Cosmos 8: Journeys in Space and Time

by Shane L. Larson

When I was a kid, I wanted to be one of four things when I grew up: an astronaut, Captain Kirk, Carl Sagan, or Indiana Jones. Now that I am (almost) grown up, it seems my chances of being an astronaut are limited by the slow pace of progress in space tourism, and as far as I know the Enterprise is not being constructed as we speak in some remote cornfield in Iowa. I’m still working on learning to communicate like Carl Sagan. What about Indiana Jones? Well, let’s see: Bullwhip? No (sigh).  Cool hat? Cool enough hat.  Professor?  Check!  Archaeologist, adventure fraught life?  Yep!

Wait, what? I’m an astronomer, not an archaeologist! Archaeology is the study of human activity in the past. In a very similar way, astronomy is the study of the Cosmic past, not entirely limited to the human perspective. (So basically, astronomy and archaeology are the same; that is to say, me and Indiana Jones, we’re the same!)

My childhood ambitions, left to right: (1) astronaut [this is Heidemarie Steefanyshyn-Piper on STS115], (2) Captain Kirk, (3) Carl Sagan, (4) Indiana Jones

Archaeology is often described primarily as being conducted through the study of artifacts. Astronomy also sometimes relies on artifacts (such as meteorites), but more often than not astronomy is about images captured by telescopes. But herein lies a fundamental difference between archaeology and astronomy: archaeology is about the interpretation of artifacts in an attempt to understand the past, whereas astronomy is a direct observation of the past. The light we see from the Cosmos beyond the shores of the Earth takes time to make the journey across the deeps of space.  At the moment we detect the light from some remote shoal of stars, the image we see is a snapshot taken millions of years before, when the light left its point of origin.

Consider this supernova (known as SN 2014g), detected only one week ago on 14 January 2014, in the galaxy NGC 3448, located just under the bowl of the Big Dipper.  Astronomers call this particular type of supernova explosion a “Type II Supernova” — an explosion from a massive star imploding on its core, then bouncing back from the enormous pressure in an explosion that, for a time, will outshine all the other stars in the parent galaxy.  When did this occur?  NGC 3448 is 157.9 million lightyears away, so this explosion occurred 157.9 million years ago!  When this star exploded and the light started travelling toward Earth, there were still dinosaurs on our planet (it was the Jurassic Period!); but we just heard the news of the supernova this week.

(L) NGC 3448 as seen by GALEX and Spitzer [Image, JPL/NASA], (R) Supernova SN2014g.

Even stuff “close” to us is far away.  During the month of January, if you go out shortly after it gets dark, you will see the constellation Orion.  The belt stars point down and to the east toward a brilliant blue-white star called Sirius, the brightest naked eye star in the sky.  Sirius is 8.611 lightyears away.  When the light you see tonight left Sirius, it was June 2005 (assuming you are reading this in January 2014).  What was going on then?  The San Antonio Spurs had beaten the Detroit Pistons 4 games to 3 in the 59th NBA Championship; Batman Begins had just been released; Watergate informer “Deep Throat” was revealed to be former Associate Director of the FBI, Mark Felt; and NASA was less than a month away from Discovery’s STS-114 flight, the first flight of the space shuttle since the loss of Columbia in 2003.

Some of the happenings 8 years ago, in June 2005. Left to Right: (1) Robert Horry’s game winning 3-point jumper during overtime in Game 5 of the NBA Finals; (2) Batman Begins was dominating the box office; (3) former FBI Associate Director Mark Felt was revealed to be the Watergate informant “Deep Throat“; (4) Discovery returned the Space Shuttles to flight, just over 2 years after the Columbia disaster.

Looking across space is looking back in time. The farther away from the Earth you look, the farther back in time you look.  This connection between distance and time may seem a bit esoteric and weird at first sight, but in fact we are used to conflating space and time; you do it every day and don’t even realize it! Just go around the house and ask different members of the family how far it is to the grocery store across town. Some of them will say 10 minutes, and some of them will say 3 miles. There is no difference in the amount of space between you and the store — these are just two different ways of saying the same thing, and you understand them both!

Even Google Maps knows you can understand travelling either in terms of space, or in terms of time, telling you both!

But travelling into space is not the same thing as travelling to the supermarket. The distances are far vaster, so the travel times are far longer. It took Apollo astronauts about 4 days to fly across the gulf of space to the Moon; that’s about the same time it takes to drive from New York to Los Angeles (assuming you aren’t doing some kind of Cannonball Run thing).  It has taken Voyager 36 years to reach the boundary of our solar system, where the interstellar winds from all the stars in the galaxy dominate over the faint, fading stream of wind of our own Sun; light takes 35 hours to make the round trip out to Voyager and back.  Voyager is the fastest object ever built by humans; if we could travel with Voyager, it would take an entire human lifetime to travel to the edge of the solar system and back.  The stars are vastly farther away; travelling by the means we know today would seem to preclude us ever travelling to the far corners of the Cosmos.

Despite the fact that we often conflate time and space, we perceive them to be very different, as evidenced by the tools I use to measure them — we use rulers to measure space, and we use wristwatches (if you’re old enough; otherwise you use a cell phone!) to measure time.  We see space and time as different because we only travel through space when decide to get up and go somewhere, but we are always travelling through time — there seems to be no way to avoid getting to next Tuesday.

