Does Light Experience Time?

Have you ever noticed that time flies when you’re having fun? Well, not for light. In fact, photons don’t experience any time at all. Here’s a mind-bending concept that should shatter your brain into pieces.

As you might know, I co-host Astronomy Cast, and get to pick the brain of the brilliant astrophysicist Dr. Pamela Gay every week about whatever crazy thing I think of in the shower. We were talking about photons one week and she dropped a bombshell on my brain. Photons do not experience time. [SNARK: Are you worried they might get bored?]

Just think about that idea. From the perspective of a photon, there is no such thing as time. It’s emitted, and might exist for hundreds of trillions of years, but for the photon, there’s zero time elapsed between when it’s emitted and when it’s absorbed again. It doesn’t experience distance either. [SNARK: Clearly, it didn’t need to borrow my copy of GQ for the trip.]

Since photons can’t think, we don’t have to worry too much about their existential horror of experiencing neither time nor distance, but it tells us so much about how they’re linked together. Through his Theory of Relativity, Einstein helped us understand how time and distance are connected.

Let’s do a quick review. If we want to travel to some distant point in space, and we travel faster and faster, approaching the speed of light our clocks slow down relative to an observer back on Earth. And yet, we reach our destination more quickly than we would expect. Sure, our mass goes up and there are enormous amounts of energy required, but for this example, we’ll just ignore all that.

If you could travel at a constant acceleration of 1 g, you could cross billions of light years in a single human generation. Of course, your friends back home would have experienced billions of years in your absence, but much like the mass increase and energy required, we won’t worry about them.

The closer you get to light speed, the less time you experience and the shorter a distance you experience. You may recall that these numbers begin to approach zero. According to relativity, mass can never move through the Universe at light speed. Mass will increase to infinity, and the amount of energy required to move it any faster will also be infinite. But for light itself, which is already moving at light speed… You guessed it, the photons reach zero distance and zero time.

Photons can take hundreds of thousands of years to travel from the core of the Sun until they reach the surface and fly off into space. And yet, that final journey, that could take it billions of light years across space, was no different from jumping from atom to atom.

There, now these ideas can haunt your thoughts as they do mine. You’re welcome. What do you think? What’s your favorite mind bending relativity side effect? Tell us in the comments below.

Destructive Interference

Destructive Interference Image Credit: Science World

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Sound travels in waves, which function much the same as ocean waves do. One wave cycle is a complete wave, consisting of both the up half (crest) and down half (trough). Waves also have a certain amplitude which is the measure of how strong the wave is; the higher the amplitude, the higher the crests and deeper the troughs. Waves don’t usually reflect when they strike other waves. Instead, they combine. If the amplitudes of two waves have the same sign (either both positive or both negative), they will add together to form a wave with a larger amplitude. This is called constructive interference. If the two amplitudes have opposite signs, they will subtract to form a combined wave with lower amplitude. This is what is called Destructive Interference, which is a subfield of the larger study in physics known as wave propagation.

An interesting example of this is the loudspeaker. When music is played on the loudspeaker, sound waves emanate from the front and back of the speaker. Since they are out of phase, they diffract into the entire region around the speaker. The two waves interfere destructively and cancel each other, particularly at very low frequencies. But when the speaker is held up behind baffle, which in this case consists of a wooden sheet with a circular hole cut in it, the sounds can no longer diffract and mix while they are out of phase, and as a consequence the intensity increases enormously. This is why loudspeakers are often mounted in boxes, so that the sound from the back cannot interfere with the sound from the front.

Scientists and engineers use destructive interference for a number of applications to levels reduce of ambient sound and noise. One example of this is the modern electronic automobile muffler. This device senses the sound propagating down the exhaust pipe and creates a matching sound with opposite phase. These two sounds interfere destructively, muffling the noise of the engine. Another example is in industrial noise control. This involves sensing the ambient sound in a workplace, electronically reproducing a sound with the opposite phase, and then introducing that sound into the environment so that it interferes destructively with the ambient sound to reduce the overall sound level.

For a hands-on demonstration of how destructive interference works, click on this link.

We have written many articles about destructive interference for Universe Today. Here’s an article about constructive waves, and here’s an article about the Casimir Effect.

If you’d like more info on destructive interference, check out Running Interference, and here’s a link to NASA Science page about Interference.

We’ve also recorded an entire episode of Astronomy Cast all about the Wave Particle Duality. Listen here, Episode 83: Wave Particle Duality.

Sources:
http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29
http://en.wikipedia.org/wiki/Loudspeaker_enclosure
http://en.wikipedia.org/wiki/Sound_baffle
http://www.windows2universe.org/earth/Atmosphere/tornado/beat.html
http://library.thinkquest.org/19537/Physics5.html
http://zonalandeducation.com/mstm/physics/waves/interference/destructiveInterference/InterferenceExplanation3.html