What do we mean when we say “now”? How long is a “now”? It feels pretty quick, so quick that by the time you’ve read the word “now” it’s already past.
But in fact “now” actually encompasses everything that happened in the last 80 milliseconds. This timespan is important for connecting the cause and effect of our own actions, and to some extent our understanding of time, our sense of self and our inability to tickle ourselves.
80 milliseconds is approximately the time it takes to integrate the sensory input from all the different parts of your body. If you touch your nose and your toe simultaneously, you feel them happening at the same time, even though the nerve signals take longer to come from your foot than your face – up to 80 milliseconds longer.
As another example, our brains can actually process sound quicker than they can process images, yet when a hand claps we see it move and hear the sound simultaneously. That is of course, until the person clapping gets so far away that the difference between the speed of sound and light causes them to be more than 80 milliseconds out of synch, at which point they suddenly become disconnected.
You can compare this with film and television. Video is typically screened at 25 frames per second, which means that 80 millisecond roughly corresponds to 2 frames. This is actually quite helpful for video editors and broadcasters, as it gives some leeway for synchronisation of sound and vision before it looks weird.
But this 80 millisecond span is not totally fixed: it can also be sped up and slowed down. I don’t mean some sort of slow motion, Keanu Reeves in The Matrix, bullet time sort of thing. Although subjectively you may think time slows down in stressful situations, that’s not really the case. Think about it: in, say, a car crash, do you hear people’s voices in slow motion?
This was actually demonstrated experimentally by David Eagleman and colleagues, who tossed volunteers backwards off a 45 metre tower. The subjects had devices strapped to their wrists that showed numbers alternating at varying rates. The hypothesis was that if people’s brains worked faster under stress then they would be able to read numbers oscillating at a quicker rate. (See Stetson C, Fiesta MP, Eagleman DM 2007, “Does time really slow down during a frightening event?”, PLoS ONE 2(12): e1295, doi:10.1371/journal.pone.0001295)
What they found was that the experimental subjects – when they were able to actually concentrate on the watch – weren’t thinking any faster when falling then they were standing still. But afterwards, when they were asked to estimate the time of their fall, they recalled it as being at least a third longer than the time they guessed for other people falling.
The theory is that time seemed to move comparatively slower in their memory of the event because of the rapid rate of stimuli that their brains had to process in such a short time. This could go some way to explain how the years seem to go by faster and faster, because as you get older there are fewer new experiences.
Despite this, there are in fact ways you can train your brain to speed up and slow down beyond the 80 milliseconds. In another study, David Eagleman got people to push a button that made a light go on, but with a short delay (Stetson C, Cui X & Eagleman DM 2006, “Motor-sensory recalibration leads to an illusory reversal of action and sensation”, Neuron 51, pp. 651–659, DOI 10.1016/j.neuron.2006.08.006 [PDF 509 KB]).
As you’d expect, when the delay was less than 80 milliseconds, people thought the button-clicking and the light-lighting happened at the same time. But when the delay was consistently increased, the subjects’ internal chronometers could be recalibrated; they interpreted flashes up to 135 milliseconds later as being simultaneous with the click.
Then the researchers did something tricky: they suddenly decreased the delay to 44 milliseconds. When this happened, the people whose brains were recalibrated saw the flashes as coming before they pressed the button.
This breakdown in causality has led David Eagleman to the idea that schizophrenia may be a problem with perception of time. If, say, you were to “hear” yourself thinking something before your intention to think it, then it would seem like voices in your head coming from somewhere else. Or if you were to think about what you’re seeing on TV before your eyes register it, then it would seem like they’re broadcasting your thoughts. And indeed, in exercises with video games, Eagleman has found that schizophrenics have more difficulty recalibrating their brain clocks.
Tickling is a slightly more commonplace example. In 1998, scientists from University College in London showed that it was possible to tickle yourself by introducing a time delay (Blakemore S-J, Wolpert DM & Frith CD 1998, “Central cancellation of self-produced tickle sensation”, Nature Neuroscience vol. 1, no. 7, pp. 635-640 [PDF 271 KB]).
They did this by creating a mechanical tickle device that people could use to touch themselves. The greater the delay, the more tickly they found the touch. This is consistent with the notion that a disconnect between an impulse and an action makes them seem unrelated, or coming from someone else.
But as well as helping us to keep track of our own actions, the ability to connect cause and effect is the basis for our understanding of how the universe behaves in time. So you could argue that this 80 milliseconds of assembling data is essential for making sure we experience most things in the right order.
It’s curious that we puzzle over the unexpected physics of time – one of the most curious results of Einstein’s Special Relativity being that simultaneity isn’t the same for all observers – when our subjective definition of “now” is inherently fuzzy.
Can we really hope to understand the whole universe when we don’t truly understand how we experience it?