What If? Lab
What if we sent a message to the stars?
People have already sent messages into space! A radio message zooms as fast as light, but stars are so far away that it takes years to get there. Let's explore with Olly.
Radio messages travel at the speed of light, about 300,000 kilometers every second. Even so, a hello to the nearest star beyond the Sun would take more than four years to arrive. Explore the real messages people have sent, from Voyager's golden records to the 1974 Arecibo message, and decide what you would say.
Radio signals move at c, so interstellar messaging is limited by light-travel time: 4.24 years to Proxima Centauri, about 25,000 years to the cluster M13 targeted in 1974. Compare radio messages with the slow Voyager golden records, and separate what is known from the open question of whether anyone will ever receive them.
The big question
Imagine sending a hello to another star. Would anyone hear it?
How long would your message take to get there? Let's find out.
Imagine you could send a message to another star. What would you say? How would you send it? And how long would you have to wait for an answer?
People have actually tried this. Scientists have beamed a radio message toward a distant cluster of stars, and two spacecraft are carrying records with pictures, sounds and music out of our solar system. This thought experiment explores what we know about those messages, and what nobody knows yet.
Sending a message to another star is not just imagination: a radio message was transmitted toward the globular cluster M13 in 1974, and the two Voyager spacecraft each carry a golden record of sounds and images from Earth.
This thought experiment examines the physics that limits interstellar messages, chiefly light-travel time, and the choices people made about content. Facts come from NASA and Astronomy magazine; whether any message will be received or answered is an open question, labeled as an idea.
Explore the idea
Make a prediction
A radio hello zooms at light speed. How long until it reaches the nearest star after the Sun?
A radio message travels at the speed of light. How long would it take to reach Proxima Centauri, the nearest star beyond the Sun?
What is the minimum one-way travel time for a radio signal to Proxima Centauri?
- About 4 seconds
- About 4 years
- About 4,000 years
-
Established fact
Light and radio messages are the fastest things there are.
Radio messages travel at the speed of light, and nothing is faster, but stars are light-years away.
Radio signals travel at c; interstellar distances make one-way delays years to millennia (NASA).
-
Established fact
People have already sent messages to space.
The 1974 Arecibo message and the Voyager golden records are real messages from Earth already traveling outward.
The Arecibo transmission (1974) and the Voyager records (launched 1977) are real interstellar messages (Astronomy; NASA).
-
An idea scientists discuss
Nobody knows if anyone will ever get them.
Whether anyone will ever receive or answer these messages is unknown; NASA says no evidence of life beyond Earth has been found so far.
Reception and reply are open questions; to date there is no evidence of life beyond Earth (NASA).
-
Established fact
Radio messages travel as fast as light.
Radio waves travel at the speed of light, about 300,000 kilometers every second, and nothing travels faster.
Radio waves, like all electromagnetic radiation, travel at c, about 186,000 miles per second; nothing travels faster (NASA).
-
Established fact
The nearest star after the Sun is more than four light-years away.
The nearest star system beyond the Sun, Proxima Centauri, is about 4.24 light-years away.
Proxima Centauri and its one or more planets lie about 4.24 light-years (40.15 trillion km) away (NASA).
-
Established fact
The golden record has hellos in 55 languages.
The golden records carry greetings in 55 languages, natural sounds, music and 115 pictures of life on Earth.
Contents: 115 images, natural sounds, 90 minutes of music, greetings in 55 languages and printed messages (NASA).
-
Established fact
Voyager will take a very, very long time to reach another star.
Voyager 1 is heading toward a star more than 17 light-years away and should pass within about 1.7 light-years of it in 40,000 years.
Voyager 1 is headed toward Gliese 445, more than 17 light-years away, and is expected to pass within about 1.7 light-years of it in 40,000 years (NASA).
-
An idea scientists discuss
Maybe someone will find the record one day. Nobody knows.
Carl Sagan said the record would only be found and played if there are advanced space-traveling civilizations out there. Whether that will ever happen, nobody knows.
As Sagan noted, the record will be played only if spacefaring civilizations exist and encounter it; that is unknown, and no evidence of life beyond Earth has been found so far.
-
Established fact
A light-year is how far light goes in one year.
A light-year, the distance light travels in a year, is about 9.46 trillion kilometers.
One light-year ≈ 5.88 trillion miles (9.46 trillion km) (NASA).
-
Established fact
In 1974, scientists sent a radio message to faraway stars.
On November 16, 1974, the Arecibo radio telescope sent a message toward M13, a star cluster about 25,000 light-years away.
The Arecibo message targeted M13, about 25,000 light-years away in Hercules; arrival takes thousands of years (Astronomy).
