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merle van osdol
merle van osdol
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Do atoms of matter repel

Do atoms of matter repel atoms of antimatter?
I am talking about things with significant mass rather than photons and such.

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

— Salman Rushdie

Mike Hewson
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RE: Do atoms of matter

Quote:
Do atoms of matter repel atoms of antimatter?
I am talking about things with significant mass rather than photons and such.


ie. does antimatter fall upwards ?

This is has been directly tested with anti-hydrogen atoms I believe, but inconclusively ie. neither up nor down was seen. This is because of the extreme technical difficulties inherent in this type of thing, basically having antimatter in free-fall without hitting matter and annihilating.

Theoretically there are ( at least ) two significant problems :

- major conservation laws ( energy/momentum ) would be breached. For these to be preserved there would have to be some other new principle, or breach of yet another existing one, to save the day. In other words one can't 'just' tweak antimatter behaviour in that one aspect and have business as usual. Remember that by definition and observation ( identically charged matter and antimatter accelerate in the same direction under electromagnetic fields ) : antimatter is not negative mass, which is what is really being implied here. The proposal would put antimatter into equations with a negative sign anywhere that matter is put in with a positive sign. So IF one chooses to do that consistently there is a ripple effect through other theory and clear contradiction with the observations upon which they are based. If not applied consistently, then you have to explain the choice/mode of inconsistency as well ie. create 'epicycles'.

- the Weak Equivalence Principle would be invalidated. This principle is at the core of GR and states that inertial mass is the same as gravitational mass. If the WEP is wrong then, for instance, the otherwise excellent agreement b/w GR and the observation of Mercury's orbit needs re-framing. Or one would have to posit that the very high precision torsion balance experiments were just accidentally in line with theory. Or that the Taylor-Hulse pulsar has some other factor/effect perfectly mimicking GR ..... you get the idea.

That's why I am so harsh on

Quote:
... until we can get an experimental (or observational) response ...


and the 'dissipate some prejudices' byline. So what next : experiment shows that the Universe hates you ? If reality contradicts your own head, get over it or not as you please, but let's not blame some rupture of science practices or etiquette of colleagues. Why refer the issue to a discussion of the sociological milieu of the craft group at all ? Why did he let drop that he had written a science fiction novel explaining the theory ? :-))

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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Further thoughts. There are

Further thoughts. There are deeper issues again.

(A) Antimatter was theoretically predicted by Dirac ( then promptly observed by Anderson ) based upon combining special relativity with quantum mechanics ie. relativistic quantum mechanics. Feynman later had his own quite clever derivation of the same by interpreting anti-matter as matter going backwards in time. Specifically you can't combine SR with QM and not get antimatter.

So Special Relativity combining with Quantum Mechanics has worked out very well giving a quantum field theory framework into which many force types now sit and are united. But not gravity. The trouble has been that when one mutually accelerates reference frames, compared to the constant relative frame velocities of SR, then relativity as GR 'breaks free' as it were from quantum interpretation. For myself I find that fascinating.

The key, as always with The Relativities, is time. Acceleration is absolute in the sense that a reference frame in and of itself can detect that it is doing that. Clocks in such a frame that have been synchronised ( via averaging the delay of two-way photon transit ) will slip out of synchrony. In practice two clocks linked by a given 'rigid body' will progressively increase in their disagreement despite prior adjustments. You would have to keep passing photons on round trips to keep the clocks agreeing. Without that some clocks will be measurably slower/faster than others within that frame - to be distinguished here from clock rate disagreements across frames.

If one takes the view that the physical signature of gravity to observers is tidal behaviour, or induced differential force on objects, then that is where the quantisation problem is manifest. That makes some component of particle energies dependent on where they are in the frame, because the frame itself is accelerating. So energy differences for that component are not localisable. You have to compare across a frame to get that energy differential due to said frame acceleration. If you like, identical rest mass objects will acquire different kinetic energy increments because of the frame behaviour. They have suffered different ( apparent ) force values and thus change their mutual separation in a manner distinct from what they would have done if not in an accelerated frame. No locality, no quanta, and jumping to another frame won't help.

