Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: barneyboy on 03/11/2013 21:20:10
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Apart from the red shift, what other evidence is there for saying the universe is expanding at an ever increasing rate please as you could get the red shift from a shrinking universe as well. please keep the answers fairly simplistic as im no prof ;D
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Apart from the red shift, what other evidence is there for saying the universe is expanding at an ever increasing rate please as you could get the red shift from a shrinking universe as well. please keep the answers fairly simplistic as im no prof ;D
The only evidence that the universe is expanding comes from cosmological redshift
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A century before Hubble, Olbers' Paradox (http://en.wikipedia.org/wiki/Olbers%27_paradox)* was an early indicator that something was wrong with a steady-state model of the universe. The most widely-accepted explanation of this paradox also refers to redshift.
*Why doesn't the sky glow with a uniform intensity of the surface of the Sun?
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In formulating his theory of relativity, Einstein realised that the steady-state universe was not a stable solution to his equations.
The universe could be contracting, or expanding. Without a carefully tuned cosmological constant (http://en.wikipedia.org/wiki/Cosmological_constant), a steady-state universe would be at best a passing phase in the life of the universe.
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Apart from the red shift, what other evidence is there for saying the universe is expanding at an ever increasing rate please as you could get the red shift from a shrinking universe as well. please keep the answers fairly simplistic as im no prof ;D
A shrinking universe would produce an increase in the frequency whereas the change observed is a decrease, showing that whatever is looked at is moving away from us. This happens in very direction where we look.
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Apart from the red shift, what other evidence is there for saying the universe is expanding at an ever increasing rate please as you could get the red shift from a shrinking universe as well. please keep the answers fairly simplistic as im no prof ;D
A shrinking universe would produce an increase in the frequency whereas the change observed is a decrease, showing that whatever is looked at is moving away from us. This happens in very direction where we look.
There is a difference in the fact that things moving away are redshifted and things moving toward us are blue shifted. Doppler shift is a different phenomena than cosmological redshift. The former means that objects emitting light are moving away from us where as the later means that the distance between us and the object moving away from use is increasing because the size of the universe is increasing.
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The former means that objects emitting light are moving away from us where as the later means that the distance between us and the object moving away from use is increasing because the size of the universe is increasing.
If I understand you correctly then, you are saying that the Metric, "space-time fabric" is expanding and the expansion is not only the result of changing distance between material objects within the universe???
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The former means that objects emitting light are moving away from us where as the later means that the distance between us and the object moving away from use is increasing because the size of the universe is increasing.
If I understand you correctly then, you are saying that the Metric, "space-time fabric" is expanding and the expansion is not only the result of changing distance between material objects within the universe???
Yes. To be more precise - The distance between galaxies is increasing because space is being created between them continuosly.
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The distance between galaxies is increasing because space is being created between them continuously.
Is space (spacetime?) being created or is it "stretching"?
Is there a difference?
If so, how would we know which was happening?
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The distance between galaxies is increasing because space is being created between them continuously.
Is space (spacetime?) being created or is it "stretching"?
Is there a difference?
If so, how would we know which was happening?
Good question Bill. I'm not sure we can know at this juncture with our present technology. The difference between "stretching and the creation of new space" may be difficult to define anyway. What do we mean when we speculate that space-time has been stretched? Is there really any difference between that notion and the creation of new space? Very curious indeed!
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After thinking a bit about your question Bill, it appears there may be a difference after all.
Adding new space would probably not effect material objects present before the addition of the new space. However, if we consider the consequences of stretching the Metric, we discover a flaw with that reasoning. This is true because; If every dimension is strecthed, and BTW, that would include the space within all particles too, every dimension would appear to remain the same to us, the observer. We would never see any change. Because we do observe the expansion, it would be safe to say that it is only an addition of new space-time that is occurring.
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Is space (spacetime?) being created or is it "stretching"?
My first reaction is to why bother asking since it seems nothing more to me than mere semantics. In the end we must be able to perform an experiment to determine which is which and from what I’ve gathered so far this won’t get us anywhere.