-
An idea scientists discuss
If someone answered, we might wait a very long time.
If anyone did answer a message, we would have to wait at least twice the light-travel time, and nobody knows whether anyone is listening.
Any reply would take at least the round-trip light time; whether a receiver exists at all is unknown.
-
Established fact
The 1974 message was a picture made of tiny dots.
The Arecibo message was 1,679 bits in 73 lines of 23, making simple pictures of a person, DNA and Earth's place in the solar system.
1,679 bits in 73 lines of 23 characters, encoding a human figure, the DNA double helix and Earth's position in the solar system (Astronomy).
-
Established fact
Our radio and TV signals are already traveling out into space.
In 1978, Carl Sagan wrote that Earth's radio and TV signals were already spreading out into space, in a bubble about 30 light-years thick.
Sagan (1978, quoted by NASA's Voyager FAQ) described a sphere of radio transmission about 30 light-years thick expanding at the speed of light.
-
An idea scientists discuss
Your message could show what you love about Earth.
There's no right answer: a good message might use pictures and sounds that don't need words. What would you include?
Content choices are open: pictures, physics, sounds or music might each be easier or harder to decode, which is debated and untested.
Messages near home
Radio messages zoom as fast as light. They are the fastest messages there are.
A radio hello reaches the Moon in about one second!
Nothing in the universe travels faster than light, NASA explains, and radio waves travel at exactly that speed, about 300,000 kilometers every second. That's fast enough to reach the Moon in just 1.3 seconds.
Farther away, even light takes a while. Sunlight takes about 8 minutes and 20 seconds to reach Earth, and a radio message to Mars takes between 3 and 22 minutes, depending on where the two planets are in their orbits.
All electromagnetic radiation, including radio waves, travels at the speed of light, about 186,000 miles (300,000 km) per second, and nothing travels faster (NASA). Light crosses the 384,400 km to the Moon in 1.3 s and the 150 million km from the Sun in about 8 minutes 20 seconds.
Radio signals between Earth and Mars take 3 to 22 minutes as the planets' separation changes, and Neptune is about 4 light-hours and 10 light-minutes away. Within the solar system, radio communication is slow but practical.
Aiming at the stars
Stars are much, much farther away than planets.
Even at light speed, a hello to the nearest star takes more than four years. An answer would take four more years to come back!
Stars are so far away that we measure their distance in light-years: how far light travels in a year, about 9.46 trillion kilometers. The closest star system beyond the Sun, Proxima Centauri, is about 4.24 light-years away.
So a radio hello sent today would arrive there more than four years from now. If someone answered straight away, their reply would take another 4.24 years to come back. That's more than eight years for one 'Hi!' and one 'Hello!'
A light-year is about 9.46 trillion km. Proxima Centauri, the nearest star beyond the Sun and host to at least one planet, is about 4.24 light-years away, so the shortest possible exchange takes about 8.5 years. M13, the target of the 1974 Arecibo message, is about 25,000 light-years away, and the Andromeda galaxy about 2.5 million.
No technology can beat these delays: they come from the speed of light itself. Whether anyone would be listening is a separate question, and an unanswered one.
Why messages take so long
Light is super fast. But space is super, super big.
Even the fastest message takes years to cross the space between stars.
The problem isn't that radio is slow; it's the fastest thing there is. The problem is that space is enormous. NASA uses Voyager 1 to show this. Launched in 1977 and now traveling at about 61,000 kilometers per hour, Voyager 1 is expected to reach one light-day from Earth, the distance light travels in 24 hours, on November 18, 2026, NASA says: more than 49 years after launch.
A light-day is about 26 billion kilometers. The nearest star beyond the Sun is more than 1,500 light-days away.
Interstellar distances dwarf even our farthest spacecraft's journey. NASA calculated that Voyager 1, launched in 1977 and cruising at nearly 61,000 km/h, would reach one light-day (about 25.9 billion km) from Earth on November 18, 2026, after 49 years, two months and 11 days. Proxima Centauri lies about 4.24 × 365 ≈ 1,550 light-days away.
Radio messages cover that gap in years rather than millennia, but no signal can arrive sooner than light allows, and the reply takes just as long to return.
Messages already on their way
On November 16, 1974, scientists sent a radio message from a giant telescope in Puerto Rico. It had a picture of a person in it!
Two spacecraft, the Voyagers, carry golden records with sounds and pictures from Earth.
On November 16, 1974, scientists used the Arecibo radio telescope in Puerto Rico to send a message of 1,679 bits: simple pictures of a person, the shape of DNA and Earth's place in the solar system. It was aimed at M13, a star cluster about 25,000 light-years away, so it was really a demonstration.