Then via the Equivalence Principle a frame acceleration can be interchanged locally with gravity. Locally here means both over a small enough distance and for a short enough time.

(B) Now there is a sense in which an anti-gravity of sorts can apparently exist. Go far out in space and then start rotating some circular object around its centre of mass. You could visualise being attached to some wheel-like space station here if you like. The force you feel, and very real at that, will be towards the 'outer' and not the 'inner'. The further you go from the centre of rotation the more g's you will feel. The slower clocks are off hub. But this is simply understood with GR where we invoke a negative curvature to spacetime* and not the usual positive one around gravitating bodies. You certainly don't need negative masses or anti-matter for this. We are still using masses with a plus sign in the relevant equations.

To explain this in rough terms : with a greater radial distance from centre the tangential speeds ( going around at a fixed radius ) are increasing. By SR ( which you can apply closely for short enough periods/distances ) then the clocks will get progressively slower as you recede from centre. Likewise the measured circumference at some radius will be progressively less, with distance from centre, than it ought be because of Lorentz contraction in the tangential direction. Note that a ruler aligned in the radial direction has no Lorentz contraction, as it's movement is perpendicular to it's long axis. So that means PI = circumference / diameter will be less than the value if the rotation was not present. You may wish to compare that with the converse, that being around a 'normal' gravitating body : PI is greater than flat spacetime and the slower clocks are closer in. So that is another demonstration that acceleration is absolute, can't be transformed away, and can be quantified by observations.

[ Now if you're a really sharp punter you might think : this dark energy stuff which is pushing galaxies away, might that mean the entire universe is rotating and we are seeing an apparent/inertial effect of that ? It's an intriguing idea, but you'd need a centre for that and that would induce lack of homogeneity/isotropy in structure and observations on that. So now it becomes a scale question. Can we see enough of the universe to know that to be true, or not ? After all two ants on our Ferris Wheel Station that are only slightly apart and haven't been around long enough for light to travel to them from distant parts of the FWS, may not be able to measure any difference from isotropy/homogeneity. That is, the Weak ( or Strong ) Equivalence Principle is a local one. Now the inertia concept goes way back to the time of Ernst Mach and Newton's Pail experiment, and while inertia has a quantum answer there is no classical equivalent of the Higgs field. ]

Cheers, Mike.

* This is not as fictional is seems. Some nuclear subs have inertial guidance based upon laser devices that leverage the very GR effects that we discuss here. This only relies upon measurement within the submarine and is not related to GPS usage, except maybe to fix position on occasions from which the laser ring will measure acceleration. Thus acceleration is integrated to velocity, velocity is then integrated to give a nett displacement from some base point. In all three spatial dimensions. So such a submarine, even if motionless with respect to the sea floor, will deduce the sum of the presence of the Earth ( gravitational 'attraction' ) and rotation of the Earth ( centrifugal 'repulsion' ) ! :-) :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
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RE: My flying car will of

Quote:
My flying car will of course use antimatter to produce lift.
I will name the first prototype the Hindenburg in honor of another aircraft that produced lift using totally safe means.


LOL. Oddly enough given the nature of matter/anti-matter annihilation it would be the case that absence of evidence would be evidence for the absence ! :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

merle van osdol
merle van osdol
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Why couldn't the missing dark

Why couldn't the missing dark matter be made up of trillions and trillions of very small black holes created early after the big bang?

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

— Salman Rushdie

Mike Hewson
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RE: Why couldn't the

Quote:
Why couldn't the missing dark matter be made up of trillions and trillions of very small black holes created early after the big bang?


That is one thesis for sure. Next problem : how would one demonstrate that in distinction from other possibilities OR if you like what is the unique observational signature of that to base a search upon ?

