Why Are We Limited To Only Seeing The Past?

We are moving from A to B. Yet everywhere we look, we are going backwards! I mean the direction we are moving along [trajectory] must [?] have something in front of earth-solar system-galaxy. Yet it is all the past. No future! Now could this be because we are at the event horizon [so to speak] the very edge of the beginning? However one ‘material’ that has moved faster-than-light is space. Light is still catching up. Why can’t we see even this?
Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan

Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan

Originally posted on Forbes!

There’s a couple things blurring together here, but the fundamental thing here is the distinction between the observable universe and the universe which exists, independent of our ability to observe it.

You’ve got a good handle on the observable universe. This is the universe that we see, where everything away from our own planet has some kind of time lag to it. We see our Sun as it was eight minutes ago, in the past. We see Jupiter as it was, about 30 minutes ago in the past. We see the stars as they were, a few years to a few thousand years ago. We see the Andromeda galaxy as it was 2.5 million years ago. As we look farther out into the universe, and strain our technology to the limits, we push further and further back into time, capturing light which has traveled longer and longer stretches of time to reach us.

But because we can only see the universe as it was, some varying degree of delay later, doesn’t mean the universe is actually delayed the farther away from us we go. Mars is a few minutes away from us, but that doesn’t mean that Mars’s “now” is actually a few minutes behind our “now” – that few minutes’ delay is just the quickest any Mars-related information can get to us.

So space hasn’t really traveled faster than light to get “ahead” of our ability to see the Universe. Space describes what and where the universe is, and is not particularly concerned with how well we can observe it.

Think of it this way. We’re sort of walking down a road, but backwards. We can see all the things we’ve passed by, all of the pieces that we know about. Now, if the road is straight, and we know where the road is supposed to go, and where we are, we can pretty safely assume that walking backwards in a straight line will keep us walking along the roadside. We might be able to predict how long it will take us to get there, walking backwards. So it is with the universe. We can see where the universe has been, and we can roughly figure out what the rules are which govern its changes. So we can predict where we will go, and check our predictions.

So maybe the road has a bend in it. You might notice that near you, the shape of the curb is different from what it has been, and that will clue you that maybe, if you want to stay near the road, you should bend that direction as well. We can change our ideas of how this particular road goes, and similarly we can constantly check how well our models of the universe’s evolution predict what we should see, versus what we actually see.

This image shows New Horizons’ current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft’s future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is slightly above the orbital plane of the planets. Image credit: NASA/JHU

This image shows New Horizons’ current position along its full planned trajectory. The green segment of the line shows where New Horizons has traveled since launch; the red indicates the spacecraft’s future path. Positions of stars with magnitude 12 or brighter are shown from this perspective, which is slightly above the orbital plane of the planets. Image credit: NASA/JHU

We do this kind of prediction in all kinds of ways — we assume that the physics of the world around us are stable, so that when I step forward onto concrete, the concrete will still be there when I put weight on it, even though I can’t see into the future to check that it will be. Any kind of rover delivered to the surface of another world, or a spacecraft very carefully planned to swing past a planet, requires these predictions of what the space outside our range of sight will look like. And they work — New Horizons, arriving at Pluto, took only one minute less to travel there than was predicted in 2006, at the start of a ten year journey.

So while we’re not able to see into the future – the speed of light simply will never allow this – we’re not going totally blind into the future. From looking into the past, we have learned the direction our future is unspooling.

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How Can We See The Largest Amount Of The Smallest Universe?

Since light has a speed, the further out we look, the further back in time we look. But the further out we look, the greater the distance, and the “sphere” of observation is larger. But the further back in time you go, the smaller the universe as the universe is expanding. – So, how can the farthest observational sphere be both the largest we see yet represent the smallest it was?
This is the deepest image of the universe ever made at optical and near-infrared wavelengths. Image credit:  NASA  ,   ESA  , S. Beckwith and the HUDF Team (  STScI  ), and B. Mobasher (  STScI  )

This is the deepest image of the universe ever made at optical and near-infrared wavelengths. Image credit: NASA, ESA, S. Beckwith and the HUDF Team (STScI), and B. Mobasher (STScI)

Originally posted at Forbes!

You’ve got the observational part of this spot on – at greater distances from Earth, we’re observing the universe at a point where it was physically smaller than it currently is. And, because light takes so long to get to us, the objects which we can observe back in time are the ones which are very distant from us. And, if we recall from a previous post on what the observable universe is, our most distant observable galaxies are in a shell surrounding us, which contains quite a large volume of space. So how do we manage to reconcile the fact that we’re seeing a lot of a very small universe?

