Here we go...

Let's talk about the speed of light.

I'm serious.  I know this may seem remedial and basic, but our perception of the universe is entirely dependent on this seemingly simple concept.

(The following paragraph is paraphrased from our book, so if you read chapter 3, feel free to skip ahead)

It was in 1675 that the speed of light was first measured with some accuracy.  The scientist was Ole Roemer who, in an attempt to predict future eclipses of Jupiter's moons, determined that it took 22 minutes for light to travel across the diameter of the earth's orbit.  Roemer's approximation was a bit generous as we now know that it takes light 16.5 minutes to travel 2 AU.  Just to remind you, the distance between the earth and the sun is defined as 1 astronomical unit (AU).  The speed of light in a vacuum (c), as measured in 1983, is exactly 299,792,458 m/s.  We can say this value is exact because this constant is actually the number on which the meter is defined.  In terms of miles: light travels 186,282 miles/second.

We've all heard it: light speed is the cosmic speed limit.  Nothing can travel faster.  I think it's tough to put this idea into perspective because in day to day experience light seems to travel instantaneously.  There's no lag time when we turn on a light or when we talk on the phone.  Consequently, we regard the speed of light as infinite.  But in an astronomical sense, the speed of light is really kind of slow.  Let me show you.  It takes light 1.3 seconds to travel from earth to the moon, our closest celestial object.  Another rover, Curiosity, is scheduled to land on Mars less than seven months from now.  When it does arrive, communication to or from the machine will take 4 minutes.  Now at the edge of our solar system, it takes over 29 hours to receive data from Voyager 1.  The closest star (that we know about), Proxima Centauri is 25 trillion miles away and its light arrives here on earth in 4.2 years.  If these are our closest neighbors, you can understand why astronomers would want to define a more efficient unit of distance.  So the light year was born and it is defined as the distance that light travels in a year.  Got it?  It is not a measure of time.  Don't be fooled.

1 ly = 9,460,730,472,580.8 km ≈ 5,878,625,373,183.6 mi ≈ 63,241.1 AU ≈ 0.306601 pc

Clearly there is no shortage of ways to express large scale distances.  But no matter how I say 9,460,730,472,580.8 km, understanding a magnitude that large is simply beyond my grasp.

The entire field of astronomy is based on the fact that when we look at distance objects we are looking into the past.  Because light is not instantaneous, we can catch a glimpse of the beginning of the universe, we can watch galaxies form, and ultimately, begin to understand how we came to be.  The constant speed of light is the thing that allows us to examine our very existence.

This constant is just so cool.  I almost feel like I don't have the words to really do it justice.  But I'm going to try to make you care anyway.

So, light is an aspect of nature that dictates how we view the universe.  What are its ramifications?  Take a look:

• Red Shift 
• This important concept was studied by Edwin Hubble during the first half of the 20th century.  As he looked out at the universe (this happened to be on the 100" Hooker telescope) he noticed that many objects were shifted toward longer wavelengths thus appearing more red.  He determined that this redshift, as it was called, was due to the expansion of space itself and that galaxies are flying away from one another at an ever increasing rate.  Objects receding slowly, he discovered, had a small redshift and those with a larger redshift are not only more distant, but they are flying away at even higher rates.  Hubble suggested a rate of expansion now known as Hubble's constant.  By using (wait for it) the 200" Hale telescope, Allan Sandage was able to measure redshifts with considerable accuracy thus confirming his mentor's prediction.  We now know the universe to be some 13.7 billion years old.

• Light Horizon
• We have this gift of a constant speed of light.  There's a catch though; this same gift is also our ultimate limitation.  If the universe is 13.7 billions years old, that would imply that it would take that amount of time for the earliest light to reach us.  Right?  Well, due to the expansion of space, it is estimated the we can detect signals of light from anywhere within a radius of about 46 billions light years.  This is often referred to as our light horizon or the observable universe.  It is the sphere around us that is the absolute limit on how far we can see.  Does space end there?  Probably not, so any observer anywhere in the universe has his own light horizon.  

• Cosmic Microwave Background
• The most distant thing we can currently detect is remnant light from shortly after the big bang itself.  Called cosmic microwave background radiation, this light played a crucial role in determining a theory of an expanding universe.  Several missions have studied and cataloged this ancient radiation with interesting results.  This will be the topic of my next blog, so stay tuned.

• Biology 
• The value 'c' refers to speed of light in a vacuum, the vacuum part being key.  186,282 miles per second is actually too fast for our eyes to respond to light.  Conveniently enough, we know that light slows as it travels through things like glass or water.  This is because as it travels through these materials it gets absorbed and then re-radiates, causing a delay that slows the light to about 124,000 miles per second (still fast).  This is why a lens works; it can bend and concentrate large quantities of light.  It's incredible to think that our eyes have evolved to perfectly accommodate this quality of light.

• Interstellar Travel 
• Few ideas excite us like that of traveling through space-time.  Highly imaginative entertainment outlets have glamorized the adventure that would be extragalactic travel.  I think a lot of us hang on to the hope that perhaps one day the impossible will become a reality.  Even if it remains an impossibility forever, space travel is a fascinating thought experiment.  How would we do it?  Even light is too slow to travel any distance in a human time scale.  We've all heard of Einstein's famous equation, E = mc², right?  It basically says that as speed increases towards that of light, mass also increases to infinity.  Meaning, it would take an infinite amount of energy for an object with mass (like a spaceship) to travel anywhere near the speed of light.  And we know that to be impossible. But,what if we could exploit the very nature of space-time to aid us in our quest?  In other words, maybe we can cheat the cosmic speed limit.  Wormholes seem to be a favorite of scientists and non scientists alike.  By punching a hole into the very fabric of space-time, it's been suggested that we can travel to another place and/or time in the universe.  Of course manipulating a wormhole to a.) appear and b.) do as you like presents a whole other slew of problems.  I don't think we'll find ourselves traveling to some fantastic exoplanet via wormhole any time soon.  My favorite concept of interstellar travel involves a special kind of spaceship that would fold space to propel it to speeds beyond that of light.  Of course, all these concepts are speculation.  I wouldn't be surprised if there are scientists who have seen the end of their careers because of overzealous deliberation of these matters.  This doesn't mean this topic is void of real scientists.  You can check it out yourself here.

Anyways...

What about neutrinos?  You think those suckers were traveling faster than light?



Next time: The cosmic microwave background.  Stick around.