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# 15.20: Light Travel Time

In the everyday world, as perceived by the human senses, light seems to travel instantaneously from one place to another. In fact, the speed of light is not infinite, and light doesn't instantly jump from your ceiling light to your desk and then to your eye. We perceive light as moving instantly because its actual velocity is almost unimaginably high; light travels at 300,000 km/s, denoted c. Using the equation Rate × Time = Distance, you can divide any distance by this number to figure out the time it would take light to cross that distance. In this way, we can see that light takes 1.5 × 108 / 3 × 105 = 500 seconds to reach Earth from the Sun, or just over 8 minutes. It takes light about 40 times longer (Pluto at a distance of 39.4 A.U.) to leave the solar system, or about 5 hours.

The speed of light is a built-in quality of our universe. All evidence to date indicates that light has always travelled at this speed, that the speed is exact, and that the same speed is observed for all observers. The vast size of the universe, coupled with the finite (albeit large) speed of light, means that as we look out in space, we look back in time. Distant light is old light.

The 5 hours it takes light to travel across our Solar System may seem like a short period to cross such a large distance, but we have to think about scale. While distances within the Solar System are large to us, they are dwarfed by the distances between the stars. Considering larger regions of the Milky Way, a natural distance unit is the distance light travels in one year. This is called a light year. We can easily calculate the size of this unit by remembering that distance has the units of velocity times time. So:

Dly = vt = c x 1 year = 3 × 105 x (3600 × 24 × 365) = 9.5 × 1012 km

A light year is the typical distance between stars in the neighborhood of the Sun. It is nearly 10 trillion kilometers or 6 trillion miles! The fundamental unit of distance defined by geometry is the parsec, equal to 3.1 × 1013 km. This is described in more detail in the article on parallax. Geometrically, one parsec is the height of a right-triangle with an angle of 1 arcsec describing its apex, and a distance of 1 AU describing its base. The units are related by a small numerical constant Dly = 3.26 Dpc. So to roughly convert from parsecs to light years, multiply by 3.3.

The following list gives the distance to various points within the Milky Way and beyond, both in terms of parsecs and the light travel time in years (which is also the distance in light years or 3.3 times the distance in parcsecs). To fully appreciate how isolated we are in space, remember that light is the fastest thing we know of. The fastest spacecraft can not reach 1% of the speed of light. So you would have to multiply the numbers on the right hand side of the table by at least 100 to estimate how long it would take to send a probe through the Milky Way and into the Local Group with current technology.

• Nearest star (α Centauri) - 1.3 pc, 4.2 years
• Sirius - 2.7 pc, 8.8 years
• Vega - 8.1 pc, 26 years
• Hyades cluster - 42 pc, 134 years
• Pleiades cluster - 125 pc, 411 years
• Orion nebula - 460 pc, 1500 years
• Nearest spiral arm - 1200 pc, 3900 years
• Center of the galaxy - 8500 pc, 29,000 years
• Far edge of the galaxy - 24,000 pc, 78,000 years
• Large Magellanic Cloud - 50,000 pc, 163,000 years
• Andromeda galaxy (M31) - 670,000 pc, 2.2 million years

Since light moves at a finite speed, we can use telescopes as time machines to see the universe as it appeared in the past. Astronomers take will actually refer to distant galaxies as having a large "look-back time," meaning that we are looking far back in time when we see them. Locally this has interesting side effects. When we look at the Orion Nebula in the night sky the light we see is 1500 years old and left Orion when Europe was in the Dark Ages. Light from the Large Magellanic Cloud has been traveling for just over 160,000 years, since before all human civilizations. If you observe the Andromeda galaxy with binoculars or a small telescope that light is nearly 2.25 million years old. In other words, we see Andromeda as it was before humans evolved from apes on the Earth! This is particularly interesting to think about when we look at the iconic "Pillars of Creation" in the Eagle Nebulae. We see shock waves moving through the system as they appeared 6500 years ago. Were we able to see this volume of space as it is today, the Pillars would actually have crumpled under the onslaught of these shocks. Think about this: We are looking at structures that don't actually still exist!

What does Andromeda look like now? Nobody knows. Since nothing travels faster than light (and this applies to all the colors of light across the electromagnetic spectrum), there is no quicker way to send information from one place to another. We are stuck with collecting and measuring "old" light. While this seems like a limitation, scientists actually find that it turns out that light travel time is a wonderful tool. By looking further out in space we look further back in time. In this way astronomers get to explore the earlier stages of the universe seeing first hand (on delay) what the early universe looked like.