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ROBERT SIEGEL, host:

For all of you out there who are 12 and under, this next item has nothing to do with you, yet. But everybody else should lean in.

Here's NPR science correspondent Robert Krulwich, with one of life's more intriguing mysteries.

Unidentified Child: (Unintelligible).

Unidentified Man #1: Make a wish.

Unidentified Group #1: Yay.

(Soundbite of music, "Happy Birthday to You")

ROBERT KRULWICH: Everybody knows that as you get older, your birthdays and your school years, your holidays, the events that come round and round and round seem to come faster and faster and faster as we age. So by the time people hit their 40s or 50s, says professor Warren Meck of Duke University...

Professor WARREN MECK (Psychology and Neuroscience, Duke University): They just have this sense, they have this feeling that time is going faster than they are.

KRULWICH: But why? Why does time move faster as we get older?

Prof. MECK: That's an interesting question.

KRULWICH: And the answer is, nobody knows. There are theories, of course, from psychologists, from neuroscientists who've been doing experiments. For example, one thought is maybe as we age, something changes in our brains so we lose the ability to measure time.

Prof. MECK: I'm thinking here of a clock idea - that each of us have a clock in our brain, and that does slow down over time.

KRULWICH: What do you mean, a clock in our brain? What in our brain would tick or tock?

Prof. MECK: Well, the neural conduction velocity slow down, but the...

KRULWICH: The neural conduction velocity. What is that?

Prof. MECK: Oh, it's the speed at which our brain cells beat or pulse.

KRULWICH: Meaning that time flows through us differently when we're older. Evidence for this comes from a classic experiment, which in a very rough way, we did right out on the streets of Washington, D.C. I asked my colleague Jessica Goldstein to go out and to stop pedestrians who looked either very young or very old.

MARGARET(ph): I'm Margaret, and I'm 90.

Ms. CATHERINE INGARD(ph): I'm Catherine Ingard. I'm 22.

Ms. MIRANDA GIBBONS(ph): I'm Miranda Gibbons. I'm 19.

MARK(ph): My name is Mark. I am 82.

KRULWICH: So Jessica then asked these two groups, the older ones and the younger ones, to close their eyes and then do a very simple time measurement.

JESSICA GOLDSTEIN: What I'd like you to do is to tell me when you think one minute has passed.

MARGARET: OK.

GOLDSTEIN: And we can start now.

MARGARET: All right.

(Soundbite of music)

KRULWICH: All over the world, whenever you do this experiment correctly...

Prof. MECK: You ask people to close their eyes and just feel the duration.

KRULWICH: ...you will find that the younger people will normally say, OK, I think the minute is up.

Ms. INGARD: I'm guessing about now.

Ms. GIBBONS: I think now.

Unidentified Man #2: I would say now.

KRULWICH: And when you check, what they have just called a minute will turn out to be 55, 60 or 65 seconds on the clock - which, of course, is very, very close to an actual, on-the-clock minute.

(Soundbite of laughter)

Unidentified Man #2: Oh, yeah? That's awesome.

KRULWICH: But the older people who are 60 and up, on average, says professor Meck...

Prof. MECK: They should wait longer before they say that your minute has elapsed.

KRULWICH: OK.

MARGARET: I think it's about time.

KRULWICH: How much longer does it take the older people? Now, this is weird.

MARK: What? God.

KRULWICH: It's a big difference. More like 90 seconds instead of 60 seconds.

Prof. MECK: You know, it takes their brain that long to accumulate these pulses.

KRULWICH: So when you're older, your brain's idea of a minute often stretches out, which creates a very paradoxical feeling. As your brain slows down, you get this strange sensation that around you, things are speeding up. Now, here is why.

If an older person were to stand on the street, you know, and this is on average, and count - anything really, but let's make it car honks. So how many car honks would they hear in a minute?

(Soundbite of car honks)

KRULWICH: Well, there's a honk, another honk; three, four, five, six. By the time what they call a minute is up, they will have heard so many honks, so much stuff.

Prof. MECK: More things pass by than they expected them to.

KRULWICH: So they're going to think in 60 seconds, so much happens.

Prof. MECK: Exactly. And the reason why a lot of things happen is they actually counted for 90 seconds. So in that sense, more - it does seem like more things are happening. And therefore, it seems the world is going faster.

KRULWICH: It's like when you walk more slowly, everybody else seems to be going faster. So one reason, then, that birthdays and holidays come around faster and faster as you age is simply, Robert Siegel...

