How much faster do we age when we get older?
When I was a child, my mom told me that once your 18th birthday passes, half of your life will be over. Since that day almost no day passed without me thinking about this.
Of course, her statement wasn't based on any scientific research, but still, it scared me. Her path of thought was that life goes by so much faster when we are older, so by the time you are 18, you’d already have reached 50% of your perceived time on earth.
My 18th birthday was getting closer and one of my biggest goals before that date was to have an in-depth conversation with my mom about whether she was afraid of her own mortality. A topic I had trouble talking about at the time.
Every older person I talked to in my life agrees with the above statement, that life goes by faster once you get older. Even my friends and I that are in our mid-20s feel this phenomenon. We aren’t as bored as when we were kids and a 3-month summer break doesn’t feel like a whole century anymore.
I started my research for finding the reasoning behind this and whether you can answer the question of when exactly a human being hits 50% of their perceived lifespan.
The Reasons
Proportional Theory
The first intuitive reason I found was that when you are 10 years old 10% of your life is only 1 year, but if you are 100 years old the same percentage of your life is 10 years.
If you go solely by this theory, 50% of a 100-year human life is over when you are just 7 years old.
For other math geeks, here is the explanation:
The method assumes that the passing of subjective time relative to the actual time for an interval is inversely proportional to the real time. So let's say R is the total real time (basically your age), dR is some interval of real time and dS is your subjective/perceived time of that interval. The formula for calculating how much subjective time passed for an interval dR is the following:
So for a 50-year-old, 1 year passes 5x the speed of a 10-year-old.
Another way to put it is that your subjective time intervals from
5-10
10-20
20-40
40-80
all feel like the same length.
So apparently my mom was even too optimistic and could have just told me that half my life is already over, the first time she told me about this phenomenon.
Maximilian Kiener made an awesome interactive website based on this theory that you should definitely check out: Why Time Flies.
It also states an interesting fact, that if you subtract the first 3 years of your life, since most people don't remember much from it anyways, 50% of your perceived life is over by the age of 18 (kudos mom!).
In 1975 Robert Lemlich1 proposed a new hypothesis that the subjective time relative to the actual time for an interval is inversely proportional to the square root of your age. This would mean that at a quarter of your current age the time was passing half as fast for you (1/√4). At half your current age the time was passing 0.71 times as fast (1/√2).
This theory seems to be closer to the truth of how people actually perceive time over a longer period of time, as also shown by his experimental results.
Physics
But what is the physical basis for the impression that time seems to go by faster as we age?
Adrian Bejan from Duke University demonstrates in his paper that the changes of new mental images that we perceive decrease with age.2 A young person at the beginning of their life receives more new images than an old person in the same amount of actual time (clock time units). That’s why time seems to pass more quickly when we age (perceived time units):
According to Bejan the reasoning for receiving fewer new images when we get older is that our nerves and neurons grow in size and complexity, resulting in longer paths for the signals to travel until we receive them as mental images in our mind. Additionally, these paths degrade as we age, resulting in signals being transmitted at slower speeds.
One piece of evidence for this is that children move their eyes more frequently than adults, which results in processing more images and acquiring more information.
The conclusion is that as we get older, we are viewing fewer new images in the same amount of actual time, creating the phenomenon that time passes more quickly.
How to prevent it?
As Albert Einstein said:
Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute.
This could be one way of artificially extending/shortening your life. But are there other techniques to prolong one's perceived life?
Awe and Fear
Research from Stanford suggests that awe expands people’s perception of time. Awe brings people into the present moment and being in the present moment can apparently prolong your perception of time.3
Another feeling that can slow down your perceived time is fear. A paper by David Eagleman4 points out that "time-slowing" is a function of recollection, not perception. So if we have more memory about an event that happened, it will seem as it lasted longer.
As an experiment for this paper, his research group let participants of the study jump down from a 46m tower. Before and after their fall, they were asked to estimate the duration via a stopwatch. The participant’s duration estimates increased by an average of 36% when they recalled their own fall. During their fall, the participants had to read numbers on a digital wrist to measure whether they actually perceived time to be slower during the fall. Since no increase in the performance of number identification could be measured, they found no evidence for a fear-induced increase in the temporal resolution. Thus, this study supports the hypothesis that our subjective time doesn't run in slow motion during a frightening event, but that we retrospectively perceive it as a longer time.
Seek Novelty
Repetition can speed up your perception of time.
As an illustration of this phenomenon look at this little GIF of a series of images.
Which one of the images was shown the longest? You probably think it was the "Life" image, but actually, all the images were shown for the same length of time. The "Life" image was simply novel to you and therefore your brain had to spend more energy processing it, which made it seem longer to you.
To slow down your perception of time David Eagleman also advises in this TED Talk to seek novelty and change your life as much as possible every day:
Life extension
So how about Life extension? If you know me, you probably know that I'm a huge fan of the Longevity research of scientists like Aubrey de Grey with his SENS Foundation or the Harvard Professor David Sinclair.
Based on the assumption we looked at before that your perceived time shrinks every year, we will have to expand the human lifespan quite a bit so that it makes a significant difference in your perceived lifespan.
Since Longevity research tries to stop or even reverse the aging process itself, the hope can be that it also stops the processes that are causing us to receive fewer new images. As a result, stopping or reversing aging could potentially also increase the perceived lifespan.
Conclusion
Time flies.
It’s not only that over 90% of the time you will spend with your parents is already over when you move out to go to college as this WaitButWhy post claims: The Tail End. It’s also that the last 10% will go by faster.
So don’t waste your time, do the things that you always wanted to do in your life and spend it with people that are worth your time!
Disclaimer
This is just a small part of an even more interesting research field called Time Perception. Wikipedia or the article Brain Time by David Eagleman is a good starting point to get to know more about this.
👋🏼 Endnote
Thanks for reading!
Cheers,
Johannes
Lemlich R. Subjective Acceleration of Time with Aging. Perceptual and Motor Skills. 1975;41(1):235-238. doi:10.2466/pms.1975.41.1.235
(the paper has a paywall but I hope that you never heard of sites like sci-hub!!!)
Bejan, A. (2019). Why the Days Seem Shorter as We Get Older. European Review,27(2), 187-194. doi:10.1017/S1062798718000741
Rudd M, Vohs KD, Aaker J. Awe Expands People’s Perception of Time, Alters Decision Making, and Enhances Well-Being. Psychological Science. 2012;23(10):1130-1136. doi:10.1177/0956797612438731
Stetson C, Fiesta MP, Eagleman DM (2007) Does Time Really Slow Down during a Frightening Event?. PLOS ONE 2(12): e1295. https://doi.org/10.1371/journal.pone.0001295