Cosmology

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Wed, 30 Sep 2009 22:38:48 GMT

We never said matter is expanding into space.

blast-of-giant-atom-created-our-universe

The above link is to 3 pages from Popular Science magazine, from December 1932.

At the beginning of the article it looks like they are talking about some sort of an explosion, but on page 2 we have the following:

"The nebulae are not running away from us. Their recession is due to expansion of space. This may, perhaps, seem to be quibbling over terms, since it amounts to the same thing in the end. Nevertheless, the distinction is worth keeping. According to the relativity theory, there is a difference between the running away of the nebulae and expansion of the medium in which they are embedded."

The idea that the Earth is apparently receding from a distant galaxy faster than c does not mean measurements are skewed from our vantage point as relativity tells us that all vantage points are essentially equivalent.

Apart from the relatively small peculiar motions of galaxies with respect to their close neighbours due to gravitation, all galaxies can consider themselves at rest and that the rest of the universe is expanding away from them. Wherever you are, the further you look the faster a galaxy apparently recedes.

Re: Our observable universe is not 30 to 38 billion light years in distance.

brooksquest — Mon, 28 Sep 2009 02:30:37 GMT

If greater than c expansion is possible then, given that we are not at the center of it all, the Earth is travelling "relative" to some other place in the universe" faster than the speed of light. If we are in motion in some direction at this speed then all of our measurements are skewed from our vantage point. We cannot assume to be standing still in the middle of it all.

Honestly, we have got to come up with the real reason we see cosmological red shift. First we said matter is expanding into space, then we said that space itself must be expanding. Whats next?

The answer could have more to do with the effects of gravity of dark matter on light than with all this make believe expansion BS. Hubble was wrong and someday we will know the truth about redshift.

San Mig anyone?

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Wed, 01 Jul 2009 21:42:32 GMT

So then really we have almost no speculation in either area, only a difference of opinion on how the term observable should be used. Yes, I did miss that qualifier didn't I?

Very well.

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Wed, 01 Jul 2009 16:33:23 GMT

Dusty_Matter:
SpeedFreek:
Any "speculation" about the current size of the observable universe is as valid as any "speculation" on the current age of the observable universe. We cannot see the passage of time...

I totally disagree with you there. The size of the universe is unknown. The age of the universe has now been pegged down pretty precisly from what I’ve heard and read.

I said the size of the observable universe! (which has been pegged down just as much as the age of the universe, as it falls out of the same equations)

Dusty_Matter:
So what you’re saying in essence by saying that the universe might only have a radius of 50 billion light years, is that, what we’ve seen so far, might just about be all there is to our universe? If that is possibly the case, then what is there really for sure to differentiate from?
There is stuff that we have seen, there is stuff that we can no longer see, and then there might be stuff that we have never seen? It doesn’t seem to me that the way they have chosen to use the word observable really differentiates anything concrete at all. It also seems to me that, whoever chose the to use the word “observable” in this fashion, really didn’t understand what he was doing in the first place.

However as you say, “… that term has been chosen.” and I know that I cannot change that. I can only resolve myself not use that term in that manner, because I think it is wrong.

What I meant is that the observable universe seems to homogeneous and isotropic at the largest scales - we have no indication of there being any physical end to it, so we can assume the whole universe is larger than the observable universe, but we have no idea how much larger it might be. We assume there will be galaxies currently further than 46 billion light years away, outside of our observable universe, in regions of the universe from which we have never received a photon.

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Wed, 01 Jul 2009 04:31:48 GMT

SpeedFreek:
What do you mean when you use time to define how big, or how large, the observable universe is?

I meant that I was not giving it’s size in terms of light years or a physically measured radius, but it terms of how far back into the past we can see. We can see over 13+ billion years of our universe’ history, which is almost all of it in terms of it‘s age.

SpeedFreek:
How do you know that the observable universe is 13.7 billion years old?

Because I’ve read about it, and gained a basic understanding on how that conclusion has been arrived at.

SpeedFreek:
What do we observe that gives us that figure? How do you think we can "truthfully observe" the age of the universe?

