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Fulmen
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Not quite. Special Relativity came first (1905) and explained the constant speed of light, the matter/energy-duality and such. General Relativity came
later (1916) and incorporates gravity/space-time into SR.
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LearnedAmateur
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Quote: Originally posted by Melgar  |
General relativity covers scenarios at more ordinary velocities, and predicts that one consequence of gravity bending space-time is that there should
be no observable difference between an acceleration due to gravity and an acceleration due to accelerating. Any phenomenon you'd see if you were in a
constantly-accelerating elevator, you should also see while under the effects of a gravitational field. So far, this has proven to be true in every
experiment that's been done. |
But isn't there also no discernible difference for electromagnetism? I mean charged objects still experience an exponential increase in acceleration
with decreasing radius, like masses with gravity, yet is many orders of magnitude more powerful. Mass is simply exchanged for charge - the equations
are even the same format albeit with different constants, however we know that electromagnetism is facilitated by photons and not space time
curvature.
Consider an observer sitting in a positively charged sphere several thousand kilometres from either a planet or a highly negatively charged metal
sphere with a negligible mass. They wouldn't be able to tell whether they were accelerating towards a massive object because of gravity, or a far
lighter charged object because of an electromagnetic attraction. In either case, they would simply be accelerating towards it, and assuming that both
objects are very large, they would have similar force-distance relationships at a sufficient radius from the surface.
In chemistry, sometimes the solution is the problem.
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Melgar
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| Quote: Originally posted by JJay | You know, a rock actually falls faster than a feather in a vacuum due to the greater gravitational force exerted by the rock (if you're confused,
assume that the rock is the size of Jupiter).
I know absolutely nothing about relativity. I always had a hard time seeing how I would ever use anything beyond Newtonian physics.
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Ah, but the "rock" would then have the lowest velocity of any of the objects. The feather would actually fall faster, assuming that "fall" wasn't so
narrowly-defined as to exclude certain components of motion that were a result of gravity.
I never much cared for physics because the easy stuff is too intuitive, and even the hard stuff is usually just proving stuff that's already
intuitive. And with the really hard stuff, there's usually not much you can do with it on an amateur budget.
Also, physics is perhaps the one branch of science that's nearly exhausted, with the possible exception of astrophysics. We're still learning plenty
of new stuff in the field of chemistry. Even amateurs here have learned things that have previously never been documented, using only their own
resources.
The first step in the process of learning something is admitting that you don't know it already.
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Melgar
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| Quote: Originally posted by wg48 | Ok ignoring your reference to a cartoon joke.
I perplexed, what then did you mean by your statement:
"the universe can and does EXPAND faster than the speed of light"
In particular what did you mean by "the universe" and "EXPAND" ? |
I was referring to the expansion of space, according to Hubble's law. After all, if the expansion of the universe was limited by the speed of light,
it could only be 27 billion light years across, since the universe is only 13 odd billion years old.
https://en.wikipedia.org/wiki/Observable_universe#Misconcept...
| Quote: Originally posted by Fulmen | | Not quite. Special Relativity came first (1905) and explained the constant speed of light, the matter/energy-duality and such. General Relativity came
later (1916) and incorporates gravity/space-time into SR. |
Eh, I never actually contradicted any of that, but I concede that I could have explained it better.
| Quote: Originally posted by LearnedAmateur | | But isn't there also no discernible difference for electromagnetism? I mean charged objects still experience an exponential increase in acceleration
with decreasing radius, like masses with gravity, yet is many orders of magnitude more powerful. Mass is simply exchanged for charge - the equations
are even the same format albeit with different constants, however we know that electromagnetism is facilitated by photons and not space time
curvature. |
Not really. The force is squared for mass and cubed for electromagnetism. That odd power actually makes a pretty big difference, since it allows for
negative forces and charges. It also makes the force increase a lot faster with smaller distances. I think (though I could be wrong) that the strong
nuclear force is to the power of 4.
