Categories: Astronomy

When Light Just Isn’t Fast Enough

Take a speed of light trip across the solar system starting at the Sun

We’ve heard it over and over. There’s nothing faster than the speed of light. Einstein set the speed limit at 186,000 miles per second (299,792 km/sec). No material object can theoretically travel faster. For all practical purposes, only light is lithe enough to travel at the speed of light.

Moving in such haste, a beam of light can zip around the Earth 8 times in just one second. A trip to the moon takes just 1.3 seconds. Fast for sure but unfortunately not fast enough. Hit play on the video and you’ll soon know what I mean. The view begins at the Sun and travels outward into the solar system at the speed of light.

Planet           Distance in AU            Travel time
....................................................................
Mercury              0.387        193.0 seconds   or    3.2 minutes
Venus                0.723        360.0 seconds   or    6.0 minutes
Earth                1.000        499.0 seconds   or    8.3 minutes
Mars                 1.523        759.9 seconds   or   12.6 minutes
Jupiter              5.203       2595.0 seconds   or   43.2 minutes
Saturn               9.538       4759.0 seconds   or   79.3 minutes
Uranus              19.819       9575.0 seconds   or  159.6 minutes
Neptune             30.058      14998.0 seconds   or    4.1 hours
Pluto               39.44       19680.0 seconds   or    5.5 hours
...................................................................

Distances and light times to the planets and Pluto (from Alphonse Swinehart)

You might first think that moving that fast will get us across the orbits of the eight planets in a hurry. I shouldn’t have been surprised, but I found myself already getting impatient by the time Mercury flew by … after 3.2 minutes. Earth was still 5 minutes away and Jupiter another 40! That’s why the video cuts off at Jupiter – no one would stick around for Pluto’s appearance 5 1/2 hours later.

As the video tediously but effectively demonstrates we live in a solar system where a few planets are separated by vast spaces. Not even light is fast enough to satisfy the human need for speed. But just to put things in perspective, the fastest current human-made objects is NASA’s Voyager I spacecraft, which recently reached interstellar space traveling at 38,000 mph (17 km/sec) or nearly 18,000 times slower than light speed.

Let’s explore further. Any material object, a Skittle for instance, moving that fast would become infinitely massive. Why? You’d need an infinite amount of energy to accelerate the Skittle to the exact speed of light. Since matter and energy are two faces of the same coin, all that energy creates an infinitely massive Skittle. Sweet revenge if there ever was.

You can however accelerate the pill-like candy to 99.9999% light speed with a finite if incredibly large amount of energy. Einstein’s cool with that. Here’s the weird thing. If you were travelling along at the speed of light it would look like a perfectly normal piece of candy, but if you were to look at it from the outside world, the sugary treat would be the entire universe. Both viewpoints are equally valid, and that’s the essence of relatively.


Wave-particle duality of light

To better imagine a day in the life of a photon, let’s go along for the ride. Photons are the particle form of light, which for a long time was only understood as waves of electromagnetic energy. In the weirdness of quantum world, light is both a particle and a wave. From our perspective, a photon rip by at 186,000 miles per second, but to the photon itself, the world stands still and time stops. Photons are everywhere at once. Omnipresent. No time passes for them.

In relativity theory, the movement of anything is defined entirely from an observer’s point of view. From the photon’s perspective, it’s at rest. From ours, it’s moving across time and space. We all have our own “coordinate frame”, so that wherever we are, we’re at rest. That’s relativity for you – all frames are equally valid.

Let say you’re in a plane. That sad bag of pretzels you were just handed is at rest because it’s in your coordinate frame. The person next to you is likewise at rest (and hopefully not snoring). Even the plane’s at rest. According to Einstein, it’s just as valid to picture the world outside the airplane window moving while the plane itself remains at rest. Next time you fly, close your eyes once the plane reaches altitude and a constant speed. You’ll hear the noise of engines, but there’s no way to know you’re actually moving.

Diagram showing how an object (sphere) contracts in the direction of motion as its speed increases. At far left, its velocity (V) is 0.3 times the speed of light. Credit: Askamathematician.com

Relativity also predicts that objects contract in the direction of their motion. Strange as it sounds, this has been verified by many experiments. The faster things travel, the more they contract.

The effect doesn’t become noticeable until an object approaches light speed, but the Apollo 10 service and crew modules reached a velocity of 0.0037% the speed of light. From the perspective of someone on the ground, the 11.03-meter-long module shrank by approximately 7.5 nanometers, an exceedingly tiny but measurable amount. (A sheet of paper is 100,000 nanometers thick). Likewise, distances contract, bottoming out at zero at light speed.

Length contraction occurs because a stationary observer sees a speedy spaceship traveler’s time tick by more slowly. Since light is measured in time units – light seconds, light years – in order for the two to agree on the speed of light (a constant across the universe) the traveler’s “ruler” has to be shorter. And it really is from your stationary perspective if you could somehow peer inside the ship. Traveling at 10% light speed, a 200-foot spaceship shrinks to 199 feet. At 86.5%, it’s 100 feet or half the size and at 99.99% only 3 feet!

We’ve traveled far today – sitting quietly in our frames of reference.

Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

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