Well, until I had just recently read Bob Berman’s article, and having learned some more from you, I had only assumed that it was the red-shift that explained this growth.

SpeedFreek:
Any "speculation" about the current size of the observable universe is as valid as any "speculation" on the current age of the observable universe. We cannot see the passage of time...

I totally disagree with you there. The size of the universe is unknown. The age of the universe has now been pegged down pretty precisly from what I’ve heard and read.

SpeedFreek:
... I am surprised you haven't focussed on the angular diameter distance...

I didn’t even know about such a thing, until Bob’s article, and then from reading your posts.

SpeedFreek:
how large it actually looks tells us how far away it was, in the same way that you can estimate how far away a car, or a house is, based on its apparent size.

I have only just learned about it, but I am grateful for the instruction. I have argued with many on the topic of the Big Bang and it’s consequential growth, but I only really had red-shift as an explanation of proof. I argued red-shift until I was blue-shifted in the face, but now with this new tool I am better armed for it’s defense. Thank you for teaching me about angular diameter distance.

SpeedFreek

"...you have misunderstood the scientific meaning of the term "the observable universe", which I admit has a definition that is somewhat counter-intuitive."

I think it fools a lot of people. It really should be explained more thoroughly whenever those statements are made.

SpeedFreek

"But as you look deeper into this subject, you find that all the distance measures are required to comprehend the evolution of the universe."

I totally agree with that, and thank you for the examples given.

SpeedFreek

"I know you argue that we shouldn't apply the term "observable" to the universe as we think it is today, but that term has been chosen. It differentiates the parts of the universe that we have seen from the parts we have never seen, and it accepts that all the parts are becoming more distant from each other.

We have received light from a region of space that is now 46 billion light-years away, but that region was a lot closer when the light was emitted. For all we know the whole universe might only have a radius of 50 billion light-years (or it might even be infinite!), but we think the parts we have seen are currently not less than 46 billion light-years in radius."

So what you’re saying in essence by saying that the universe might only have a radius of 50 billion light years, is that, what we’ve seen so far, might just about be all there is to our universe? If that is possibly the case, then what is there really for sure to differentiate from?

There is stuff that we have seen, there is stuff that we can no longer see, and then there might be stuff that we have never seen? It doesn’t seem to me that the way they have chosen to use the word observable really differentiates anything concrete at all. It also seems to me that, whoever chose the to use the word “observable” in this fashion, really didn’t understand what he was doing in the first place.

However as you say, “… that term has been chosen.” and I know that I cannot change that. I can only resolve myself not use that term in that manner, because I think it is wrong.

Thank you speed for your time, patience, and knowledge. And about Bob Berman. His articles are the first ones I turn to in "Astronomy" magazine. I might disagree with some of the things he says, but sometimes his articles are more informative to me than the rest of the whole magazine. I respect him anyway. You should read his articles speed. Who knows, he might even teach you something some day.

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Tue, 30 Jun 2009 18:09:04 GMT

Dusty_Matter:

I guess another way of looking at it would be akin to the event horizon of a Black Hole.

It has been claimed that we have seen our own central galactic black hole swallow up a star that was once in orbit about it. Yet no one claims to be able to observe what happened to this star beyond the event horizon of our black hole, the point at which photons are no longer in contact with us.

The expansion of our universe at great distances does in essence the same thing. We are cut off due to time, speed of recession, and distance, to objects that we are viewing as having existed 13 billion years ago. So why claim that they are observable to us today beyond this obvious barrier beyond which, we will never see them again?

Nobody claims they are observable to us today beyond that barrier, you have misunderstood the scientific meaning of the term "the observable universe", which I admit has a definition that is somewhat counter-intuitive. But as you look deeper into this subject, you find that all the distance measures are required to comprehend the evolution of the universe.

Let me give you an example to show you why all three distance measures have to be considered.

Lets look out into the universe and describe what we see at further and further "distances", using the three distance measures.

Light-travel time.

z=0.1 - a galaxy whose light is 1.2 billion years old.

z=1.4 - a galaxy whose light is 9.1 billion years old.

z=7 - a galaxy whose light is 13 billion years old

z=1089 - the CMBR, which is 13.7 billion years old.

