|MadSci Network: Astronomy|
Hi Michael, It seems to me as if you've made a close association between two ideas which are actually separate. Let me explain the way I see it. The first concept you mentioned is that of "lookback time": the further away an astronomical object is, the longer it takes light from that object to reach us. Despite what many people think, this phenomenon crops up whenever we look at something. We just don't notice it every day because light crosses the distance between fishes, Ferraris and other earthbound objects almost instantaneously. Only when light has to cross enormous regions of space does it become a concern. This means that when we look at distanat galaxies, we see them not as they appear today, but as they were when the photons arriving now left their source. In a sense, you are right about being uncertain about the present state of a star or galaxy. If the light from the star Vega took 26 light years to reach us, how do we know that Vega is still there? Twenty-six years is a long time. What if Vega went supernova in that time, or aliens fed it into a big star-compacting machine? The answer to this is an easy one. Astronomers assume, based on the million or billion-year lifetimes of stars and galaxies, that nothing much has gone on since the light we're now seeing left its source. But this assumption breaks down for very distant objects such as quasars, which are billions of light years away. A lot more could have happened in a billion years, so it's not safe to assume that quasars are still as we see them. Instead, we assume that the quasars have evolved into galaxies resembling our own. But we're more uncertain about this than we are about nearby stars, hence the greater uncertainty that you describe. Yet this is *not* linked to Heisenberg's uncertainty principle, which describes the probabilistic behaviour of very small particles like protons and electrons. This tenet of quantum mechanics does have to do with light and how we sense things, it is on an entirely different scale from lookback time. Now, that that's been cleared up, the final part of your question awaits. The answer is no, it's impossible to really know what those very distant quasars look like right at this minute. All we can do is assume that they have evolved into galaxies similar to those in the Local Supercluster. My answer is based on the fact that light travels at a constant speed for all comoving observers. Nobody has yet thought of a way to get around this. If you want to win a Nobel Prize, see if you can come up with a way to see the far reaches of the universe as they appear right now. I, for one, would be really interested.
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