The difficult part for physics is that mass is responsible for both gravitational force and inertia, and yet there isn't really a good reason that
those two should be exactly proportional like they are. So mass plays two totally different roles in two different models. And for some reason, the
rules are very different in the two models, but it's virtually impossible to tease them apart, because the only experiments that could be done to test
hypotheses would require the use of a black hole. I think. This is not my area of expertise, and maybe someone else understands it better.
| Quote: Originally posted by LearnedAmateur | | Consider an observer sitting in a positively charged sphere several thousand kilometres from either a planet or a highly negatively charged metal
sphere with a negligible mass. They wouldn't be able to tell whether they were accelerating towards a massive object because of gravity, or a far
lighter charged object because of an electromagnetic attraction. In either case, they would simply be accelerating towards it, and assuming that both
objects are very large, they would have similar force-distance relationships at a sufficient radius from the surface. |
Here's a clue: if you're disintegrating because of the electrical repulsion caused by the ionization of your entire body, it's the electrical one.
[Edited on 10/20/17 by Melgar]
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Fulmen
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I could have been clearer myself it seems, saying that GR deals with "more ordinary velocities" is wrong. GR covers all of SR plus gravity.
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LearnedAmateur
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I was under the assumption that for electromagnetic interactions, F=k*Qq/r^2 where k is the reciprocal of 4π(ε0), hence follows the inverse square
law like gravity does (F=-G*Mm/r^2). Negative charges are accounted for in the equation, if both Q values have opposite signs then the force is
negative/attractive, whereas the same sign indicates a positive/repulsive force. For gravity, the value of G is always negative hence gravity always
attracts since you can never have a negative mass without negative energy.
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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wg48
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Melgar wrote:
"I was referring to the expansion of space, according to Hubble's law. After all, if the expansion of the universe was limited by the speed of light,
it could only be 27 billion light years across, since the universe is only 13 odd billion years old".
That’s just an other way concluding that very distant galaxies are moving fast than light.
I can not give you the full version but the short version is:
Thanks to relativity in an expanding universe there are no common sets of coordinates of position and time that allow you to do the every day simple
velocity distance time calculations between different parts of the universe and get correct answers. Its much more complicated than that.
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Fulmen
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IIRC space isn't limited by the speed of light, so space could expand (and according to inflation theory, already has expanded) faster than the speed
of light.
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Melgar
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Well, they actually are, and relative to us, no less. Special relativity doesn't hold true for objects so far distant that the expansion of the
universe is significant. Here is an article that I skimmed and seems to explain it well:
http://curious.astro.cornell.edu/about-us/104-the-universe/c...
Also, do you understand the "dark energy" problem very well? Because that's one of the few big problems that physicists don't know what to make of
yet.
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wg48
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| Quote: Originally posted by Melgar |
Well, they actually are, and relative to us, no less. Special relativity doesn't hold true for objects so far distant that the expansion of the
universe is significant. Here is an article that I skimmed and seems to explain it well:
http://curious.astro.cornell.edu/about-us/104-the-universe/c...
Also, do you understand the "dark energy" problem very well? Because that's one of the few big problems that physicists don't know what to make of
yet. |
The article you reference explains very little. I would give it a popsci score of nine out of ten. It did caveat the statement “there are galaxies
moving away from each other faster than light” but failed to detail the caveat. However when discussing receding galaxies approaching the speed of
light he did make the following statement “After that, we will observe them to freeze and fade, never to be seen again”, presumable SR effects.
That statement could be reworded to: “in our inertial frame we observe no galaxies moving faster than the speed of light”.
We are apparently making no progress in our discussion I can do no more than repeat my previous statements with more detail and you could stand at the
interacting point of beams of the LHC and say see the protons that don’t collide are moving away from each other at almost twice the speed of
light just like some galaxies do.
My understanding of dark energy is that it’s the mechanism that is accelerating the expansion of universe.
Is space expanding at the LHC? If its diameter is expanding will it end up in my garden at some point?
Sorry I could not resist adding the sarcastic and humorous comment.
