1

Physics, Astronomy & Cosmology / Re: So.. what precisely is Gravity?? Does anyone know?

« on: 08/01/2013 08:04:48 »

Hmmm it would appear that, in fact, no one does really 'how' gravity works.  As Phracticality says... we use the term 'field' which is nothing but a mathematical description and yet evan_au talks about fields 'vibrating' which seems to imply a real thing.
It seems 'gravity' and 'field' and 'graviton' etc are terms like 'dark energy'  - place holder words almost - which don't have any 'real' meaning at all. I guess the predictive power of a theory is virtually (or perhaps really ;-)) paramount.  I guess this is true in much of physics where terms ( in relation to particles) are used such as colour / spin / up / down etc which don't actually mean what you might assume from the normal meaning of the words. Again they are placeholder words for some property that I guess has no analogy to anything we can relate to in a real  /  meaningful way.

2

Physics, Astronomy & Cosmology / So.. what precisely is Gravity?? Does anyone know?

« on: 07/01/2013 08:00:08 »

So does anyone know just what gravity is??
Gravitons? are they a real thing? If spacetime really is curved (the marble on a sheet of rubber analogy) there isn't any need for gravitons as the marble just sits at the lowest point. 
An object orbiting another object just goes along the curved path that result in net zero 'force'.
Curvature of spacetime makes orbiting object want to move towards the object being orbited and its 'straight line' motion makes it want to move away.. they balance and it goes around - why do we need gravitons at all.?
Alternatively if spacetime isn't curved or you just cant analogise this way then how do gravitons (theoretically) work?
As I understand it if we detect gravitational waves one day it will not be gravitons we are detecting but rather we will notice effects on other particles (photons say or variations in pulsar timings etc) from which we can infer the existence of gravitational waves as opposed to a 'direct' detection.

So what would it be about gravitons that makes the earth want to move towards the sun for example?

Anyone?

3

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 20/09/2009 09:21:52 »

OK. Great - Really the first response that addresses the issue.
The summary of your answer is that it is the 'emission distance' - which, as I've discovered since grokking all the terminology, is the only answer that makes any sense.

Still it's annoying that in basically every place I've looked the way the 'horizon problem' is stated is technically insufficient (at best) or wrong..

Guess it's time to put this one to bed..
Have to think up another good question.. LOL.

4

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 01/09/2009 13:40:21 »

Yes that is indeed generally how I view the situation. 
My problem comes when the horizon problem is stated and then 'inflation' is used to fix the problem.  In the (fast getting infamous) wikipedia article the problem I'm having is the 20 billion light years. Quite simply is this the 'emission' or the 'reception' distance.  Or in 'balloon' terminology - is that the distance between the emission point (A) and the reception point (B) as would have been measured over the surface of the balloon at the instant the photon was emitted? (in which case there is a horizon problem) Or is it the distance between A and B at the instant the photon is received.  If so then I do have an issue with the horizon problem.  One must specify what this 20 billion light years is.. It is certainly not.. the distance between A at the time of emission and B at the time of reception as this is simply meaningless - bit like asking how many metres it is to yesterday.

Basically if we see the cosmic background radiation some 13+ billion light years from us (in both directions) and this is the emission distance then we have two points 16+ billion light years away only a few hundred thousand years (whenever exactly the universe became transparent to photons) after the big bang and clearly some explanation (such as inflation) is required to explain that size at that age.  On the other hand if the distance are the reception distance then, it seems to me, there is no horizon problem.

Cheers.
JS

5

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 01/09/2009 05:16:00 »

OK.  A more reasoned response but still not one which answers my specific question. 
Is the ten billion the emission distance or the reception distance.  To simply state a number (especially for distant objects) is meaningless - you might as well ask the distance to yesterday , or last year etc.  The terms 'emission distance' and 'reception distance' are defined cosmological terms that specifically cater for time and the expansion of the universe.

I agree that many things do appear to be poorly presented and this hinders true understanding (perhaps for the sake of simplifying (overly) for those who have not the desire or, ultimately, the wherewithal to understand fully).

Anyway.. to explain somewhat..

The "emission distance" is the distance (if you could freeze time or somehow measure instantaneously) that would be measured between the receiving point and the emitting point - at the cosmological time of emission of the photon. 