Angular diameter distance.

z=0.1 - a galaxy that was 1.2 billion light-years away

z=1.4 - a galaxy that was 5.7 billion light-years away

z=7 - a galaxy that was 3.5 billion light-years away

z=1089 - a CMBR photon that was emitted 40 million light-years away.

Comoving distance.

z=0.1 - a galaxy that has receded to 1.35 billion light-years away.

z=1.4 - a galaxy that has receded to 13.7 billion light-years away.

z=7 - a galaxy that has receded to 29 billion light-years away.

z=1089 - a CMBR photon whose emission point has receded to 46.5 billion light-years away.

The angular diameter is what is actually seen. The dimmest, most redshifted galaxies look larger in the sky than brighter, less redshifted galaxies. It is only when the redshift drops below z=1.4 that galaxies look closer in distance as they get closer in time. Above that redshift, galaxies look closer in distance as they get further away in time! This is because they were closer to us, as the universe gets smaller and smaller, the further back in time we look.

For a z=7 galaxy to look a lot closer than a z=1.4 galaxy makes no sense if you simply use the redshift as an indicator of the age of the universe without also considering where the z=7 galaxy must have receded to by the time its light finally passed that z=1.4 galaxy. When the z=7 galaxy emitted the light we see, the z=1.4 galaxy (or whatever was in that region of space at that time) was a lot closer to us than the z=7 galaxy. When the z=1.4 galaxy emitted the light we see, the z=7 galaxy (or whatever was in that region of space by then) must have been a lot further away than the z=1.4 galaxy.

I know you argue that we shouldn't apply the term "observable" to the universe as we think it is today, but that term has been chosen. It differentiates the parts of the universe that we have seen from the parts we have never seen, and it accepts that all the parts are becoming more distant from each other.

We have received light from a region of space that is now 46 billion light-years away, but that region was a lot closer when the light was emitted. For all we know the whole universe might only have a radius of 50 billion light-years (or it might even be infinite!), but we think the parts we have seen are currently not less than 46 billion light-years in radius.

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Tue, 30 Jun 2009 17:30:40 GMT

Dusty_Matter:
SpeedFreek:
So, you can understand how large it is by knowing how old it is?

No. I have not been talking about it’s size, nor can that connection be made.

SpeedFreek:
Our observable universe has a radius of 13.7 billion years then. But what does that actually mean, in terms of how big, or how large, it is?

You seem better adapt at answering that question. I was not making a comment on how big or large it is. That is a different topic. I was only commenting on what is observable.

Well I must have misunderstood you then, when you said:

Dusty_Matter:
I maintain that the time definition for how big is our observable universe is, is the most accurate statement for how large the observable universe is.

What do you mean when you use time to define how big, or how large, the observable universe is?

Dusty_Matter:

SpeedFreek:
Nobody is saying we can see the edge of the observable universe as it is today.

Then why say that we can see 30 to 40 billion light years away? If it is not true, then people shouldn’t be saying that.

Nobody is saying that we can see 30 to 40 billion light years away! What we say is that the most distant parts of the universe that we have seen are "now" up to 46 billion light-years away, but we are not saying we have seen across a distance of 46 billion light-years.

Dusty_Matter:
But when you speak of objects that are 30 billion light years away, you know that they are no longer connected to us by photons. Time and the expansion of the universe has ended our relationship with them. You can extrapolate their distance, but that is all. You have no idea how many collisions they may have had merging with other galaxies, how big they may be now, nor any of their existing traits today. The only information that you have on these distant galaxies is from 13 billion years ago. The only other piece of information you can give on them, is that they are long gone now, at 30+ billion light years away, and we have no way of knowing what they look like now. Therefore our observable universe goes back about 13+ billion years into the past. This is not a comment on it’s size or how far distant objects may be, but on only what we can truthfully observe. Everything else is speculation.

The objects that are now 30 billion light-years away are connected to us by photons, emitted when those objects were only 3.5 billion light-years away. What has happened to those photons helps us understand what has happened to those objects. Of course, those objects likely do not exist in anything like the same form that they were when their light was emitted, but whatever has happened since, in that region of the universe, happened in a region of space that is now around 30 billion light-years away.

