Ages 8–12 · about 6 minutes

How Scientists Decide What Is True

How do I know if a big science story is real?

The big idea

Every idea sits on a ladder, and only a careful measurement can move it.

claim: Something a person says is true, before anyone has checked it.
evidence: What you find when you go and look, so other people can look too.
prediction: Saying out loud what you will see, before you see it.

How do you know if a big science story is real?

Picture two grown-ups arguing at a table. One says empty space is full of hidden energy. The other says that is nonsense. Both of them sound completely certain. Neither one has shown you anything you can check.

That is an uncomfortable place to be. Scientists have been stuck there too. So they built a tool for it. They stopped sorting ideas into true and false. Instead they put every idea somewhere on a ladder.

An old brass instrument hanging from a thin thread inside a glass jar, on a heavy wooden bench in warm lamplight. — This is what knowing looks like. A brass arm hangs from a thread so fine that the tiniest push turns it. Scientists build machines like this to measure forces far too small to feel.

What we know for sure

The bottom rung is an idea nobody has tested yet. It might be a wonderful idea. It just has not been anywhere.

One rung up, somebody has tested it once. Higher still, teams in different countries have tested it. They all got the same answer. The top rung holds things measured so many times that they go in every textbook.

On this website you will meet ideas from every rung. Each one carries a label. The label is not a scolding. It simply says how far that idea has traveled so far.

A clue counts most when only one story expected it. If both stories expected it, the clue tells you almost nothing. That is true even when the clue looks dramatic.

Here is the sneaky rule. Ten people telling you the same story is still one story. If they all heard it from one person, you have one clue. Not ten. So good scientists walk back to where a claim began. They read the first paper themselves.

What scientists are testing right now

The best researchers do something brave. Before running an experiment, they say out loud what result would prove them wrong. Then they run it anyway.

Chance Glenn did exactly that. He published how his bench works and set a date to test it. A company called Exodus Propulsion promised a public test in orbit. Scientists are testing these ideas right now. Nobody has the answers yet. Whatever the machines do, the numbers will be there for everyone to read.

Why it matters

Enormous claims will find you for the rest of your life. Some will be about space. Some will be about medicine. You cannot test them all yourself.

But you can always ask three questions. Who actually measured it? Did anyone else get the same answer? What result would show it was wrong? Those three questions are a tool. You can carry them anywhere.

Your turn

Think of something you believe but have never checked. Where did you first hear it? If you wanted to find out yourself, what would you measure? And how would you know if you were wrong?

For the biggest ideas on this website, nobody knows the answer yet. The experiments are still running. Maybe one day you will be the one running them.

Try this at home

Guess first, then count

You need: A cup, a spoon, a bag of dry rice or lentils, paper and a pencil

1.Write down your guess for how many spoonfuls of rice fill the cup. Write it before you start.
2.Now fill the cup one spoonful at a time. Count out loud.
3.Ask someone else to guess, then let them count too. Compare all four numbers.

Notice: Your guess will be close but not exact, and that is fine. What matters is that you wrote it down first. A written guess can be checked. A guess kept in your head quietly turns into whatever the answer was.

For grown-ups and older readers

This lesson comes from Chapter 1: The Question and the Evidence Ladder and the map unit The evidence ladder.