[Edited on 21-10-2017 by wg48]
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Melgar
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| Quote: Originally posted by LearnedAmateur | | I was under the assumption that for electromagnetic interactions, F=k*Qq/r^2 where k is the reciprocal of 4π(ε0), hence follows the inverse square
law like gravity does (F=-G*Mm/r^2). Negative charges are accounted for in the equation, if both Q values have opposite signs then the force is
negative/attractive, whereas the same sign indicates a positive/repulsive force. For gravity, the value of G is always negative hence gravity always
attracts since you can never have a negative mass without negative energy. |
Err, oops. I was wrong. I was thinking of magnetic attraction. I guess the only real difference with electrostatic attraction is that it actually
can be negative, unlike gravity that's only positive. Also, electrostatic forces tend to cancel each other out on macroscopic scales, and would
destroy the matter that they occupied if they became too large, due to the large value of the constant.
| Quote: Originally posted by wg48 | Is space expanding at the LHC? If its diameter is expanding will it end up in my garden at some point?
Sorry I could not resist adding the sarcastic and humorous comment. |
Large Hadron Collider? Um, no. Space is expanding uniformly, and the expansion is only noticeable for points that are separated by very large
distances. Seriously, I think you're dismissing the expansion of the universe as just being an illusion due to relativistic effects, when that isn't
the case at all. This is a very real phenomenon that nobody completely understands to this day. And the cosmic microwave background radiation? And
dark energy? And the fact that the universe* is proven to be 100 billion light years across, more or less, despite being less than 14 billion years
old? This all has to do with actual, real phenomena regarding space expansion; it's not just some delusion of physicists that don't understand
relativity.
* meaning everything that came into being during the Big Bang, including photons and the space that everything occupies
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wg48
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| Quote: Originally posted by Melgar |
Large Hadron Collider? Um, no. Space is expanding uniformly, and the expansion is only noticeable for points that are separated by very large
distances. Seriously, I think you're dismissing the expansion of the universe as just being an illusion due to relativistic effects, when that isn't
the case at all. This is a very real phenomenon that nobody completely understands to this day. And the cosmic microwave background radiation? And
dark energy? And the fact that the universe* is proven to be 100 billion light years across, more or less, despite being less than 14 billion years
old? This all has to do with actual, real phenomena regarding space expansion; it's not just some delusion of physicists that don't understand
relativity.
* meaning everything that came into being during the Big Bang, including photons and the space that everything occupies |
Don’t you find it odd that the LHC can get protons to separate at more than the speed of light without expanding space or violating SR but the
universe has to create space and ignore SR?
Have you ever read a technical paper explaining the difference or discussing the details of expanding space or how it stretches photons? Popsci does
not count. It’s not hard to find ones calling it a myth and bitching about it.
These are rhetorical questions, which hopefully kick start the critical thinking parts of your brain. This is pervasive myths it will not die easily.
It will be like admitting you believe in spontaneous human combustion or perpetual motion.
Incidentally the size of the universe at present was perplexing to me. Not because it expanded (separated) faster than light but because the numbers
did not make sense. I thought it was a linear but apparently it expanded exponentially similar to how hot gas would. That should not have been a
surprise. There is nothing mysterious about it, no violation of any physics including sr. But it is a lot more complicated to explain or understand
than the myth.
If your serious comment was serious I wasted my words on you. Perhaps they will help others exorcise the myth.
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Melgar
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| Quote: Originally posted by wg48 | | Have you ever read a technical paper explaining the difference or discussing the details of expanding space or how it stretches photons? Popsci does
not count. It’s not hard to find ones calling it a myth and bitching about it. |
Well, here's a classic paper on the subject. (Attached)
I sort of understood your joke, but you seemed to be referencing some event at the LHC that I'm unfamiliar with, involving speeds faster than light.