What is important is what it all means in the end. The universe is said to expanding because there is more and more room being created. In essence all that really means is that we have more places to put things. Nothing else.
Is there a difference?
No. It’s useful to think in terms of analogies such as the balloon one which is of the surface area of an expanding sphere. Just make sure you don’t take that old adage of “the fabric of spacetime” and think it’s some sort of “fabric” that’s being manipulated. Just like the cheese between my toes. :)
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I came up with a simple model which will illustrates the ideas behind an expanding universe and the idea that space is being created. I call this model The Circle Universe and it has a radius R which parameterizes the size of the Circle Universe. Let theta represent the relative angular displacement of two “observers” on the circumference” of the Circle Universe. The metric which determines the distance between two such points is L = g(theta) = R*theta. If the two observers remain at rest then the rate change of the distance between them is dL/dt = (dR/dt)*theta.
Notice that when R = 0 then size of the universe was also zero and all points were at the same point. Also there is no point in the Circle Universe that can be said to be at rest. Does this mean that there is space being created/is expanding in my model of a Circle Universe or does it mean that the distance between the two observers is being stretched? Notice that it really doesn't matter which is the most correct. It only matters that we know how to describe it to others.
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thanks for the replies, but if the universe was shrinking surely you could also have redshift in all directions, stretching/more space being created and an increase in the speed that galaxy's moving apart. When matter is pulled towards an event horizon it speeds up in a similar way fluid speeds up the closer to a plug hole it gets. As it does, it stretches away from the surrounding matter as its accelerates so from the view point on any piece of matter all other matter is moving away as space stretches.
so if the universe is collapsing from the centre outwards (like a collapsing star) you would not need to use dark matter to explain the speeding up/expanding universe. ???
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thanks for the replies, but if the universe was shrinking surely you could also have redshift in all directions, stretching/more space being created and an increase in the speed that galaxy's moving apart.
I don't understand where this is comming from. Are you now changing the subject totally around to be something entirely else?
"surely"? Why "surely"? A shrinking universe would be a contracting universe and the result would be a cosmological blue shift, not a cosmological redshift. What's leading you to believe that it would still be a redshift?
When matter is pulled towards an event horizon it speeds up in a similar way fluid speeds up the closer to a plug hole it gets.
But there's no event horizon here and no cosmological contractions/expasions in that case. Where did you all of a sudden bring up event horizon's?
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The red shift from other galaxies show that other galaxies are moving away from us so the universe is expanding.
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I asked the original question like I did first to see if there was any other indicator that the universe was expanding other than just the red shift. I apologise if this has annoyed anyone but I know that I would be shot down if I just said "how do you know that the universe is shrinking". so :
If the universes was contracting from its outer regions back to the centre point then there would be a blue shift as galaxy's would be "squashing" back together but if the universe was collapsing from the centre outwards the distance's would increase as the nearer to the centre they were, the faster they would go, as I said, like water getting pulled towards the plughole or like matter falling into a singularity.
if you don't like my analogies im sorry but its the simplest way I could think of to ask my question
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Sorry pmb I though my first reply was clearer than it was, I was using the event horizon analogy just to show that things can shrink to a central point yet accelerate away from us and stretch the space between two points yet still be collapsing.
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Nice thought. Are you imagining a universe in where everything (of matter?) sort of are becoming 'black holes', although as you include the 'space' around it possibly make it keep a equilibrium? Then you will need a lot more dimensions for it, I think? Or you can possibly use (non-classical) 'locality' to define it. Alternatively described it's the idea in where everything are shrinking in a fixed 'volume' of a container-universe, giving us a redshift. If the universe can be defined as a 'container' then the last is as possible as one in where that 'container' is expanding producing a redshift. From my point of view (non classical 'locality') 'c' is communication, and although a shared 'constant', theoretically observer dependent. To see that one just think of how light always will be measured at 'c', doesn't matter how or on what you measure, as long as we're not discussing accelerations/decelerations. That's what makes it a 'constant', and to my eyes, observer dependent. And from such a view distances becomes a very fluid definition, as well as speeds.
