City Universe

Look into the past.


Imagine how a city would look if it took light ten years to travel each city block.


To Do and Notice

Imagine you are standing on the main street in your town looking away from the town center.

Imagine also that it takes light 10 years to travel each block.

Draw what you would see.

Ask those around you what the streets were like in the past, check out history books.

If there are a group of people have each one draw a city block.

For example If I stand on mythical Universe Avenue in San Francisco.

Imagine you turn around and look the other way, what do you see? answer

Turn to the left and right, what do you see? answer

Of course what you see is what was not what is.

One block away, in the first street block, you see it is 1990, but, it is actually 2000 there just as it is at your location. You see the past because it takes light ten years to reach you.

Someone standing on first street in the year 2000 and looking one block away toward where you are standing, would see your block as it was in 1990.

Now imagine you can walk almost as fast as light.(You are made of matter so you cannot travel at the speed of light, but you can go as close to lightspeed as you like.)

Take a walk toward the 1990 block.
You'll get there in 10 years.
When you get there it is 2010.
As you travel you keep running into the light coming toward you. When you had gone half a block, after five years, you see the light from the block that left in 2000, at the same time you left. You see time passing twice as fast in the blocks ahead as you run into the light signal.
When you get there you find out that in the year 2000 they had a Starbucks too!

Turn around to look back at the block you left in 2000 and you see that it is still 2000 there! You surfed the light wave traveling just as fast as the light wave that left your home in 2000, so that is the light you see.

Look a block further on, to second street, and you see it is what year there? You are in 2010 at first street looking a block away. Make a stab at the answer before checking it.

You see the light arrive from the year 2000 at second street.

Looking on down the street as far as you can what do you see?

The street was built in 1860 and that is 15 blocks away from 2010, so you see the street stretching 15 blocks from first street, you can see to 16'th st.

Turn around and look toward the end of the street way beyond zeroth street.

It looks the same as it did when you left zeroth street, you surfed the light wave moving one block further away and taking ten years to do it.


What's Going On?

When we look out in the universe we see the past, light travels at a fast but finite speed.

1 light year per year

Thus we see the history of the universe as we look out to further distances.

When we look out far enough we see the birth of the Universe.

That is what we see, but it is not what is there now.

When we saw supernova 1987a explode in the Magellanic cloud it had actually exploded 176,000 years before and the light was traveling to us all that time.

If we were to travel to the location of the supernova at nearly the speed of light we would get there in 176,000 years and find little trace of the ancient explosion left, it happened 360,000 years in the past after all. Look closely and find a spinning neutron star the remnant of the supernova.

Turning around we'd see the earth just as we left it. After all, we surfed along at the speed of light. Actually the earth itself has aged and moved on.

Given 14 billion years we could travel to where we see the big bang. When we got there it would be 28 billion years since the big bang if we looked outward we'd see a much bigger universe, the distance to the big bang would be 28 billion light years.

Even though we are surrounded by the appearance of the big bang, we can't ever get there.

Going Further

I have assumed that every block has a clock that reads the same time now. We can all do this by looking back until we see the street appear. Define that time as 1860 and count the blocks adding ten years for every block. Thus every clock will read the year 2000 at the same time.

When I move from one place to another, the time measured by my clock is predicted by the equations of special relativity. The faster I move, the slower my clock runs relative to all the stationary clocks mounted on the corners. As I approach the speed of light, my clock slows to a stop as viewed by those cllocks. So that when I travel from 0 street with a clock that reads 2000 and travel to 1st street at near the speed of light my clock will still read 2000 while the clock at the corner reads 2010.


You see the exact same thing, more or less, whichever way you look.

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Scientific Explorations with Paul Doherty

© 2001

3 April 2001