How do you know that the observable universe is 13.7 billion years old? What do we observe that gives us that figure? How do you think we can "truthfully observe" the age of the universe?

Any "speculation" about the current size of the observable universe is as valid as any "speculation" on the current age of the observable universe. We cannot see the passage of time, we can only infer it, in exactly the same way we infer the co-moving radial distance. This is why I am surprised you haven't focussed on the angular diameter distance, which is the one thing that can thought of as being "truthfully observed" in a far less abstract way than the other two. Whilst theoretical physics leads us to the derivation for the age or the comoving radius of the universe, empirical measurements tell us how far away a galaxy was when it emitted the light we see.. how large it actually looks tells us how far away it was, in the same way that you can estimate how far away a car, or a house is, based on its apparent size.

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Tue, 30 Jun 2009 12:22:23 GMT

I guess another way of looking at it would be akin to the event horizon of a Black Hole.

It has been claimed that we have seen our own central galactic black hole swallow up a star that was once in orbit about it. Yet no one claims to be able to observe what happened to this star beyond the event horizon of our black hole, the point at which photons are no longer in contact with us.

The expansion of our universe at great distances does in essence the same thing. We are cut off due to time, speed of recession, and distance, to objects that we are viewing as having existed 13 billion years ago. So why claim that they are observable to us today beyond this obvious barrier beyond which, we will never see them again?

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Tue, 30 Jun 2009 00:27:18 GMT

SpeedFreek:
So, you can understand how large it is by knowing how old it is?

No. I have not been talking about it’s size, nor can that connection be made.

SpeedFreek:
Our observable universe has a radius of 13.7 billion years then. But what does that actually mean, in terms of how big, or how large, it is?

You seem better adapt at answering that question. I was not making a comment on how big or large it is. That is a different topic. I was only commenting on what is observable.

SpeedFreek:
Nobody is saying we can see the edge of the observable universe as it is today.

Then why say that we can see 30 to 40 billion light years away? If it is not true, then people shouldn’t be saying that.

SpeedFreek:
the observable universe can be defined as the history of the space-time from which we have received information.

We have received information almost all of the way back to the big bang. Is this not true?

SpeedFreek:
By accepting that cosmological redshift tells us how long the light has been travelling, you are accepting that the universe has expanded during that journey, and that the emitter of that light is now further away than it was.

Yes.

SpeedFreek:

Basically, cosmologists like to take what the astronomers have seen and analyse it using the best cosmological theory we have, to understand the evolution of the universe from what it was to begin with to what it is today, and it has been given the term "the observable universe" as it is based on the region of the universe to which we have been connected by photons.

True. But when you speak of objects that are 30 billion light years away, you know that they are no longer connected to us by photons. Time and the expansion of the universe has ended our relationship with them. You can extrapolate their distance, but that is all. You have no idea how many collisions they may have had merging with other galaxies, how big they may be now, nor any of their existing traits today. The only information that you have on these distant galaxies is from 13 billion years ago. The only other piece of information you can give on them, is that they are long gone now, at 30+ billion light years away, and we have no way of knowing what they look like now. Therefore our observable universe goes back about 13+ billion years into the past. This is not a comment on it’s size or how far distant objects may be, but on only what we can truthfully observe. Everything else is speculation.

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Mon, 29 Jun 2009 17:25:15 GMT

Dusty_Matter:
I maintain that the time definition for how big is our observable universe is, is the most accurate statement for how large the observable universe is.

So, you can understand how large it is by knowing how old it is? Our observable universe has a radius of 13.7 billion years then. But what does that actually mean, in terms of how big, or how large, it is?

Nobody is saying we can see the edge of the observable universe as it is today. We can see the edge, as it was 13.7 billion years ago (the CMBR), and its redshift (derived using WMAP) tells us how much the universe has "stretched" since. The redshift tells how the scale of the universe has changed between the emission of that light and our detection of it. The CMBR has a redshift of z=1089, which means the universe is 1090 times larger "now" (1+z), than it was "then". The place where that light was emitted from is now 1090 times further away than it was. 42 million light-years has turned into 46 billion light-years.