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Observer dependent relative the idea of a 'commonly shared universe' that is. Doesn't mean I envision a 'universal container' filled with 'time dilations and Lorentz contractions'. Such a universe that I'm considering here is just a way of communicating, defined through commonly shared principles and constants (always locally defined). The 'container' we see is of our own making, defined through those local experiments.
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Thanks for that, Yor-on, but rather than all matter becoming singularity's, I meant that if the universe came from a single point and "the big bang" event was able to fling out all the material that makes up this universe, why not have a big crunch.
I know that this theory has been looked at already but under the premise that the matter at the furthest most reaches of the universe will start to collapse inwards.
My thought is that at the same point that the big bang occurred so to could the big crunch, a point in space that is pulling all matter back to its origin therefor stretching space back towards its origin, keeping the redshift intact. Like you said, all matter from a single container but now the container is pulling it all back.
The outer edges of the universe could be still expanding or even stationary as long as every thing else is moving back towards the original start point, the nearer to the original start point the faster they go, distances increase even as the universe decreases.
At the point that the universe came into being the laws of physics as we know it did not apply did they? The matter thrown out exceeded the speed of light, vast quantity's of material appeared and exotic forces came into being, even if it was only fleetingly, all impossible in our understanding of physics so maybe at that point in space/time where it all started, physics still do no apply, as they do not in a singularity, and all matter is being pulled back to its original state, at its original point.
please see my other post for my explanation why I do not ask questions in the manor that you are probably accustomed to :I
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hmm :)
Are you suggesting a shrinking container, with a even faster shrinking matter content? Or that the matter content stops shrinking at some time while the container itself keeps on shrinking?
The first case will produce a red shift, but it can't become a 'big crunch, as the container always must be bigger than its matter content to keep a redshift.
The second should mean a blueshift.
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I was actually meaning that the container is a fixed point, the original container that held all matter infinitely small and that all matter that has escaped is now being pulled back into it, increasing in speed as it approaches (and if ive got this right, the closer to the speed of light matter travels at the smaller it becomes) cramming its self back into the container that it originated from.
the increase in speed the closer to the container matter got would continue to stretch space the closer it got and this would account for the red shift :o)
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Thanks for that, Yor-on, but rather than all matter becoming singularity's, I meant that if the universe came from a single point and "the big bang" event was able to fling out all the material that makes up this universe, why not have a big crunch.
The universe coming out of a single point is not part of the big bang theory (BBT). The BBT can only traced back so far and then we don’t know what happened before that.
Physicists used to believe that the universe might stop expanding and reverse to a big crunch. That was only one of the possibilities. Now that it appears that the universe is expanding at an accelerating rate that does not seem possible.
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I know that this theory has been looked at already but under the premise that the matter at the furthest most reaches of the universe will start to collapse inwards.
The matter everywhere is expanding or collapsing at the same time regardless of how far away it is.
My thought is that at the same point that the big bang occurred so to could the big crunch, a point in space that is pulling all matter back to its origin therefor stretching space back towards its origin, keeping the redshift intact.
There is no place in the universe like that. People get the wrong idea quite often that the big bang occured at a particle point in spacel which is not true.
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Barney, I lose you there. Either you define the universe as a container, containing it all, matter, light and energy and .... Or you define a universe as a result of local experiments, sharing common principles and constants. To do both simultaneously becomes a very tricky proposition as you now on one hand define a 'empty container (universe)' with what, outside it? Another container containing those stars etc 'rushing back' into this original container?
Why would they need to do that?
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Are you considering some 'center'? That somehow 'calls back' all matter? Then that's not a 'container' as I'm thinking of it. A 'container' is to me just a universe, and all it contains. And there is no center to it, using the model we have today. The universe expands equivalently from all fixed positions, aka planets, you measure that expansion from. And that is also a very strong argument for this 'container' to be considered 'infinite'. As it otherwise should be so that at some position xyz and time you would find a 'uneven expansion', if the universe has borders. That doesn't necessarily mean that the universe can't be 'contained' as described from some 'outside', only that it to us inside is 'infinite' and 'seamless'.
