“Looks like we’ve got us a dragon by the tail,” Pettit announced from 250 miles up once he locked onto Dragon’s docking mechanism.

“You’ve made a lot of folks happy down here over in Hawthorne and right here in Houston,” radioed NASA’s Mission Control. “Great job guys.”

NASA controllers clapped as their counterparts at SpaceX’s control center in Hawthorne, Calif. — including Musk — lifted their arms in triumph and jumped out of their seats to exchange high fives. The two control rooms worked together, as equal partners, to pull off the feat.

Although cargo hauls have become routine, Friday’s linkup was significant in that an individual company pulled it off. That chore was previously reserved for a small, elite group of government agencies.

Not only that, the SpaceX Dragon is designed to safely return items, a huge benefit that disappeared with NASA’s space shuttles. The vessel is the first U.S. craft to visit the station since the final shuttle flight last summer.

The unmanned capsule is carrying 1,000 pounds of supplies on this unprecedented test flight.

Two hours after the capture, the crew attached the Dragon to the space station as the congratulations poured in. They’ll start unpacking the capsule on Saturday.

“Now that a U.S. company has proven its ability to resupply the space station, it opens a new frontier for commercial opportunities in space — and new job creation opportunities right here in the U.S.,” NASA Administrator Charles Bolden said in a statement.

After this test flight, SpaceX — officially known as Space Exploration Technologies Corp. — has a contract to make a dozen delivery runs. It is one of several companies vying for NASA’s cargo business and a chance to launch Americans from U.S. soil.

SpaceX launched the capsule from Cape Canaveral on Tuesday with its Falcon 9 rocket. On Thursday, the Dragon capsule came within 1½ miles of the space station in a practice fly-by. It returned to the neighborhood early Friday so Pettit, along with Dutch astronaut Andre Kuipers could capture it with the station’s robot arm.

First, the capsule went through a series of stop-and-go demonstrations to prove it was under good operating control.

NASA ordered extra checks of the Dragon’s imaging systems as the capsule drew ever closer to the space station, putting the entire operation slightly behind schedule. At one point, SpaceX controllers ordered a retreat because of a problem with on-board tracking sensors.

Given that the Dragon is a brand new type of vehicle and this is a test flight, the space agency insisted on proceeding cautiously. A collision by vehicles traveling at orbital speed — 17,500 mph — could prove disastrous for the space station.

President Barack Obama is pushing commercial ventures in orbit so NASA can concentrate on grander destinations like asteroids and Mars. Obama’s chief scientific advisor, John Holdren, called Friday’s linkup “an achievement of historic scientific and technological.”

“It’s essential we maintain such competition and fully support this burgeoning and capable industry to get U.S. astronauts back on American launch vehicles as soon as possible,” he said in a statement.

Without the shuttle, NASA astronauts must go through Russia, an expensive and embarrassing situation for the U.S. after a half-century of orbital self-sufficiency. Once companies master supply runs, they hope to tackle astronaut ferry runs.

Musk, who founded SpaceX a decade ago and helped create PayPal, said he can have astronauts riding his Dragon capsules to orbit in three or four years.

The space station has been relying on Russian, Japanese and European cargo ships for supplies ever since the shuttles retired. None of those, however, can bring anything of value back; they’re simply loaded with trash and burn up in the atmosphere.

By contrast, the Dragon is designed to safely re-enter the atmosphere, parachuting into the ocean like the Mercury, Gemini and Apollo capsules did back in the 1960s. Assuming all goes well Friday, the space station’s six-man crew will release the Dragon next Thursday after filling it with science experiments and equipment.

Going into Tuesday’s launch of this Dragon, NASA had contributed $381 million to SpaceX in seed money. The company has invested more than $1 billion in this commercial effort over the past 10 years.

___

Online:

SpaceX: http://www.spacex.com

NASA: http://www.nasa.gov/offices/c3po/home/

Now, more than 2½ years after Bob Stupak’s death, an attorney for his estate has sent to NASA officials in Houston a tabletop display featuring the four gray chips the size of grains of rice. They’re magnified in a Lucite dome about as big around as a U.S. 50-cent piece set with a small blue and white Nicaraguan flag. Combined, the chips weigh 0.05 grams.

Renee Juhans, NASA inspector general executive officer, confirmed Tuesday that the agency was “taking steps to authenticate” the display it received from attorney Richard Wright. Juhans declined to say what would happen after that.