Earlier, I said that the observable universe can be defined as the history of the space-time from which we have received information. We have seen information from these regions of the universe, and the redshift/luminosity/surface brightness information tells us how long ago the light was emitted, where the light was emitted, and how much the universe has expanded since. All these measurements are on an equal footing as all are derived from the same information, using the same mathematics.

If the universe were not expanding, putting more distance between the source and the eventual detector of light, there would be no cosmological redshift. By accepting that cosmological redshift tells us how long the light has been travelling, you are accepting that the universe has expanded during that journey, and that the emitter of that light is now further away than it was.

Basically, cosmologists like to take what the astronomers have seen and analyse it using the best cosmological theory we have, to understand the evolution of the universe from what it was to begin with to what it is today, and it has been given the term "the observable universe" as it is based on the region of the universe to which we have been connected by photons.

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Mon, 29 Jun 2009 07:21:17 GMT

SpeedFreek:
No, of course we cannot see those galaxies where they are now.

You and I both know that we cannot see objects that are 46 billion light years away. Looking at the fuller description that wiki gives for the observable universe then, I can only say that it’s definition conflicts with itself and it is in error then.

A definition for observable is that you can see it, observe it, study it, or examine it. I like that definition.

SpeedFreek:
It is only observable theoretically, using the mathematics of the Lambda-CDM concordance model. Unfortunately, a lot of mainstream media articles are not very good at explaining this properly.

What does observable theoretically then mean? It means that you are pretending that you can see it? So for any mainstream articles, or articles period, that are making the statement that the observable universe is about 38 billion light years in distance, they need to include the statement: “We are only pretending to be able to observe that far. We can’t really do that.” That’s what should be included in those statements.

I maintain that the time definition for how big is our observable universe is, is the most accurate statement for how large the observable universe is.

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Sun, 28 Jun 2009 20:27:28 GMT

Dusty_Matter:
The observable universe (according to wikipedia) consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because the signals from those objects has had time to reach us…
This contradicts your definition. The signals or light from the galaxies and other matter that is inferred to be 30+ billion light years away cannot be observed in the present day on Earth because their signals have not reached us, and they never will.

This is the meaning of the term observable that most people are referring to, and this is the definition that I am referring to.

The further away that we view an object, the older our visual information is, and the greater our discrepancy with a commoving radial distance reading. (What we see doesn’t match it’s actual location.)

It comes down to not actually seeing it’s location in space, but actually it’s location in time.

So if you want to stick to our visual distance as being 5.7 billion light years. So be it.

If others want to say that our observable universe is 30+ billion light years in distance. They are misleading, and in my opinion wrong.

Our observable universe goes to about 13+billion years into the past, and that’s how far we can see.

Thank you for such a lively discussion though Speed Freek. It was very invigorating, and I did learn a few things. I hope others did too.

It may be the definition that you are referring to, but it is not the definition used in science. You will notice that the wiki article also says "The age of the universe is about 13.7 billion years, but due to the expansion of space we are now observing objects that are now considerably farther away than a static 13.7 billion light-years distance. The edge of the observable universe is now located about 46.5 billion light-years away." In fact, the article does not contradict me at all.

You do realise that we do not use redshift alone to work out the distance at emission? We also use a test for surface-brightness which is an indicator of angular diameter. The angular diameter (how big something looks, in the sky) tells us how close it was when the light was emitted, as you would expect.

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Sun, 28 Jun 2009 19:06:57 GMT

Please excuse me for missing this. So our visible universe, only goes out to a distance of 5.7 billion light years?

What I meant was; Comoving distance is more accurate to our visual information at closer distances. For instance

Nearby galaxy - distance now 1.35 Gly But when light was emitted 1.22 Gly. A discrepancy of .13 Gly

Further galaxy - now at 28 Gly but when light was emitted 3.5Gly A discrepancy of 24.5Gly

Which reading is more closely aligned to our visual information?

The one that is closer. It has nothing to do with which is more “real”.

