We’ve all played with hand shadows—the classic bird, the barking dog, the occasional “I’m not sure what that is, but it’s definitely blocking the light.” We think of a shadow as a “nothing.” An absence. A void where light isn’t. But what if I told you that a shadow is actually a lack of physical pressure? And that by standing in one, you are technically lighter than you were a moment ago?
It sounds like a silly question: How much does a shadow weigh? But the answer reveals that light is far more than just “visibility.” It is a physical force, a constant bombardment of energy that shapes our world in ways we rarely notice.
Or do we?
1. Chicago Weighs More on a Sunny Day
We usually think of light as a ghost—something that moves through us, illuminating but never touching. But light has… oomph. It has momentum. When sunlight hits a surface, it exerts “light pressure.” On Earth, this push is about one-billionth of a pound per square inch.
That sounds like a negligible amount of weight. Which is… fun. But over a large enough area, “negligible” becomes substantial. On a clear, sunny day, the entire city of Chicago weighs roughly 300 pounds more simply because the sun is pushing down on it.
On Earth, this is mostly a curiosity. But in the vacuum of outer space, where there is no atmosphere or magnetic field to filter solar wind and radiation, this push becomes a literal trajectory shifter. A spacecraft traveling to Mars can be pushed 1,000 km off course by the pressure of light alone. To navigate the cosmos, engineers have to account for the weight of the sun’s “touch.”
We’ve even built solar sails—giant mirrors that use nothing but this light push to move through space.
“In a way that is calculable, though difficult to measure, an area covered in shadow technically weighs less than surrounding areas being pushed by light.” — Vsauce Transcript
2. The Speed of Push vs. the Speed of Light
Imagine you have a board that is one light-year long. At one end is you; at the other, a button.
If you push your end of the board, does the other end immediately hit the button? Have you just sent information across a light-year faster than c, the universal speed limit?
The answer is no. Because you aren’t moving a “solid” object. You are starting a conversation.
When you push a rigid object, you’re actually compressing the first layer of molecules. Those molecules push the next, and the next, creating a compression wave. This wave travels at the speed of sound through that material.
Information travels through the board by telling the next atom to move—and that “telling” takes time.
You can see this delay with a Slinky. Hold a Slinky vertically and then drop it. The bottom of the Slinky does not move immediately. It hangs in midair, seemingly defying gravity, until the compression wave reaches it. Until that wave arrives, the bottom of the Slinky literally doesn’t know it has been dropped.
3. Photonic Booms and Blue Light in Your Eyes
We are taught that nothing can go faster than the speed of light, c (299,792,458 meters per second). But that speed limit applies only in a vacuum. In a medium like water or glass, light actually slows down.
This creates a loophole.
A charged particle, like an electron, can travel through water faster than light travels through that same water. When this happens, we get a “photonic boom” called Cherenkov radiation.
Normally, as a charged particle moves, it polarizes nearby molecules. These molecules then emit photons as they return to their original state. If the particle is moving slowly, those photons spread out and destructively interfere with one another. The light cancels itself out.
But when the particle outruns light in that medium, those photons cannot escape each other’s path. They pile up and create constructive interference, producing a ghostly blue glow.
Astronauts traveling to the Moon have reported seeing flashes of light inside their own eyes. High-speed particles pass through the liquid of the eyeball faster than light can travel through it, triggering tiny photonic booms directly on the retina.
You aren’t just seeing the universe.
The universe is happening inside you.
4. The Elusive Venusian Shadow
We tend to think there are only two celestial bodies capable of casting shadows on Earth: the Sun and the Moon.
But there is a third candidate.
Venus.
Venus is incredibly bright, but for it to cast a shadow, its light must be focused and the environment must be extremely dark. Researcher Pete Lawrence demonstrated this using a tube with a cutout shaped like the astronomical symbol for Venus (♀).
When the tube was pointed at an empty patch of sky, nothing happened.
But when it aligned with Venus, the light was intense enough to project the symbol’s shadow onto a surface.
This works because light travels in straight lines—or at least, our brains assume it does. Our eyes are sensitive enough to detect photons that have traveled millions of miles across space, tracing them back to a single, tiny point of light in the sky.
5. Why Transparent Water Casts a Shadow
If water is transparent, why does a glass of it cast a shadow on your table?
It seems paradoxical, but the answer lies in refraction and lensing.
Water is denser than air, so it bends light. A stream of water acts like a lens, focusing light into bright rippling patterns known as caustics. Because light is being concentrated into those bright regions, it must come from somewhere else. The areas that lose that light become darker—those are the shadows you see.
Water is also less transparent than we tend to think.
Scientists measure how much light a material absorbs using something called an attenuation coefficient. Pure water has a coefficient between 0.004 m⁻¹ and 2 m⁻¹. Air, by comparison, is between 0.00001 m⁻¹ and 0.0001 m⁻¹.
That means air is roughly 1,000 times more transparent than water.
This difference is so predictable that engineers use it in industrial fluidic systems. A precisely directed stream of water can act as a physical off switch for a light sensor. When the water flows, its refractive and absorptive properties interrupt the beam, triggering a response in manufacturing or security systems.
Water isn’t just something we drink.
It can also be a physical gate for information.
A Universe of Straight Lines
We perceive the world as real because our brains are master geometers.
When you see a pencil appear bent inside a glass of water, your brain is being fooled. The light rays are refracting—changing direction—but your brain traces them backward as straight lines, placing the pencil where it appears to be rather than where it actually is.
Light defines our reality.
But it’s a reality built on pressure, speed limits, and the constant push of trillions of photons.
As you finish reading this, light is hitting your screen and your skin. It is pushing you. It is making you heavier.
How many other invisible forces are pressing against you right now, waiting to be measured?
And as always, thanks for reading.