Quick Question Thread

[guymillion]

Hello! The idea of this thread is to post quick questions that you have. The questions can be on any topic, although they usually should be science related. Anyone can contribute to this thread. Here a couple of guidelines:

1. Try to make sure every question is discussed and answered before it is buried.

2. The thread is intended for simple questions. If a longer discussion is anticipated, start a new thread.

3. If a question unexpectedly gets a good discussion going, then a new thread should be made to allow for a longer discussion and to allow for more new questions to be asked on this thread.

4. Thank the people who are giving you answers.

5. Ask lots of good questions!

Thanks to Harold14370 and Chrisgorlitz for helping with these guidelines.

[zinjanthropos]

It's been nearly a day since this thread was introduced. So far no questions which kind of makes one ask what the definition of quick applies to here, but never mind, I suppose someone needs to get this thing kick started.......

Pertaining to science, what is a simple question?

[guymillion]

Pertaining to science, what is a simple question?

Well, here's one:
Why is the sky blue?

Or:
What chemicals make up DNA?

Or:
How does the double slit experiment work?

As you can see, they can be very simple, or a little bit more complex, like the last one. Any question that could potentially be answered in a paragraph or less, really.

[zinjanthropos]

Pertaining to science, what is a simple question?

Well, here's one:
Why is the sky blue?

Or:
What chemicals make up DNA?

Or:
How does the double slit experiment work?

As you can see, they can be very simple, or a little bit more complex, like the last one. Any question that could potentially be answered in a paragraph or less, really.

Simple is relative to the enquirer. However I think most people believe every science question has a complex answer. I guess what I'm saying is that nearly, if not all, science questions do have complex answers. I mean every answer can be taken down to the quantum or up to the universal level if you want. Anyway I don't really want to piss all over this thread and I for one endorse it wholeheartedly and I hope it gains popularity.

[guymillion]

Simple is relative to the enquirer. However I think most people believe every science question has a complex answer. I guess what I'm saying is that nearly, if not all, science questions do have complex answers. I mean every answer can be taken down to the quantum or up to the universal level if you want.

Good point, but when I ask you whether it is going to be windy, you don't give me a lecture on the hairy ball theorem.

Anyway I don't really want to piss all over this thread and I for one endorse it wholeheartedly and I hope it gains popularity.

Thanks!

[Markus Hanke]

Why is the sky blue?

Or:
What chemicals make up DNA?

Or:
How does the double slit experiment work?

As you can see, they can be very simple, or a little bit more complex, like the last one. Any question that could potentially be answered in a paragraph or less, really.

1. Rayleigh scattering. Short wavelength get scattered differently from longer ones in our atmosphere, thus our sky appears blue.
2. Polymers, sugars, phosphates and esters. Quite a complex structure.
3. By exposing the wave-like property of photons, thus creating interference patterns at the detector.

[zinjanthropos]

Here's one that I have wondered about for years.....If all the particles in the universe are in motion and have been so since the BB then relative to one another, does the possibility exist that no two particles are of the same age?

[Markus Hanke]

If all the particles in the universe are in motion and have been so since the BB then relative to one another, does the possibility exist that no two particles are of the same age?

I don't understand what you mean by this. Can you elaborate a bit further ?
If all particles originate at the BB ( which, btw, is not the case ), then they must of necessity be of same age, unless I am missing something in your question.

[MeteorWayne]

Here's one that I have wondered about for years.....If all the particles in the universe are in motion and have been so since the BB then relative to one another, does the possibility exist that no two particles are of the same age?

Your font is unreadably small to me.

[zinjanthropos]

If all the particles in the universe are in motion and have been so since the BB then relative to one another, does the possibility exist that no two particles are of the same age?

I don't understand what you mean by this. Can you elaborate a bit further ?
If all particles originate at the BB ( which, btw, is not the case ), then they must of necessity be of same age, unless I am missing something in your question.

I'm thinking time dilation, same as an astronaut returning to Earth slightly younger than those he/she had left behind.

[Markus Hanke]

I'm thinking time dilation, same as an astronaut returning to Earth slightly younger than those he/she had left behind.

I think I get it now. If by 'age' you mean the reading of a hypothetical clock which moves in the same frame as the particle, then yes, not all of these clocks will agree because some particles will accelerate and decelerate during their lifetimes.
However, if one knew their speeds and acceleration histories, one could then calculate backwards, and would arrive at the same clock reading for all particles at some point in the distant past - that's the BB.