Wright said he expects that if the chips are authentic, they’ll be returned to the people of the Central American country. If not, he said they should be sent back to him.

“I told them it was either Stupak’s or Nicaragua’s,” said Wright, who said he counseled Stupak when ownership questions were raised more than a decade ago not to try to sell or auction the display.

The tiny rocks can be considered priceless or worthless, said Joe Gutheinz, a retired NASA investigator and moon rock hunter who has spent decades on a quest to find 160 missing moon rock samples around the world.

“In a sense, they’re worthless because you can’t sell them,” Gutheinz said by telephone this week from his law office in Friendswood, Texas. “But for people who love space, you can’t put a price on it.”

They’re part of a limited supply of about 842 pounds of rock collected by U.S. astronauts in six missions between Apollo 11 in 1969 and Apollo 17 in 1972. The Soviet Union collected about 300 grams of rock, or about two-thirds of a pound, during unmanned probes to the moon.

Gutheinz said the U.S. distributed 270 moon rock samples in the 1970s as a goodwill gesture to countries around the world. States received 100 samples and territories received six. The United Nations received a sample from the Apollo 11 mission.

NASA has conceded it lost track of some of the 26,000 samples of moon rock and other space material loaned to researchers and museums. The agency inspector general said last December that more than 500 pieces were reported missing since 1970.

The tiny chips that made their way to Stupak by 1987 apparently were a gift to Nicaragua.

Stupak was a wheeler, dealer and gambler of the first order. He won a $1 million wager on Super Bowl XXIII and a World Series of Poker championship bracelet, both in 1989; nearly died in a motorcycle crash in 1995; and lost a bid in 2006 to become lieutenant governor of Nevada.

For a time, Wright said, the lunar stones were displayed at the Moon Rock Cafe at Bob Stupak’s Vegas World casino, which featured a rocket ship logo and big sign declaring “Sky’s the Limit.” The display went into storage after Stupak replaced the place with the tallest structure on the Las Vegas Strip, the 1,149-foot Stratosphere tower resembling the iconic Space Needle in Seattle.

Stupak bought the rocks for $10,000 and 200,000 shares in his casino from Arizona preacher and businessman Harry Coates, according to documents provided by Wright and the recollections of Coates’ widow, Silvina Coates.

Harry Coates, a Baptist minister and missionary, met his wife in 1985 during a mission to Costa Rica. He died in July 2005 in Arizona at age 85.

Silvina Coates, of Casa Grande, Ariz., recalled Tuesday that her husband had lots of side business deals, including one with a man in Costa Rica for the moon rock display. She couldn’t remember what the trade involved.

Wright has a copy of Stupak’s $10,000 check to Harry Coates’ business, Midway Development Inc., along with an affidavit describing how Coates acquired the display from a man named Bob Stone of Golfito, Costa Rica.

The display had been picked from a pile of looted items by an unnamed Costa Rican mercenary fighting with Nicaraguan soldiers when a Somoza compound dubbed “El Retire” was sacked “at the time of the revolution in Nicaragua,” according to the affidavit. It said the mercenary later switched sides to fight for the Contras, before returning to Costa Rica in 1979.

“Bob bought it in good faith,” Wright said.

Stupak wanted to sell the display a little more than a decade ago. He offered Wright 10 percent of the proceeds if he could help, then upped the offer to 25 percent.

Wright counseled him that he couldn’t auction or sell it, because whether it had been lost or stolen, it wasn’t clear that Stupak had any legal right to own it.

After Stupak died, Wright contacted NASA and the Nicaraguan consulate about returning the display.

Wright obtained a written promise in April from NASA attorney Cedric Campbell that if the rock display is authentic, “NASA will return the rock to the people of Nicaragua.”

A Las Vegas Review-Journal columnist who wrote about Stupak’s moon rocks in March 2001 wrote about them again for a Sunday column. After an interview on Friday, Wright sent the display off to NASA.

Gutheinz, who teaches an online University of Phoenix course and enlists student sleuths to find missing moon rocks, said the sample sounded authentic. He said he expects an ownership fight in Nicaragua.

But that’ll just provide another chapter for one of the many stories Gutheinz tells about moon rock samples. He said his students have helped find 79 displays since 2002.

Governors took them home in Colorado, West Virginia and Missouri, he said. A display given to then-Arkansas Gov. Bill Clinton turned up in archived materials after Clinton became president.