SIEGEL: Mm-hmm.

KRULWICH: ...it's physiological. It's maybe that your brain, my brain, too, are just pulsing differently round now.

SIEGEL: The theory is that because our brains are - we're thinking more slowly; the world seems to be going by us faster.

KRULWICH: Which makes a kind of sense. It's never only one - there are many explanations for this feeling about time getting faster.

SIEGEL: I'm up for another explanation.

KRULWICH: All right. Then, how about this one? How about the proportional explanation? When you're 6, two years is a big wad of your life.

SIEGEL: Third of your life.

KRULWICH: Third of your life.

SIEGEL: Probably half of your speaking life.

KRULWICH: But when you're 63, then the same, exact two years is one-thirty-second of your life. So that's a smaller slice. So proportionally, it should feel quicker, right?

SIEGEL: This is more simple arithmetic...

KRULWICH: Yeah.

SIEGEL: ...and makes a certain amount of sense to me, yes.

KRULWICH: OK, now let me try you on one further explanation.

SIEGEL: Oh.

(Soundbite of music)

Unidentified Group #2: (Singing) Happy birthday to you. Happy birthday to you.

KRULWICH: Think back to one of your very early birthdays, with the cake and the candles and the presents, the guests, very new and exciting. According to neuroscience professor David Eagleman...

Dr. DAVID EAGLEMAN (Neuroscientist, Baylor College of Medicine): When something is new to you, your brain writes it down in a lot of detail.

KRULWICH: Particularly when you've never had the experience before.

(Soundbite of puppy barking)

KRULWICH: Like hugging a new puppy for the very first time. You have so many things to think about - the big eyes, and the dog, his tongue on your face, the warm feel of his fur - so you notice more and you feel more and therefore, you write more down in your brain. At Eagleman's lab, they have measured new experiences.

Dr. EAGLEMAN: It turns out that your brain has to use up more energy to represent the new object.

KRULWICH: And because your brain is putting in all these new details, when you think back on it later, there's so much more to remember, it just seems slower.

Dr. EAGLEMAN: Yeah, I mean, I do know in the sense that when I think back on a childhood summer, it seems to have lasted forever. When you've had all these great new experiences, so many things to think about...

KRULWICH: So years later, when you're thinking back to the summer and thinking, oh, this thing happened, oh, and that thing happened, and the other thing happened, you get the illusion so much happened, time really - therefore was slower.

Dr. EAGLEMAN: It will feel to you that way. Your perception will conclude it, yeah.

KRULWICH: All right, now let's fast-forward to your - let's make it your 45th birthday.

(Soundbite of music)

KRULWICH: So now, you know all about cakes and all about presents and all about candles.

Dr. EAGLEMAN: And when you've seen something a lot before, your brain can get away with encoding that with very little effort.

KRULWICH: Because now your brain goes yeah, yeah, yeah.

Dr. EAGLEMAN: Yeah, I know what this kind of situation is, and it doesn't have to spend much effort, and it doesn't write down much memory about it.

KRULWICH: So that leads us to the neurological explanation for why life seems to get faster as you get older. When you're young, that's when you have all those new experiences, and your brain writes it all down. So when you think back to that first summer...

Dr. EAGLEMAN: It seems to you reasonable. It seems to your brain that the whole thing must have taken a long time because look at how much got written down here.

KRULWICH: But that's a trick, really. It's your brain painting lush scenes when you're 10 years old, and sort of quickly sketching them in when you're 40 years old, so you skip past your 40s.

Dr. EAGLEMAN: That's the idea. It's a construction of the brain. The more memory you have of something, when you read it back out, you think wow, that really took a long time. So when you're reading them out in retrospect, you think the whole event lasted longer.

(Soundbite of music)

KRULWICH: So why, then, does life speed up as you get older? Well, maybe there's more new in your life when you're young, or maybe your brain pulses differently when you're young, or maybe when you're young, each year matters more. Whatever the reason, sooner or later, everybody gets this feeling.

MARGARET: I could have told you that in the beginning.

KRULWICH: Yeah, I know. But the weird thing is, we still aren't sure why.

Robert Krulwich, NPR News.

(Soundbite of music)

SIEGEL: Our science team has turned David Eagleman's theory of dense memory into a beach film. And you can see it on Robert Krulwich's page, at npr.org.

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