[zinjanthropos]

Thanks Markus. One other thing, you intimated earlier that not all particles originated at the BB. Were you referring to virtual particles or what?

[Markus Hanke]

Thanks Markus. One other thing, you intimated earlier that not all particles originated at the BB. Were you referring to virtual particles or what?

No, it's simply that a given particle might be the result of reactions ( or decay ) between other particles, i.e. they came into existence long after the BB.

[guymillion]

I have a question. How can zero-dimensional point particles cause the third dimensional macroscopic world?

[Markus Hanke]

I have a question. How can zero-dimensional point particles cause the third dimensional macroscopic world?

They don't, they only exist as entities within our universe.

[zinjanthropos]

Is it possible that Curiosity or previous Martian rovers actually brought microbic life to the planet?

[MeteorWayne]

It's certainly possible. They do everything they can to prevent that, sterilizing the craft, but is it perfect???

[Strange]

I have a question. How can zero-dimensional point particles cause the third dimensional macroscopic world?

Do you mean, how can these point particles form matter with volume? The reason is the the space and forces between them. The reason that a table feels solid is not because you are touching the electrons in the atoms but because the electrostatic fields of the molecules in the table are interacting with those in your skin.

[guymillion]

Do you mean, how can these point particles form matter with volume? The reason is the the space and forces between them. The reason that a table feels solid is not because you are touching the electrons in the atoms but because the electrostatic fields of the molecules in the table are interacting with those in your skin.

Thanks Strange, that's cool.

[KALSTER]

I moved my stuff here: http://www.thescienceforum.com/new-h...-universe.html

Thanks

[Lynx_Fox]

I chuckled when I read this thread because I know very few scientist who can give a quick answer to anything.

[epidecus]

Prove that the product of any two integers is always an integer. The proof alone explains itself so that's all I need. Thanks.

[guymillion]

I don't know whether you mind that it is wikipedia, but here it is:

Integer - Wikipedia, the free encyclopedia
Closure (mathematics) - Wikipedia, the free encyclopedia

I don't know how to explain this as a proof mathematically. Sorry. But, I hope it answers your question!

[John Galt]

I chuckled when I read this thread because I know very few scientist who can give a quick answer to anything.

Me.

[arKane]

I chuckled when I read this thread because I know very few scientist who can give a quick answer to anything.

Me.

I'm sure you can always give a short answer, but is it going to always be the right answer?

[John Galt]

Yes.

[arKane]

Yes.

But that answer is only a biased opinion in your favor and may not be correct from another point of view.

[Raziell]

I read a theory that black holes may create new universes. http://www.insidescience.org/?q=content/every-black-hole-contains-new-universe/566

Hypothetical question: If this is the case, how would that affect one black hole swallowing another?

[arKane]

I read a theory that black holes may create new universes. http://www.insidescience.org/?q=content/every-black-hole-contains-new-universe/566

Hypothetical question: If this is the case, how would that affect one black hole swallowing another?

When galaxies collide it results in a larger galaxy and is usually characterized as the larger eating the smaller one. When BH's merge a larger BH is the result. But it seems a bit premature to speculate on that kind of detail on the other side of an event horizon.

[Strange]

I read a theory that black holes may create new universes. http://www.insidescience.org/?q=content/every-black-hole-contains-new-universe/566

Hypothetical question: If this is the case, how would that affect one black hole swallowing another?

It would require someone with more knowledge of GR than me to really answer this, but my understanding is that in this scenario, the "new" universe is separated from the black hole that formed it by an event horizon (in the same way the we are separated from the inside of the black hole - it is effectively no longer part of our universe). Therefore anything that happens to the black hole later will have no effect on it.

On the other hand, things like "later" don't always have the expected meaning when it comes to black holes...

[Markus Hanke]

I read a theory that black holes may create new universes. http://www.insidescience.org/?q=content/every-black-hole-contains-new-universe/566

Hypothetical question: If this is the case, how would that affect one black hole swallowing another?

I don't think it would have any effect at all for an outside observer, simply because whatever happens beyond the event horizon is not causally connected to the rest of our universe.
Bear in mind though that this "new universe" model is purely hypothetical.

[zinjanthropos]

Yes.

But that answer is only a biased opinion in your favor and may not be correct from another point of view.

Hey Arkman.....Yes is an acceptable reply when a quick & short answer is required. I thought it was witty and funny.

[Raziell]

I have some more quick questions regarding the eyes.

