Massless momentum
QI
HOW CAN A PHOTON OF LIGHT CARRY ENERGY BUT ALSO BE CONSIDERED MASSLESS? DOESN’T EINSTEIN’S E = MC2 MAKE THAT IMPOSSIBLE?
Robert Bobo Pullman, Washington
AI
Einstein’s famous mass-energy equivalence equation, or E = mc2, is actually a special case of a slightly longer formula known as the energy-momentum relation, which is written out as E2 = p2c2 + m2c4.
This equation relates energy (E) to rest mass (m), the speed of light (c), and momentum (p), which is the key to how photons can carry energy but have no mass. When a particle is at rest, it has no momentum and the equation simplifies to the more familiar E = mc2. But if a particle has no mass, the equation becomes E = pc.
But wait, you might be asking, how can a particle have momentum without mass? That’s where light’s duality as both a wave and a particle comes into play. Unlike a particle, whose momentum is related to its mass, a wave’s momentum comes solely from its motion, meaning that it can carry momentum even without mass.
Interestingly, something that has neither mass or momentum has no energy, which means it is nothing at all — i.e., it cannot exist. But photons do exist, so it follows that they can never be at rest. And the only speed that remains the same in every reference frame is the universal speed limit (c). Light isn’t the only massless particle, however. Gluons, massless particles inside atoms, also travel at the
speed of light.
Caitlyn Buongiorno Associate Editor
QI
IF THE SOLAR SYSTEM IS ONLY 4.6 BILLION YEARS OLD BUT THE UNIVERSE HAS EXISTED FOR 13.8 BILLION YEARS, ISN’T IT LIKELY THAT OTHER FORMS OF LIFE EXISTED WELL BEFORE US IN THE UNIVERSE?
Bob Spangler Fruita, Colorado
AI
With estimates suggesting there are more than 10 billion terrestrial planets in the Milky Way and several hundred billion galaxies in the observable universe, it seems statistically unlikely that lightning only stuck once when it comes to life. Because we only currently know of one planet able to sustain life, scientists base their searches off Earth, looking for small, rocky worlds in the habitable zone — where surface liquid can exist — around stars with a few common key elements needed for life: carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur.
Even limiting ourselves to those conditions, as you point out, the universe is significantly older than the Sun, meaning that some intelligent civilization should have existed long before humanity. So, where are all the aliens? Why haven’t we received any messages? Scientists call this disparity between the apparent likelihood of the abundance of life versus our utter lack of evidence the Fermi paradox.
We don’t yet know why the cosmos appear so
deafeningly silent, but plenty of people have proposed hypotheses. One, known as the Great Filter, claims that although intelligent life may evolve frequently, some factor prevents it from lasting long enough for us to observe. This may be the case even for microbial life. Take Mars, for instance, which shows evidence that it could have once hosted such life long ago. Or maybe intelligent life inevitably develops technology faster than it can evolve the ability to use it responsibly, causing advanced civilizations to eradicate themselves. Or random chance may annihilate life — if a nearby supernova, gamma-ray burst, or giant asteroid were to strike Earth, there would be nothing we could do to stop it.
But astronomers are still looking for life because of other arguments like the Drake equation, which estimates the number of active extraterrestrial civilizations at any given time with the capability to communicate. Though the Drake equation can never be accurately calculated, a paper published in 2016 in Astrobiology found that as long as the odds of a civilization developing on a habitable planet are greater than about 1 in 10 billion trillion, humanity is not alone in the universe.
Caitlyn Buongiorno
Associate Editor
QI
HOW DO SCIENTISTS WEIGH CELESTIAL OBJECTS?
Mike Sackheim Evanston, Illinois
AI
It’s true that you can’t simply place a planet or galaxy on a scale to measure how heavy it is. Luckily, astronomers have a few tricks up their sleeves.
The first trick is understanding that gravity and mass are inherently linked. It’s important to note that weight — which measures the strength of your local gravitational pull on an object — can change, while mass does not. For example, if you step on a scale on Earth and weigh 150 pounds (68 kilograms), that same scale would read 379 pounds (172 kg) on Jupiter. Your personal mass isn’t what’s changing, but your weight changes because more massive planets exert greater gravitational pull on you than less massive ones.
So, to find the mass of an object, astronomers can simply look at how long it takes nearby bodies to orbit that object. Provided they know the distance between the bodies, they can calculate the mass of the central body. In the case of binary stars, astronomers can observe the stars orbiting each other to determine their combined mass. If the stars are nearby and astronomers can see how closely each star orbits their common center of mass, they can determine each star’s individual mass. For galaxies it’s a little different, but by examining how fast a galaxy is rotating, researchers can similarly determine its mass.
There’s another common trick astronomers can use to estimate mass: luminosity. In most cases, a star or galaxy’s luminosity — how brightly it shines — is roughly proportional to its mass. So, provided you know one, you can solve for the other. In the case of stars, scientists use computer simulations of how these objects evolve to better understand the relationship between a star’s mass and its luminosity — as well as other parameters that can be observed, like temperature and composition.
Caitlyn Buongiorno Associate Editor