If Only I .....

Mike Hewson
Mike Hewson
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Topic 198206

.... had gamma photon detectors for eyes and a high resolution timing circuit in my brain and an enormous amount of patience and never blinked and held my breath in space for long enough, then I could have discovered this :

Seriously I have the closely read the abstract for the recently announced discovery and it occurs to me to emit some comments without polluting that thread with my armchair based fevered delirium. So here's my musings :

- a single gamma photon gets spat out of an electromagnetic maelstrom surrounding a gigantic conducting flywheel. It could have gone in any one of very many directions away from that dynamo gadget, but by chance came over our way ( most of its fellow photons have gone well away from us ). It didn't hit the Earth. It did miss alot of other things along the way though. It did run into/through a much smaller gadget placed nearby though. The FermiLAT. Fortunately the incident was recorded and amongst many more sibling photons made up a slab of data for Earthlings to later analyse.

- inverse square law. We have ordinary terrestrial seeing eyes. Maybe three photons of approximately the right energy could transition a single pigment molecule inside a cell upon your retina. Maybe if enough of those molecules do that within a single retinal cell, more or less within about a 50 millisecond span of time, then that set of events triggers the relay of an electrical message to the rear part of your brain. You would perceive and probably describe it as a momentary "flash of light" in your field of view.

But what would happen if you could get one flash per photon ? Well you could "see in the dark" for starters! But apart from that, and maybe restricting the input to the eye, most of life would be much the same.

Now imagine a friend walking away from you in the night-time holding a torch and pointing it back toward you. He could walk along way away before you would say "I can't see the light from the torch". But before that happened you would see the torch turn "on and off". There would be short periods of time when you would not receive any photons, and you would know that to be true because those single-photon-activated eyes that you now have are good enough to back that claim. The torch seems to sputter where before it was constant. You could check with your friend and confirm that it was still being held steady pointing toward you, the battery hadn't gone flat, the switch wasn't dicky etc ....

What we have here is the inverse square law but with quantum mechanics applying. Mr Einstein hit the centre of the target exactly when he invented the photon concept with his study of the photoelectric effect. If there are a gazillion photons per second blazing out from some source then they will distribute themselves collectively as if they constituted a classical ( spherical ) wave travelling outwards. If ones' photon receiver doesn't do one-at-a-time but can only respond to large bunches arriving sufficiently close in time then you won't know any different. There is an integration of signal and then a threshold is applied. But the further away you go each photon does not divide into fragments, they are all or none. What reduces with distance to source is the probability that a photon will arrive at a certain point. Sometimes that will resolve to be a 'yes' and sometimes to be a 'no'. With more no's happening further out than closer in.

[ if you really want to do your head in then consider that the probability/potentia must span across light years ..... for that matter who or what might catch gamma photons sprayed from the source in those other directions ? ]

You know this punchline ..... put your friend on a turntable and spin him up. About nine full rotations per second will do it nicely for this example. Now he is a lighthouse/pulsar beacon and you are FermiLAT. Only during some rotations yield a photon that you will see. Very many rotations have no photon that we have caught.

- there's a glitch !

Not only was it a detection but it showed the not uncommon finding of a sudden change in the rotation rate, presumably from some rapid shifting/settling of the stars' material. [ I hadn't realised this also involved ( doh ! ) a second derivative too, a transient in the transient as it were, which in my understanding ought say something of interest about the equation of state .... ??? ]. For this one needs a longer record spanning either side of the glitch. I think the article implies a pretty severe standard for the timing solution on this : one must be able to count each and every pulse ie. so that even when the source is not being looked at, when one comes back to observation there can be a firm claim as to how many rotations have occurred meantime*. IIRC the Taylor-Hulse system has had this degree of certainty since discovery, so that one can be given an exact integer count of rotations from then. It is this level of precision which embeds certainty into any theoretical understanding. This is the 'celestial clock' idea that pulsars are credited with. Imagine knowing exactly how many heart beats you have had since your birth ! Not one beat more and not one beat less .... :-)

- separating the influence of radiation from 'adjacent' cloud. For this one looks at the photon energies and finds that the pulse profile really sticks out more with higher energy. [ left side of top/first graphic ]. This means there is a general fuzzy glow about, uncorrelated with the pulsars' rotation, but the pulsar produces relatively higher energy photons than the glow does. You can think of this as a color comment if you like ie. translating this to human domain light frequency range : imagine looking at diffuse red glow in the sky but seeing within that a bright blue object pulsing. Indeed that glow being supernova associated it could also be the system from which the pulsar came from. But there are several nearby candidates for that, and that hypothesis in turn implies a kick-out from said system.

