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Hubble vs. Webb - How Far Back Will They See?
Roland Piquepaille writes "According to Forbes, reporting in "Peering Back At The Universe's Past," space telescopes are really acting as time machines. They can watch objects which are so far from us that light has taken billions of years before reaching their mirrors. The Hubble telescope is able to look at events that took place 13.3 billion light-years ago. But the James E. Webb space telescope, currently under construction, and scheduled to be launched in 2011, will be able to see even further and catch phenomena which happened 13.5 billion light-years ago. The astronomers think the Webb telescope might even be able to see up to 13.7 billion light-years ago, when our universe was just 200 or 300 million years old. We are used to see fantastic images from Hubble, without paying too much attention to the characteristics of the telescope itself. So here is a thorough comparison between the two space telescopes."
is the fact that while Hubble can view things in the optical, James Webb will be looking at things in the infra-red. The two Wiki links (from the article) provide much more information.
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/. is screwing up the text, but the links should still work.
http://en.wikipedia.org/wiki/James_Webb_Space_Tel
http://en.wikipedia.org/wiki/Hubble_Space_Telesco
Grr...
It's rather more complicated than you think. The light reaching the telescopes is x billion years old, meaning that the objects that emitted the photons have long since moved elsewhere and are no longer there where the telescope sees them. So, when looking out into the universe, you are seeing mirages of the past. The more distant the object, the older its light. So yes, telescopes are time machines in that regard because such is the nature of spacetime - if you look over any given distance you are in effect looking into the past.
----- One learns to itch where one can scratch.
Wouldn't that be 3 %?
I worked on the Webb telescope project for a short period of time (back when it was Next Generation Space Telescope) and, believe me, they had a hard enough time scrounging up the money to create what they have now. Making the mirror "a little bigger" or increasing the size of the infrared array would require much more effort than you might think.
http://www.astronomycafe.net/qadir/acosmbb.html
Just for the record, the Big Bang theory is becoming as accepted in cosmology as the theory of evolution is in Biology.
There will eventually be a limit to how far back we can look in time. The Big Bang itself will just appear to be an incredible brilliance everywhere.
That same brilliance has cooled to the point that nowadays, it's only detectable as an almost-universal background microwave radiation.
The detection of that radiation is considered one of the strongest "proofs" of the Big Bang theory, by the way.
There is an optical limit or boundary which cannot be seen past - the surface of last scattering - preventing you from actually seeing right to the beginning.
Batman: "Slake your thirst. You'll have worse than a parched sensation when we're through with you!"
It will be located at Lagrangian point L2 which as you say is a million miles from Earth. The logic being that gravity is equalised there so it wont move and its deep enough in space to reduce heat interference on the IR camera. Part of the project goal is to reduce operational costs as Hubble incurs 230-250 Million US a year to run so there are no service missions envisaged, it will be a standalone effort.
Do not try to read the dupe, thats impossible. Instead, only try to realize the truth
What truth?
There is no dupe
13.7 / 13.3 = 1,030075188 => 0.03 % performance increase with the new, latest, more expensive system.
As another poster has pointed out, it's actually a 3% improvement.
The point is, that's only 200 or 300 million years from the very beginning of the universe, and it gets exponentially more difficult the further back you want to see.
Rather than 13.7 vs. 13.3 billion years back from now, think 200/300 million years from the start versus 600/700 years from the start. That's a pretty good improvement.
A light year is a valid distance measurement since the speed of light is a constant. It's as valid as defining the distance between home and work as "10 minutes in my car travelling at a constant 60 mph".
There are 2 main methods:
the 1st one is called parallax (or triangulation) and consists on measuring the position of the star from different points of the earth's orbit (i.e., at different times of the year). The differences in the angular position are then used to calculate the distance of the object.
For objects (stars) that are too far away to give a measureable parallax (more than 400 light years), an indirect technique is used. It is known that different kinds of stars have different emission spectra (colors), and every kind of star has a characteristic brightness. This has been proven by observation of close stars. This way one can analzye the spectrum of a given star and guess how bright it should be. Since the light emission of a star is a spherical wave, the theoretical attenuation of its intensity can be used to calculate the distance. This does not mean that single photons lose energy on their way: they don't. A photon's energy is related with light's frequency (color), while the apparent brightness of the star is related to the number of photons that get here. Since thay propagate as the surface of a sphere, the further you are the fewer photons you get per unit area.
