Dinner with three
very smart people.

Physics was about
to win anyway.

Okay. I have a
relativity question.

Kevin is 12 years
old and lives 5
light-years away.

Bolt races straight
toward him at
almost light speed.

At departure, Bolt
sees light Kevin
sent 5 years ago.

And Heimdall floats
nearby, stationary
relative to Kevin,
watching the whole trip.

So Bolt initially
sees 7-year-old
Kevin.

That part makes
sense. The image is
5 years old.

But after Bolt
travels for 5 years…

Hold on. Which
Kevin is this?

Home Alone
Kevin?
Or Wonder
Years Kevin?

Neither. Same
Kevin.
Different light.

Then why
does
he look like
he switched
franchises?

Because you
switched
subjects.

age 8

age 12

…Bolt arrives and
sees 17-year-old
Kevin.

Wait. Kevin looked 7
at the beginning 
and 17 at the end.

Bolt watched Kevin
age 10 years during
a 5-year trip?

Easy. 
Bolt is moving
so fast that Kevin's
time is dilated.

Kevin should look
like he ages slower,
not faster.

So 7 to 17 
cannot be
right.

But Kevin is
staying
home.

Heimdall says
Kevin actually
becomes 17
while Bolt travels.

Then
Heimdall's
calendar is
suspicious.

Your
calendar
is not
the 
suspicious
one.

You're both mixing
up Kevin's age with
the age of the light
Bolt receives.

Bolt is chasing
Kevin's old light
from behind.

Maybe Bolt keeps
seeing the same
7-year-old Kevin for
almost the whole
trip.

Then, near the end,
all the newer images
arrive at once.

Like Kevin suddenly
fast-forwards from 7
to 17.

That sounds less
like physics and
more like bad
streaming.

So does Kevin age
slowly, normally, or
all at once?

Slowly!

Normally… but
visually fast
at the end?

My brain is
receiving delayed
signals.

Why are you arguing
with light travel
time over dinner?

Move over.

Grand Relative,
Kevin appears to age
10 years in 5 years.

Kevin does not age
twice as fast in his
own life.

He ages normally:
from 12 to 17 while
Bolt travels for 5
years in the home
frame.

Bolt is seeing Kevin
through light
signals arriving
more and more
quickly.

At departure, Bolt
receives light Kevin
emitted when he was
7.

As Bolt approaches
Kevin, each later
birthday signal has
less distance left
to travel.

Those signals arrive
closer together than
they were emitted.

That is the
approaching
Doppler effect.

Bolt sees Kevin's
clock running faster
because Bolt is
rushing toward the
incoming light.

It is not just time
dilation. It is time
dilation plus
changing light-travel
delay.

Heimdall, near
Kevin, sees the
ordinary story:
Kevin goes from 12
to 17.

Bolt begins by
seeing 7-year-old
Kevin because that
is the first old light
reaching him.

By arrival, Bolt
sees 17-year-old
Kevin standing
there now.

So yes: Bolt
observes Kevin go
from 7 to 17.

That is 10 years of
visible aging during
5 years of Bolt's
travel.

The faster Bolt
approaches, the
more compressed
Kevin's incoming
timeline
appears.

For several seconds,
the gangs had no reaction.

So Kevin does
not secretly
age twice
as fast.

Bolt
receives
the years
of Kevin's
life faster
because
he is
approaching
the signals.

And “what time
is it there?”
is different
from “what
am I
seeing now?”

Finally.

Next time you chase
someone across five
light-years…

…remember that their
old light is coming
toward you too.

That is how you
earn a cosmic
speed ticket.

Can the ticket
be paid in
dumplings?