The questions that kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places that adults forget to explore. With that in mind, we’ve started a series called Science Question From a Toddler, which will use kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. The answers are for adults, but they wouldn’t be possible without the wonder that only a child can bring. I want the toddlers in your life to be a part of it! Send me their science questions, and they may serve as the inspiration for a column. And now, our toddler …
Q: What’s above space? Does it have a ceiling? — Hector B., age 51
It’s easy to look up and feel like a sort of cosmic nesting doll — I am in my house, which is inside the envelope of Earth’s atmosphere, which is inside the twirling commune of the solar system, which is inside the Milky Way galaxy, which is inside the Virgo supercluster, which is inside the Laniakea supercluster, which is inside … what, exactly? How many layers does the universe have? Am I really supposed to be able to contemplate “infinity” without the aid of religion and/or philosophy? And, if so, at what point would a hypothetical outbound spaceship cartoonishly smush — recycled soda can-like — against an Acme-brand Celestial Construction Barrier?2
The truth is that science doesn’t have a way to answer this 5-year-old’s question — and it might never find one. Scientists can tell Hector how far back in time we can see: approximately 14 billion years. But that’s not the same thing as saying that the universe ends in a brick wall 14 billion light-years from here. (A light-year is the distance light travels in one year.) Instead, 14 billion years ago is when the farthest light we can see began to travel toward where we now are.
But, like a middle-aged waistline, the universe has been expanding and is now bigger than it used to be. That means that the farthest sources of light we can see are now even farther away — possibly as far as 46.5 billion light-years, if they still exist. But because light can only go so fast, we can only see the part of the universe whose light has reached us in the 14 billion years of the universe’s existence. “We can only see so far into the past,” said Chanda Prescod-Weinstein, a theoretical physicist at the University of Washington, Seattle. “And we can only see so far into the universe.”
Measuring distance in space is not like measuring distance on Earth — you can’t place a billion yardsticks end-to-end through the cosmos. Instead, almost everything is done by proxy, measurements of something else that can be used to extrapolate distance. But those proxies must be very, very good. Millions of dollars’ worth of high-tech equipment depends on them — you wouldn’t want your rover to miss Mars. Lives depend on them — distance proxies got astronauts to the moon. Getting accurate distances out of our proxies is also incredibly important because the mathematical relationships between distance, time and other variables mean that we need distance in order to know other important bits of information — like the size of a planet or the mass of a star.
The closest thing we have in space to a running car odometer, reporting back distances as it traverses the heavens, is Voyager I — the first human-made object to leave our solar system. As of the very moment this sentence was written, Voyager I was 20,632,942,755 kilometers from Earth, traveling through the space between stars, where our sun no longer holds sway over physics. We know where it is because we can measure the amount of time it takes for a radio signal to travel from Earth to the spacecraft and then ping back to us, said Eric Christian, a NASA astronomer who has worked on the Voyager program. Once you have that measurement, calculating the distance between Voyager and Earth becomes a relatively simple math problem … you know the speed at which radio waves travel,3 you know the amount of time it took for this particular radio wave to make its circuit, now just multiply to solve for X.
This is, essentially, radar, and it’s how we’ve measured most distances within our solar system since 1961. But the deeper you go into space, the worse radar functions. When we ping-pong with Voyager I, we’re communicating with what is basically a 1970s ham radio strapped to a structural beam. And it works! But only to a point.
As of this writing, the round-trip light time between Earth and Voyager is 38 hours, 14 minutes and five seconds. So the data we get back from the spacecraft — including information about magnetic fields and plasma ions — doesn’t arrive in perfect real time, but it’s close enough. There are, however, things scientists want to study that are much farther away. Say you want to study an interstellar gas cloud, a nebula, that you think is around 50 light-years from Earth — that’s the same as 50 years of travel time, one way, for your radio signal. Most scientists can’t sit around and wait 100 years to get an accurate distance measurement.
Enter the concept of the cosmic distance ladder — basically a fancy way of saying that the farther you get from Earth, the more likely you are to need a new proxy for how you estimate distance. Every giant leap requires mankind to purchase a new ruler. And sometimes it has worked the other way around: Discovering the ruler has led to a broadened understanding of how big space is.
Less than a century ago, for instance, scientists thought that the “ceiling” of space was the edge of our own galaxy and that our sun was the center of the Milky Way. And a key part of proving that neither of those things was true was the discovery of a way to measure distance at an intergalactic scale. That breakthrough came from an unlikely source, Prescod-Weinstein told me — a woman with a generalist undergraduate degree who was employed at servants’ wages as a sort of cosmic bookkeeper, cataloging the brightness levels of stars from data collected by a “real” astronomer.
Her name was Henrietta Swan Leavitt, and in the early 20th century, she discovered more than 2,400 stars while going photographic plate by photographic plate through images taken by the Harvard College Observatory. She was particularly interested in variable stars, which change in brightness over time. Through painstaking, arm-chewing drudgery, Leavitt spotted something that nobody else had the patience to notice — a correlation between the length of time it took a certain kind of star to go through its brightness cycle and how bright the star actually was. If that sounds less than impressive, remember all the mathematical threads connecting lots of different variables in space. Turns out that Leavitt’s correlation was just the string scientists needed to pull to ring the bell on distance.
Leavitt’s correlation was published in 1912. Before it, scientists could use other mathematical proxies to measure distances of, at most, about 100 light-years. Leavitt’s variable stars — rare, but scattered throughout space — gave scientists a signpost to see much farther. After her discovery, the edge of our observable universe extended to 10 million light-years away. Within just a few years, Edwin Hubble used Leavitt’s work to show that what people had previously called the Andromeda nebula was an entirely separate galaxy — and that the universe, as a whole, was expanding. “She should have gotten the Nobel,” Prescod-Weinstein said. Instead, Leavitt’s boss published her work under his name, and she went largely unknown for decades.
It’s possible, Prescod-Weinstein and Christian said, that somebody will, someday, come up with a way to measure distance in space beyond the 14 billion light-year mark. As history has shown, just because we have reached the limit of our ability to measure distance in space doesn’t mean there isn’t distance beyond that. But, for now, most of what we have is speculation — we can’t make light go faster and get to us from 16 billion or 17 billion light-years away any sooner. “It’s fun to speculate,” Christian said. “But it’s not real science if we can’t prove it. And I don’t see any other way other than waiting more billions of years to get more data.”