The Space of Now

space-of-now-varies-by-perceptual-modeThe other night I was having dinner with my wife and my neighbor, discussing the concept of “living in the now”. Bah! My cold physicist brain immediately dismissed such new age nonsense.  We cannot possibly live in the “now” because all of the sensory input of our surroundings comes from the past, not the present. Information travels at a finite speed, be it via light (300,000 kilometers per second, or roughly 1 foot per nanosecond1), sound (340 meters per second, or about 1 foot per millisecond 1), or someone’s very strong perfume (about 3 millimeters in an hour if it is simply diffusion, but nearly instantaneous if you are on a plane). Looking at my wife, I was seeing her as she appeared 3 nanoseconds in the past; listening to my neighbor, I heard a story she told 3 milliseconds ago; and the full moon bearing down on us was merely a mirage from the distant past – all of 1 second ago. My experience at that dinner table was a hodge-podge of the asynchronous past.

Then the warm, fuzzy, “living in my skin” side of me kicked in and realized: wait, we cannot possibly detect differences of time on nanosecond or microsecond scales!  Our visual and auditory systems react far more slowly. To be precise, in 1875 Sigmund Exner found that our visual system can only sense changes in our visual environment on timescales of 20 milliseconds, a discovery that motivated both the Phi phenomenon and the current 50-60 frames per second“ high end” HDTV frame rate. In 1933, Ulrich Steudel found a similar “event limit” of 50 milliseconds when distinguishing different sounds, making rhythms in 1/64 time difficult to detect by all but the most highly trained ear.2  Events that happen on shorter timescales blur these senses, appearing or sounding to us to be simultaneous.

So while physics has a strict and instantaneous concept of “now”, humans perceive now in finite chunks.  Perceptually, our dinner conversation was in fact “in the now” – synchronous and real-time.  But certainly there are other kinds of events –  international phone calls, the rumble of an approaching storm – that are perceptually asynchronous. Where do we draw the line?

These musings tickled my transperceptional funny bone – can we translate “now” from a time to a space?  How big is our now?  Where is the boundary between the present that we perceive in “real time” and interact with, and the past that is forever lost to us?

Let’s go back to light.  Our 20 millisecond visual “blur” means that two simultaneous light events – say the flash of your friends camera and the flash of a lighthouse in the distance – will be perceived as simultaneous as long as their light travel time doesn’t differ by more than 20 milliseconds.  With light traveling 1 foot per nanosecond, this distance is immense: 20,000,000 feet or about 38,000 miles.  In fact, this is larger than the circumference of the Earth, which essentially puts the entire planet in the visual “now-space”.  Of course, seeing events halfway around the world is made difficult by the darn planet being in the way, but it is possible in principle to be in California and see things in India that are happening “now”.

Sound is a little more restrictive.  Because sound moves a million times slower, the space of sound is almost a million times smaller. With our 50 millisecond auditory blur, we can only hear “now” out to about 50 feet.  Beyond that, we detect asynchronicity.  50 feet is on the scale of large auditoriums, theaters and stadiums. Next time you go to a baseball or football game, see if you can detect the slight delay between seeing “the wave” on the far side of the stadium and hearing it. (Physicist tip for the arts: always buy seats up front if you really want to hear “live music”.)

The delay between sight and sound is a familiar phenomenon – lightening and thunder, a plane flying overhead – but in the framework of “now space” these ideas take on a slightly different meaning.  Go to an open window and take in the scene – perhaps there are cars driving by, people talking, children playing, motorized lawn equipment (always).  Now close your eyes and focus only on the sound.  Visually, all of this scene was in the present; now, the soundscape you hear is a mix of present and past, a simultaneous blend of synchronous and asynchronous information.

Now consider that this is the world of the blind: a blended temporal environment of many pasts converging in sound.

The visual space of now is the ultimate boundary, for as far as we know nothing can travel faster than the speed of light.  Our Earth is enveloped in this now-space, but besides an occasional meteor nearly every other celestial object is beyond it; we are forever asynchronous with the Universe. Astronomers are thus scientists of history, studying events that occurred in the distant past, often before humans, Earth or our Galaxy even existed. Astronomers have no ability to communicate or interact with the Universe in the “present moment”3, a fact that puts them at odds with other scientific endeavors. Astronauts who travel into deep space also venture into this outside-now-space, although so far this has included only 24 men – the Apollo astronauts – who circled the Moon and in doing so separated their personal now-space from the now-space of humanity, each of us seeing the other as we were 1-2 seconds in the past. As we contemplate and plan for future human exploration of space, we might consider what it means to have part of our species reside in a permanent synchronal schism for the rest of us.

For now, we share our now-space4, but only insofar as we participate in it.  So stow these thoughts, dive back into whatever it was you were doing, and enjoy living “in the moment” – that is, in the here and now.

– Adam


1 1 millisecond is 1 thousandth of a second, or 0.001 seconds; 1 nanosecond is 1 billionth of a second, or 0.000000001 seconds

2 A very good discussion on sound thresholds of perception (with demonstrations!) has been posted by Justin London of Carleton College at

3 This does not mean we cannot predict events, such as an approaching comet, a solar storm or the transit of Venus in front of the Sun. These motions are much slower than the speed of light, so we can see that they will happen in the future and plan accordingly. Nevertheless, observing these phenomena still means watching events that happened seconds, minutes or hours in the past.

4 In my lifetime, noone has left the now-space of humanity.

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