# The Twin Paradox: Part III

This is the third in a series on the "Twin Paradox".

Just to remind you of the important facts:

The Earth Federation has created a series of artificial planets stretching away in space in both directions. The planets are called "P1", "P2", "P3", etc., in the positive direction (right), and "M1", "M2", "M3" and so forth in the minus direction (left). Each planet is exactly one year (or light-year) from its neighbor in each direction. The planets keep their clocks synchronized with each other.

The Federation Navy has a special fleet of red supply ships which travel through space with a constant speed v = 0.9c. There are many, many ships, all moving in a line, one after the other. These red ships keep their clocks synchronized with each other.

There is also a second fleet of green supply ships which travel through space in the OPPOSITE direction, moving with constant speed v = 0.9c. These green ships keep their clocks synchronized with each other (but not with the red ships).

Joe and Bob are twins living on Earth. On their twentieth birthday, Jan 1, 2500, Joe joins the Space Navy. He takes a shuttle into space to catch the Red Flagship, one of the Red Fleet ships. The clock on board that ship also reads Jan 1, 2500, as Joe climbs aboard.

Joe's plan is to travel to planet P27, then return home to Earth by switching to a green supply ship.

Now, on planet P27, something is happening on Jan 1, 2500. A young man who is 20 years old decides NOT to join the Space Navy, but instead, to become a Boot Salesman.

#### Joe's trip -- according to Bob and other observers on Earth

Joe and Bob say good-bye in the year 2500, as Joe prepares to leave the Earth. We could draw a space-time diagram showing Joe at the origin, since Earth is at position zero, and since we're measuring time since 2500.

The blue line shows the location of events which are simultaneous, according to Earth-based observers with Joe's departure. In other words, the blue line shows all events which take place on Jan 1, 2500, according to the Earth's time system.

Here are the ships of the Red Fleet which happen to be passing Earth and its neighboring planets on Jan 1, 2500, according to the reference frame of the Earth.

Joe takes the shuttle to the Red Navy ship Red Flagship and flies to planet P27. Since he is moving at v = 0.9c, and must travel 27 light-years, he arrives 30 years later. Again, the blue line indicates all events which occur when the planetary calendars read "2530".

Joe then hops onto the Green Navy ship Green Flagship and comes back to Earth. It will take another 30 years for this return trip, according to people on Earth, so he arrives in 2560.

#### Joe's trip -- the first half, according to Red Navy sailors

Joe and Bob say good-bye in the year 2500, as Joe prepares to leave the Earth. The Red Navy ship Red Flagship, which picks Joe up, has a calendar which also (by coincidence) reads Jan 1, 2500. If we draw a line showing the location of all events which are simultaneous according to Red Navy observers, it looks like this:

In other words, events which occur anywhere along that red line will have a Red Navy ship flying past with a calendar reading "Jan 1, 2500." For example, here are the ships of the Red Fleet which happen to be passing Earth and its neighboring planets on Jan 1, 2500, according to the reference frame of the Red Navy.

When Joe jumped from the planetary frame of reference to the Red Navy frame of reference, his notion of events which are "simultaneous" changed drastically. Look what happened to the Boot Salesman on planet P27:

As Joe and the ship travel towards planet P27, time passes on board. The rate at which time passes is slower for Joe than for Bob back on Earth. When Joe and the ship reach planet P27, only about 13 years have passed. The red line in the diagram below shows the location of events which occur when Red Navy calendars read "2513".

```

Q:  The Red Navy ship "Red Condor" flies
past the Earth just as the Red Condor's
calendar reads "2513".  If the sailors
ask people on Earth what year
it is, what will the Earth people say?

Q:  How old is Bob when the "Red Condor"
flies past the Earth?

```

#### Joe's trip -- a brief shopping excursion

Joe's ship reaches planet P27, and he hops onto a shuttle which takes him down to the planet. He goes shopping. As he purchases some boots, he asks the salesman what year it is.

"Why, it's the year 2530," says the Boot Saleman.

"Hmmm," replies Joe, "I need to move my watch ahead by 17 years."

Joe's notion of "simultaneous" changes again.

```

Q:  At this moment, according to the
Boot Saleman, back on Earth,
how old is Bob?

```

Joe goes to the spaceport to purchase a return ticket to Earth aboard the Green Navy fleet. He asks the ticket agent about the Green Navy. "Oh, it's a fine fleet," say the Ticket Agent. "One of their best ships, the Green Monkey, should be passing by the Earth right now."

#### Joe's trip -- the return segment, according to Green Navy sailors

Joe takes the shuttle from Planet P27 up into space and hops aboard one of the Green Navy ships, the Green Flagship. The Green Navy flies at the same speed, v = 0.9c , but in the opposite direction.

By a nice coincidence, the calendar aboard the Green Flagship just happens to read Jan 1, 2530, which is the same as the calendars on Planet P27.

After Joe has boarded the ship, his notion of "simultaneous" shifts once again. The green line in the diagram below shows the location of all events which occur when Green Navy ships read "Jan 1, 2530".

```

Q:  The Green Navy ship "Green Dragon" flies
past the Earth just as the Green Dragon's
calendar reads "2530".  If the sailors
ask people on Earth what year
it is, what will the Earth people say?

Q:  How old is Bob when the "Green Dragon"
flies past the Earth?

```

As he boards the ship, Joe asks the purser about the Green Navy ship which is currently passing the Earth. "Oh, that would be the Green Dragon," says the purser.

Time passes aboard the Green Flagship. In fact, about 13 years go by for Joe as he waits to get back to Earth. When he arrives, the Green Flagship's calendar reads 2543.

#### Joe's arrival back home -- according to Bob

Joe takes the shuttle from the Green Flagship back down onto Earth. As he lands, he asks a spaceport worker what year it is here on Earth.

"Why, it's 2560, of course," says the Spaceport Worker.

"Gosh," thinks Joe, "I must re-set my watch forward by 17 years again."

Now that he is back in the planetary frame of reference, his notion of "simultaneous" events shifts a final time. The blue line below shows the events that he now considers "simultaneous."

```

Q:  How old is Bob when the twins meet again?

Q:  How old is Joe when the twins meet again?

```