Why do the eyes need time to adapt to light changes?
Why isnt the effect immediate and painless?
Is the pain/slow adaption beneficial in any way or just a "flaw"?
What exactly causes the stinging pain when going from pitch black after sleeping to bright daylight?
Can it cause damage to the eyes?

[John Galt]

As you move from light to dark the following comes in to play: "In biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as they absorb light. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. Exposed to a spectrum of light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully, but most of the adaptation occurs within the first five or ten minutes in the dark." (Source: Night vision - Wikipedia, the free encyclopedia)

As you move from dark to bright light you overload the photoreceptors before your iris has had time to close and reduce the amount of light admitted. They then take a short time to recover.

I'm not aware of any pain associated with either of these processes. Others will need to comment on that.

[sigurdW]

As you move from light to dark the following comes in to play: "In biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as they absorb light. Rhodopsin is the chemical that allows night-vision, and is extremely sensitive to light. Exposed to a spectrum of light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully, but most of the adaptation occurs within the first five or ten minutes in the dark." (Source: Night vision - Wikipedia, the free encyclopedia)

As you move from dark to bright light you overload the photoreceptors before your iris has had time to close and reduce the amount of light admitted. They then take a short time to recover.

I'm not aware of any pain associated with either of these processes. Others will need to comment on that.

So thats why I use only one eye at the time!

[Strange]

I'm not aware of any pain associated with either of these processes.

Me neither.

Raziell, do you sneeze when exposed to light? (Photic sneeze reflex - Wikipedia, the free encyclopedia) And when did you last have your eyes checked?

[guymillion]

How can spacetime be quantized like in LQG and be relative at the same time?

[Raziell]

I'm not aware of any pain associated with either of these processes.

Me neither.

Raziell, do you sneeze when exposed to light? (Photic sneeze reflex - Wikipedia, the free encyclopedia) And when did you last have your eyes checked?

Nope. Dont think thats something I have.

But what do you mean with "I'm not aware of any pain associated with either of these processes."

If youve been up all night in a dark room, and then exit the front door and its mid-day and sunny - everyone gets that stinging pain in the eyes which makes it almost impossible to open them for a minute? (Well can physically open them but DAMN it hurts)

[Markus Hanke]

How can spacetime be quantized like in LQG and be relative at the same time?

Quantization of space-time only happens at very high energies; at such domains Lorentz invariance may no longer be valid or required.

[guymillion]

How can spacetime be quantized like in LQG and be relative at the same time?

Quantization of space-time only happens at very high energies; at such domains Lorentz invariance may no longer be valid or required.

Thanks!

[Martinaston]

Why does angular momentum occur ?

[guymillion]

Why does angular momentum occur ?

Here is a simple video describing angular momentum and gyroscopic precession:

Gyroscopic Precession - YouTube

[Martinaston]

Gyroscopic precession is a product of angular momentum, but why does it happen ?

[KALSTER]

Angular momentum is a consequence of inertia, which is a consequence of mass, which is mediated by the Higgs boson.

[Martinaston]

Mediated ?

[KALSTER]

Or imparted by the Higgs field. Higgs boson - Wikipedia, the free encyclopedia

[Harold14370]

There is a fairly straightforward explanation of precession here: HowStuffWorks "How Gyroscopes Work"

Let's say you are sitting on a swivel chair, facing north, holding a spinning bicycle wheel by the axle. The top part of the wheel is moving north, the bottom south. You apply a torque by turning the axle counterclockwise, as if steering a vehicle left. The force you apply will try to make the top part of the wheel change direction toward the left (westward) and the bottom part of the wheel to the right (east). After your chair precesses by swiveling to the left, the top part of the wheel ends up going more to the west and the bottom to the east, just as you would expect.

[Martinaston]

Again, both, products of angular momentum. Regardless of mass.But why does it happen?

[Markus Hanke]

Again, both, products of angular momentum. Regardless of mass.But why does it happen?

Because of inertia, since, to get angular momentum, you need to perform an acceleration, i.e. add energy to the system.

[Martinaston]

Ok, so you add energy, you get acceleration, that gives you inertia.But why do you get angular momentum?
http://<a href="http://en.wikipedia....mation.gif</a>

[Markus Hanke]

Ok, so you add energy, you get acceleration, that gives you inertia.But why do you get angular momentum?File:Torque animation.gif - Wikipedia, the free encyclopedia

Because the acceleration in this case is perpendicular to the direction of motion, thus there is a change of trajectory, thus angular momentum. It's basic Newtonian mechanics.
Where exactly are you going with this ?

