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

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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.

What is Wassailing, Anyway?

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It’s easy to think that wassailing is some cozy wintertime tradition that’s fun for the whole family. After all, there’s a jaunty, wholesome Christmas carol about it! But the truth is, if you ever see a minor out wassailing, you may want to call his or her parents.

The word wassail has many meanings. For centuries, it was a way to toast someone’s good health. Before the Battle of Hastings in 1066, English soldiers reportedly sang:

Rejoice and wassail!

(Pass the bottle) and drink health.

Drink backwards and drink to me

Drink half and drink empty.

But, in England, wassail also denoted the alcoholic beverage you imbibed during that toast—an elixir of steamy mulled mead or cider. Sometimes, wassail was a whipped dark beer flavored with roasted crab apples.

Wassail was usually slurped from a communal bowl before, during, and after big events and holidays. It was supposedly on the menu during Lammas Day, a pagan autumnal harvest holiday that involves transforming cornhusks into dolls. It was also imbibed on Twelfth Night, a January holiday that involves lighting a fire in an orchard, dancing, and singing incantations to apple trees in hopes of encouraging a bountiful harvest.

By the Middle Ages, the practice of sharing a giant bowl of wassail—that is, the practice of wassailing—evolved from a holiday celebration to a form of boozy begging. “At Christmastide, the poor expected privileges denied them at other times, including the right to enter the homes of the wealthy, who feasted them from the best of their provisions,” Robert Doares, an instructor at Colonial Williamsburg, explained. The poor would either ask to sip from their rich neighbor’s wassailing bowl or would bring their own bowl, asking for it to be filled. According to Doares, “At these gatherings, the bands of roving wassailers often performed songs for the master while drinking his beer, toasting him, his family, his livestock, wishing continued health and wealth.” The original lyrics of Here We Come a-Wassailing are quite upfront about what’s going on:

We are not daily beggars

That beg from door to door

But we are neighbours’ children

Whom you have seen before.

Not all rich folk were happy to see wassailers at their doorstep. One 17th century polymath, John Selden, complained about “Wenches … by their Wassels at New-years-tide ... present you with a Cup, and you must drink of the slabby stuff; but the meaning is, you must give them Moneys.”

Misers like Selden may have had a point: Since alcohol was involved, wassailers often got too rowdy. “Drunken bands of men and boys would take to the streets at night, noise-making, shooting rifles, making ‘rough music,’ and even destroying property as they went among the wealthy urban homes,” wrote Hannah Harvester, formerly the staff folklorist at Traditional Arts in Upstate New York. In fact, boisterous wassailers are one reason why Oliver Cromwell and Long Parliament passed an ordinance in 1647 that essentially banned Christmas.

By the 19th century, wassailing would mellow. Beginning in the 1830s, music publishers started releasing the first commercial Christmas carols, uncorking classics such as God Rest Ye Merry Gentlemen and The First Noel. Among them were dozens of wassailing songs, including the circa 1850 Here We Come a-Wassailing and dozens of others that are now, sadly, forgotten. As the custom of caroling became the dominant door-to-door pastime, alcohol-fueled begging dwindled. By the turn of the 20th century, carolers were more likely to sing about libations than actually drink them.

But if you’re interested in engaging in some good, old-fashioned wassailing, the original lyrics to Here We Come a-Wassailing are a helpful guide. For starters, ask for beer.

Our wassail cup is made

Of the rosemary tree,

And so is your beer

Of the best barley.

Don’t be shy! Keep asking for that beer.

Call up the butler of this house,

Put on his golden ring.

Let him bring us up a glass of beer,

And better we shall sing.

Remind your audience that, hey, this is the season of giving. Fork it over.

We have got a little purse

Of stretching leather skin;

We want a little of your money

To line it well within.

Screw it. You’ve sung this far. Go for it all, go for the gold, go for ... their cheese.

Bring us out a table

And spread it with a cloth;

Bring us out a mouldy cheese,

And some of your Christmas loaf.

Thirsty for your own wassail? Stock up on sherry and wine and try this traditional recipe from The Williamsburg Cookbook.

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Are There Any Synonyms for the Word Synonym?

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Some of the most frequently used words in the English language must have been created by someone with a devilish sense of humor. The word monosyllabic isn’t one syllable, long is only four letters, lisp is difficult to pronounce if you have a lisp, and synonym doesn’t have any synonyms. Or does it?

The answer to that last question is a bit complicated. Thesaurus.com lists metonym as a synonym of synonym, but their meanings aren’t exactly the same. The Oxford Dictionary of English Grammar defines synonym as “a word or phrase that means the same, or almost the same, as another in the same language.” Metonym, on the other hand, is defined as “a word or expression which is used as a substitute for another word or expression with which it is in a close semantic relationship.” For example, the crown can be used to refer to the queen, and Washington sometimes refers to the U.S. government.

There is another possibility, though: poecilonym. This is probably the closest synonym of synonym, although it’s antiquated and rarely used. David Grambs, a lexicographer for American Heritage and Random House, included it in his 1997 book The Endangered English Dictionary: Bodacious Words Your Dictionary Forgot. The word is pronounced PEE-si-lo-nim, according to Grambs, who pays homage to its obscurity. “Maybe we could all use a few spanking old poecilonyms,” Grambs writes. “Poecilonym? It's an old synonym for synonym that you'll find in these pages. But many words in this dictionary have no real counterparts in today's English.”

Allen’s Synonyms and Antonyms from 1920 also lists poecilonym and another word—polyonym—as synonyms of synonym. However, it says both of these terms are rare. So technically, there are two other words that have the same meaning as synonym, but it’s a tough position to argue when those words are no longer in modern usage.

To add another dimension to this question, some have argued that there are no true synonyms at all, as every single word carries a different shade of meaning. “Even though the meanings of two words may be the same or nearly the same, they almost never are the same in connotation, distribution, and frequency,” according to Dictionary.com. “House and home may be offered as synonyms for each other, but we all know that they are not the same.”

So if you want to start using poecilonym or polyonym in place of synonym, you’d technically be correct—but don’t expect anyone else to know what you’re talking about.

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