How Much Does a Thought Weigh?

  • By Greg Kestin
  • Posted 12.07.17
  • NOVA

Anywhere information goes, even in your mind, energy goes with it—bottle up that energy, and it has mass. Find out how to weigh a thought in this episode of What the Physics?!

Running Time: 03:58


How Much Does a Thought Weigh?

Published December 6, 2017

The thought that you're having right now, how much does that weigh?

As much as an electron?

As much as a water molecule?

Or, as much as a mosquito?

Because anywhere information goes—even in your mind—energy goes with it.

Bottle up that energy and it has mass...weight.

That's heavy.

If you've seen any movie, you've probably seen an animation like this. Nerve impulses shooting through brain neurons, but how much energy is actually in that “neuro-electricity”?

This is about a hundred million neurons. It's a part of the brain that lit up in you when you saw my face. It's called It's the fusiform face area. And the neuron in it weigh about as much as a paperclip. But we are interested in the weight of a thought going through those neurons.

And it's hard to define a thought. Is a dream a thought? Is a reflex a thought? But we can all relate to recognizing somebody and having the mental image of them in your mind.

In the moment you recognize somebody, that about 1/10th of a second, even if they don't recognize you, the electricity that surges through those neurons has a combined energy of about a billionth of a joule. That's the amount of energy to lift an eyelash just a few inches, and make a wish.

The sun's fusion converts atomic mass to light energy? What if we do that in reverse, and take that brain energy, and bottle it up into mass, it would weigh less than a trillionth of a trillionth of a pound.

About the weight of a water molecule. So that's the weight of the energy in that section of the brain.

How much would a thought outside a brain weigh, just disembodied.

What's a thought but information?

And there's a precise relationship between information and energy. The smallest amount of information is one bit: 0 or 1, yes or no, happy or sad. To record 100 million bits of information at room temperature, you'd need the energy of one visible photon.

More on that in a future episode on the physics of memory.

One of your current thoughts is probably the mental image of what's on this screen – how much information is that?

Each colored pixel has 24 bits of information, and your screen has about 5 million pixels. That's 100 million bits of information in the image that you're seeing right now. And to record that much information, you'd need the energy of a million visible photons, that's the number of photons you'd get if you looked at a star in the night sky for about a minute or two.

If you literally bottled up those photons and weighed them, they would weigh about a millionth of a trillionth of a trillionth of a pound. That means a single thought outside of a body weighs about as much as one single electron.

No, that's not heavy.

So a single fleeting thought isn't that heavy.

But your brain usually does a lot of things all at once. And whether you're solving a puzzle or dreaming, your brain runs on a fairly steady 20 watts of power, that's the same as a single compact florescent light bulb.

Only about 10 percent of that is being used for electrical signals in your brain. But if we assume the rest of it is in some way supporting your ability to have thoughts. Then, over a lifetime, that adds up.

A lifetime of thought, bottled up, weighs about as much as one mosquito.


Host, Producer: Greg Kestin

Researcher: Samia Bouzid

Writer: Samia Bouzid and Greg Kestin

Animation & Compositing: Danielle Gustitus and Greg Kestin

Contributing Writer: Lissy Herman

Scientific Consultants: Frank Haist and Murti Salapaka

Filming, Writing, & Editing Contributions from: Samia Bouzid, David Goodliffe, and Brian Kantor

Guest Appearances: Lindsey Chou, Ana Aceves, and Drew Gannon

Editorial Input from: Julia Cort and Anna Rothschild

Special thanks: Ari Daniel, Allison Eck, Fernando Becerra, Gil Kaplan, Eric Brass, Lauren Miller, and the entire NOVA team

From the producers of PBS NOVA © WGBH Educational Foundation

Funding provided by FQXi

Music provided by APM

Sound effects:

Neuron & Digital Brain footage: Pond5

Related Links