[arKane]

Again, both, products of angular momentum. Regardless of mass.But why does it happen?

Let me try and use an example of what you are asking. When gas is coming together under gravity to form a new star, why does it spin?

I'd like to know that answer too.

[Strange]

Let me try and use an example of what you are asking. When gas is coming together under gravity to form a new star, why does it spin?

Good question (it isn't clear if that is what the original question was about; I thought it might be about conservation).

The only way there could be no rotation as the gas falls is if it is all falling perfectly radially to the centre of mass (or, equivalently, that the distribution of velocities were perfectly equally distributed) and there were no external influences at all. Both of these are extremely unlikely (basically impossible, from what we know of chaos theory) and so there is always going to be some net lateral velocity in some direction. As the cloud collapses, this rotation will get faster due to conservation of angular momentum.

[Strange]

Ok, so you add energy, you get acceleration, that gives you inertia.But why do you get angular momentum?File:Torque animation.gif - Wikipedia, the free encyclopedia

I'm not quite sure what you are asking, but surely the fact that there is also a force towards the centre causes the sideways force to become angular momentum?

[Martinaston]

Because the acceleration in this case is perpendicular to the direction of motion, thus there is a change of trajectory, thus angular momentum. It's basic Newtonian mechanics.

Newton basicaly states A+B=C, how does the trajectory know its changed ?

Where exactly are you going with this ?

Everywhere

[Harold14370]

Because the acceleration in this case is perpendicular to the direction of motion, thus there is a change of trajectory, thus angular momentum. It's basic Newtonian mechanics.

Newton basicaly states A+B=C, how does the trajectory know its changed ?

Where exactly are you going with this ?

Everywhere

The quick answer to the quick question has been given. If further explanation is still needed, please start a new thread.

[guymillion]

Is the Theory of Everything more likely to have one "ultimate" equation, or made up of a series of equations, each describing a different thing? For example, in order to understand quantum mechanics, you need to know a lot of different equations, not just one "super" equation. However, since it is a theory of everything, would it be different in this case?

[sigurdW]

Is the Theory of Everything more likely to be one "ultimate" equation, or made up of a series of equations, each describing a different thing? For example, in order to understand quantum mechanics, you need to know a lot of different equations, not just one "super" equation. However, since it is a theory of everything, would it be different in this case?

Did I miss the theory of everything in this thread?
Its not possible, there IS no everything that CAN be described by a theory.

[Martinaston]

Everything = Everything.

Oh wait, don't they cancel each other out ?

[sigurdW]

Everything = Everything.

Oh wait, don't they cancel each other out ?

The semaniacs of the Ancient concepts of
"Everything" and "Nothing" interests me...go on!

[Martinaston]

Would you want a theory of everything ?
If you had it, would you publish it ?

[guymillion]

Everything = Everything.

Oh wait, don't they cancel each other out ?

The semaniacs of the Ancient concepts of
"Everything" and "Nothing" interests me...go on!

Not looking to start a long philosophical discussion here, guys. Just wondering whether or not it is possible to describe the entire universe in one equation. (It sounds a lot more complicated when you put it like that, doesn't it?)

[Martinaston]

Yes

[sigurdW]

Everything = Everything.

Oh wait, don't they cancel each other out ?

The semaniacs of the Ancient concepts of
"Everything" and "Nothing" interests me...go on!

Not looking to start a long philosophical discussion here, guys. Just wondering whether or not it is possible to describe the entire universe in one equation. (It sounds a lot more complicated when you put it like that, doesn't it?)

1 Is the future included? then no!
2 Is the present included? then again no!
3 Is the past included? then maybe!
Since the past limits the present.

But the past changes very quickly
so the equation, or at least its inputs,
grows outmoded equally quickly.
To me the answer seems to be: NO!

[guymillion]

Everything = Everything.

Oh wait, don't they cancel each other out ?

The semaniacs of the Ancient concepts of
"Everything" and "Nothing" interests me...go on!

Not looking to start a long philosophical discussion here, guys. Just wondering whether or not it is possible to describe the entire universe in one equation. (It sounds a lot more complicated when you put it like that, doesn't it?)

1 Is the future included? then no!
2 Is the present included? then again no!
3 Is the past included? then maybe!
Since the past limits the present.

But the past changes very quickly
so the equation, or at least its inputs,
grows outmoded equally quickly.
To me the answer seems to be: NO!