The 'reception distance' is the distance (if you could freeze time or somehow measure instantaneously) that would be measured between the receiving point and the emitting point - at the cosmological time of reception of the photon.

Clearly they are different (ignoring local movement - or more correctly 'peculiar' movement) by the amount the universe expands between the time of emission and the time of reception.

Perhaps you can also see that the horizon problem only really seems to be a problem if the '10 billion LY' distance is the emission distance.

Maybe I will find this in my growing pile of texts on the topic.. That said, I look forward to further discussion.

Cheers.
JS

6

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 01/09/2009 00:53:13 »

... Groan....

Not the answers to any questions I asked.

I know I shouldn't do this as I don't want to go off topic... but if your post reflects your opinion...
Why are you even here??

Looking forward to more meaningful comment.
JS

7

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 31/08/2009 13:14:34 »

OK.. having done some further research into cosmology in general I am now able to refine my question somewhat.
In the way the horizon problem is typically stated...

Here is a direct extract from wikipedia regarding the horizon problem:

"If one were to look at a galaxy ten billion light years away in one direction, say "west", and another in the opposite direction, "east", the total distance between them is twenty billion light years."

Is the stated distance of twenty billion years the 'emission distance' or the 'reception distance' - In my opinion it must be the emission distance or else the horizon problem should not be a problem. 

Please, you must technically understand the terms used to be able to make meaningful comment.

Assuming that it is the emission distance why is this never stated? Is this implied or thought to be so obvious that it does not need stating? Or too complex that it is assumed the reader would not understand anyway?

If it is the emission distance then I understand the 'horizon problem' but not the reason for it being poorly/incorrectly/incompletely worded almost everywhere.  If it is the reception distance.. then my problem with THE problem remains - LOL.

Look forward to comments.

Cheers.
JS

8

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 01/08/2009 02:16:42 »

It's all interesting stuff but my core question remains unanswered.
I know it's complicated and no example or analogy I give will exactly match someone's or anyone's or everyone's own mental image but
nevertheless my belief that the way the Horizon Problem is stated is incomplete / oversimplified / maybe wrong - remains.  The 'distances' referred to appear to be incompletely specified (sorry I'm an engineer - I can't help it).

I've ordered myself a copy of "Cosmology: The Science of the Universe" a book recommended and, from the looks of it, a degree level textbook.
This may get me to where want to be in my understanding of the "Horizon Problem" (maybe) and if not it will be an interesting read anyway.

Cheers.

9

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 31/07/2009 16:35:18 »

OK.  As I understand it.

There is no reason things should be different - in fact the issue is that areas of space so far apart that they could NOT have had causal contact are SO similar that they MUST have had causal contact!!! 

These two statements (or the two parts of that one statement) seem to contradict each other but it comes down to the words 'causal contact'.  Normally this (loosely) means areas of space where a photon (or anything travelling at the speed of light) CAN travel from the one area and eventually get to the other area.  Or... if point B is within the Hubble Sphere of Point A then they are in causal contact and information etc can travel between the two and temperatures can equalise and so on...

Now the 'Horizon Problem' says (sort of - and I have my problem here) that there are parts of the universe so far apart that they can NOT have had causal contact (the speed of light thing) and yet they are so similar in temperature that they must have been..

(let's ignore your contention that they all came from the BB so what's the problem - for now) 

Cosmology solves the contradiction above by simply saying that there must have been a time when the universe expanded at some phenomenal rate (superluminal expansion) and that's how two areas of space so far apart can be so similar in temperature.. This is the theory of inflation.

In respect of your BB comment... my understanding is that quantum fluctuations way back then - even after the expansion of the universe should result in (for areas that can [and again my problem surfaces here] not have had causal contact) larger variations in temperature than we actually see.  So.... they must have had contact and we need inflation.. yada yada.

So.. my issue (in the way the horizon problem seems to be stated) is , in some ways similar to yours, that when wikipedia (say) says words to the effect of - look east and see some object 10B LY away and look opposite to West and see some other object 10 LY away and so those object are 20B LY apart and the universe is only 13.7B LY old and therefore we need inflation.

I say.. hang on... what is the 10B LY distance they are talking about.. 'proper' distance, 'comoving' distance or what? (please refer to my earlier posts regarding these terms)  without this clarification the Horizon Problem seems, at best, to be improperly stated.  I do not believe that East and West are, in effect 'opposite' when you take the effect of time and the expansion of the universe into account.

