and hosts to many known exoplanets in the habitable zone.

The authors also note that advanced civilizations might be aware of this effect and transmit signals only during quiet periods of stellar activity. However, if a transmitter operates continuously—such as an automated beacon—or if a civilization does not account for stellar weather, the signals might never reach us in a detectable form.

“Now that we understand how stellar activity can distort narrowband signals, we can design searches that better match what actually reaches Earth, rather than what might have been transmitted,” said Grace Brown.

In brief

A new study from the SETI Institute suggests that space weather—stellar winds and coronal mass ejections—can strongly smear narrowband alien radio signals, making them invisible to current searches. Simulations show that 70% of stars broaden signals by more than 1 Hz, 30% by more than 10 Hz, and powerful flares can exceed 1000 Hz, effectively hiding the signal from narrowband detection. This phenomenon could be one explanation for the Great Silence.

The research was published on March 5 in The Astrophysical Journal.