... information wants to be forwarded
It's the "Woodlands" theme/stylesheet by Bryan Bell.
You are missing the fact that NASA spends a lot of money making housecalls on Hubble, which have greatly extended its lifetime. This will not be possible with Webb because it is much further out.
Running Webb at L2 will save money. It's difficult and expensive to run a large space telescope in low Earth orbit (LEO). Observations have to be planned carefully since the Earth gets in the way for most of the sky every 90 or minutes. The satellite also has to have batteries to power the systems when the satellite/telescope is eclipsed by the Earth. Batteries are heavy, have to be recharged and they fail. Hubble's are failing. Large satellites in LEO slowly see a degeneration of their orbits because of drag from the very highest parts of the Early atmosphere. This requires them to be reboosted very so often. Any future service mission to HST needs to also reboost it.
Finally, satellites in LEO - least ones in orbits like the one HST is in - have to travel through a radition belt every orbit that can cause electronics to fail and bits to flip. This sometimes causes the telescope to go into safe mode and ruins observations. While in safe mode, operations crews are standing around and more observations have to be either cancelled or rescheduled.
Many of these problems are avoided at L2 or similar locations. Webb's life will be limited by the amount of sensor coolant on board, but space telescopes like the International Ultraviolet Explorer have operated for 20 plus years. IUE used a small crew, was easy to operate and produced more then 3,000 papers at a very low cost - a great return in value for tax payer.
No, sorry. There is a limit to how far we'll ever to able to see, and it's called our "light cone".
John Barrow's book "Impossibility" has a nice description of this (and other limits).
The orbit is about 1.5 million km distance from the earth, at something called the L2 Lagrangian. The Webb wiki page has a link to the Lagrangian page, but for the lazy people, it's here. The orbit was chosen to keep the position of the sun constant relative to the telescope, so that the big 'parasol' can be used to shield the infra-red sensor.
As for Hubble, it's been able to give some awesome images, but it has its limits. I was hoping that the JW (henceforth called J-Dubya?!) would be able to start spotting planets around other stars, but it's not designed for that. I'd like to know if it's theorically possible to keep both in orbit and use them in parallel somehow, in the same way that ground-based radio telescopes have been linked together in arrays. Probably not worth the hassle?
The 'infra-red only' sensor troubles me. Since the telescope's aim is to study the Big Bang, the light/photons it'll be receiving will have travelled for a long time/distance and I guess be red-shifted way down to the IR band. This is all very well, but it means that the telescope shouldn't be considered as a replacement for Hubble, which carries out a wider range of observations.
As an aside, I believe that there is a limit to how far back we can look. At some point, probably less than 1 million years (a guess, can anyone help?), the universe was just too dense for photons to travel around unhindered as they seem to these days. Who said it was better back in the old days eh?
Now two questions. First why beryllium? I know that it's lightweight so easier to lift into orbit. Any other reasons? And secondly what happens if a micro-meteor hits this shield? Do we get a permanent bright spot on all subsequent images, like a broken pixel on an LCD display?
This is not a sig
mankind has divided time and space, cause the concept of spacetime hurts their little brain...
according to einstein space and time should be space-time, so it is definately a measurament for distance, and time.
mass "curls" space around it, creating time in the process.
Actually, the speed of light is not constant. They have done various tests and proved that light an slow down.
Light can slow down. In an open vacuum it is at it's highest speed. Going through materials it slows down a little. The speed change is different for different materials.
An example is that light slows down going though glass.
Evolution or ID?
It's not inconceivable to use it as a measure of the radius of a 'cone' of space time which can be viewed from a certain point. Kind of a synthesis of distance and time.
In that sense, it's implied in almost ALL astronometrical comments like "we saw this 15 light years away"; it's are really saying "we saw this event happening 15 years ago because that's as recent as we can see anything from that target".
So yeah, basically you're right, but it's faintly arguable.
-Styopa
Any service missions would need to be entirely automated, which probably makes them impossible.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
Actually, you do. It's called Cherenkov radiation, and it's very similar to the way a sonic boom forms, with waves piling up. It's a kind of eerie blue light, I believe.
Trees everywhere, and not a forest in sight.
You can't use parallax to get the distance of a galaxy!
