Does Einstein's Theory of Relativity Imply That Interstellar Space Travel is Impossible?

iStock
iStock

Does Einstein's theory of relativity imply that interstellar space travel is impossible?

Paul Mainwood:

The opposite. It makes interstellar travel possible—or at least possible within human lifetimes.

The reason is acceleration. Humans are fairly puny creatures, and we can’t stand much acceleration. Impose much more than 1 g of acceleration onto a human for an extended period of time, and we will experience all kinds of health problems. (Impose much more than 10 g and these health problems will include immediate unconsciousness and a rapid death.)

To travel anywhere significant, we need to accelerate up to your travel speed, and then decelerate again at the other end. If we’re limited to, say, 1.5 g for extended periods, then in a non-relativistic, Newtonian world, this gives us a major problem: Everyone’s going to die before we get there. The only way of getting the time down is to apply stronger accelerations, so we need to send robots, or at least something much tougher than we delicate bags of mostly water.

But relativity helps a lot. As soon as we get anywhere near the speed of light, then the local time on the spaceship dilates, and we can get to places in much less (spaceship) time than it would take in a Newtonian universe. (Or, looking at it from the point of view of someone on the spaceship: they will see the distances contract as they accelerate up to near light-speed—the effect is the same, they will get there quicker.)

Here’s a quick table I knocked together on the assumption that we can’t accelerate any faster than 1.5 g. We accelerate up at that rate for half the journey, and then decelerate at the same rate in the second half to stop just beside wherever we are visiting.

You can see that to get to destinations much beyond 50 light years away, we are receiving massive advantages from relativity. And beyond 1000 light years, it’s only thanks to relativistic effects that we’re getting there within a human lifetime.

Indeed, if we continue the table, we’ll find that we can get across the entire visible universe (47 billion light-years or so) within a human lifetime (28 years or so) by exploiting relativistic effects.

So, by using relativity, it seems we can get anywhere we like!

Well ... not quite.

Two problems.

First, the effect is only available to the travelers. The Earth times will be much much longer. (Rough rule to obtain the Earth-time for a return journey [is to] double the number of light years in the table and add 0.25 to get the time in years). So if they return, they will find many thousand years have elapsed on earth: their families will live and die without them. So, even we did send explorers, we on Earth would never find out what they had discovered. Though perhaps for some explorers, even this would be a positive: “Take a trip to Betelgeuse! For only an 18 year round-trip, you get an interstellar adventure and a bonus: time-travel to 1300 years in the Earth’s future!”

Second, a more immediate and practical problem: The amount of energy it takes to accelerate something up to the relativistic speeds we are using here is—quite literally—astronomical. Taking the journey to the Crab Nebula as an example, we’d need to provide about 7 x 1020 J of kinetic energy per kilogram of spaceship to get up to the top speed we’re using.

That is a lot. But it’s available: the Sun puts out 3X1026 W, so in theory, you’d only need a few seconds of Solar output (plus a Dyson Sphere) to collect enough energy to get a reasonably sized ship up to that speed. This also assumes you can transfer this energy to the ship without increasing its mass: e.g., via a laser anchored to a large planet or star; if our ship needs to carry its chemical or matter/anti-matter fuel and accelerate that too, then you run into the “tyranny of the rocket equation” and we’re lost. Many orders of magnitude more fuel will be needed.

But I’m just going to airily treat all that as an engineering issue (albeit one far beyond anything we can attack with currently imaginable technology). Assuming we can get our spaceships up to those speeds, we can see how relativity helps interstellar travel. Counter-intuitive, but true.

This post originally appeared on Quora. Click here to view.

Celebrate the Holidays With the 2020 Harry Potter Funko Pop Advent Calendar

Funko
Funko

Though the main book series and movie franchise are long over, the Wizarding World of Harry Potter remains in the spotlight as one of the most popular properties in pop-culture. The folks at Funko definitely know this, and every year the company releases a new Advent calendar based on the popular series so fans can count down to the holidays with their favorite characters.