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What such a reasoning does is bringing us back to what a 'infinity' consist of? Using a 'one to one correspondence' to define it, can a infinity expand? Isn't all infinites the same? Or put in another manner. If the universe has a 'outside', being a 'container' of us/it all measurable. Is that a 'real' infinity, using a one to one correspondence?
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(A 'one to one correspondence' is you putting one thing beside another, ad infinitum, comparing them (their position) with each other. If you have no end to that, you should have yourself a infinity... As taking all primes, placing them aside all natural numbers... In a 'system' limited by your defined 'borders, cut offs' there will be less primes than natural numbers, but as both, each by itself, seem to be able to be counted up 'ad infinitum', you should be unable to end a 'one to one correspondence' placing numbers aside each other. In reality making them equivalently 'infinite', if you see how I mean.)
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Yes I did mean a sort of centre, the original expansion point and I see what you mean about my container usage, sorry :I
what I meant was that at the moment of the big bang, all matter in the universe today was released (all be it in a different form). for a very small period of time, there was an unimaginable force that flung all the matter outwards. it flung it so hard that it expanded outwards faster than light (impossible within our {yours more than mine :o)} understanding of physics) until matter slowed and the laws of physics came into play.
so expansion started fast then slowed and now is speeding up again?
whats to say that that unimaginable force is not now drawing all matter back.
Please bare with me on this next bit.
one theory of the big bang, so ive read, is that two "wavy branes" of a multiverse touched and at the point that they touched our universe was created. two ultra massive bodys with masses of energy released matter and energy in an instant to create our universe. now the branes are pulling apart again and their drawing their energys and matter back out at the point that they first touched.
an unimaginable force inflating then deflating our universe.
now whether its two wavy branes separating, an uber super massive black hole or just GOD with a big vacuum cleaner whats to say that the generally accepted unimaginable force that blew us apart isn't now drawing us back in, ever faster the closer to original start point you get.
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Thanks for the one to one explanation :) it infinitely helped ;D
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Please bare with me on this next bit.
one theory of the big bang, so ive read, is that two "wavy branes" of a multiverse touched and at the point that they touched our universe was created. two ultra massive bodys with masses of energy released matter and energy in an instant to create our universe. now the branes are pulling apart again and their drawing their energys and matter back out at the point that they first touched.
an unimaginable force inflating then deflating our universe.
Interesting thought barney..............
At present, we suspect the increase in expansive velocity is being caused by the presence of dark energy. Notice,.........I use the word "Suspect" because we still don't understand much about this phenomenon. Not only don't we know much about dark energy, we don't have a good explanation for why it should even exist. So......, until we do, using the term "dark energy" has very little to do with the scientific method, repeatable experimental results. I submit that at present, using the term "dark energy" should be classified as merely a hypothesis and not a theory. Likewise, your estimation of this phenomenon is also lacking in the scientific method. Nevertheless, your thoughts are intriguing.
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Since so many people are so easily confused by the concept of expanding space, even when the balloon analogy is used, that I took that balloon analogy and simplified it. Since the balloon analogy relies on two particles moving on a great circle I looked only at the circle and created a page called the The Circle Universe which is now at
http://home.comcast.net/~peter.m.brown/gr/circle_universe.htm
The purpose of this page was to show how a calculation like this is done in lower dimensions. Please keep in mind that this is the balloon analogy, just looking at a slice of the balloon on which two particles are moving on.
Noitice the resulting equation Eq. (2). It shows that the relative speed of two "galaxies" depends on their (angular) seperation. The angular coordinates are referred to as comoving coordinates.
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I don't know what your equations mean but if I get your explanation then what your saying is that as points b, and c, move away from their original start point a, the distance increases but at the same time the distance between b, and c, also increases showing that they must be expanding. But if I have understood that correctly then there would only be a red shift in the universe and if there was only a red shift in the universe, there would not be galaxy's crashing into each other and the Andromeda galaxy would not be aiming to take a chunk out of the milky way as if they were getting closer then there would be a blue shift here and there and if there was a blue shift here and there then b, and c, could be heading back towards a, even if d, and e, was not.