It's way too far away. It's done by finding the redshift. Light waves from a distant galaxy are stretched as they travel, due to the expansion of the universe. The factor by which the wavelength increases (minus 1) is termed the redshift. The most distant galaxies known have redshifts 6-7. Cosmologists almost always use redshifts rather than times. The redshift is measured generally by looking at the spectral lines in the light from the galaxy, and comparing the wavelengths of those lines with those in a non-redshifted spectrum.
The world is everything that is the case
BTW, this is where the term parsec comes from. An object in space is considered to be one parsec away if it appears to move 1 parallax-second in six months (when the the two observations are 2 A.U. apart because of the Earth's orbit). One thing that tends to confuse people about parsec measurements is that it's actually a reciprocal measurement. That is, an object that moves a 1/2 parallax-second is said to be 2 parsecs away, etc.
Anybody else notice that Webb is expected to have a lifetime ten years shorter than Hubble?
I'd have expected a more recently built telescope to last longer than an older one.
Also, anybody have a clue exactly what happens when a telescope dies?? (Visions of Hubble slowly growing incontinent etc.....)
I'm going to butcher the explanation, but modern cosmology posits that there is no center to the universe in the way you mean.
It's important to remember that at the moment of the big bang, there wasn't a universe outside of it. That is to say that when the big bang occured, it didn't expland into some already exisiting space, rather it was the space that was expanding. As such, all objects are moving away from all other objects.
http://www.astro.ucla.edu/~wright/nocenter.html
has a decent drawing to illustrate how this leads to no "real" center.
The other explanation that has always helped me picture it is to imagine the universe as an un-inflated balloon. In this model, we've reduced the universe to a two-dimensional, unbounded, infinite space in order to help us visualize this principle. Before inflating the balloon, mark several points with permanent marker, Now, when you inflate the balloon, you can see that each point grows more distant (over the surface of the balloon) from every other point you've marked and that the farther one mark is from another, the faster it moves away from it. From the point of view of a given mark, everything else is moving away from it, which would give the impression that it's at the "center" of the balloon's surface. At the same time, however, that impression would appear to be true for every other mark.
... that we'll eventually see the big bang?
Nope. In the very early Universe, all the matter was so hot that it was completely ionised. That is, there were lots of protons flying about and lots of electrons, just doing there own thing. It turns out, that light interacts very strongly with free electrons, so any light that was around at this early stage (such as from the big bang...) would've bounced around so much that it no longer carried any useful information about earlier times. Kind of like trying to see what the moon looks like through a really dense cloud.
Incidently, once the Universe cooled enough, light was able to pass through it. The light that started at this time is the oldest in the Universe and is what we now see as the Cosmic Microwave Backgound - far from being useless, this tells us huge amounts about the early Universe.
NASA's WMAP Mission site has a very good explanation.
But what I wanna know is, does this mean we are looking away from the center of the universe?
Not as such. To picture the expansion of the univsere, think of all the galaxies, stars etc as small dots on the surface of a baloon. As the balloon is inflated, the area of it's surface, and the separation of the dots, expands. You can rotate the balloon so that you're looking at any dot you choose, and everything looks the same - there is no real centre to the 2 dimensional surface of the balloon. The only sensible definition of a centre is at point in 3D space where the expansion of the balloon started.
Similarly, there is no point in 3D space in our universe that could be considered it's centre; the only true centre of the universe must be the position in 4D space-time in the past, from which the expansion started. i.e. the big-bang is the centre of the universe.
Is there some crazy ball of energy still expanding outward or something?
Yes, but we can only see so far back as the universe was opaque very early in it's history; we can see the remnants of the big bang, but not the fireball itself.
Ahh but it is related to the distance from the object to the observer by Hubble's law (velocity is proportional to distance).
Essentially both you and the parent are partly right. Redshift is a reasonable proxy for distance when you are sufficiently far away that your random relative motion (proper motion) is small relative to your Hubble expansion velocity. The problem is you have to know Hubble's constant very well in order to turn a redshift into a physical distance.
Thus there's a degeneracy where you have to measure distances to a bunch of objects to find Hubble's constant.
That's why you still need other methods of measuring distance. The "distance ladder" builds from very well measured distances using geometric parallax (only good very nearby), then further out using objects whose luminosities are known (from nearby objects whose distances are measured using lower rungs of the "ladder") like variable stars (Cepheids) and type Ia supernovae.
Doug
Venn ist das nurnstuck git und Slotermeyer? Ya! Beigerhund das oder die Flipperwaldt gersput!
