In science fiction, spaceships moving at or beyond lightspeed enable all manner of universal exploration. But in Earth-bound reality, traveling at the speed of light (299,792,458 meters per second, or 670,616,629 miles per hour, in a vacuum) in a clunky rocket is a physical impossibility. “It’s the speed at which massless things travel,” says Gerd Kortemeyer, an associate professor emeritus of physics at Michigan State University. So, anything with mass cannot reach that speed. And even massless particles are limited by lightspeed. “It’s often called the cosmic speed limit because nothing will go faster than that,” Kortemeyer says.
Doubly unfortunate for those among us eager to visit galaxies far, far away–even moving at near lightspeed isn’t in the cards. “It’s neither possible nor survivable to travel near the speed of light relative to our good old Earth,” the physicist says.
Wonky theoretical explanations aside, it would simply take far too much fuel and energy to propel any human-bearing spacecraft up to that speed. By Kortemeyer’s calculations, reaching 99% of lightspeed in a vessel weighing 10 metric tons (significantly lighter and smaller than most spacecraft), while accelerating at a tolerable g-force would use more than 200 times the amount of energy consumed on Earth in a year. And that’s assuming a perfectly efficient fuel, where mass converts to propulsive energy without any heat loss–another physical impossibility according to the second law of thermodynamics.
The closest we’ve come to lightspeed is accelerating teeny tiny individual atomic particles to 99.99999896% the speed of light in the Large Hadron Collider.
But let’s ignore all of that and imagine, for a moment, that we could get close to lightspeed. If we had the perfect, efficient fuel source, a ton of it, a vessel crafted to withstand it, and the gumption–what would near-lightspeed travel be like?
Well, perhaps unsurprisingly, things would get weird.
What’s so special about lightspeed?
First, it’s important to understand a few of the quirks of lightspeed. It isn’t just a speed, it’s also “one of the fundamental constants of nature,” explains Kortemeyer. Since the 1600s, celestial observations of planetary movements have hinted at the speed of light. And in 1865, James Clerk Maxwell deduced that light was an electromagnetic wave and calculated its speed, with close agreement to the present-day, known value in his landmark physics paper, “A dynamical theory of the electromagnetic field.”
Then, Einstein turned our understanding of physics inside out. His theory of special relativity, presented in 1905, conceives of space-time as a unified universal fabric, connected via the constant, “c”, which defines the relationship between energy and matter. This value, when he calculated it, just so happened to be equal to the speed of light. This is the famous E=mc^2 equation.
In the most basic terms, special relativity states that lightspeed doesn’t change, but rather time– the fourth dimension–bends relative to objects’ movements. Therefore, objects in motion experience time differently from objects at rest. At most conceivable speeds on Earth, this isn’t noticeable. But at near light speeds, it would be, in a phenomenon known as time dilation (we’ll come back to this in the next section).
Plus, because of lightspeed’s unique relationship to time-space, it remains the same no matter the speed of an observer. Imagine, for a moment, that you’re in a car on the highway. If you’re traveling at a constant 30 miles per hour and a car in front of you passes you at 60 miles per hour, then that faster car is moving away from you at 30mph, relative to your speed. However, if you were racing to catch up with a photon, even if you reached 50% the speed of light, that same photon would still be moving away from you at the speed of light. “It’s always the speed of light, independent of how you are moving, which is very different from anything else,” Kortemeyer says.
Together, these concepts combine to make approaching lightspeed a wild ride.
What would near lightspeed travel be like?
Colors and brightness would distort and look very different, as illustrated in this 2012 simulation developed by Kortemeyer and collaborators at MIT. The simple game is meant to illustrate the relativistic effects of moving near light speed and is premised around a universe where light moves much more slowly, and constantly slows as you navigate the world. In that universe, though one still wouldn’t be able to reach or exceed lightspeed, you would be able to approach it at a brisk walking pace.
As you did, you’d experience a visual doppler effect–similar to how an ambulance speeding by with its siren blazing seems to change its tune as it moves. Moving towards an object would make it appear bluer, as its wavelength visually shortens. Moving away from an object would do the opposite, shifting its appearance redder.
Speeding towards something would amplify your perception of its brightness, in a spotlight effect. Kortemeyer compares this phenomenon to running through the rain. Moving fast through a downpour ensures that more drops hit your front, and thus that your shirt is soaked through more quickly. In the simulation game, the proverbial water droplets are photons. Running in a beam of light at near lightspeed means that more light particles would hit your eyes at once.
If that’s not strange enough, consider what would happen to time. Remember the concept of time dilation? Because space-time warps to accommodate the constant speed of light, a person traveling through space at near lightspeed would age more slowly than all the other humans waiting back on Earth. This idea is exemplified in the twin paradox thought experiment. Time dilation, and a change in time at rest relative to a change in time while in motion at a certain velocity, can be precisely calculated.
If you managed to reach 299,792,450 meters per second (i.e. just under lightspeed), two minutes of travel at that speed through space would be equivalent to about six days of time passing on our planet.
Often, this concept of warped time is used to explain how hyper-lightspeed travel might work. “I’m a very big fan of Star Trek, so I don’t want to trash talk,” says Kortemeyer. However, the show’s sci-fi idea of “warp speed” to outrun the speed of light is all fiction and no science, he says. “Warping space is a physical reality,” but there’s no way to force or control the warping of space to manipulate speed. “There is no such thing as a warp drive in physics. I wouldn’t know what physics principle would make that possible,” he says.
And to take things back down to Earth even more, reaching 299,782,450 meters per second would be a trial all on its own. When it comes to high speeds, the biggest barrier to contend with isn’t cruising at a constant velocity, but rather the acceleration. Already, we’re moving far faster than you might expect. All of us on Earth are hurtling around the sun at about 67,000 miles per hour. But, because that speed doesn’t change, we don’t feel it. Yet reaching lightspeed relative to Earth would be a different story. “You can’t just take off and reach the speed of light. You would be flattened,” says Kortemeyer.
The g-force of getting to near-lightspeed would be monumental, unless you sped up very carefully. Humans are adapted to survive at 1 g, the gravitational force on Earth. Most people can withstand 4-6 g in short bursts of a few seconds to minutes. But for longer periods of time or at higher intensity, g-force becomes fatal as our body’s internal fluid dynamics get gunked up.
It would take a person about one year to accelerate to lightspeed, if you wanted to keep the acceleration force under 3 g, per Kortemeyer’s calculations. But we don’t know how long-term exposure to even that level of acceleration force would impact the body, he notes. It’s possible that 12-straight months of acceleration force beyond what we’re built for would test, not just the limits of physics, but our own physical limitations.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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