{ I was discussing a recent medical 'breakthrough' with a colleague recently. The issue in logic here is that - for a given hypothesis and collected evidence - "is consistent with" is not even an adjacent state of "has been shown". These two are quite different and very frequently confused ( sometimes deliberately and other times just stupidly ) with the basic confounding factor being the total set of hypotheses being considered. Many so-called modern 'sciences' manage this area simply by a priori exclusion of theses. As an analogy modern media outlets use the practice of 'producer capture' : so higher management can say they don't influence production decisions. However producers are selected, in the full knowledge of all parties, of what their output is bound to be. Alas much of alleged independent scientific 'evidence' is of this nature. It's an ancient technique : he who sets the agenda wins the day. }

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

merle van osdol
merle van osdol
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Antimatter is the

Antimatter is the inverseUniverse. Gravitationally not only does it repulse normal matter, it repulses it increasingly with distance. AntiElectromagnetism does not emit radiation but absorbes it.

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

— Salman Rushdie

Ole
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As I understand the expansion

As I understand the expansion of the universe is due to the energy density.
And how about energy and gravitational waves. Does a gravitational wave have impulse? How large can a gravitational waves be? Is gravitational waves a compression of space or time or both? If I have a liter og milk in the supermarked and a gravitational wave passes through will the density of the milk change? And can a gravitational wave burst a soap bubble?

Bill592
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RE: So such a submarine,

Quote:
So such a submarine, even if motionless with respect to the sea floor, will deduce the sum of the presence of the Earth ( gravitational 'attraction' ) and rotation of the Earth ( centrifugal 'repulsion' ) ! :-) :-)

If the Earth 'stopped' rotating (not suddenly) how much more
would we weigh ? without the centrifugal repulsion ?

Bill

Mike Hewson
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RE: As I understand the

Quote:
As I understand the expansion of the universe is due to the energy density.


Two parts :

- the fact that it is expanding at all we put down to initial conditions ie. the Big Bang. Why that happened at all is another question.

- the fact that the expansion is accelerating we attribute to the vacuum energy density. That's essentially a deduction from the mathematics of GR though.

Quote:
And how about energy and gravitational waves. Does a gravitational wave have impulse?


Impulse* meaning the integral of a force over time ? Yep. I could be sitting around with not alot happening, with my handy LIGO device monitoring stuff, and then a wave turns up because say, two black holes have collided and the wave from that reaches here. If I choose to interpret the test mass movements in a classical force paradigm ( which I may in a particular sense ) then I could sum that over some relevant interval.

Quote:
How large can a gravitational waves be?


In terms of amplitude - generally quoted as spacetime strain or fractional deviation from 'flat' - no theoretical upper limit. It will be due to whatever kicked the wave off and thus is mechanism dependent. What is measurable will depend upon proximity & orientation to the source ie. it will be more closer in.

Quote:
Is gravitational waves a compression of space or time or both?


SHORT ANSWER : Yes :-)))

LONGER ANSWER : Relativity has this core concept of 'frame of reference', essentially each one of these is a ( perhaps hypothetical but feasible ) data collection system constructed in a certain manner. Often 'reference frame' is substituted with the word 'observer' though. In fact, and in theory, we don't have to assume the presence of a thinking being { there's a similar issue with the language of quantum mechanics which has triggered no end of confusion eg. Schrodinger's Gerbil }. A frame is not some guy sitting at a some coordinate origin with a clock and binoculars. The key issue becomes how to compare/translate the measured values in one frame to another.

The reason why we have to stuff about with this at all is because of the speed of light. It is a finite value. So it takes time - whomsoever is measuring - for any effect to travel. Light is another way of saying 'the electromagnetic force' and to date has been found to be the fastest of all force types ( although gravity is equally paced ). Light also travels at the same speed regardless of which frame we choose to test that in. That yields the specifics of the coordinate transforms because we can use the travel of a particular light beam to calibrate frames against each other.