With galaxies moving away from us faster than light (when universe expansion is added into their velocity relative to us), they actually seem to be
moving away from us more slowly than they are, in a way that's remarkably similar to what an object would look like as it crossed the event horizon of
a black hole. If you witnessed that, the object would actually never seem to cross the event horizon, rather, it would stop there, then it would
become redder and redder, until it was only visible in infrared, then microwave, then radio spectra. Finally, there would be no electromagnetic
radiation coming from it that would be detectable, although this would take a very long time to happen. Galaxies that are moving away from us faster
than light show the same phenomena, just on a longer time scale. They just become redshifted out of the visible spectrum. Because of how much light
galaxies emit though, they'd probably be visible it at least some spectra for trillions of years.
Attachment: hawking1982.pdf (189kB)
This file has been downloaded 351 times
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wg48
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| Quote: Originally posted by Melgar | | Quote: Originally posted by wg48 | | Have you ever read a technical paper explaining the difference or discussing the details of expanding space or how it stretches photons? Popsci does
not count. It’s not hard to find ones calling it a myth and bitching about it. |
Well, here's a classic paper on the subject. (Attached)
I sort of understood your joke, but you seemed to be referencing some event at the LHC that I'm unfamiliar with, involving speeds faster than light.
With galaxies moving away from us faster than light (when universe expansion is added into their velocity relative to us), they actually seem to be
moving away from us more slowly than they are, in a way that's remarkably similar to what an object would look like as it crossed the event horizon of
a black hole. If you witnessed that, the object would actually never seem to cross the event horizon, rather, it would stop there, then it would
become redder and redder, until it was only visible in infrared, then microwave, then radio spectra. Finally, there would be no electromagnetic
radiation coming from it that would be detectable, although this would take a very long time to happen. Galaxies that are moving away from us faster
than light show the same phenomena, just on a longer time scale. They just become redshifted out of the visible spectrum. Because of how much light
galaxies emit though, they'd probably be visible it at least some spectra for trillions of years. |
Yes I was giving what I thought was a clear example of how objects, in this case protons, can separate faster than c. In fact about 1.9999c. Just as
distant galaxies on opposite sides of the earth do. However in either case in the initial frame of either the proton or the galaxy the other is moving
at less than c that’s the Einstein velocity addition at work.
The reference you gave seems not contain that quote??? or I have misunderstood and its your writing? Its sloppy and inaccurate for Hawking compare it
the reference.
I have been a bit unfair I introduced the myth so I should be prepared to defend and explain it. I did make some key points but did not address every
misconception given in support of the expansion of space. There is google. Well I searched with google but failed to find a clear explanation but lots
of references to expanding space. The myth really is very deeply embedded. So here goes my explanation. It will probably be in parts and it probably
should be put in its own thread.
First I will define the myth. The myth is that the apparent motion of distant galaxies away from us is not caused by the motion of the galaxies but by
the space increasing or expanding between the galaxies.
The usual justification for the myth is the observable universe is larger than the distance light could have travelled since the big bang so it must
be expanding faster than the speed of light. As the galaxies of the universe cannot travel faster than light and as space is not a thing subject to
the speed limit of light it must be the space between the galaxies that’s expanding.
I think there are two main reasons why the myth persists even now.
1) For more than fifty years the myth has been perpetuated in lots of popsci article news papers and even many technical papers. Only occasionally is
a caveat added such as “sort of space expansion”.
2) Relativity is complicated and very counter intuitive, add cosmology with its peculiar terms (no pun intended) and it gets really tough.
All I can say about 1) is it’s a deeply embedded myth. Have the courage of your understanding of physics (SR in particular) and the scientific
principal. Its ridiculous that phenomena based on false premises has persisted for over fifty years. I doubt very strongly good cosmologists or
physicists don’t know it’s a myth. I suspect it is a convenient name to avoid a complicated explanation.
The principle difficulty of 2) is with two of the distance definitions used in cosmology. They are proper distance and comoving distance.
Proper distance is the normal every distance that can be measured with a rigid measuring stick and with a stopwatch, determine the velocity of moving
things. However thanks to relativity distance and velocities (even time) are not fixed things. They must be specified in a particular initial frame as
they can be very different in a different intial frames. That is a vitally important point.