“Here, the attorney did the right thing,” Gutheinz said. “He told NASA, and they’re in the process of turning it over properly. We can only hope that Nicaragua gets its property back.”

The most likely culprits are so-called Earth-crossing asteroids— objects with highly elliptical orbits that cross the plane of Earth’s more circular path around the Sun. At least three hundred of these potential killers are known, and in the next few decades some of them will pass uncomfortably close. On February 22, 1995, a just-discovered asteroid with the benign name 1995 CR whizzed by within a few Earth-Moon distances. On September 29, 2004, asteroid Toutatis, an elongated 1.5-by-3-mile object, passed even closer. And in 2029 asteroid Apophis, a 900-foot diameter rock, is predicted to cross much closer still, well inside the Moon’s orbit. That unsettling encounter will irrevocably alter the Apophis orbit and possibly bring it even closer in the future.

For every known Earth-crossing asteroid, there are probably a dozen or more yet to be spotted. And when one of these projectiles is finally observed, it will likely be much too close for us to do much about it. If we’re the bull’s-eye, we may only have a few days’ warning to settle our affairs.

The consequences of an impact will vary according to the size and location of the impact. A ten-mile boulder would devastate the globe just about anywhere it hits. (By contrast, the dinosaur-killing asteroid of 65 million years ago is estimated to have been about six miles across.) If a ten-mile object hits the oceans—a 70 percent chance, given the distribution of land and sea—then all but Earth’s highest mountain peaks will be swept clean by immense globe-destroying waves. Nothing will survive up to a few thousand feet above sea level. Every coastal city will utterly disappear.

If such a ten-mile asteroid hits land, the immediate devastation may be more localized. Everything within a thousand miles would be obliterated, and massive fires would sweep across whatever continent is the unlucky target. For a short time, more distant lands might be spared the violence, but such an impact would vaporize immense quantities of rock and soil, sending Sun-obscuring clouds into the high atmosphere for a year or more. Photosynthesis would all but shut down. Plant life would be devastated and the food chain would collapse. A few humans might survive the horror, but civilization as we know it would be destroyed.

Smaller impactors would cause less death and destruction, but any asteroid over a few hundred feet, whether it smacks the land or the sea, would cause a natural disaster greater than anything we have known. What to do? Should we ignore the threat as too remote, too insignificant in a world that has so many more immediately pressing problems? What could we do to divert a big rock?

The simplest, first step in avoiding such an event is to look as hard as we can for those elusive Earth-crossing destroyers—to know the enemy. We need dedicated telescopes, automated with digital processors, to locate the Earth-crossing projectiles, to plot their orbits and predict their future pathways. Such an endeavor is relatively cheap and already under way. More could be done, but at least the effort is being made.

And what if we found a large rock that is projected to smash into us a few years from now? For the late Carl Sagan, along with others in both the scientific and the military communities, asteroid deflection is an obvious strategy. If initiated early enough, even a small nudge by a rocket engine or a few well-placed nuclear explosions could shift an asteroid’s orbit sufficiently to change a collision course to a near-miss. Such an eventual necessity is reason enough for a robust program of space exploration, he argued. In a prescient 1993 essay, Sagan wrote, “Since hazards from asteroids and comets must apply to inhabited planets all over the Galaxy, if there are such, intelligent beings everywhere will have to unify their home worlds politically, leave their planets, and move to small nearby worlds around. Their eventual choice, as ours, is spaceflight or extinction.”

Spaceflight or extinction. To survive in the long run, we must journey outward to colonize neighboring worlds. First will come bases on the Moon, though our luminous satellite will long remain a hostile place to live and work. Next is Mars, where more abundant resources— especially lots of frozen subsurface water, but also sunlight, minerals, and a tenuous atmosphere—are at hand. It won’t be easy or cheap; nor is Mars destined to become a thriving colony anytime soon. But settling, and perhaps terra-forming, our promising neighbor may well be the next essential step in our species’s evolution.

Two obvious obstacles will probably delay, if not prevent, the establishment of a Mars base. The first is money. The many tens of billions of dollars it will take to design and implement a Mars landing is outside the most optimistic NASA budget, even in the best of financial times. A cooperative global effort may be the only option, but such a massive international program has never been attempted.