The Voyager spacecraft, launched in 1977, each carry a 12-inch gold-plated copper record with greetings in 55 languages, natural sounds, music and 115 images. NASA says no one has found evidence of life beyond Earth so far.
The Arecibo message (November 16, 1974), crafted by a team led by Frank Drake and including Carl Sagan, was 1,679 bits in 73 lines of 23 characters, heard as nearly three minutes of two tones. Aimed at M13, about 25,000 light-years away, it was more demonstration than attempt at contact (Astronomy).
The Voyager golden records, chosen by a committee chaired by Sagan, hold 115 analog images, natural sounds, music from many cultures and eras, and greetings in 55 languages, with symbolic playing instructions and a pulsar map on the cover. NASA notes that thousands of exoplanets are confirmed but there is, so far, no evidence of life beyond Earth.
Amazing facts
-
The golden record says hello in 55 languages.
The Voyager golden records start with greetings in 55 languages, beginning with Akkadian, spoken about 6,000 years ago, and ending with Wu, a modern Chinese dialect.
The spoken greetings run from Akkadian, spoken in Sumer about six thousand years ago, to Wu, a modern Chinese dialect (NASA).
-
The Arecibo message sounded like beeping tones.
To the people at the 1974 ceremony, the Arecibo message sounded like nearly three minutes of two tones.
The 1,679-bit message was heard as nearly three minutes of two alternating tones (Astronomy).
-
A light-day is a very long way.
A light-day, the distance light travels in 24 hours, is about 26 billion kilometers. NASA expects Voyager 1 to get that far on November 18, 2026, more than 49 years after launch.
One light-day is 25,902,068,356 km (NASA); NASA's calculated date for Voyager 1 to reach it is November 18, 2026, 49 years, 2 months and 11 days into its flight.
Words to know
- light-year
- the distance light travels in one year, about 9.46 trillion kilometers
- radio telescope
- a large dish that collects or sends radio waves
- interstellar
- between the stars
- bit
- the smallest piece of digital information: a 0 or a 1
- golden record
- a gold-plated record of sounds and pictures carried by each Voyager spacecraft
- exoplanet
- a planet that orbits a star other than our Sun
Discovery challenge
-
How fast do radio messages travel?
How fast does a radio message travel through space?
At what speed do radio signals propagate through space?
- At the speed of light
- At the speed of sound
- As fast as a rocket
Show the answer
At the speed of light
As fast as light, the fastest thing there is!
At the speed of light, about 300,000 kilometers every second.
At c, like all electromagnetic radiation (NASA).
-
How long would a radio hello take to reach the nearest star after the Sun?
How long would a radio message take to reach Proxima Centauri?
What is the one-way light-travel time to Proxima Centauri?
- About 4.24 years
- About 8 minutes
- About 1.3 seconds
Show the answer
About 4.24 years
More than four years!
About 4.24 years, because it is 4.24 light-years away. 8 minutes is the Sun; 1.3 seconds is the Moon.
4.24 years; 8 min 20 s is the Sun and 1.3 s the Moon.
-
What do the Voyager spacecraft carry?
What message do the Voyager spacecraft carry?
What is the Voyager message?
- A golden record of sounds and pictures
- A letter in English only
- A video screen
Show the answer
A golden record of sounds and pictures
A golden record with music, sounds and pictures.
A gold-plated record with 115 pictures, sounds, music and greetings in 55 languages.
A gold-plated copper phonograph record of images, sounds, music and greetings, with symbolic instructions.
-
Has anyone found life on other planets yet?
According to NASA, has evidence of life beyond Earth been found?
What is NASA's current statement on life beyond Earth?
- Not yet
- Yes, on Mars
- Yes, near Proxima Centauri
Show the answer
Not yet
Not yet. Scientists keep looking!
Not yet: NASA says there is no evidence of life beyond Earth so far.
No evidence of life beyond Earth so far, despite thousands of confirmed exoplanets.
Experiments to try
- Watch What If? Lab What if we could travel at the speed of light? Light travels about 300,000 kilometers every second. At that speed you could reach the Moon in 1.3 seconds, but the nearest star after the Sun would still take more than four years. And physics says nothing with mass can ever quite reach light speed. Explore why.
- Watch What If? Lab What if we found life on another world? So far, the only life we know of is here on Earth. Scientists search for signs of life with rovers on Mars, spacecraft heading to icy moons and powerful telescopes. Find out how they look, why they test every clue so carefully, and imagine what a first discovery might be like.