The early universe was initially so hot and bright that it was opaque much like the flame of a candle. Photons could not travel far before they were scattered. As the universe expanded it also cooled. Around the time it was 200 million years old (I think), the universe cooled to a point where it suddenly changed from opaque to transparent and all the bouncing photons suddenly started travelling in straight lines. These are the earliest photons we can see since they must travel a straight distance of 13.8 billion light years or so to reach us. The only remnant we can see of the opaque early universe is the cosmic microwave background.
This is a tough one to comprehend but heres a shot. It doesnt matter where you look. Its everywhere, and here is why:
When you look away from the earth, you are looking back in time. This is due to the fact that photons travel at the speed of light. So if you look at the moon, you see the moon a half a second ago. Mars is several minutes ago. Alpha Centari is about a year ago. So the futhrer out you see, the further back in time.
Now think of the universe as expanding. If you look out a to a distance where the light is half as old as the universe, you see the universe as it was at that time. But the universe was much smaller then so the galaxy you look at seems bigger than it should given how they look today. So the expansion of the universe and the traveling of photons acts as a lense making things look bigger as you look back further (theres less universe to fill the sky so objects look bigger).
OK so then you look all the way back. The big bang then fills the sky. It is everywhere. And we see it. Its what is refered to as the 3 degree Kelvin background radiation. And in the radio, no matter where you look, you see it.
Now this is not actually the big bang itself. The universe was too dense for anything to be seen. So what we see is what is referred to as the universe at the time of last scattering, when the light from the big bang was finally able to escape as the universe had expanded enough that it was not so dence to capture all the light. So when you hear about people studying the fluctuations in the background radiation, they are actually studying this period of the universes expansion.
Today is a gift. Save the receipt.
Astronomers have a whole range of different ways to measure distances, each of which works in a different regime. They form a "cosmological distance ladder" - you attempt to calibrate each new method during its overlap region with the previous method.
Parallax is the method for the very shortest distances (nearby stars).
For intermediate distances (distant stars in our own galaxy, relatively nearby galaxies), most of the methods come down to finding some sort of "standard candle" - something that you know the intrinsic brightness of, so you can use its apparent brightness and the inverse square law to calculate its distance. Astronomers tend to use particular types of variable stars (stars with a well-defined cycle of brightness changes) for this purpose. For galaxies, you can sometimes use averaged properties of all the stars to estimate the distance.
For cosmological distances (very distant galaxies) the most common trick is to use redshift. Because of the universe's expansion, an object twice as far away is receding from us twice as fast, and so its light is Doppler-shifted twice as much. Ideally, you look for known features of the object's spectrum and see what wavelength they have ended up at. This is what people are talking about when they measure the distance to Hubble's latest find.
There is also a complementary method that uses standard candles at cosmological distances. In this case, you use Type Ia supernovae, a particular type of exploding star that looks pretty much the same every time. They're bright enough to be seen very far away, and again you can get the distance using the inverse square law (modified by general relativity). It's the difference between this method and the redshift method that provides the strongest evidence for dark energy - it shows us that the universe is expanding faster than we expect, and that this expansion is accelerating.
Heh. A cute idea.
On point 3) though, you'll have a big problem. The diffraction limit of an apertures defines the smallest angular detail you can see, and for any appreciable distance from the earth, you rapidly lose any interesting information. You also have the problem that planets which are illuminated by their parent stars, which are up to ten billion times brighter than the light reflected from the planet's surface towards you.
This is what the Terrestrial Planet Finder mission is trying to do - it is trying to see the light of other earth-like planets around other stars, and the diffraction effect for finite sized mirrors means that the light of a planet is buried within the diffraction halo from the parent star, by a few million times. Two proposed techniques to improve detection of planet light include nulling interferometry, and coronagraph optics.
Interferometry takes the light from two widely separated telescopes and combines them such that the parent star light is nulled out whilst the planet light passes through (essentially a fantastically accurate spatial filter) and the coronagraph has a black disk flying in front of the telescope blocking the light from the central star.
Dr Fish
No. The furthest we can hope to see back in UV/optical/IR light is the "recombination era". That's the point at which the universe had cooled enough (by expanding) for the electrons to recombine with the atomic nucleii. Before that, the matter was ionized and therefore opaque to the kinds of radiation that JWT can detect. The recombination era occured a few minutes after the big bang (IIRC and in the standard models, both of which might be wrong).