Astronaut survival is an equally daunting challenge, for it’s next to impossible to ensure a safe round-trip to Mars. Space is harsh, with myriad sand-size meteorite bullets to pierce the thin shell of even the most armored capsule and unpredictable solar bursts of lethal penetrating radiation.

What’s more, no rocket technology on the books would allow a spaceship to carry enough fuel to get to Mars and make it back. Some inventors talk of processing Martian water to synthesize enough fuel to refill the tanks, but that technology is only a dream and probably a long way off. Perhaps the more logical option—one that flies in the face of NASA mores but is increasingly promoted in passionate editorials—is a one-way trip. Were we to send an expedition with years of supplies instead of fuel, with sturdy shelter and a greenhouse, with seeds, with a lot of oxygen and water, and with tools to extract more life-giving resources from the red planet, then an expedition might just make it.

Fifty million years from now, Earth will still be a vibrant living world, its blue oceans and green continents shifted but recognizable. The fate of our human species is much less certain. Perhaps we will be extinct. But it is also possible that humans will survive and evolve, moving outward to colonize first our neighboring planets, then our neighboring stars. If so, if our descendants make it into space, then Earth will surely be treasured as never before—as a preserve, as a museum, as a shrine and place of pilgrimage. Perhaps only by leaving our world will humans ever fully appreciate the place of our species’s birth.

Reprinted by arrangement with Viking, a member of Penguin Group (USA) Inc., from The Story of Earth by Robert M. Hazen. Copyright (c) 2012 by Robert M. Hazen

Life is extremely resilient once it takes hold, but it requires rich chemistry, large energy sources, and stability, right from the beginning. The comparative planetology of our Solar System makes it seem like those initial conditions are hard to come by. Earth seems perfect, whereas the rest have obvious defects. Mars is on the smallish side, lacks a substantial atmosphere and water, and is very cold (although we still hope to find life there). Jupiter is too big; its crushing pressures and element-poor environment make interesting chemistry impossible. The trouble with such a comparative analysis, however, is that it leaves out a crucial class of planets that, purely by happenstance, doesn’t occur in our Solar System.

A super-Earth is a planet that is more massive and larger than Earth, although still made of rocks — perhaps with continents and oceans — and an atmosphere. There is no such planet in our Solar System, but we know that they must be common in other planetary systems. Moreover, theory predicts that they might have all the nice attributes of Earth, and, in fact, provide a more stable environment on their surface. True super-Earths!

A super-Earth is a planet defined by its mass — between 1 and 10 ME (where ME stands for Earth mass). The Earth’s mass is 6,310₂₄ kg, or a 6 with 24 zeros. That’s pretty big, but the Sun’s mass is 2,310_₃₀ kg — a million times larger — lest we lose perspective.

The first super-Earth was found by the serendipitous work of Eugenio Rivera, Jack Lissauer, and the California-Carnegie team in 2005. It orbits the small star Gliese 876 and is about seven times more massive than Earth. It is also very hot because it orbits very close to its star — only seven stellar radii, or approximately 1.7 million kilometers, away! (For comparison, this is only 3 percent of the distance from our Sun to Mercury.) Another, much colder super-Earth that is about 5 ME was discovered by J. P. Beaulieu and his team at the Paris Institute of Astrophysics.

In early 2007, Michel Mayor’s team in Geneva spotted at least three planets orbiting the star Gliese 581; two of them are super-Earths with minimum possible mass of 5 and 8 ME each, and orbital distances that are 7 percent and 25 percent of the distance from Earth to the Sun, respectively. A year later the same team reported several more super-Earths, some orbiting stars as big and hot as our Sun. The first transiting super-Earth was discovered by the CoRoT space mission in 2009: CoRoT-7b, a very hot small planet, probably similar to Gliese 876.

The preplanet structure — the “seed” of a planet — consists of solids (mostly silicates) and volatiles (such as water and ammonia), with trace amounts of hydrogen and noble gases. Due to the energy of the accretion process and the constant collisions with large solid bodies, this seed is thoroughly molten. (Some of Earth’s internal heat is a relic of this process.) In this state the structure differentiates. Iron and siderophile elements (high-density transition metals that like to bond with iron) precipitate from the silicate mix and sink under their own weight to form the core in the center. The remaining silicate minerals will remain in a mantle with the less dense ones closer to the top. Volatiles that are left over after hydrating the mantle minerals will rise to the surface and atmosphere.

