Tuesday, July 1, 2008

Distant stars

We are sometimes amazed that we are seeing the distant past when we look at distant stars. But just about always when we look around, we see the past--light takes time to come to us from the objects. Why is it more amazing when we see the more distant past? And anyway, isn't the measure of temporal distance relative to reference frame?.

8 comments:

Beancan Tatterpants said...

Are you asking rhetorically because you're not as impressed or are you really looking for the "why?" in this case?

If the latter, I have one small thought - that we're impressed by large amounts of time based on our own perspective of how long things take. The concept of discipline and loyalty also come into play.

For example, I'm impressed that my parents have been married for 30 years. I haven't even been alive that long! In my perspective, that's an eternity. And even more impressive is my grandparents who just celebrated their 51st.

It's incredible because I can't imagine something that long (since I haven't experienced anything that long). So the concept of something taking one million years to get here is mind-boggling.

Plus, we're used to being the largest focus in our lives - so when we come across something that makes us feel incredibly small, it's awe-inspiring. Perspective-altering. Mostly because our brains can't process the information.

This works for most anything - distance, size, weight.

Or, if we're egotists, we might just be amazed at our own ability to see things differently than we normally do. :)

Alexander R Pruss said...

This is very helpful, thanks.

Gorod said...

Time and space go hand in hand in our perception.

If it is far, it takes a long time to get there...

If I have little time, then my radius of action is small...

So, I would say that awe at the remoteness in time of distant stars is equivalent to awe at their remoteness in space. Stars are unattainable both in time and in space.

The slight pastness of nearby things is as unexciting as their material proximity. I can deal with it easily because I can also reach out for it easily, it is attainable.

The exciting thing about the pastness of distant stars is not that it is a pastness, but that it is an immense pastness.

Enigman said...

Alexander, I wonder what your last question means... I read somewhere that relativity means that for the photon no time at all elapses as it travels such vast distances. If so, that would make the time of your perception equal to the time of the photon equal to the time of the distant star. It would make seeing a distant star like touching it, maybe (but I may have misread something :)

Alexander R Pruss said...

If there is no elapsed time for photons, then one does wonder about the sense in which one is seeing the past, as is often alleged.

PFS said...

You mentioned reference frames. I have never fully understood how these should really be conceived. Doesn't it show that I'm in the same reference frame with my friend John, who is on the other side of the universe (or really far away), when I poke a message on his side using morse code with a stick that I was holding that spanned the distance from me to John? I'm sorry that this is off topic, but I thought you be the guy to straighten me out on this.

Alexander R Pruss said...

It's a good question. A lot of the paradoxes of relativity theory are related to this. The thing to remember is that relativity theory is incompatible with the existence of perfectly rigid objects. What happens when you have a long stick and you poke someone with it is that you push on the molecules that are right by your hand. These molecules then move a little, and exert a force on the molecules next to them. Those molecules then move, and exert a force on the next ones. If you could watch the whole process in slow motion, you'd see a wave of compression traveling over the stick from your hand to the other side. But because this happens so fast, we normally don't notice it. But if you had a stick that was a mile long, and two good clocks, I bet you'd notice that the other end moves a little after you pushed your end. I don't know how fast this movement propagates over the stick, but it's definitely slower than the speed of light.

So, in answer to your question, if you use a stick to poke someone who is in orbit around alpha Centauri, the poke takes more than four years to arrive, and so you can't use it to set up simultaneities.

Alexander R Pruss said...

(That's more than four years in your reference frame. It'll be different in the reference frame of the compression wave traveling through the stick.)