Help needed

[samsam]

Hi guys
stupid homework question i for one don't understand.
M13 is 21.00 Light years away! a signal travelling at the speed of light will arrive at M13 21.000 years from day sent! What would the reply time be ???

[tiny-tim]

Hi samsam!

:confused: You do realise that's 21000 light-years, and 21000 years?

It's 21000 light-years away, so light takes 21000 years to get there.

So the reply time is … ? :smile:

(if you're interested, it's the Great Globular Cluster in Hercules, and it's actually 25000 light-years, see http://en.wikipedia.org/wiki/Messier_Object_13)

[samsam]

I am lost now ? please help!!:cry:

[tiny-tim]

Hi samsam!

M13 is a cluster of stars.

Its distance from us is 21000 light-years.

(do you know what a light-year is?)

So if we broadcast a very strong signal (probably a radio signal) towards M13, it wil arrive in 21000 years time.

The homework question is asking, how long would it take to have a conversation, sending one message to M13 and getting one message back. :smile:

ok?

(this isn't a merely theoretical question - a message was actually sent to M13, in 1974, from the Arecibo radio telescope)

[samsam]

Being not English! tutor says this is answer !!! Is it ?


So M13 is 21.000 light years away, that means the signal traveling at the speed of light will arrive at M13 21.000 years from the day it was sent, so only
20.973 years to go before the signal gets to M13, and 21.000 years for a
reply

[samsam]

right or wrong ? So M13 is 21.000 light years away, that means the signal traveling at the speed of light will arrive at M13 21.000 years from the day it was sent, so only
20.973 years to go before the signal gets to M13, and 21.000 years for a
reply

[tiny-tim]

Yes, that's right! :smile:

(Except it would be the same time there as back - I wonder why your tutor says it's 27 years less on the way there? hmm … maybe he thinks Arecibo sent the message in 1981.)

But the important point is - do you understand why that's the right answer? :smile:

[samsam]

Thankyou so much, yes understand now. many thanks . Sam