Differentiation is an orderly and predictable process thanks to our knowledge of chemistry and mineral properties under pressure. Some super-Earths, the rocky ones, develop quite similarly, although the pressure in the mantle is almost tenfold higher and different varieties of minerals form. Other super-Earths, the oceanic ones, are totally exotic beasts, with oceans that are 100 kilometers deep overlying a dense hot solid water, called ice VII.

It might seem ridiculous to refer to this water as ice, given that it is at a searing temperature of 1,000 K, but under such high pressures, it forms. Water — H2O — has a familiar structure and formula, but our familiarity with it can make us overlook the fact that it is actually very complex. In common ice, the weak bonds between the molecules cause the molecules to form rings (mostly hexagons) that leave lots of empty space in between. The empty space gives it a lower density than liquid water, and so — as you know from a glass of ice water — it floats. Under high pressure the density of water increases as the molecules are forced closer together; the bonds are bent to form tighter rings, which also interpenetrate. That makes the water solid, almost irrespective of the temperature, and much denser than the liquid phase. The high-pressure ices that exist at high temperatures are known as ice VII, X, and XI; these are the ice phases we expect to find inside oceanic super-Earths. These ices are still less dense than rocks, however, so an oceanic super-Earth will be less dense than a rocky one of the same mass.

Ocean planets might be very common in the Universe because water is very common in the low-temperature environments where planets form and evolve. This might be especially true for super-Earths, which can retain volatiles more easily thanks to their larger mass and surface gravity. In order for a planet to become an ocean planet, it should form with or obtain at least 10 percent of its mass in water. Ammonia could be mixed in, but water is by far the dominant volatile chemical we see among the materials in proto-planetary disks. For comparison, Earth’s oceans are just about 0.02 percent of its mass.

An ocean planet, regardless of its surface temperature, should have the same layers inside: an iron core surrounded by a silicate-rich mantle that transitions into the hot water ice. The latter will become liquid water near the surface (the last 100 kilometers or so). The surface of the liquid water ocean will be covered with ice Ih, if the planet is far from its star and cold, like Jupiter’s moon Europa. If the planet is close to its star and hot at the surface, the liquid ocean will transition into a thick hot steam atmosphere. If the planet has moderate temperatures such as we have on Earth, the water ocean will resemble Earth’s, but there will be no continents or basalt tectonic plates under it. The interior of the ocean planet will remain under the control of the planet’s internal reservoir of heat. The transition between silicate mantle and hot water ice happens with a small change in density but no change in temperature, and the two materials have similarly high viscosity. Like the silicate mantle, the hot water ice “mantle” convects slowly.

The two families of super-Earths have planets that are diverse in size and amount of water. These characteristics depend on the mixture of elements present as the planet forms. From studying the spectra of many stars, we know that the amount of iron and other heavy elements will be different in different planetary systems. We already know that where in the proto-planetary disk a planet forms also matters. So, among the rocky planets we could find super-Mercurys — planets that have as much as 70 percent of iron core inside, like our planet Mercury. Or we could find super-Moons — planets that have no iron core, just an iron-rich mantle and perhaps a water layer.

There is a third possible family of super-Earths and terrestrial planets— carbon planets. These would be extremely rare, as they require more carbon than oxygen to be present in the planet-forming mixture. Normally carbon is half as abundant as oxygen. But astronomers have observed rare stars in which carbon is more plentiful than oxygen. A planet that forms from such a mixture will be different — it will have a mantle rich in silicon carbide and graphite in its interior. It will still have a precipitated iron core, but its overall size and the chemistry on its surface and crust will be very different. Silicon carbide is a very hardy substance — we use it to make durable ceramics, the disk brakes of sports cars, and tools for other high-stress environments. So volcanism, tectonics, and weathering are going to be minimal on carbon planets. Also, carbon planets are likely deficient in water.

Astronomy has always been about big numbers — astronomical numbers — and experience with big numbers has taught us that they do not guarantee inevitability. We have to go and find out for ourselves. Still, it seems likely that on some of those Earth-like planets, we will find signs of life. When we discover New Earth — a planet we could call home — the question of the “plurality of worlds” will come front and center, reminding us yet again that we are not the center of the universe.

Reprinted from “Life of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on Our Planet” by Dimitar Sasselov. Available from Basic Books, a member of the Perseus Books Group. Copyright © 2012.

