Session 11: The Great Silence
There are up to 400 billion stars in our galaxy.
Many of those stars have planets.
Some of those planets could be like Earth.
The universe is over 13 billion years old.
Start thinking: If there are so many stars and planets, where is everybody?
The answer? We don’t really know.
Maybe they are too far away.
Maybe they no longer exist.
Maybe we just haven’t looked hard enough.
Today’s Big Question:
If the universe is so vast, why haven’t we heard from anyone?
If the universe is so vast, why haven’t we heard from anyone?
PATHWAY 1
Use Less Space
10 mins
PATHWAY 2
Feeding The World Differently
10 mins
Finished? Complete the ‘Guessing the Galactic Odds’ Minigame
Can We Calculate Aliens?
Drake’s equation has eight variables. We can group them into three categories:
Scientists can estimate this confidently | Scientists have some idea of this | Scientists have almost no evidence for this |
Can We Calculate Aliens?
Group the seven variables into the three categories.
Scientists can estimate this confidently | Scientists have some idea of this | Scientists have almost no evidence for this |
| 2. The number of stars with planets | 3. The number of habitable planets | 4. The number of planets where life begins |
5. The number of planets where civilisations evolve | 6. The number of civilisations that can communicate with us | 7. The length of time for which those civilisations send signals |
Can We Calculate Aliens?
Scientists can confidently estimate these figures using telescopes to count everything in a tiny snapshot of space, then multiplying that number across the whole sky. Because the universe is spread out fairly evenly in every direction, this gives a remarkably reliable count. Since planet-hunting telescopes have shown that almost every star has at least one orbiting world, scientists can simply multiply that total again to estimate the planets! |
stars formed | 2. The number of stars with planets |
Can We Calculate Aliens?
3. The number of habitable planets |
Scientists have some idea of the number of habitable planets. By watching thousands of stars, telescopes found a clear pattern: about one out of every five Sun-like stars hosts a rocky planet in the "Goldilocks Zone". However, it is only a partial estimate because being in the right location is just the first step; we can't yet easily check if those distant worlds have the other things needed to be truly habitable. |
Can We Calculate Aliens?
4. The number of planets where life begins | 5. The number of planets where civilisations evolve | 6. The number of civilisations that can communicate with us | 7. The length of time for which those civilisations send signals |
Scientists have almost no idea about these because Earth is our only example. We don't know if the chemical spark that creates life is common or a fluke, whether building civilizations is a rare accident (since only humans did it out of billions of Earth species), or if aliens would even invent radio technology rather than living quietly in dark oceans. On top of that, because humans have only used radio for a century, we have no clue whether technological civilizations survive for millions of years or quickly wipe themselves out, making the timing and existence of their signals a guessing game. |
Can We Calculate Aliens?
There are many possible explanations for why we haven’t yet found other intelligent life.
For each explanation, decide whether you think it could be true, and pick out the flaw in each explanation - why is not a satisfactory explanation?
Can We Calculate Aliens?
Could it be true? What is the flaw?
| 2. Intelligent life is extremely rare. | 3. Civilisations destroy themselves (The Great Filter). |
4. Space is too big. | 5. Aliens deliberately avoid us (the Zoo Hypothesis). | 6. We haven’t been searching for long enough. |
Can We Calculate Aliens?
| 2. Intelligent life is extremely rare. | 3. Civilisations destroy themselves (The Great Filter). |
The flaw: Why would Earth chemistry be a complete one-off in a universe full of the same ingredients?
The flaw: If dinosaurs didn't get hit by an asteroid, would they have evolved intelligence? Is it a fluke?
The flaw: Does every single civilisation collapse? It only takes one to survive and colonise the galaxy.
Can We Calculate Aliens?
4. Space is too big. | 5. Aliens deliberately avoid us (the Zoo Hypothesis). | 6. We haven’t been searching for long enough. |
The flaw: Even at slow speeds, a civilisation could cross the galaxy in a few million years. The universe is billions of years old.
The flaw: Could a galaxy-wide empire really keep a secret that well? No leaks?
The flaw: We have only been listening for 60 years. Isn’t that just a blink of an eye in cosmic time?
Can We Calculate Aliens?
| 2. Intelligent life is extremely rare. | 3. Civilisations destroy themselves (The Great Filter). |
4. Space is too big. | 5. Aliens deliberately avoid us (the Zoo Hypothesis). | 6. We haven’t been searching for long enough. |
Which explanation is most convincing, and why?
WE HAVE COVERED:
Learner Journal
Part 1: Three Things I Learnt Today
Open your Learner Journal. Write down three things you learnt. These can be facts, ideas, or things that surprised you.
Write your response in your Learner Journal, then finish sections 3-5.
Learner Journal
Part 2: Reflective Question
What is the most likely explanation for the Great Silence? Why?
“Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying.” Arthur C. Clarke, author, reflecting on what it could mean to find other life, or not (1991)
Drake built a tool to organise what we do not know. Fermi asked a simple question that has never been answered: where is everybody? Both of these scientists showed something important: the best response to a genuinely hard question is not to pretend you know the answer, but to think carefully about what would change your mind. After 60 years of listening to the universe, the Great Silence remains exactly that: a silence, waiting to be explained.
Lock-In Quote
Extension Activities
Extension 1
Extension 2
How Bats Use Sound
Impossible to Intercept