The observable universe (according to wikipedia) consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because the signals from those objects has had time to reach us…

This contradicts your definition. The signals or light from the galaxies and other matter that is inferred to be 30+ billion light years away cannot be observed in the present day on Earth because their signals have not reached us, and they never will.

This is the meaning of the term observable that most people are referring to, and this is the definition that I am referring to.

The further away that we view an object, the older our visual information is, and the greater our discrepancy with a commoving radial distance reading. (What we see doesn’t match it’s actual location.)

It comes down to not actually seeing it’s location in space, but actually it’s location in time.

So if you want to stick to our visual distance as being 5.7 billion light years. So be it.

If others want to say that our observable universe is 30+ billion light years in distance. They are misleading, and in my opinion wrong.

Our observable universe goes to about 13+billion years into the past, and that’s how far we can see.

Thank you for such a lively discussion though Speed Freek. It was very invigorating, and I did learn a few things. I hope others did too.

Re: Our observable universe is not 30 to 38 billion light years in distance.

SpeedFreek — Sun, 28 Jun 2009 16:14:45 GMT

Dusty_Matter:

The question remains one of observation. If it is answerable in terms of distance, then please, you tell us how big our observable universe is. How far can we observe? Not how far are the objects now. Can you give us a specific distance?

Yes I can give you a specific distance, and I have done so already. We have seen out to a distance of 5.7 billion light-years, the Hubble distance as we see it. Not where it is thought to be "now" (around 14 billion light-years), but where it was "then".

Dusty_Matter:
The commoving radial distance is good for anything nearby but becomes more difficult with distance. Again, can you give us a specific outer visual distance for our observable universe?

Why do you say comoving radial distance is good for anything "nearby"? We see everything at the distance it was when the light was emitted. Why is it any better to say a nearby galaxy with a redshift of z=0.1, whose light was emitted only 1.28 billion years ago, was originally only 1.22 billion light-years away but is now 1.35 billion light-years away? How is that any more "real" than saying a z=7 galaxy was 3.5 billion light-years away but is now 28 billion light-years distant?

Dusty_Matter:
I agree that you do need spatial dimensions to build a meaningful model, but that was not part of the question. How far can you actually see, not infer, is how I see the question, and how most people would ask it.

Dusty_Matter:
Yes, the commoving distance does come out of look back times from the red shift, but can you actually observe them as they are now in our present time?

No, of course we cannot see those galaxies where they are now. The maximum proper distance we have seen is 5.7 billion light-years, and what we see there are galaxies that were receding at the speed of light when they emitted the light we see. That light was emitted 9.1 billion years ago. That is the edge of our Hubble Sphere as we see it, and it is often referred to as the visible universe.

Our observable universe, on the other hand, is defined by the Cosmic Microwave Background Radiation, its emission distance (the CMBR photons we currently detect were originally emitted only 42 million light-years away, in all directions), and the distance that the emission coordinates would have receded if they were comoving with the expansion of the universe (46.5 billion light-years), which is known as the particle horizon.

What you are looking for, is the observed or visible universe, not the "observable" universe, which is a cosmological definition, based in general relativity. The observable universe considers the history of the space-time from which we have received information. It is only observable theoretically, using the mathematics of the Lambda-CDM concordance model. Unfortunately, a lot of mainstream media articles are not very good at explaining this properly.

Re: Our observable universe is not 30 to 38 billion light years in distance.

Dusty_Matter — Sun, 28 Jun 2009 14:59:34 GMT

The question remains one of observation. If it is answerable in terms of distance, then please, you tell us how big our observable universe is. How far can we observe? Not how far are the objects now. Can you give us a specific distance?

The commoving radial distance is good for anything nearby but becomes more difficult with distance. Again, can you give us a specific outer visual distance for our observable universe?

I agree that you do need spatial dimensions to build a meaningful model, but that was not part of the question. How far can you actually see, not infer, is how I see the question, and how most people would ask it.

Yes, the commoving distance does come out of look back times from the red shift, but can you actually observe them as they are now in our present time?

13 billion years ago. - Time

30 billion light years away - Distance

“One is just as “real” as the other.” Yes, they are, but one is observable (can be seen) and the other is not.