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The universe coming out of a single point is not part of the big bang theory (BBT). The BBT can only traced back so far and then we don’t know what happened before that.
Physicists used to believe that the universe might stop expanding and reverse to a big crunch. That was only one of the possibilities. Now that it appears that the universe is expanding at an accelerating rate that does not seem possible.
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My thought is that at the same point that the big bang occurred so to could the big crunch, a point in space that is pulling all matter back to its origin therefor stretching space back towards its origin, keeping the redshift intact.
There is no place in the universe like that. People get the wrong idea quite often that the big bang occured at a particle point in spacel which is not true.
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I'm sorry, I didn't realise that this had been proven as a scientific fact rather that an unproven theory
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I don't know what your equations mean…
What is the problem that you’re having understanding them. If you explain that then I’ll help you understand what they mean.
.. but if I get your explanation then what your saying is that as points b, and c,…
What are the points b and c that you’re talking about?
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My thought is that at the same point that the big bang occurred so to could the big crunch, a point in space that is pulling all matter back to its origin therefor stretching space back towards its origin, keeping the redshift intact.
There is no place in the universe like that. People get the wrong idea quite often that the big bang occured at a particle point in spacel which is not true.
I'm sorry, I didn't realise that this had been proven as a scientific fact rather that an unproven theory
The point is that current big bang theory is about the expansion of spacetime itself, not like an explosion that occurs within some greater space. The whole universe was very dense and hot, and the whole universe is expanding from that condition. This idea fits what we see now very well. The idea of there being some point in space where the expansion began does not fit well with what we observe, and the maths doesn't work out.
Science doesn't really generate 'proven scientific facts'; theories are provisional explanatory models, open to refinement or replacement. Newtonian physics was a pretty good fit with observation, but Einstein's relativity is even better. The current big bang theory is not the whole story, but it's hard to see how a 'one point in space expansion' theory could be made to fit.
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Science doesn't really generate 'proven scientific facts'; theories are provisional explanatory models, open to refinement or replacement. Newtonian physics was a pretty good fit with observation, but Einstein's relativity is even better. The current big bang theory is not the whole story, but it's hard to see how a 'one point in space expansion' theory could be made to fit.
Good explanation dlorde, and the casual scientific observer only see's portions of the whole picture. It is becoming more popular among physicists today to talk about concepts like The Bulk. The Bulk refers to what some might call Hyperspace where many different universes could exist. This lends support to understanding how our particular universe could be expanding with it's occupants riding along as on the surface of an expanding balloon with no central point of expanding origin. Unlike the former concept of unlimited space, this idea places our finite universe within an infinite Hyperspace where many different universes could possibly exit. There would be of course no opportunity for adjacent universes to ever communicate with each other because escape from any such individual time/space continuum could not be possible.
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Hi PMB
I don't know what your equations mean…
What is the problem that you’re having understanding them. If you explain that then I’ll help you understand what they mean.
you equations seem simple to you and your esteemed academics but to me the are complex and alien , this does not mean im thick, it means that I have not been trained to understand them, that is why I asked for simplistic terms on explanations. My analogies, to me seem very simple yet you do not seem to get them because they are alien to you and yet I think that they both can show the same result.
.. but if I get your explanation then what your saying is that as points b, and c,…
What are the points b and c that you’re talking about?
I meant that if you wrote the letter b, and the letter c, on the surface of a rubber balloon and then inflated that balloon not only would they move away from the mouth piece (lets call that a,) but b, and c, will also move away from each other
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Hi Dlorde
I apologise, I don't get what you mean "The whole universe was very dense and hot, and the whole universe is expanding from that condition".
why is that statement different from the universe expanding from a single point.
