What’s the first thing you remember about the night sky? Maybe it was the brightness of the full Moon. Maybe it was the miriads of stars in the Milky Way on a moonless night. The way we perceive our surroundings, along with the questions we received from our school visits, inspired us to create this resource.
The Universe does not arrive at us as facts. It arrives as light in our eyes, pressure on our skin, and motion through space. From these experiences, we begin to build understanding. In this activity, students retrace that journey. We are switching definitions with perception, to uncover a simple way to organise the cosmos: matter, space, and radiation, the three baskets that make the observable Universe.
Big idea
We can understand the Universe by organising it into:
- Radiation (what travels and allows us to see)
- Matter (what things are)
- Space (where things are)
Students begin with their own experiences and build toward a scientific model of the Universe.
This activity builds a conceptual model rather than a complete physical description.
Teacher Notes
Students (and most people) may initially think we “see objects directly.”
Guide them toward the idea that we see light interacting with objects. At nighttime, for example, as the light gets dimmer, we can see less than we do in daytime. Turn the light off, and it would be very hard to see anything.
NASA astronaut Christina Koch is illuminated by a screen inside the darkened Orion spacecraft on the third day of the agency’s Artemis II mission. To the right of the image’s center, CSA (Canadian Space Agency) astronaut Jeremy Hansen is seen in profile peering out of one of Orion’s windows. Lights are turned off to avoid glare on the windows. Credit: NASA
The core idea
Instead of starting with physics, we can start with “How do we know anything about the world?” And let the students reverse-engineer reality.
Activity Structure (3 Phases)
Phase 1: “What can we detect?”
“Imagine you are on an alien world. What are all the different ways you can detect or experience things?”
Students brainstorm:
You’ll get things like:
- touch
- seeing
- hearing
- movement
- temperature
Write everything up.
Group their answers into 3 clusters:
| 1. Things we can touch | 2. Things about where things are | 3. Things we detect without touching |
| solid objects surfaces resistance | distance direction movement | light heat signals |
Do NOT name the baskets yet.
Phase 2: “What must exist for these to happen?”
So the question is…
“If these are the ways we experience the world… what must exist out there for this to be possible?”
This is how physics begun too
Guide them toward:
For touch → something must exist
“stuff”
For distance/movement → there must be separation
“space”
For seeing/heat/signals → something must travel
“something moving through space”
Drumroll …
Phase 3: Naming the Universe
The great reveal:
“Physicists organise the Universe in a very similar way.”
Introducing:
Matter Space Radiation (Light) → the “stuff” we can touch → the “between” → what travels and lets us see
Cool fact
Radiation (light) is one of the main ways we detect the Universe — it travels through space and allows us to see. We don’t actually see objects. We see light coming from them or bouncing off them. We perceive heat coming from a certain source.
Ask the students:
“So what are we really seeing /feeling?”
Answer:
radiation
How can you tell? If you turn the lights off or at night, it is very hard to distinguish – see, objects.
Next level additions
- add a misconception trap (“Do we see objects directly?”)
- use a dark room + torch demo
- integrate a brain/perception discussion
For older students
Sometimes light can turn into matter – this is where the good old T-shirt with E=mc^2 makes sense.
Briefly describe:
- gamma ray → electron + positron
No equations needed — just the idea.
Variant: Physical role-play
Assign roles:
- some students = matter (stand still, form objects)
- some = radiation (move across the room carrying “information”)
- the room itself = space
Then:
- radiation “bounces” off matter
- “observer” students only see via radiation
This isn’t the only way to describe the Universe but it’s a powerful way to begin understanding it.
