We all have these thoughts, the ones that come to us in the wee hours of the night. Who am I? Why are we here? What if my cell phone worked with vacuum tubes instead? Randall Munroe has the answer to, well, just one of those questions, but also the answers to a whole host of others collected in What if? 2: Additional serious scientific answers to nonsensical hypothetical questions. Yes, it’s a T-Rex eating a plane. In the excerpt below, Munroe examines what it would take to transport an average-sized human to a chair on top of Australia’s tallest skyscraper, using only the power of pigeons. A lot and a lot of pigeons.
Extract of What if? 2 by Randall Munroe. Copyright © 2022 by Randall Munroe. Excerpted with permission from Riverhead, an imprint and division of Penguin Random House LLC, New York. All rights reserved. No part of this excerpt may be reproduced or reprinted without the written permission of the publisher.
How many pigeons would it take to raise an average person and a throwing chair to the height of Australia’s Q1 skyscraper?
In a 2013 study, researchers from Nanjing University of Aeronautics and Astronautics, led by Ting Ting Liu, trained pigeons to fly up to a perch while wearing a weighted harness. They found that the average pigeon in their study could take off and fly upwards while carrying 124 grams, or about 25% of its body weight.
The researchers determined that the pigeons could fly better if the weights were slung under their bodies rather than on their backs. So you would probably want the pigeons lifting your chair up from the top rather than supporting it from the bottom.
Suppose your chair and slings weigh 5 kilograms and you weigh 65 kilograms. If you used the pigeons from the 2013 study, it would take a flock of around 600 of them to lift your chair and fly uphill with it.
Unfortunately, flying with a load is a lot of work. The pigeons in the 2013 study were able to carry a 1.4 meter load upwards to a perch, but they probably couldn’t have flown too high. Even unencumbered pigeons can only sustain intense vertical flight for a few seconds. A 1965 study measured a rate of climb of 2.5 m/s for unencumbered pigeons,* so while we’re optimistic, it seems unlikely that pigeons will be able to lift your chair more than 5 meters.†
No problem, you think. If 600 pigeons can lift you the first 5 meters, you only need to take another 600 with you, like the second stage of a rocket, to carry you the next 5 meters when the first flock is tired. You can bring another 600 for the 5 meters after that and so on. Q1 is 322 meters high, so around 40,000 pigeons should be able to take you to the top, right?
No. There is a problem with this idea.
Since a pigeon can only carry a quarter of its body weight, it takes four flying pigeons to carry a resting pigeon. This means that each “stage” will need at least four times as many pigeons as the one before it. Lifting a person can only take 600 pigeons, but lifting a person and 600 resting pigeons would take another 3,000 pigeons.
This exponential growth means that a 9-story vehicle capable of lifting you 45 meters would require almost 300 million pigeons, roughly equivalent to the entire world population. Reaching halfway would require 1.6 × 1025 pigeons, which would weigh about 8 × 1024 kilograms, more than the Earth itself. At this point, the pigeons would not be pulled down by Earth’s gravity – the Earth would be pulled up by the gravity of the pigeons.
The complete 65-story craft to reach the top of Q1 would weigh 3.5×1046 kilograms. It’s not just more pigeons than there are on Earth, it’s more mass than there is in the galaxy.
You could make things more efficient by reusing pigeons. In the 2013 study, the researchers gave the pigeons 30 seconds to rest on the roost before bringing them down for another try. If each “step” lasts two seconds and the pigeons are refreshed after 30 seconds, you could fly arbitrarily high with a 15-step craft, but that would still require trillions of pigeons.
A better approach might be to avoid carrying the pigeons with you. After all, the pigeons can climb to the top of the skyscraper themselves, so you might as well send them to wait for you there instead of letting their friends carry them with you. If you could train them well enough, you could drag them to the appropriate height, then grab and pull yourself up for a few seconds when you reach their altitude. Keep in mind that pigeons cannot grab and carry objects with their feet, so they would need small harnesses with aircraft carrier style hooks to intercept you.
With this arrangement it is possible that you could fly to the top of the tower with only a few tens of thousands of well-trained pigeons. You should probably make sure you have some kind of security system that will keep you from plunging to your death every time a hawk flies in and scares the pigeons away.
The craft would not only be more dangerous than an elevator, it would also be much more difficult to choose your destination. You could to plan to get to the top of Q1, but once you get airborne… you’ll be completely under the control of anyone with a bag of seeds.