"IF" the universe is expanding, then it was smaller to start with, either as a infinitely small and infinitely dense point that went bang or as a huge dense point that went bang.
or do you mean that the physical size of the "shell"of the universe is fixed and the hot dense material held within as it cools and coagulates into galaxy's then the more space would be created between galaxy's creating the red shift.
if you started with a set amount of space that could not either increase or decrease, lets say a 1 litre bottle and in that bottle was 1 litre of hot dense gas. as the gas cools it turns into different liquids that seperate so it doesn't take up as much space as the gas, but as the bottle cannot shrink, so some space is created. Then as the liquids turn into solids they take up even less space therefore creating MORE space because yet again the "universe" bottle cannot change size. you would end up with a 1 litre volume bottle and 1 litre of matter and a vacuum of space that had being created had this been in zero gravity then the little lumps of coagulated solids would have show an increase of space between them without the need for inflation or dark matter.
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hi ethos
[/quote]. There would be of course no opportunity for adjacent universes to ever communicate with each other because escape from any such individual time/space continuum could not be possible.
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why, of course, and how can you be so sure that it would not be possible. Are you that sure that we (you rather than me) have such a good grasp on all things physicy that you can say with no doubt that this is a science FACT and not "Science doesn't really generate 'proven scientific facts'; theories are provisional explanatory models, open to refinement or replacement"
(this one could be messy :o) sorry)
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hi ethos
Greetings barney.........
why, of course, and how can you be so sure that it would not be possible.
Nothing is an absolute surety barney, but there is some evidence to support these possibilities.
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I didn't mean about the bulk theory, I meant regarding the cross universe communication.
There would be of course no opportunity for adjacent universes to ever communicate with each other because escape from any such individual time/space continuum could not be possible.]
why would it not be possible to escape individual time/space continuum.
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why would it not be possible to escape individual time/space continuum.
Even though travel thru a Worm Hole may be theoretically possible someday in the very distant future, travel outside of our universe is thought by just about every credible scientist to be impossible. Travel by means of shortcut wormhole method is only theoretically possible within our own universe. And as we speak, there remains no method by which this journey outside our universe is thought possible.
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I apologise, I don't get what you mean "The whole universe was very dense and hot, and the whole universe is expanding from that condition".
why is that statement different from the universe expanding from a single point.
As I understand it, at the big bang, the universe was arbitrarily large (possibly infinitely large). Since that time, it has expanded. It's reasonable to say that the currently observable universe would have been extremely small, but the observable universe is probably only part of the whole universe.
"IF" the universe is expanding, then it was smaller to start with, either as a infinitely small and infinitely dense point that went bang or as a huge dense point that went bang.
OK - in physics (geometry), a point has no size, so you can't have a 'huge' point. If you're saying the universe may have been huge, very hot, and very dense, then expanded rapidly, that's basically what I'm saying - although I don't think it is believed to have been infinitely dense.
... do you mean that the physical size of the "shell"of the universe is fixed and the hot dense material held within as it cools and coagulates into galaxy's then the more space would be created between galaxy's creating the red shift.
No. Not sure what you mean by 'physical shell', but the physical size of the universe is expanding, and every point in it is moving away from every other point, so the distance between objects generally increases over time except where some force holds them together.
if you started with a set amount of space that could not either increase or decrease, lets say a 1 litre bottle and in that bottle was 1 litre of hot dense gas. as the gas cools it turns into different liquids that seperate so it doesn't take up as much space as the gas, but as the bottle cannot shrink, so some space is created. Then as the liquids turn into solids they take up even less space therefore creating MORE space because yet again the "universe" bottle cannot change size. you would end up with a 1 litre volume bottle and 1 litre of matter and a vacuum of space that had being created had this been in zero gravity then the little lumps of coagulated solids would have show an increase of space between them without the need for inflation or dark matter.
Possibly; but that's not a realistic model for the big bang universe.The universe is a closed system, it can only cool by expansion, and a history of that is what we see when we look out there. The current big bang model was developed to account for what we actually observe.
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As I understand it, at the big bang, the universe was arbitrarily large (possibly infinitely large).
I disagree. Where did you get that idea from?
OK - in physics (geometry), a point has no size, so you can't have a 'huge' point.
I believe that he was talking about a dynamical property of the point, not the size of the point.