- they have bagged a most-wanted suspect using a blind search. While not perhaps Al Capone it is at least a Dillinger. :-)

- the team at AEI are writing the textbook as we speak for this sort of thing. I get the sense that this was a planned approach to localise this source in sufficient detail, with the disambiguation as outlined. Hidden In Plain Sight is quite a apt moniker. It was there all along but camouflaged.

Cheers, Mike.

( edit ) I can't wait for when we do this type of thing and much more with 'for real' gravitational wave data from LIGO et al .... intersecting studies from several observation modes on the one and the same object. Warts and all ! :-) :-0

( edit ) * Full-horror/key-point as follows ( my emphasis in blue ) :

Quote:
... When timing radio pulsar glitches, Yu et al. (2013) noted that unique solutions for glitch epochs could not be found for large glitches occurring during an interval between two radio observations. We observe a similar effect here, although our limiting factor is the photon flux. When phase folding, a full rotation can be lost/gained if the offset between the model glitch epoch and the true glitch epoch is more than 1/delta_f ~ 0.3 days; however an average of only 1.4 weighted photons are observed from the pulsar within this time, making this phase wrap simply undetectable.

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

Bikeman (Heinz-Bernd Eggenstein)
Bikeman (Heinz-...
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If Only I .....

Hi Mike.

Thanks for writing this up, I think it's an excellent account on why this discovery is so cool!

BTW, people who find this equally amazing and like to play around with single photons (or at least very few photons) and quantum effects in real life will find astrophotography and even more so amateur astronomical photometry (measuring brightness of stars) a rewarding hobby.

The (apparent) brightness of stars is usually given in magnitudes, a logarithmic scale, and the well known star Vega (in constellation Lyra) has magnitude ca. 0. A zero mag. star will send us (very roughly) 1 million photons per second per square cm of aperture in the spectral range of visible light. A human eye has an aperture of ca. 0.3 cm^2

5 magnitudes , by definition, mean a factor of 100 fewer photons per time per area, so 10 mag means a factor of 10,000 and 15 mag a factor of 1,000,000, so a 15 mag star, well within the reach of amateurs with modest equipment, will yield, on average, a single photon per square cm per second. At that level you will be able to observe that the photons entering into your telescope know their quantum physics well :-) : the number of the star's photons per pixel arriving in a fixed time interval will vary in a random fashion, where the standard deviation is ca. the square root of the mean value. That's nothing else but quantum noise. So the accuracy of the measurement gets limited by quantum noise and you will have to choose your integration time long enough (and the telescope's aperture big enough) to get meaningful results.

So to do photon counting at the regime of quantum physics, all it takes is a small telescope and e.g. a DSLR. Have fun!

Mike Hewson
Mike Hewson
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Hi HB ! Cool it is indeed. In

Hi HB ! Cool it is indeed. In cricket terms I call this a 'six' or 'over the boundary'. I am absolutely fascinated by this single/few photon business. :-)

Many years ago ( l979 ? ) I had the pleasure of listening to and meeting Mr Bart J. Bok, a seriously good astronomer and very engaging fellow, at an impromptu lecture for the physics faculty at Melbourne Uni. He was passing through town, as it were, and he talked mainly of the 'globules' now named after him : the interstellar clouds from which stars form by gravitational contraction. At the end he delighted in showing us a small gadget in his hand. This was one of the first Charged Coupled Devices used in astronomy. He said this would forever change the way astronomers see the sky. I tried to look it up in the library, but being very cutting edge then, it wasn't mentioned. Finally I tracked down a reference ( courtesy of a friend in the Engineering faculty ) and read about this business of little buckets for photons and 'bucket-brigade' reading out etc. As this was prior to any of my QM courses then I only vaguely understood what was going on ( nearly all physics undergrads start with a 'classical head' because that is the way it is taught ).

Now there are pixels everywhere and each mobile phone has become a roving TV camera ! So as you say even a modest investment in optical equipment will give you far better technology than only a few generations ago. :-)

Cheers, Mike.

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

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