In their new book, “Time Travel and Warp Drives,” Tufts physics professor Allen Everett and University of Central Connecticut math professor Thomas Roman explain the science behind the fiction of time travel, and tackle the question: Is it even possible? The authors delve into the lore of sci-fi shows and books to explain how wormholes, warp drives, and parallel universes work; and what Einstein’s theory of relativity is and its relevance to time travel. They also parse through all those pesky paradoxes that arise when one tries to go back in time.

Everett spoke with Salon over the phone to discuss how we perceive time, traveling at the speed of light, and why we can’t go back in time to kill Hitler.

I’m going to jump the gun and ask: In your opinion, is time travel possible?

There are ways in which it might be possible, but there are problems with all of those. I think it’s conceivable that time travel is possible. If someone put a gun to my head and made me make a yes or no prediction, I would predict no — with what we know now.

Twenty years ago, I think the answer would have been that it just wasn’t possible. We have learned some things about general relativity in the last 30 or 40 years or so, which make us take, at least for some of us in the profession, the idea of it being possible more seriously. But, still, one would have to say that the odds are against it. You might run into problems with time travel that you don’t run into with superluminal [faster than the speed of light] travel.

The correct statement is that backward time travel is not yet possible and may never be. We can travel forward in time.

How so, theoretically?

If you get into a space machine and travel from Earth to a nearby star and return, at a high speed (near the speed of light), when you get back to Earth, you will find that perhaps the trip took one year for you, as measured by your clock on the spaceship, but when you return to Earth, you will find that you returned to an Earth which is 10 years into the future, from the time when you left. So that’s exactly what we mean by traveling into the future. The possibility of doing that is well-established physics. As a practical matter it’s hard to do with people because the effect only becomes noticeable when the speed approaches the speed of light and it takes a prohibitive amount of energy to accelerate a people-carrying rocket ship to the speed of light and then slow it down again.

You can accelerate elementary particles (which decay radioactively to speeds very close to the speed of light). There are abundant observations to the fact that particles live longer when they are traveling at speeds close to the speed of light than they do when they are at rest. That is part of everyday observations in elementary particle laboratories. So you can certainly travel, in principle at least, into the future and return, but you can’t go back and listen to Mozart play a piano concerto. The chance of doing that is iffy at best.

You write that “it would be a bit tough to organize any kind of galactic federation” because of a phenomenon called “time dilation.” Can you explain what that is?

Suppose there’s a nearby galaxy which belongs to the emperor’s galactic empire. The emperor says, “I’m going to change the law in the empire. Henceforth, everyone on getting up in the morning must devote five minutes to praising the emperor.” So he sends one of his deputies off to a nearby star system with instructions to institute his new law, and in order to get there in a reasonable length of time, he’d have to be traveling at nearly the speed of light. He will get to the planet [in the nearby star system] in, let’s say, 100 years in the future, as far as the people on the planet are concerned. So they will say the law went into affect in the year [2111], but the Emperor’s deputy will take, say, only 10 years to travel to the planet, and another 10 years to travel back to Earth because of the time dilation effect. The law will be different at every point in the galactic empire because different observers will think the law went into effect at different times. I think that would make it difficult to run a reasonable empire.

Good point. So the person in the spaceship is experiencing time at a different rate?

Essentially, clocks in the spaceship are running more slowly than clocks that are at rest [outside the spaceship].

Does this have to do with our subjective experience of time?

No. If they don’t look outside, the people on the spaceship will not be aware that the time is moving slowly for them, but they will become aware of it when they return to Earth and find out that while 20 years has elapsed for them, 200 years has elapsed on Earth.

OK, if there are two different times, inside and outside time machine, then doesn’t time travel occur around us all the time? You mention in the book the idea of a bullet being fired. Theoretically, is the time inside the bullet slower than our time outside the bullet?

Yeah. If you imagine you had a tiny little watch in the bullet and you fired the bullet at a target, let’s say, 10 miles away. The bullet would take something like 10 seconds to get there, as seen by people on the Earth, but the little watch in the bullet, when it hits the target, will not read 10 seconds, but, say, 3 seconds. So the time as read by the clock in bullet will not be the same as the clock at the target, when the bullet get to the target — that’s provided you have synchronized the clocks.

[Going back to our previous example with the emperor], people on Earth and on the [other] planet think their clocks are running at the same rate, but then the clock on spaceship, when it arrives at the other planet, will not be synchronized with the clock on [that] planet because the clock in the spaceship (just as the clock in the bullet) has been traveling at high speed and has undergone time dilation.