- Watch What If? Lab What if telescopes found planets around every star? Astronomers have confirmed thousands of planets around other stars, called exoplanets: 6,451 in NASA's Exoplanet Archive in October 2026. Most were found by watching a star dim slightly as a planet passed in front. Imagine planets around every star, meet some worlds a bit like Earth, and see how a tiny dip in starlight reveals a planet.
- Watch What If? Lab What if you could see Earth from the Moon? From the Moon, Earth hangs in a pitch-black sky, glowing with blue oceans and white clouds. Like the Moon, it goes through phases: when we see a crescent Moon, Earth looks nearly full from the Moon. Discover Apollo 8's famous Earthrise, Earth's phases and the photos astronauts took of our blue planet.
- Watch What If? Lab What if meteor showers happened every night? A few shooting stars streak across any clear night sky, but during a meteor shower you might see dozens an hour. Showers happen when Earth passes through dust left by a comet, which is why they come back on the same dates every year. Imagine a shower every night, and find out what that would take.
Explore next
- Watch What If? Lab What if we could travel at the speed of light? Light travels about 300,000 kilometers every second. At that speed you could reach the Moon in 1.3 seconds, but the nearest star after the Sun would still take more than four years. And physics says nothing with mass can ever quite reach light speed. Explore why.
- Watch What If? Lab What if we found life on another world? So far, the only life we know of is here on Earth. Scientists search for signs of life with rovers on Mars, spacecraft heading to icy moons and powerful telescopes. Find out how they look, why they test every clue so carefully, and imagine what a first discovery might be like.
- Watch What If? Lab What if telescopes found planets around every star? Astronomers have confirmed thousands of planets around other stars, called exoplanets: 6,451 in NASA's Exoplanet Archive in October 2026. Most were found by watching a star dim slightly as a planet passed in front. Imagine planets around every star, meet some worlds a bit like Earth, and see how a tiny dip in starlight reveals a planet.
- Watch What If? Lab What if you could see Earth from the Moon? From the Moon, Earth hangs in a pitch-black sky, glowing with blue oceans and white clouds. Like the Moon, it goes through phases: when we see a crescent Moon, Earth looks nearly full from the Moon. Discover Apollo 8's famous Earthrise, Earth's phases and the photos astronauts took of our blue planet.
- Watch What If? Lab What if meteor showers happened every night? A few shooting stars streak across any clear night sky, but during a meteor shower you might see dozens an hour. Showers happen when Earth passes through dust left by a comet, which is why they come back on the same dates every year. Imagine a shower every night, and find out what that would take.
- Watch Try it What If? Lab What if Earth had no Moon? The Moon's pull stretches Earth's oceans and gives us most of our tides. Take the Moon away in our tide model and see what is left: the Sun still makes tides, but they are less than half as big as the Moon's.
- Watch Try it What If? Lab What if Earth had two moons? Our Moon goes round Earth in a neat, repeating loop. Add a second moon in our orbit model and watch what changes: the two moons tug on each other as well as on Earth, so their paths start to wobble.
- Watch Try it What If? Lab What if the Sun disappeared? The Sun cannot vanish, and it will keep shining for billions of years. But imagining it gone answers a real question: what keeps Earth going round? Switch off the Sun's pull in our model and Earth stops curving and travels on in a straight line.
For parents & teachers
What children will learn
- Radio messages travel at the speed of light, but stars are light-years away.
- A light-year is a distance: how far light travels in one year.
- People have sent real messages: the 1974 Arecibo message and the Voyager golden records.
- Whether anyone will receive them is unknown; no life beyond Earth has been found so far.
Recommended ages: 9 to 17 · About 15 minutes
Talk about it
- If you could include one sound and one picture in a message to the stars, what would they be, and why?
- How could you explain something to someone who doesn't share any language with you?
- Should people send messages to the stars at all? What are good reasons on each side?
Try at home or in class
- Design a 'picture message' on squared paper: color squares to make a simple picture, then write it as rows of 1s and 0s and see if a friend can rebuild it.
- Say 'hello' and count 'one-one-thousand': that's about how long a radio hello takes to reach the Moon (1.3 seconds). A hello to the nearest star beyond the Sun would take more than four years. Talk about what you might be doing when it arrived.
Key vocabulary: light-year, radio telescope, interstellar, bit, golden record, exoplanet
Where these facts come from
- Voyager 1: What Is a Light-Day , NASA Science
- Voyager 1 , NASA Science
- Voyager Golden Record Contents , NASA Science
- Voyager Golden Record Overview , NASA Science
- Voyager Frequently Asked Questions , NASA Science
- Nov. 16, 1974: Arecibo sends a message , Astronomy magazine
- Are We Alone in the Universe? , NASA Science
Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.