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As I understand it, at the big bang, the universe was arbitrarily large (possibly infinitely large).
I disagree. Where did you get that idea from?
Stuff I've read in various places. Some say it may have been infinite, others that it was finite in size, but that the observable universe is likely only part of a larger whole. From my reading, most don't think that it was an infinitesimal size. So if it wasn't infinitesimally small, it was either infinite or some other size. YMMV.
OK - in physics (geometry), a point has no size, so you can't have a 'huge' point.
I believe that he was talking about a dynamical property of the point, not the size of the point.
I don't understand - in what sense can a point be huge?
Oh wait - do you mean that he meant 'hugely dense'? If so, I wouldn't argue with that.
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I don't understand - in what sense can a point be huge?
Oh wait - do you mean that he meant 'hugely dense'? If so, I wouldn't argue with that.
Yes. That’s exactly it. In fact that’s what he was talking about, i.e. a huge mass density. If it’s actually a point then the mass density would be infinite.
Think of the difference in mass between an electron and a muon. The masses are finite yet they have no size.
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Think of the difference in mass between an electron and a muon. The masses are finite yet they have no size.
Hmm; I know in they're treated as 'point particles' in QM, but it seems debatable whether they have spatial extent - in classical relativistic physics they have a classical radius, e.g. as used in Thomson Scattering calculations; in quantum field theory, they're excitations of a field - does size have meaning for that? It seems to me that both cases are abstract models for whatever is 'really' there... I suppose it depends how you look at it.
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Hmm; I know in they're treated as 'point particles' in QM, but it seems debatable whether they have spatial extent - in classical relativistic physics they have a classical radius, e.g. as used in Thomson Scattering calculations; in quantum field theory, they're excitations of a field - does size have meaning for that? It seems to me that both cases are abstract models for whatever is 'really' there... I suppose it depends how you look at it.
What is it about the electron being a truly point particle that bothers you? It's my understanding that the electron is truly a point particle.
Suppose we were to model the electron as a classical solid sphere or radius r, mass m, with angular momentum hbar/2. If we assumed that the classical electron radius was less than r = 10^(-16) cm then the sphere would be rotating with points on the surface moving faster than the speed of light. Faster if the radius was smaller. It we require the spinning electron model to hold then we know this can’t work.
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Suppose we were to model the electron as a classical solid sphere or radius r, mass m, with angular momentum hbar/2. If we assumed that the classical electron radius was less than r = 10^(-16) cm then the sphere would be rotating with points on the surface moving faster than the speed of light. Faster if the radius was smaller. It we require the spinning electron model to hold then we know this can’t work.
Yes, of course we know that macro-scale concepts don't really apply; electrons aren't little billiard balls rotating, but sometimes size, spin and angular momentum, etc., are useful metaphors for how they seem to behave, and sometimes not; sometimes we have to treat them like waves. Sometimes familiar names are given to properties that are nothing like their macro-scale namesakes, e.g. 'spin', 'color', 'flavor', etc.
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Yes, of course we know that macro-scale concepts don't really apply; electrons aren't little billiard balls rotating, but sometimes size, spin and angular momentum, etc., are useful metaphors for how they seem to behave, and sometimes not; sometimes we have to treat them like waves.
That’s not quite an accurate statement. Point particles are not treated like waves. There is only a wavelike property associated with their behavior. By this I mean that they have a property which gives rise to probability distribution. That means that there is a distribution of where they’re detected given an ensemble of particles or measurements.
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Yes, of course we know that macro-scale concepts don't really apply; electrons aren't little billiard balls rotating, but sometimes size, spin and angular momentum, etc., are useful metaphors for how they seem to behave, and sometimes not; sometimes we have to treat them like waves.
That’s not quite an accurate statement. Point particles are not treated like waves. There is only a wavelike property associated with their behavior. By this I mean that they have a property which gives rise to probability distribution. That means that there is a distribution of where they’re detected given an ensemble of particles or measurements.
It's accurate enough to make the point; they have both wave-like properties and particle-like properties, as professor Norton explains (http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_waves/index.html).