# Explaining Light Travel Time

1. Dec 3, 2009

### brentefs@comc

Imagine an event occuring on the moon that is not visible to the human eye, but with a high powered telescope it is easily viewed. Did the telescope see it happening in real time or did it see the event 1.32 seconds after it occured. Based on common sense you can imagine it as realtime, but I know better. How can this be explained to a very bright 11 year old.

2. Dec 3, 2009

### fawk3s

If you want to take it that way, we see nothing in "real time." Light takes time to travel.

Though you should probably wait for the pros of this forum to answer it since Im a twat.

3. Dec 3, 2009

### mikelepore

Take the other person outside to a large field with a pair of walkie-talkies. Get about 100 meters apart. Yell "hello" loudly into the walkie-talkie. The other person will your voice through speaker, and then, after a short delay, hear your voice through the air. Explain that the speed of sound causes a greater delay than the speed of light does. But when people are a quarter of a million miles apart (earth to moon), the delay due to the speed of light is noticable.

4. Dec 3, 2009

### Staff: Mentor

What common sense? The only difference in the path the light had to take to get to your retina happened in the last few feet. For the other 239,000 miles, the path was the same. Common sense tells me if there is no substantial difference in the path, there is no substantial difference in the time.

5. Dec 3, 2009

### Staff: Mentor

That's not what the OP was asking.

6. Dec 3, 2009

### Drill

The telescope has no effect on the time travel , it has just magnified the event for a human eye to view/witness. A superman with an ideal vision can view the event with no need to a telescope

the situation can be rephrased as two stars collide and, we view the event with our naked eyes at the moment when the traveled light hits our eyes , after traveling so many light years

:)

7. Dec 3, 2009

### Neo_Anderson

If an advanced civilization that's 41 light years away from earth were to train their hi-power telescope at our moon, they'd see Neil Armstrong taking his first step off his capsule right now; not 41 years ago.

They'd murmur to each other, "The Earthlings just landed on their moon! The Earthlings just landed on their moon!" To us, however, the event took place 41 years ago.

8. Dec 3, 2009

### mikelepore

What does the word "that's" in your post refer to? The OP shows a lack of practical experience with the fact that waves have finite speeds, and the resulting delays. I described an activity that can produce a personal experience with wave travel time. That's how I recommend teaching the concept to an 11-year-old.

9. Dec 3, 2009

### Staff: Mentor

"That's" means "that is". Ie, "That is not what the OP was asking".
I'm not seeing that. What the logic is in the second case, I'm not sure, but in the first case it is clear that the OP understands that light takes 1.32 seconds to travel the distance from the earth to the moon.

10. Dec 4, 2009

### brentefs@comc

Thanks guys or ladies; I must confess that the "superman " idea didn't impress my grandson after I let him read all the answers you were kind enough to suggest. The "light waves" was the one he focused on the most. I was able to get him to understand it for awhile, but he called me this morning, since he thinks grandpa knows everything, and now wants me to explain how a laser works. This, I believe, will be much easier. Thanks again, B.L.