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's the Difference Between Stuffing and Dressing?

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For carbohydrate lovers, nothing completes a Thanksgiving meal quite like stuffing—shovelfuls of bread, celery, mushrooms, and other ingredients that complement all of that turkey protein.

Some people don’t say stuffing, though. They say dressing. In these calamitous times, knowing how to properly refer to the giant glob of insulin-spiking bread seems necessary. So what's the difference?

Let’s dismiss one theory off the bat: Dressing and stuffing do not correlate with how the side dish is prepared. A turkey can be stuffed with dressing, and stuffing can be served in a casserole dish. Whether it’s ever seen the inside of a bird is irrelevant, and anyone who tells you otherwise is wrong and should be met with suspicion, if not outright derision.

The terms are actually separated due to regional dialects. Dressing seems to be the favored descriptor for southern states like Mississippi, Tennessee, South Carolina, and Georgia, while stuffing is preferred by Maine, New York, and other northern areas. (Some parts of Pennsylvania call it filling, which is a bit too on the nose, but to each their own.)

If stuffing stemmed from the common practice of filling a turkey with carbs, why the division? According to HuffPost, it may have been because Southerners considered the word stuffing impolite, and therefore never embraced it.

While you should experience no material difference in asking for stuffing or dressing, when visiting relatives it might be helpful to keep to their regionally-preferred word to avoid confusion. Enjoy stuffing yourselves.

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Why Do Tires Have to Be Filled With Air?

BookyBuggy/iStock via Getty Images
BookyBuggy/iStock via Getty Images

Paul Misencik:

This is an issue that has perplexed me for most of my life, because pneumatic tires filled with air seem like the last anachronistic, 19th-century component of a modern automobile, and an idea which should have disappeared many decades ago. In an era where even the internal combustion engine itself is giving way to electric motors, and where a new economy hatchback has exponentially more computing power than the Space Shuttle, pneumatic tires don’t seem to make sense any longer.

(And before I get flamed, I know modern tires are vastly more advanced and reliable and capable than their 1930s counterparts. Blowouts, which were a common occurrence when I was a kid, are pretty much unheard of today. Modern tires are great, but they are still vulnerable and maintenance-intensive in a way that doesn’t make any sense to me.)

Companies have experimented with non-pneumatic passenger vehicle tires in the modern age—one of the primary drivers was Michelin. But the tires weren’t filled with solid rubber. In fact, they didn’t even have sidewalls. They were open on the sides, and they had a support lattice of structural polyester ribs, with a ton of air space between the contact patch and the (now deformable) wheel.

One of the big problems with switching from pneumatic tires to non-pneumatic tires is the fact that the current air-filled tire is an important component of the suspension of a vehicle. The flex in the sidewall is a critical part of the compliance of the suspension and substantially affects a vehicle's ride and handling. (Which is why race car drivers sweat tire pressures at each corner of the vehicle so much, as even a small change in tire pressure can have a big effect on the handling and grip of a vehicle.)

If a company like Michelin wants to make a non-pneumatic tire, they'll improve their chances of finding success with it if the new design mimics the compliance and flex characteristics of the outgoing, air-filled models as closely as possible. That way, Michelin would be able to sell the new, non-pneumatic design as a retrofit to older vehicles whose suspensions were originally designed with pneumatic tires in mind. And that is hugely important because if they can’t, it becomes much more difficult to convince manufacturers to change over to the new design—particularly after the mild debacle of Michelin’s failed “TRX” metric tire idea of the 1980s, which required the use of a special wheel and which, despite being by most accounts a superior design in almost every way, never really took off. (Owners of 1980s Ferrari 512 Berlinetta Boxers and some Saab 900 turbos will know what I’m talking about here.)

Non-pneumatic Michelin tires are also rather weird looking, and it’s not clear which manufacturers, if any, would take the risk of being the first to offer them on a new car.

So that is the real issue: Any non-pneumatic tire design must be not only clearly superior to the pneumatic designs of the past, but it must be functionally identical to the outgoing models they would replace, and they must be visually acceptable to consumers.

I hope it happens, though. I hope someone cracks the nut. Pneumatic tires are a 19th-century application still being used on 21st-century vehicles, and at some point that needs to change.

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

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