Taking my example to the extreme.. if you could , somehow, detect particles from right back to the BB (and they would be coming from EVERY direction EXACTLY the same as the CMB) then you are detecting particles, effectively, from the same point in spacetime.... despite the fact that the appear to come from 'opposite' directions.  In effect my contention is that when we look out, from our position in spacetime, in EVERY direction there is.. then wherever you look... that point.. ultimately would end up being back at the BB and would be the SAME point.  You've got to get your head around it but it means particles / photons apparently at 'opposite' end of the universe are NOT.. they are (more and more as you go back in time) actually in the same place! And so these 'supposedly' far apart sources are NOT far apart .. and why the need for inflation?!  They aren't causally seperated ... they are in the same place...(ultimately)..

Crikey I'm rambling.. hope that made some sort of sense..

Cheers.

10

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 30/07/2009 16:23:47 »

Peculiar is not my term it's a term used to describe relative motions other than caused by the expansion of the universe itself.  Just as 'comoving' is a technical term used in cosmology

If you refer to the papers I mentioned you will understand.

Reading those papers might also shed some further light on 'redshift'
None of these ideas are my approach - they are simply what I am reading in scientific papers published in recognised places.

I do not want to understand, per se,  what the universe looks like at one particular moment.  We all, unavoidably see it through a time tube as you call it.. Although many do NOT understand this.

All I wish is a fuller explanation of the horizon problem using more scientific terms than what are typically used - nothing more and nothing less.  Again reference to the papers I mentioned might further your understanding, at the least of what exactly I'm trying to get to the bottom of.

Please don't misunderstand my comments - I'm sure you are very knowledgeable in this area but it would appear that the particular question I'm trying to get to the bottom of requires  more understanding of the precise cosmological terminology than  perhaps either of us have.

We're not really making any progress.  If you were to read the two papers I mentioned we may have ground for further discussion.

Cheers.

11

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 30/07/2009 11:37:33 »

Yes I understand and pretty much agree with all of that but...
It does nothing to address my question which is in relation the the statement of the horizon problem.

For the problem to make any sense the distance must be properly stated.  If, when stating the distance as 'x' lightyears they really mean the 'comoving' distance or some other distance then this has to be clarified for the statement to have any meaning.

I've read several papers including

"Expanding Confusion:  common misconceptions of cosmological horizons and the superluminal expansion of the universe" by Tamara M. Davis and Charles H. Lineweaver

and

Distance measures in cosmology by David W. Hogg.

Both of these are very interesting read and will perhaps clarify what my 'issue' is.  It is simply not sufficient to say a 'lightyear' when you are referring to highly redshifted objects.  Perhaps there is an implied term or descriptor (as to distance) when the horizon problem is stated - if so maybe someone who understands this could explain it.

You mention redshift but even that is not quite a simple as just 'redshift'.  Redshift really consists of two components.  These two components derive from (1)the 'peculiar' velocity of the object in question and (2)the expansion of the universe (known as the cosmological redshift).  There are many situations where knowing which you are referring to or understanding the breakdown of the two components is crucial to the issue.

Similarly, I believe, the horizon problem (as stated in wikipedia for example) is either incorrectly stated or not stated in sufficiently technical terms for it (to me) to make proper sense.

I hope I get to the bottom of this... LOL.

Cheers.
JS

12

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 30/07/2009 01:37:14 »

Thanks for the reply but these are not the source of my problem.
I believe the velocity between galaxies you are referring to is known at the 'peculiar' velocity and for my example two objects with a peculiar separation velocity of zero will work just fine.
The two objects or points I was referring to were (between themselves) certainly moving apart at greater than the speed of light but not seperating from E (us) at greater than the speed of light as the points were just inside the horizon and the light got to us for us to be able to observe it.

Anyway the summary of my problem with the 'horizon' problem remains with the fact that when it is stated (everywhere) that I could find it simply talks about 10 billion light years (or whatever distance they choose) when the term 'light year' without specifing 'proper' or 'comoving' distance only makes sense on a very local scale. 

The distance needs to be properly specified when you are working on cosmological distance and time scales.  When we see the photons arrive from some very far away point (time and space) it does not make sense to simply say 10B light years. 