Comoving distance is not a everyday distance like proper distance is. It calculated by adding up half the proper distance to the adjacent galaxies
on either side of each galaxy, in the inertial frame of that galaxy along a particular line of sight. The commoving distance can not be measured by a
rigid measuring stick between the ends of the line of sight.
Comoving distance is a measure of the of the available realestate along that line. It can not be used to calculate the velocity of anything in any
intial frame. It can be used to calculat the rate at which that available realestate is increasing. You could call the available realestate avaialbe
space. But available space is not the proper space of proper distance in a particular initial reference frame. SR limits velocites to less than c but
it only applies to object in a particular inertial frame and not comoving distances. So commoving distance can expand at any speed including speeds
greater than light.
Cosmologists tend talk about the size of the observable universe in commoving distances which are not subject to SR and hence not subject to any
velocity limit.
The effect of using the commoving distance is to stretch out the universe you can see and its the universe you can observe that is limited to c.
ok I am done. It probably could be explained much better. I may improve it later. So have I killed the myth?
.
[Edited on 24-10-2017 by wg48]
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Fulmen
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I didn't know there was such a myth. Of course you can add up velocities of to bodies moving relative to the observer, their combined speed would then
be limited to 2c if they move away from each other. But that's a trick of the light, so to speak.
We're not banging rocks together here. We know how to put a man back together.
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wg48
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| Quote: Originally posted by Fulmen | | I didn't know there was such a myth. Of course you can add up velocities of to bodies moving relative to the observer, their combined speed would then
be limited to 2c if they move away from each other. But that's a trick of the light, so to speak. |
That’s not just a trick of light. The LHC collides two oppositely moving proton beams at an intersection point. Each of the beams are moving towards
the intersection point at speeds of more than 0.99c but less than c. Not all of the protons collide. So the ones that don’t collide move past each
other and separate at almost 2c. Protons are hydrogen nuclei, small bits of matter not photons. However the speed at which one bunch of protons moves
relative to the the other bunch of protons in it's inertial frame is not almost 2c it is just less than c due to SR. Nothing can ever be observed to
move faster than c in the inertial frame of the observer thats SR in action.
Good I seem to be making some head way.
So now lets take say 10 antimatter powered rockets. The rockets are mounted on top of each other. So the first rocket can accelerate its self and the
other nine to 0.9c. Then the second rocket accelerated its self and the other 8 rockets to 0.9c relative to the first rocket. Then the third rocket
… and so. The rockets would have to progressively get much smaller than the previous one for this to be remotely possible.
So now we have 10 rockets moving away from the earth. The first rocket moving away from the earth at 0.9c. The second rocket moving away from the
first rocket at 0.9c. The third moving away from the second at 0.9c…. and so on.
So the velocity between successive rocket is 0.9c. In cosmology terminology the commoving velocity between them is 0.9c Making a comoving velocity
between the earth and the last rocket 9c.
If you viewed the rockets from the earth the first rocket would be moving at 0.9c but thanks to SR the second would be observed to be at say 0.92 w
The third and at a little more and the last rocket would be moving at say 0.99c. You would also observe the distance between the rockets to
progressive reduce in the same way the velocities do again thanks to SR.
No lets extend the line of rockets to infinity. It would not matter which rocket you where riding on. In each direction you would see a line of
rockets moving away from you getting a little bit faster each time but never faster than c. You would also observe the rockets getting closer and
closer together getting infinitely close to together at some position. I am assuming your observing with a special a entanglement sensor so there is
no travel time associated with your observation.
Now lets replace the rockets with galaxies and we have to use an optical sensors for the observation so there will be delays in our observations. We
should also add gravity dark matter, dark energy and probably a few other things but they are smallish effects. So now you have a conceptual model of
universe and no part of it is moving faster than c relative any other part of it, except in a comoving sense WHICH DOES NOT COUNT.
There is also the start up problem which determines the position of the observed asymptote of distant galaxies. Perhaps I will get to that later.
Is the myth dead and leid to rest now?
Feel free to ask thoughtful questions or comment.
Edit:corrected some spelling errors and added improvments
[Edited on 25-10-2017 by wg48]
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