Generally a particular position in space at a specific time is called an 'event' ( even if nothing 'happens' ). It's a shorthand for a set of four co-ordinates ( 3 in distance and one of time ) with respect to a frame. The distance measuring does not have to be of right-handed orthogonal/Cartesian structure. Indeed most of the mathematical formulation of GR deals with frames in a generic sense ( via tensors ) with a specific choice of system inserted when hard numbers are wanted. Typically we have frames moving with respect to each other ie. we can view spacetime events using one of many possible choices of measurement system. Here we are reasonably assuming that the universe exists independently of choices of description. So a star blowing up for instance will ( eventually ) blow up in all frames. For each frame the numbers defining the 'blowing up' event will differ, but are consistently translatable amongst frames according to various rules within The Relativities.

[aside]
There are some obvious 'self checks' here. So for a given event I may translate a given set of 4 numbers ( the event coordinates ) to a new set of 4 in another frame. Then if I translate from the second back to the first I should recover the original numbers. For three frames A, B and C then the numbers ought match if I translate from A -> C directly compared to A -> B followed by B -> C.

Also the reason why you see formulae like sqrt[1 - (v/c)^2] is that we frequently apply a variant/equivalent of Pythagorus' Theorem in our derivations. Basically Pythagorus asserted that there are no shortcuts, there is a single 'shortest' route b/w two points and thus others are 'longer' than that one.
[/aside]

It is not mere measurement though. Measurement is code for reception of a force effect. A wiggle over there will be delayed in causing any response, by anything, over here. People don't have to be involved. This is why it is important to remove any anthropocentric aspect to relativity discussions. We are actually talking about the ordering of cause and effect in the universe which is independent of our presence, even though our devices bring the record of happenings back to human scale.

As an overall comment the idea of things happening 'simultaneously' is a guarded concern. There is no universal clock for everyone to refer to when determining the order of events : earlier, same or later. It depends on the frame of measurement chosen. Roughly speaking : supposing you were to receive light emanating from a given source at some place and time. I could initially be 'in' your frame but then move relative to you either toward or away from the light source and thus receive that light myself either earlier or later respectively. Because light speed is finite and I can traverse a distance while it is propagating. My movement has changed, for me, the interception of the light 'message' compared to you.

Finally the upshot is that : some degree of difference in physical distance b/w two events for you I can at least partly convert into a time difference for me, and vice versa. There is however a sort of aggregate that remains a constant for two given events. This is called the 'spacetime interval' and is almost, but crucially not exactly the same as, a Pythagorus construct extended to four dimensions.

To ground this more firmly, let's sit down at the corner station at a LIGO. Basically it counts photons and when all other disturbances are removed this count represents the phase difference in light passage along two paths, those being the interferometer arms sited at a right angle to each other. If that phase shifts then the count changes and the photodetector picks that up. Phase when referring to wave phenomena indicates at what point along a wave cycle am I ? The wave travels in space and time ( or if you like spacetime ) so I can move from one phase value to another either by waiting or by moving along. So when the photo diode output changes I am at liberty to ascribe that to an alteration of distance in the arms or an alteration in the rate of time in the arms, or some combination of those.

I do hope that helps ! :-)

Quote:
If I have a liter og milk in the supermarked and a gravitational wave passes through will the density of the milk change?


Briefly yes. I don't reckon that could ever be verified though. We have to use light to measure gravitational waves and the relative strengths of gravity vs. electromagnetism is too great for the scale of milk bottles. As we speak the LSC is intensely struggling to detect an alteration of nuclear width over the distance of several kilometers, which even factors in the 'folding' of the beam to an effective ~ 800 kms.

Quote:
And can a gravitational wave burst a soap bubble?


Yes & ditto. But if it did it would mean you were really close to some big astrophysical event. Like you could see a black hole out of your window type of thing ... :-)

Cheers, Mike.

* Technically this refers to a force times a time, and as force is the time rate of change of momentum then an impulse ought be equivalent to a nett momentum delivered. To be exact it is a little unclear to me how this is managed in GR for an extended object like an interferometer, but I'd assumed there must be some type of 'punch' - from the gravitational wave - occurring to change the state of the gadget. I could of course be wrong. :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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