0
$\begingroup$

So, I hear people say blah blah blah is 3 light years away, and here's a photo. So does that mean the photo that we see are actually at least 3 years ago since that's how long it takes that light to travel to Earth for us to take the picture?

$\endgroup$
4
  • $\begingroup$ Well, yes, it is three light-years away. Although that's not even to the nearest star, so I'm curious as to what's in the photo in question. I'm guessing it's an arbitrary number, though, right? $\endgroup$
    – HDE 226868
    Dec 24 '14 at 18:05
  • $\begingroup$ Yes. And I'll have to add some words to make it a legitimate comment! $\endgroup$
    – ProfRob
    Dec 24 '14 at 19:32
  • 1
    $\begingroup$ Looking into space is looking back into time. With light moving at only 29.98 centimeters (11.8") per nanosecond, your view of the far wall of whatever room you're currently sitting in is multiple nanoseconds older than your view of the LCD display directly in front of you. $\endgroup$ Dec 25 '14 at 15:31
  • $\begingroup$ The exact time you are looking back will depend on your relative motion w.r.t. the distant object, which gives rise to the famous Andromeda Paradox. $\endgroup$ Dec 25 '14 at 19:59
1
$\begingroup$

As the comments to your question say, the short answer is "yes". A lightyear is, by definition, the distance traveled by a photon in one year.

Note, however, that at large distances, the answer becomes more complicated. Due to the expansion of the Universe, the light received from a galaxy far, far away was emitted when that galaxy was closer to us. That means that in the beginning, the photons traverse a larger "fraction" of the Universe per time than at a later time.

Thus, to calculate the total pathlength you need to integrate its distance in an expanding coordinate system.

For instance, if a galaxy was a 4 billion lightyears away from the Milky Way when the Universe was only 1 billion years old, it is now 28 billion lightyears away. But as the Universe is 13.8 billion years old, the light received today from that galaxy has been traveling for "only" 12.8 billion years.

Objects in our local neighborhood, however, such as stars in our own galaxy and the nearest galaxies (e.g. Andromeda) are gravitationally bound and thus "resist" the expansion of space, so here your assumption is correct.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.