Nevermind, I don't want to take up any more space on this thread, I'll make a separate post.

[Ascended]

Why doesn't space expand inside things held together by gravity like solar systems? or are the things we think of, like planets and stars, being 'in' space actually sitting on top on the space like a surface?

[Strange]

Why doesn't space expand inside things held together by gravity like solar systems? or are the things we think of, like planets and stars, being 'in' space actually sitting on top on the space like a surface?

Two answers.

The simple one is simply the fact they are held together by gravity. I don't think it is right to consider the expansion of space as a "force" which pushes things apart. But in the absence of forces holding them together, the expansion of space will carry them along. But the behaviour can be counter-intuitive.

The other answer (which is pretty much equivalent) is that the solution of Einstein's equations which describes expanding space is only accurate for a homogeneous distribution of mass. This is only true on the very large scale. Locally, it is very unhomogeneous and so the expansion does not occur.

I learnt a lot from this paper: [0707.0380] Expanding Space: the Root of all Evil?
Some of if it, at least, is understandable without following all the maths (I certainly don't!).

[Ascended]

Cheers Strange, that's an interesting paper, gave me an idea for a second that if light being carried by expansion could arrive at a destination point at a super velocity it might be possible to veiw the future, that is until it dawned on me no matter how fast light travels it's still not possible to see light that hasn't left yet! I think I watch too many sci fi movies ,gives me odd ideas til I think them through at least.

[zinjanthropos]

If I have two identical hemispheres together forming a sphere and I pull them apart so that each half is moving in opposite directions at 99.9% of the speed of light then how fast is the space between them opening up?

[SpeedFreek]

If I have two identical hemispheres together forming a sphere and I pull them apart so that each half is moving in opposite directions at 99.9% of the speed of light then how fast is the space between them opening up?

From whose frame of reference?

For a distant observer, each hemisphere is moving at 99.9% of c, and as they are moving away from each other the space between them increases at 199.8% of c. There is nothing wrong with this, as neither object is moving faster than c relative to the observer.

But, if you observe the situation from the frame of reference of one of hemispheres, things are different - the other hemisphere is not moving away at 199.8% of c, as nothing can move faster than light.

We need to use the relativistic velocity addition formula:



where c=1 and the velocities of the hemispheres are represented by v and u.







So each hemisphere calculates the other to be receding at 99.99994995% of c.

[arKane]

If I have two identical hemispheres together forming a sphere and I pull them apart so that each half is moving in opposite directions at 99.9% of the speed of light then how fast is the space between them opening up?

It would seem to be almost twice the speed of light.

[KALSTER]

If I have two identical hemispheres together forming a sphere and I pull them apart so that each half is moving in opposite directions at 99.9% of the speed of light then how fast is the space between them opening up?

It would seem to be almost twice the speed of light.

Nope, check SpeedFreek's post.

[arKane]

If I have two identical hemispheres together forming a sphere and I pull them apart so that each half is moving in opposite directions at 99.9% of the speed of light then how fast is the space between them opening up?

It would seem to be almost twice the speed of light.

Nope, check SpeedFreek's post.

Yes that was a good post, but I was reading zinjanthropos post to mean the observer remained at the point of separation and 199.8 percent of c, is pretty close to twice the speed of light. However, I might wonder if an observer on one half of the sphere would even see any light coming from the other half because of how fast the space is opening up between the two halves.

[KALSTER]

The problem with your answer is that nothing can travel faster than C, even under those circumstances. That is where time dilation and length contraction comes in.

[arKane]

The problem with your answer is that nothing can travel faster than C, even under those circumstances. That is where time dilation and length contraction comes in.

But from the observer at seperation point, the half spheres are not in violation of exceeding the speed of light. But one of the things I keep hearing about our expanding universe is that space can and is expanding faster than the speed of light. Excuse me if I'm wrong here, but I don't see any difference between the expanding universe and the space between two object increasing faster than the speed of light.

[zinjanthropos]

Geez, I always end up having to clarify but that's ok, just shows my lack of understanding of things like this. I chose the sphere because I could put myself inside it, dead centre, and when the hemispheres separate I stay in position while they move away from me in opposite directions. I'm not going for a ride but staying put. Does that help or change things?

I do read up on this stuff now and again. Usually I'll read something that will baffle me and I stop to think about it. Of course I am referring to space expanding faster than c here. I was trying to think of a scenario where it would appear to me that space was expanding faster than c. This is what I came up with and so I figured I'd ask. Didn't have to be two hemispheres, could have been two sheets of whatever.

