science fiction hour

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

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.

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.

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.

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?

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.

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.

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.

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 ! :-)

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.

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. :-)