Different ways we have devised to measure space or time.

At the start of the 20th Centruy, a young Albert Einstein had a startling realization: time and space are really manifestations of a single, unified fabric that underlies the entire Cosmos, called spacetime. That space and time are inextricably intertwined is hard to see in our slow, everyday lives, but what Einstein realized is the intertwining becomes far more important and obvious when you start travelling fast, at speeds approaching the speed of light. The laws of Nature that describe how we experience and measure the world at high speeds are called “special relativity.”  The laws of special relativity are a generalization of the usual Newtonian laws of mechanics that govern cricket games, the flights of unladen swallows, and car crashes; at the low speeds of these everyday events, special relativity gives exactly the same predictions and results as Newtonian physics.

But suppose you had a fast car, and you could stomp on the accelerator, increasing your speed without difficulty. As you approach the speed of light you would notice that your observations of the world change compared to your friends who are standing still (or driving Yugos). Special relativity accurately predicts the consequences of travelling at high speeds, and foremost among these predictions is an effect known as time dilation — clocks that move at high speeds (mechanical, electronic, or biological — it matters not) tick more slowly than clocks that move slowly or remain at rest.

Free neutrons die, on average, after 881.5 seconds, decaying into a proton, an electron, and an anti-neutrino.

Changing how fast you move through space changes how you move through time. It is an altogether astonishing result, completely counter-intuitive to our everyday experiences here on Earth.  But science is a game where the proof is in the pudding — extraordinary claims require extraordinary evidence, and special relativity has been put through the wringer. The most prominent way in which special relativity has been tested is in particle accelerators.  Many species of atoms and sub-atomic particles have limited lifetimes and eventually die by breaking apart (“decaying”) into smaller, more stable pieces. The time that these particles live is called the “half-life” and it is easy to measure in the laboratory — you plunk a particle down on your bench and you watch it until it dies!  The classic example of this is the neutron, one of the fundamental particles that makes up all the atoms in your body.  If you set a neutron on your workbench, it will live for about 881.5 seconds, just over 14 minutes and 41 seconds. Special relativity tells us that all clocks moving at high speeds run slow; this includes the clock the neutron carries with it that determines when it is going to decay. If we accelerate the neutron to speeds approaching the speed of light, we find that it lives longer than the predicted 881.5 seconds, confirming that its internal clock is running slow.  Special relativity has been tested in this way billions and billions of times in particle accelerators around the world, and never once has it failed to correctly describe the outcome of an experiment.  

While special relativity explains a great many physical phenomena that can be observed in the Cosmos, there is an important realization to be had: special relativity provides a way for us to journey to the stars. If we can construct a rocketship that could travel close to the speed of light (a not implausible idea, even with today’s technology), special relativity tells us that as passengers, our biological clocks will tick slowly enough that we will live to see the journey’s end.

Nuclear powered starship concepts that, in principle, are not outside the boundaries of our technology. (L) Starship Orion; (C) Project Daedalus; (R) Bussard Ramjet. [Images from Cosmos: A Personal Voyage] For many detailed starship concepts, see the book The Starflight Handbook by Eugene Mallove.

What kind of travels could we imagine? Consider a rocket where we hold down the accelerator and never let up. We accelerate just enough that it feels like we have Earth normal gravity (“one gee”) on the ship, allowing us to pass time on our journey in comfort, pursuing ordinary everyday activities like ping-pong and shuffleboard. Our rocketship would increase its speed rapidly, ticking off the decimal places ever closer to, but never reaching, the speed of light.  The table below shows what different trips would look like.  For destinations close to Earth, like the Voyager spacecraft, our rocket travels fast enough to make the trip no more onerous than an extended vacation, but not so fast as to see the relativistic slowing of time.  But over Cosmic distances, time slows dramatically aboard the ship.  Travelling to the center of the Milky Way,to explore the wilderness near the monstrous black hole that lurks there, would take only 10.6 years, less than 22 year around trip.  But on Earth, 52,000 years will have passed, changing our planet and civilizations in unimaginable ways. We don’t even know what human life was like 52,000 years ago, because our written histories only extend back a tenth that long — who knows what 52,000 years in the future will look like!  To those of us on the voyage, scant decades will seem to have passed, but our species will have moved on without us.

Distance an measured times on relativistic rocket trips at acceleration of 1 g.

For most of these journeys, those of us who make the trip will behold firsthand the wonders of the Cosmos, but there will be none of our friends and family to hear the tales when we return home.  I often think  about these journeys, and wonder whether I would do it, given the chance.  Would I leave the Earth behind, never to walk her green meadows again?  I would never be able to Sail the Aegean Sea, or hike down Olduvai Gorge, or howl with coyotes on cool fall evenings in the Sonoran Desert.  But in return I would see the galaxy from the inside out, watch as the Milky Way black hole tears a star apart, and surf along the tendrils of molecular gas and dust that will one day become a new generation of stars.  It may be, perhaps, a fair trade — one set of Nature’s wonders for another.

The black hole at the center of the Milky Way, surrounded by a swarm of stars and the strained tendrils of gas clouds and stars that have been eaten by the black hole. [Image from European Southern Observatory]

But for now, it is only a lovely daydream, enabled and provoked by our growing understanding of the Cosmos, and how it is put together.

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This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage.  The introductory post of the series, with links to all other posts may be found here:  http://wp.me/p19G0g-dE