A commonly repeated concern when speaking of time travel (to the past especially) is the danger of changing history. Is it possible to change the past? Can you get around the paradox of your travel being assumed (i.e., you are meant to go back but would you still be able to if your history is altered)? Is that when we run into problems?

Yes. That’s when you get into logical problems. You may go back in time trying to assassinate Hitler, but on the other hand you know that Hitler wasn’t assassinated. You cannot change the past. That past has already happened. Nothing you can do about it. If you travel back in time, the only possibility is if, somehow, you’re forced to do so in some way that doesn’t alter the past. Or, there is the other possibility that when you travel backward in time you wind up in a parallel universe. So in the universe we live in where Hitler wasn’t assassinated, we could, conceivably, travel backward in time into a different universe where you would assassinate Hitler and the subsequent time evolution in that second universe would then be quite different.

Can you explain what space-time is? We know time as the fourth dimension, but can we visualize it, spatially, like the three others?

In many ways, time is similar to the three dimensions of space but in other ways it is quite different. They are connected with one another, [but] there are important differences between them. A quantity in physics is defined by how you measure it. You measure the spatial difference between two points with a meter stick; you measure the temporal distance between them with a clock. That means, to a physicist, that they are fundamentally different quantities because the experiments that you did do to measure them are different.

Are there some fundamental truths or laws that must hold for time travel to be possible?

Among the most important laws of physics are the conservation laws, with things like energy and momentum, which must remain constant in time. As far as we know there is no way of producing or getting rid of the total amount of energy in the universe.

In Madeleine L’Engle’s “A Wrinkle in Time,” the characters travel by a method they call “tesseracting,” which involves folding space. Is time folding theoretically possible, in the way, say, we’re able to fold two- or three-dimensional objects?

Yes, if you can achieve the right distribution of matter and energy which would fold space in the manner you might like it to be folded. Whether you can really achieve that distribution of matter and energy is less clear. In particular, one of the things that is true is that if you want to fold space in such a way that time travel becomes possible, for example, you need a distribution of matter and energy with the property that there is a region of space and time where the energy density is negative—negative meaning that there is even less matter and energy present than the amount in empty space.

You talk about chaos or disorder in relation to backward time travel. Can you explain their relationship?

The question is: What is it that causes a difference between the two directions in time? The basic, classical laws of physics, which are Newton’s laws and their corresponding mechanical laws, make no distinction between the two directions in time—a process which can occur in one direction, can also occur in the opposite direction.

Why is backward time direction different than forward time direction? To me, the answer to that is something called the second law of thermodynamics, which says that the total entropy of the universe increases as time increases. Entropy is, essentially, the amount of confusion — mixing — in the universe. So, it’s that law which says it’s perfectly possible to see a diver dive off a diving board into a swimming pool. The laws of physics say that, if you arrange the system just right, in fact you can find that the diver will pop, spontaneously, out of the water and land on the diving board. It is possible, but the probability of the system having just the right conditions for that to happen is so small that we will never see it in the whole history of the universe. So when we talk about the difference between going forward and backward in time, what is it that defines that difference? Physicists say: What is it that produces an arrow of time? The answer is the second law of thermodynamics.

So this law explains cause and effect and why you can’t have an effect before its cause?

Yes.

Is it theoretically possible to get stuck in a time loop, like Bill Murray in “Groundhog Day”? 

The answer is yes, but you have to assume the “many worlds” interpretation of quantum mechanics is the correct one, and then you’re not exactly stuck in a time loop — every time you go around the loop, you come back to the same starting point in a parallel universe.

So, even if time travel is not possible for humans (for now), what do you think of its future prospects? And what kind of social impact does this kind of research bear on daily life?

Well, it’s hard to say because you don’t know what the results of the research are going to be. Physicists have learned quite a bit about the consequences of relativity as the result of this kind of research, so we know a lot more than we did, say, maybe 30 years ago. A person on the street might say, “So what?”—and that’s not an unreasonable attitude to have. Will there be actual consequences on people’s lives? It’s hard to answer that question. I would hope that the evolution of human society is going to be a lot different if it turns out that in some way or other, one can travel faster than light, in which case we would probably expand into the galaxy. From the point of view of humanity, I think the question of whether superluminal travel is possible will have a lot more consequence than the question of whether time travel is possible.