Even still, I know it's not perfect because what are the two halves in? I mean I can only measure the space between the two halves, not what's above or below.

[KALSTER]

The distinction is that with movement the object moves through space, while with expansion the object moves with space. If you were sitting on one of the spheres and it was moving away through space, you would feel an acceleration from 0% c to 99.9% c, while if the half sphere was moving away from the centre with space, you'd feel no acceleration.

If the spheres were moving away from each other because the space was expanding, then you would measure each half as moving away from you at 99.9% of c initially, with the light being severely red shifted. Pretty soon though, the recession speed will exceed c, after which you will no longer be able to see them. So while the space between the halves are expanding at a constant rate, the recession speed will accelerate due to the compounding effect of more space being added per second than the previous second.

Edit: Please disregard my comments regarding acceleration of expansion, as I have been misunderstanding that bit this whole time.

[zinjanthropos]

The distinction is that with movement the object moves through space, while with expansion the object moves with space. If you were sitting on one of the spheres and it was moving away through space, you would feel an acceleration from 0% c to 99.9% c, while if the half sphere was moving away from the centre with space, you'd feel no acceleration.

If the spheres were moving away from each other because the space was expanding, then you would measure each half as moving away from you at 99.9% of c initially, with the light being severely red shifted. Pretty soon though, the recession speed will exceed c, after which you will no longer be able to see them. So while the space between the halves are expanding at a constant rate, the recession speed will accelerate due to the compounding effect of more space being added per second than the previous second.

So if in the scenario I presented where I stay put and the halves move away from me in whatever medium they're in then the space between the two objects increases by a rate greater than c? This has nothing to do with the expansion of space, correct?

[KALSTER]

Let's say two rockets come flying past you from opposite directions, each travelling at 99.9% c relative to you, then from where ever you measure it from, they will never move away from each other at c or above, as SpeedFreek worked out. This is them moving through space.

If however the space between them starts to expand at a huge rate, where at a given t=0 they recede from each other at 99.9% c, then they would not be able to see each other, while you would be able to see both from your centre point. After another second though, you will lose both of them as well.

So, expanding space is not the same as our simple day to day movement through space. The moment the apparent receding speed reaches c, light from that object will never reach you.

But one of the things I keep hearing about our expanding universe is that space can and is expanding faster than the speed of light.

To be more precise, space does not expand faster than light. It expands by a certain percentage per unit time. It is the receding speeds of far away objects that can reach and exceed the speed of light. The further away they are, the faster the receding speed.

Edit: Please disregard my comments regarding acceleration of expansion, as I have been misunderstanding that bit this whole time.

[Ascended]

Could the inertial fields within quantum mechanics that define wave functions describing physical phenomenon help us show that all physical motion can ultimately be reduced to rotation?

If so would this then help us to use Einstein's metric torsion tensor to show that spin fields twisting spacetime can point us in the direction towards a grand unifying theory?

[zinjanthropos]

Let's say two rockets come flying past you from opposite directions, each travelling at 99.9% c relative to you, then from where ever you measure it from, they will never move away from each other at c or above, as SpeedFreek worked out. This is them moving through space.

Yes but the actual space between the two moving objects is still increasing at near 2c, correct?

If however the space between them starts to expand at a huge rate, where at a given t=0 they recede from each other at 99.9% c, then they would not be able to see each other, while you would be able to see both from your centre point. After another second though, you will lose both of them as well

Kalster, hope I'm not boring you with this stuff. Would it be something like this: Two worms set out to burrow at a fixed rate from opposite sides of a pit within a fruit to the outer skin. As they burrow the fruit thickens. If an observer were to watch this from the starting point of the worms' journey then he would see them moving away from each other at a rate greater than what they started off at. So if the rate was near c then eventually they would be separating at a rate greater than c. Is this a fair analogy?

[SpeedFreek]

Wow. Firstly, I had not realised the scenario was supposed to represent the expansion of the universe, and secondly, KALSTER's recent replies are actually describing accelerating expansion, rather than constant expansion.

Firstly, we cannot use the relativistic velocity addition formula for this.

Secondly, when cosmologists refer to constant expansion (which is the state between an accelerating and a decelerating universe), it means that objects recede at a constant speed (which is actually how zinjanthropos described the scenario), NOT that the hubble parameter remains constant. This is important, because it means that we can see galaxies who have always had recession speeds faster than light.