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Right now, you can pre-order the 2020 edition of Funko's popular Harry Potter Advent calendar, and if you do it through Amazon, you'll even get it on sale for 33 percent off, bringing the price down from $60 to just $40.

Funko Pop!/Amazon

Over the course of the holiday season, the Advent calendar allows you to count down the days until Christmas, starting on December 1, by opening one of the tiny, numbered doors on the appropriate day. Each door is filled with a surprise Pocket Pop! figurine—but outside of the trio of Harry, Hermione, and Ron, the company isn't revealing who you'll be getting just yet.

Calendars will start shipping on October 15, but if you want a head start, go to Amazon to pre-order yours at a discount.

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Why Do We Say 'Trick or Treat' on Halloween?

"Give us candy, or else!"
"Give us candy, or else!"
kali9/iStock via Getty Images

Each Halloween, hordes of costumed kids trudge from door to door exclaiming the same phrase at each stop: “Trick or treat!” It’s really a treat-only affair, since adults always shell out candy and children rarely have tricks up their sleeves (except perhaps for those dressed as magicians). In other words, they may as well save half a breath and simply shout “Treat!”

So, where did the term come from?

Halloween Hijinks

Halloween wasn’t always about cosplay and chocolate bars. During the 19th century, Irish and Scottish children celebrated the holiday by wreaking (mostly harmless) havoc on their neighbors—jamming hot cabbage into a keyhole to stink up someone’s house, frightening passersby with turnips carved to look ghoulish, etc.

According to History.com, kids didn’t give up that annual mischief when they immigrated to the U.S., and Americans happily co-opted the tradition. Toppled outhouses and trampled vegetable gardens soon gave way to more violent hijinks—like the time a Kansas woman almost died in a car crash after kids rubbed candle wax on streetcar tracks, for example—and these pranks escalated during the Great Depression.

Almost as terrifying as a turnip.London Stereoscopic Company/Hulton Archive/Getty Images

In short, tricks were a huge part of Halloween throughout the early 20th century. So, too, were treats. For All Souls’ Day in the Middle Ages, people went door-to-door offering prayers for the dead in exchange for food or money, a tradition known as souling. A similar custom from 19th-century Scotland, called guising, entailed exchanging jokes or songs for goodies. While it’s not proven that modern treat-begging is directly derived from either souling or guising, the practice of visiting your neighbors for an edible handout around Halloween has existed in some form or another for centuries.

Canada Coins a Catchphrase

With tricks and treats on everyone’s minds come October, it was only a matter of time before someone combined them into a single catchphrase. Based on the earliest known written references to trick or treat, this may have happened in Canada during the 1920s. As Merriam-Webster reports, a Saskatchewan newspaper first mentioned the words together in an article from 1923. “Hallowe’en passed off very quietly here,” it read. "'Treats' not 'tricks' were the order of the evening." By 1927, young trick-or-treaters had adopted the phrase themselves.

"Hallowe’en provided an opportunity for real strenuous fun," Alberta’s Lethbridge Herald reported in 1927. "No real damage was done except to the temper of some who had to hunt for wagon wheels, gates, wagons, barrels, etc., much of which decorated the front street. The youthful tormentors were at back door and front demanding edible plunder by the word 'trick or treat,' to which the inmates gladly responded and sent the robbers away rejoicing."

The phrase appeared in Michigan’s Bay City Times the following year, describing how children uttered "the fatal ultimatum 'Tricks or treats!'" to blackmail their neighbors into handing out sweets.

Donald Duck's Endorsement

Sugar rationing brought trick-or-treating to a temporary halt during World War II, but the tradition (and the phrase itself) had gained popularity once again by the early 1950s—with some help from candy companies and a few beloved pop culture characters. Charles Schulz depicted the Peanuts gang cavorting around town in costume for a Halloween comic strip in 1951; and Huey, Dewey, and Louie got to go trick-or-treating in a 1952 Donald Duck cartoon titled Trick or Treat.

Fortunately, the treat part of the phrase has thoroughly overtaken the trick part. But if you stuff rank cabbage in your neighbor’s keyhole this Halloween, we won’t tell.

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