There's a couple of parameters of interest here.
First of all, when you're looking at objects a looooong way off, there's a question of how many photons you get to collect from that object per unit time. If you collect too few photons, anything you might see gets lost in the noise associated with your detector (your 'camera'). You can see stuff further away with a bigger primary mirror (more photons collected) and a better detector (less noise). If you know the parameters involved--brightness of object, mirror size, detector quality--you can make a reasonable estimate of the effective distance you can observe.
For these objects that are really far away and reallly old, you run into another problem that limits how far back you can see. Very distant objects are significantly redshifted. The expansion of the universe has effectively stretched out these photons to much longer wavelengths--partly through Doppler shift, partly through the stretching of space itself. This redshift is correlated with distance, and these distant objects that should glow brightly in the visible and ultraviolet actually show up well into the infrared.
Ground-based telescopes can't see this stuff at all, no matter how big their mirrors--water vapour in the atmosphere screens out infrared radation. Space telescopes can see it, but warm equipment produces infrared radiation that can swamp the signal. Consequently, the JWST carries a cargo of liquid helium (designed to last five years) to cool some of its instruments to operate at 7 kelvin.
By combining a larger mirror with cooler instrumentation, the JWST can see further back in time than Hubble. Based on their knowledge of those parameters and a smattering of astrophysics, NASA can peg a rough estimate of just how far that is.
~Idarubicin
... to drop a camera X light years from us is a horrible kludge. FTL violates causality by definition, therefore it is physically equivalent to time travel. You may as well just go back in time directly and observe our past at arbitrary closeness.
First, simplify your model. Assume someone else put a mirror far enough out to reflect the image you want to see. That gets rid of the question of what you see first (not the spaceship). It also negates the issue of the spaceship flying in an arc so you don't see it. Now, here's the problem: if you want to see 50 years back, and a mirror was put in place right now, you would have to wait 50 years to get an image returned. Total time to see image would be 100 years. If you put the mirror at 25 light years, you would see 50 years back at time of viewing, but would only see images from 25 years back at time of placement.
The solution is to look for mirrors that are already in place (or put a large number of mirrors in place for future generations). This sounds absurd, but remember this: black holes can theoretically wrap light around at exactly 180 degrees at a given point from their centre. So we already have a number of mirrors out there. Now the big problem: black holes will have huge distorions around them, and very little light reaching them in the first place, so it's doubtful that you would be able to see anything remotely useful. This is also the problem with placing artificial mirrors: the light returned would be so small, that it would be useless. So much for looking back in time.
Sure I'm paranoid, but am I paranoid enough?
Pathetic Star Wars Fanboy Mode = ON IIRC in the Star Wars Extended Universe books Kessel is a planetary system that is situated near a black hole. The Kessel run is the route to this planet that has to navigate around this black hole. A faster ship has the ability to plot a course closer to the black hole without being pulled in.
Note that in the comparison box, the launch vehicle for the Webb isn't a shuttle. It's an Ariane rocket.
As I'm sure everyone will be quick to point out, lightyears isn't a measure of time, rather of distance.
Well, in a Newtonian sense, yes...
Einstein will tell you that time = distance. You just have to use the proper conversion factor (c, the speed of light in a vacuum) to get your units right. In relativity work, we often use units where c = 1. Time and space then behave identically in the math and you don't have to do one thing for one dimension and something a little different for the other three.
c, by the way, is exactly 299,792,458 m/s. EXACTLY. The meter is _defined_ as the distance a photon travels in exactly one second. (The second has a much more complicated definition)
So yes, light-years measure distance. And they measure time.
In Soviet Russia, sig types you!
FTL travel is not required. You're assuming that space is a ball with a vacuum inside through which light travels. It is not. Here's my understanding of this - if anyone has a better explanation please let's have it :) Light is propagated along the curvature of spacetime (i know that this is vague, but without mathematics it's difficult to explain in natural language). Assume that the galaxy we'll be seeing three billion years from then is a point light source. The light travels in an expanding cone along spacetime. The universe is finite and the light will curve back as it were (some models suggest that this may not be entirely true though). The universe expands and so does the cone of light. We come into being and are in the cone of light at a certain point in (space-) time. The original source has moved but the cone has not. The difficulty here is in visualizing space expansion not as a 3d phenomenon that happens at the boundary of a sphere but also affects its contents.
----- One learns to itch where one can scratch.