Even as the expansion of the universe accelerates, the Hubble parameter is still decreasing. Confusing eh?

All the Hubble constant represents is the average of the rate at which the universe has expanded over the age of the universe. It represents how much the universe would have had to expanded, over the past 13.7 billion years, for it to reach the size it is today. It would have had to have expanded at an average of around 70km/s/Mpc to do so.

The thing to remember is that when cosmologists talk of constant expansion, it means that objects recede at a constant speed, and that speed increases with distance. The further away something is, the faster it recedes, and it continues to recede at that speed. More distant objects will recede at a faster but constant speed and closer objects will recede with a slower but constant speed.

Objects only accelerate away from us in an accelerating universe (in a universe with no cosmological constant, only gravity, what is there that would make an object accelerate away from us?), and only in an accelerating universe is there a cosmic event horizon which objects pass beyond and become invisible. That horizon is not where the object recedes at the speed of light (known as the Hubble distance), it is the distance beyond that where the light from a distant event cannot make it to our Hubble distance, and it is known as the cosmological event horizon, or the light horizon.

We can see galaxies that are, and always have been, receding faster than light, but due to the acceleration of the rate of expansion there is a limit to how far past the Hubble distance we can see an event from.

The Hubble constant only actually remains constant during certain extreme forms of exponential expansion, like for instance the inflationary epoch or the ultimate fate of an accelerating universe.

I am not denying the "expansion of space" can make objects accelerate away from us, I am only denying that constant expansion (the state between deceleration and acceleration) does so.

Not such a quick question, this one.

[arKane]

Let's say two rockets come flying past you from opposite directions, each travelling at 99.9% c relative to you, then from where ever you measure it from, they will never move away from each other at c or above, as SpeedFreek worked out. This is them moving through space.

If however the space between them starts to expand at a huge rate, where at a given t=0 they recede from each other at 99.9% c, then they would not be able to see each other, while you would be able to see both from your centre point. After another second though, you will lose both of them as well.

So, expanding space is not the same as our simple day to day movement through space. The moment the apparent receding speed reaches c, light from that object will never reach you.

To be more precise, space does not expand faster than light. It expands by a certain percentage per unit time. It is the receding speeds of far away objects that can reach and exceed the speed of light. The further away they are, the faster the receding speed.

So if you were that observer, regardless of how those two objects were being moved away from eachother, would you be able to tell the difference?

[KALSTER]

Wow. Firstly, I had not realised the scenario was supposed to represent the expansion of the universe, and secondly, KALSTER's recent replies are actually describing accelerating expansion, rather than constant expansion.

Good lord, I never realised. As I was writing that, I did start to wonder if this is starting to sound different to what I have been hearing regarding acceleration vs constant. This brings up the question, what would balance the Hubble parameter out in such a way that it reduces over time by the precise degree to keep receding speeds constant? I think I'll read up a bit!

With that, I'd rather hand over to the more capable hands of SpeedFreek or Markus. I might separate out this question, because it is getting a bit long for this thread.

[zinjanthropos]

Not such a quick question, this one.

Expansion is nothing like putting air into my tires.

Thanks for your replies. I have to read them more than once to get the gist but its worth it in the end. I thought I was getting my head around Kalster's words and now this but it's all good. I don't think I've pissed anybody off yet, at least I hope not.

[SpeedFreek]

This brings up the question, what would balance the Hubble parameter out in such a way that it reduces over time by the precise degree to keep receding speeds constant? I think I'll read up a bit!

Without a cosmological constant, in a universe with any stuff in it the expansion would never be constant, but if the universe had a certain density (the term used for this in cosmology is omega), it might tend towards a constant expansion as the age of the universe approaches infinity. With any mass in the universe, gravity will cause deceleration. The only question is how much? Will it decelerate towards a constant rate but never quite get there, or decelerate towards a halt but never quite get there, or decelerate to a halt and start to contract? That was the main question before we discovered the expansion, rather than continuing to decelerate, had started to accelerate.

The speeds at which galaxies had been receding from each other had been decreasing for billions of years, but around 5 or 6 billion years ago it started to increase.

[SpeedFreek]

Not such a quick question, this one.

Expansion is nothing like putting air into my tires.

Quite. Space doesn't "expand". That's more of a description of what dark energy does, if it acts like a cosmological constant.

You might like this paper:
[0707.0380] Expanding Space: the Root of all Evil?