If you’re familiar with redshift as a unit of distance, we can actually use that number to tell us about the size of the universe when the light from that object left its source and began its journey towards us. At a redshift of 2, we’re looking at a universe that is 1/3rd its current size. A redshift of 9 is 1/10th its current size. Effectively, add one to the redshift, and then make that into your fraction. (This math is a bit of a rough estimate, but it’s a good way to get the general scope of things in perspective).

If the universe is physically smaller, this means that the distances between galaxies are all smaller, and the entire universe is more dense than it currently is. But the critical thing to consider here is the volume of space we’re able to observe. Things that are very near us we can only see within a very small volume; at greater distances we see a much larger volume of space. But if we’re headed for smaller total volumes as we go back in time, and the observed volume is going up, there’s only one way out. We are seeing a larger fraction of the Universe, as we look further back in time.

Our local environment is only a very small fraction of the current universe; we expect the volume we see as our ‘nearby environment’ to be repeated many times over the course of the Universe’s total size (however large that might be), in pretty much every possible configuration of galaxies, no galaxies, and combinations of galaxies. As we look further and further back, we see a much larger fraction of the universe. Since we don’t know the total volume of the universe, we can’t really say how much that fraction changes, but it’s certainly a bigger number than for the nearby universe!

Being able to see a larger volume of space as we look further back in time is actually scientifically useful! If we look back and spot that there are a lot of galaxies which are sitting around in groups of 3 or 4 galaxies, we could reasonably conclude that those groups must be reasonably common, as we have a pretty good sample size to work with. Very nearby galaxies give us a much smaller set of galaxies to work with, since we have a smaller volume of space, so it’s harder to say how rare our local group of galaxies is, for instance.

The volume of space we’re able to see only helps us so far, though – ultimately we’re limited by the fraction of the Universe that we can see. If we weren’t limited by this, our studies of the universe would be very different!



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Would Any Observer Anywhere Think They're At The Center Of The Universe?

Artist's logarithmic scale conception of the observable universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic microwave radiation and Big Bang's invisible plasma on the edge.  Image credit: Pablo Carlos Budassi

Artist's logarithmic scale conception of the observable universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic microwave radiation and Big Bang's invisible plasma on the edge. Image credit: Pablo Carlos Budassi

Originally posted at Forbes!

They should! And if their understanding of physics is as advanced as ours, they should subsequently realize that what they see as being the “center” of the Universe is really just the center of the observable universe, which is necessarily centered around them, as the observers.

There are a couple of things in play that allow me to answer this question, and the first is the nature of the observable Universe. The observable Universe is best thought of as a series of spherical shells, centered around our planet, where most of our observing happens (plus or minus a few astronomical units). The amount and number of things that we see in any given direction is limited, fundamentally, by the speed of light. We cannot see things which have not had the time for their light to reach us yet.

In the local Universe, this is not a major constraint, as the time delays that light imposes are relatively manageable. The delays even to communicate with New Horizons, at the very outskirts of our solar system, are only a few hours (it’s a 9 hour round trip from Earth to Pluto and back). From Earth to Mars is a positively rapid 14 minute round trip on average. Within our galaxy, it starts to be a little more noticeable that our vision is a little more delayed getting to us; for instance, our current observations of Eta Carina indicate that that particular star is probably going to explode sometime soon. However, light takes over seven thousand years for it to reach us, so if it had exploded last week, we’d still have 7500 years before we’d be able to notice.

A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. Image credit:  Jon Morse (  University of Colorado  ), and   NASA

A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. Image credit: Jon Morse (University of Colorado), and NASA

The further out we go, the further behind our vision becomes, lagging more and more dramatically behind “now”. This lag affects light equally as it comes from all directions, so we wind up with spheres of lagged space-time in any possible direction. Because we are the receiving end of all this light, this observable sphere is by definition centered on us. Any other observer at any other position in the Universe should see the same thing; their view of the Universe will be just as limited in scope by the speed of light as our own.



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You will get FREE science art from Paige’s Photography for participating, as well as a chance to win $50! (At least two Astroquizzical readers will definitely get $50, but there are 100 $50 prizes available.) There are also t-shirts and other perks! It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

Does space go on forever?

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