In the time the photon took to get here space has expanded tremendously and that photon emitter is no longer where it was.  So the 10B refers to what? the distance between where the emitter was at time of emission of the photon and where we were at the same cosmological time?  Or is it the distance between us and where the emitter is now? at this cosmological time?  Of course the emitter may not even exist 'now' anymore but we will not know that until sufficient time as passed for photons (if any) from the emmiter 'now' reach us at some point in the future.

In summary then.. ignore peculiar velocities altogether.
When they say "10 billion light years away" what does that distance mean..?
Is it 'proper' now, 'proper' then, 'comoving' distance?

Hopefully this clears up the core of my query.. ;-)

Cheers.
JS

13

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 29/07/2009 09:41:23 »

Can anyone shed some light / more properly state or give an example of the 'horizon problem'?

Cheers.
JS

14

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 24/07/2009 07:11:01 »

So, Soul Surfer, are you able to clarify the 'horizon problem'?  Taking the wikipedia 'definition' as an example what do you believe the "10 billion light years" distance really is?  Is it the distance between where the photon started the journey and where E was at the same cosmological time?

That is, is it the proper distance from A(past) to E(past)  or is it the proper distance from A(now) (which we can't see of course) to E(now) or is it the comoving distance which , as I understand it, remains constant and can be factored up by the "scale factor" of the universe at a given time to arrive at proper distance? (somewhat simplified definition).

Thanks.

JS

15

General Science / How do we know what the exact correct time is?

« on: 23/07/2009 15:55:49 »

Well, by 'real time' I suppose you must mean at a certain precise location as it makes a difference.

Firstly if you really want to be finicky about the real time you can forget about your car clock or your time from the radio - none of these are very accurate and could be significantly incorrect at any time. 

The highest level of time keeping would be with various atomic clocks around the planet and these measure time as a function of the vibration of atoms such as cesium . 

Generally these clocks will measure a universal time (perhaps it's GMT I'm not 100% on that) from which, in theory, you should be able to get your local time if you really wanted to be super precise.  I live in Australia and I have to add 10 hours to GMT time to get my local time. 

In relation to the rest of your query there are many things that effect our measures of time such as years / months / days / hours etc as these are generally functions of the rotation of the earth on it's axis and around the sun both of which are totally local phenomena and really would have no meaning somewhere else in the universe. 

Of course the speed of rotation of the earth is changing over time as is it's speed around the sun (in accordance with well established scientific principles) and it wobbles on it's axis and so on.  These changes do add up over time and are the reason we have leap years and special rules about leap years (it's not just every 4 years), we have leap seconds also to account for these varying things.

Basically , at high levels of accuracy, all your normal measures of time Y/M/D/H etc are changing and somewhat fluid thing.  A second used to be 1/60 of a minute which was 1/60 of an hour which was 1/24 of a day and so on .. I believe a second is now defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" (from wikipedia).  What this means is that in the future a day / month / year (whatever) as per the above will have a different number of seconds than is the case now.

In short 'exact time' or 'real time' are very slippery things - your best bet is to use the time from one of the atomic clocks (many place on the web - use google)and ajust for your locality.

Hopefully this sort of clears it up for you.
Cheers.

16

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 23/07/2009 02:06:54 »

Well no I'm definitely not confusing those two things. 

The 'marble' I'm imagining is the entire universe and the circle drawn upon it is the limit of the observable universe - which would actually appear as a sphere to us - in fact the 'hubble sphere' as I now understand the terminology.

The entire marble = the entire universe at that time and my concept of the "infinite" universe (in my little mental analogy and in absence of proof which would destroy it) is that if you were able to travel around the surface of that marble then you would of course never get to any 'end' you may get back to where you started if you fully circumnavigate but there is no end.  Hence this universe is finite but unbounded - no end but not infinite.

Of course you can not actually do this as, thinking about the horizon or hubble sphere in reverse, it also represents the maximum point to which you could ever hope to travel. It is the point where the recessional speed = c and therefore (unless you can travel faster then light) it would take infinite time to get there.  In practice then - the universe does become , in a manner, infinite because most of it is too far away for us to ever reach. 