In empty space, what is there that can expand? (Nothing)

In a universe without a cosmological constant, why would galaxies accelerate away from each other? (They wouldn't)

The recent "expansion of space" pedagogy is useful in that it helps describe the accelerating expansion of the universe via a cosmological constant. In our universe, galaxies are accelerating away from each other. But it can cause confusion, for sure!

[zinjanthropos]

I might separate out this question, because it is getting a bit long for this thread.

Option #3 of Post #1

[KALSTER]

Yep. I'll get on it in the morning though. Sleepy time!

[arKane]

You might like this paper:
[0707.0380] Expanding Space: the Root of all Evil?

In empty space, what is there that can expand? (Nothing)

In a universe without a cosmological constant, why would galaxies accelerate away from each other? (They wouldn't)

The recent "expansion of space" pedagogy is useful in that it helps describe the accelerating expansion of the universe via a cosmological constant. In our universe, galaxies are accelerating away from each other. But it can cause confusion, for sure!

Thanks for the reading material. It helps a little, I think.

[zinjanthropos]

I might as well make this my own personal questions thread.....I have another:

If I hold a mirror in front of another mirror to where I can gaze upon the face of one of them, the repeated reflected image gets decidedly smaller and smaller. My question is: am I gazing upon an infinite number of reflections or is there a limit as to how small a reflected image can get?

[arKane]

I might as well make this my own personal questions thread.....I have another:

If I hold a mirror in front of another mirror to where I can gaze upon the face of one of them, the repeated reflected image gets decidedly smaller and smaller. My question is: am I gazing upon an infinite number of reflections or is there a limit as to how small a reflected image can get?

I scrounged up the following answer.

Photons travel at finite speed, so it would take an infinite amount of time to form the infinite number of reflections.

[zinjanthropos]

My next question:

If the average human being weighs 150 lbs and they are composed of 66% water, then on average each human is about 100 lbs of water. A gallon of water is approximately 10 lbs so that would equate to about 60 billion gallons worldwide contained in humans. If we take into account the water contained in every life form including all vegetation, then how much water is actually tied up in living things? If all life were to disappear tomorrow would there be enough water to raise the sea levels across the globe and by how much?

[guymillion]

If no one is answering zinjanthropos's question, I have one of my own. The electron is held to the atom through the electromagnetic force. The electromagnetic force follows the inverse square law. So, the inverse square law should determine "how strong" the electron is held to any given nucleus, right?

[arKane]

My next question:

If the average human being weighs 150 lbs and they are composed of 66% water, then on average each human is about 100 lbs of water. A gallon of water is approximately 10 lbs so that would equate to about 60 billion gallons worldwide contained in humans. If we take into account the water contained in every life form including all vegetation, then how much water is actually tied up in living things? If all life were to disappear tomorrow would there be enough water to raise the sea levels across the globe and by how much?

Without doing the math, I'm just going to say from the point of view of an off world observer. They probably wouldn't notice any rise in the ocean level.

[arKane]

If no one is answering zinjanthropos's question, I have one of my own. The electron is held to the atom through the electromagnetic force. The electromagnetic force follows the inverse square law. So, the inverse square law should determine "how strong" the electron is held to any given nucleus, right?

I might be wrong here, but I'm going to say no. It might be right for a hydrogen atom, but I'm sure when you start working with heavier atoms with more electrons in shells around many protons and neutrons in the nucleus, the interactions can get quite complex to a point where the inverse square law by itself won't work for you.

[Ascended]

My next question:

If the average human being weighs 150 lbs and they are composed of 66% water, then on average each human is about 100 lbs of water. A gallon of water is approximately 10 lbs so that would equate to about 60 billion gallons worldwide contained in humans. If we take into account the water contained in every life form including all vegetation, then how much water is actually tied up in living things? If all life were to disappear tomorrow would there be enough water to raise the sea levels across the globe and by how much?

I think 60 billion gallons world probarbly raise the worlds oceans by about 7mm, no idea how you'd work out how much water is stored in all lifeforms or vegetation though.

[guymillion]

I have one more question.

I was wondering why there is more than one type of subatomic particle. (Not exactly a quick question, but short enough I thought I should post it here.)

In string theory, different particles exist due to the frequency that their string vibrates at. Does the standard model have any explanation?

[Markus Hanke]

In string theory, different particles exist due to the frequency that their string vibrates at. Does the standard model have any explanation?

No - and that was one of the motivating forces to develop String theory in the first place.

[Ascended]

Is there any theoretical limit on just how large a size the universe can expand to?