More on the Speed of Light

This is a favorite subject of mine, so I thought I'd delve a little deeper than I did yesterday. This may be old hat to some of you, but others might find it interesting.

The question I presented yesterday was essentially, "How can extreme distance cause the past to appear as the present?" To get a better fix on the solution, imagine we're all at a baseball game, box seats. A batter comes up to the plate, we'll call him Swingy McBallsmacker. He lines up, receives the pitch and cracks a home run out into the stands.

At that moment, if we're paying attention we'll notice that the sound of the crack of the bat comes a second after we see Swingy hit the ball. We've probably all experienced that, and we likely all know the explanation. Sound travels much slower than light, so while the light bouncing off of Swingy as he hits his home run reaches our eyes nearly instantaneously, the sound takes a perceivably longer amount of time to get to our ears. And the further away our box seats are from the action the longer the delay we'll perceive between the sight of the bat striking the ball and the resulting crack. In a sense, we're hearing the past, sensing with our ears an event that has already occurred.

Sound travels at 768 mph, or roughly one mile every five seconds. So if our box moved out to ten miles from Swingy and his triumphant home run, assuming our ears were sensitive enough we'd hear the crack 50 seconds after we saw Swingy swing. We'd be listening almost a minute into the past. But, assuming the glass windows of our box allowed us to telescopically see the batter from any distance away, even ten miles away we'd see Swingy's home run in realtime. Light travels at 186,000 miles a second. At that speed, even at 1,000 miles away the light coming from the action at the plate would reach our eyes nearly instantly.

But the point here is that light, like sound, travels at a finite speed. Like sound, if we got sufficiently far away so that we outstripped the distance light can travel in a given time period, we would sense the light from events, and thus see those events delayed from their actual happening. Imagine our box tore free from its moorings, lifted off into space and traveled a million miles from earth. Because of our telescopic box windows we can still see the stadium and Swingy's home run. However, we'll see it happen almost six seconds after the event occurs. Traveling at 186,000 miles a second the light takes about that long to reach us a million miles away. And the further away we are when Swingy knocks it out of the park the longer our perception of the event will be delayed.

Now imagine that our box is somewhere out in deep space, let's say one light year from earth. A light year is the distance light can travel in a year, or 6.87 trillion miles. Let's say we don't know anything about what's going on back on earth. We point our telescopic windows in the direction of the stadium just in time to see Swingy win it for the team. To our eyes the event just happened. But in actuality the light that allows us to see the event has been traveling toward us for a year. By the time we see Swingy smack the ball out into the bleachers he has long since gone home, finished the season, and started the next. In a very real sense we've just peered one year into the past.

Finally, scrap the box. We're now alien astronomers living on a planet 1o0,000 light years from earth. In the course of our normal investigations we train our telescope on a point of space that happens to be the surface of earth, inside a sports arena, where a strange looking creature with "Swingy" emblazoned on his shirt swings an elongated cylindrical object at a sphere another creature has hurtled at him. We make notes about this latest observation. We've been following the exploits of this distant alien culture for some time. However, our insights are tempered by the fact that what we're seeing took place 100,000 years ago. It has taken that long for the light leaving the surface of the earth to reach us. We have no idea what's actually going on for earth cultures in the present. That light is just leaving the planet. In fact, given the time span its likely that humanity no longer exists. 100,000 years is a long time, and while the human cities we're studying now seem strong and vibrant, a lot has happened in the intervening time. Cultures and peoples don't last forever, and it's very possible that the culture we're studying has died off or changed radically.

The point is, we're studying the past. We are seeing, in the present what happened to humanity, if it still exists, 100,000 years in its past. Swingy McBallsmacker is long dead, as is the game of baseball. If humanity still exists 100,000 years after Swingy's exploits it's likely unrecognizable. But we, alien astronomers living 100,000 light years from earth will never know. Or rather we'll have to wait 100,000 years to find out.

That's how extreme distance allows, and in fact forces us to see the past in the present. And it's further evidence of just how cool reality is.

Fun with Space Time

Let's say that you, at age 20 go out on a cloudless night with an immensely powerful telescope, aim it at another, far distant planet 50 light years away. Light, leaving that planet will take 50 years to reach you. At the exact same time, on that far distant planet a member of another sentient species sits out on a cloudless night pointing a telescope back at you. Both of you, looking directly at the spots where each of you are sitting will see nothing. Empty space. Because the light each of you are seeing left that point on the other planet 50 years before each of you got there.

So you see nothing, but, on a lark each of you waves at the nothing you both perceive.

Fast forward 50 years. You're both 70 now, and for fun you both return to the same spot you took your telescopes to fifty years prior. Nostalgically you look back to the same point in space where you once waved to an imaginary alien, an empty spot of soil. Amazingly you now both behold a creature sitting at the spot that 50 years ago was empty. They seem to be young creatures, both peering back at you through a telescope. Excited that your sight-seeing didn't return empty-handed this time you both wave excitedly. And, in seeming recognition of your gesture, the creature on either other end of the telescope waves back.

Of course that wave happened 50 years ago, and was in response to seeing nothing. But, in a sense, there is real contact in the present. At least in the minds of each of our observers. And that's likely the closest we'll come to traveling in time. But it's enough. The farther away we peer into the cosmos, the farther back in time we're seeing. It's very possible that one day we may find another sentient civilization in some far flung part of the universe. But we may be seeing things that happened hundreds or thousands (or hundreds of thousands) of years earlier. By the time we find them, they may already be gone. But we can still watch their past. Observe them as our two stargazers observed each other fifty years in each other's pasts. We can observe but we can't participate.

Time travel is a spectator sport only.