Anything on the hubble sphere would take infinity to reach and anything beyond that... well still infinity I guess!  ;-)

Cheers.
JS

17

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 22/07/2009 15:30:23 »

I'm reading a couple of papers regarding some the fundamental topics in this discussion but from what I see so far my problem with the way the 'horizon problem' is stated remains.
 
At the very least when distances are stated it should be specified what distances are being used 'proper' or 'comoving' (or other as appropriate) as without this being specified the discussion really becomes meaningless - as you no doubt are aware.  I've discovered that distance measures in cosmology is a topic all in itself (and quite interesting). I guess what it all boils down to is that the way the 'horizon problem' is stated almost everywhere is, at best, incomplete.

Cheers.
JS

18

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 21/07/2009 09:22:08 »

I don't understand what you mean but it's clear that I was not clear enough also.
Let me rephrase.

In my expanding sphere example (where the surface of that sphere is actually 3d space and from the centre out through the surface is time) take a point on the marble (E) and say that's the point that will one day expand to become where earth is now. 

On the marble you can draw a circle which represents the point beyond which light can never reach E.  This is the horizon.  We will never see anything of the marble outside that circle.  That "circle", to us, of course is seen as a spherical shell around our position.

Now take two points  A and B on the marble which are just inside the horizon but which are on opposite sides of E.  They are just enough inside the horizon that they just happen to reach E now.  What we might now think we see are two photons coming from A and B which appear to be each 13.7B (say) light years apart. 

In fact they are NOT that far apart as the photons (when the light was emitted) were very very much closer together.  As we take this example further back to where the marble is (almost) a single point.. the two photons are ,effectively, coming from the same point.

Again we have what appears to be two photons coming from opposite sides (and in a simplistic view they have) but those two 'opposite' points were so close together in the past they , now, they are effectively coming from so close to the same direction as to not really make any difference.

If you think of the light cone which represents our horizon (the cone is what the expanding circle on the marble traces out over time) then what we actually see of the universe can be plotted as a shape within that cone staring at the centre of the expanding base (which is us)and then curving asymptotically towards the walls of the cone as we work (backwards in time) towards the apex.

Again the inevitable conclusion is that the further back we look the more we are looking at a smaller and smaller universe (and ultimately a single point) and that two photons appearing come from opposite directions each 13.7B LY away is simply an incorrect assumption based on a 3d perspective of what is really a 4d universe.

Hoping this clarifies somewhat.
Cheers.
JS

19

Physics, Astronomy & Cosmology / Do we ever face the center of the universe?

« on: 21/07/2009 00:52:04 »

Justaskin, read the new topic on the Big Bang and the Horizon Problem - some of the discussion in there may help you to visualise why the 'centre' of the universe is in all directions.

Cheers.
JS

20

Physics, Astronomy & Cosmology / The Big Bang and the "Horizon Problem"

« on: 21/07/2009 00:40:17 »

OK - I agree with that totally. 

Just like the CMB you will see the start in all directions.
This may seem counter-intuitive at first as you have (effectively) a point source (and compared to the size of the universe now the CMB is also pretty close to a point source) and yet it comes from every direction.
 
Personally I visualise this by mentally mapping 3d space onto the surface of a sphere that is expanding in time.  Then think of a point of light as it makes it's way across the surface (actually 3d space) - when it reaches it's destination the distance it has traveled is larger than the initial distance due to expansion of the universe. 

Anyway cutting that ramble short - what this surely means is that all directions from the observer (us on earth say) tend toward the exact same direction as you look further away and hence backward in time. 
At very small distances you can ignore these effects and if I walk 10km one way and someone else 10km in the opposite direction we will surely be 20km (approx) apart.  However when you start looking further in distance and time, as per the quote in my previous post from wikipedia, then surely that whole statement of the 'horizon problem' is simply a nonsense. We look 10 Billion light years in one direction and then we look 10 billion light years in the other direction and the two points we see are plainly not 20 billion light years apart - as these directions are not opposite!  We are looking in almost the same direction. 

So if we look (with our gravity scope) back 13.7B years in opposite directions we are effectively looking at the exact same point (the "marble") although the two views would be looking at (not exactly but close enough for this example) opposite sides of the 'marble'(more properly points on the marble opposite each other but within our light cone).  Does this not destroy the whole basis for the 'horizon problem'?