robin_astro

4 hours ago, Ed astro said:

Another way to display the changes is by plotting a heatmap of all the spectra

Impressive work again !   I used a similar technique to generate  the image in my avatar (but from data at a much higher frequency )

https://britastro.org/observations/observation.php?id=20100825_230000_95356e4359198628

Cheers

Robin

Hi Steve,

No need to patronise me. I am a career hardened  physicist happily living in a quantum mechanical and relativistic universe  😉  

The equipment is just an off the shelf astronomical spectrograph and my measurements were just to test its mechanical stability under different orientations, not to make some profound physical insight. I saw this thread and saw the measurements as an interesting  alternative perspective on the measurement of the speed of light which does not have the difficulties of  round trips or synchronised clocks covered in the article and videos, though it may have other difficulties which we are exploring here (It is clearly a fully one way measurement, no reflection, no return path. The light leaves one end and arrives at the other where the wavelength is measured from the diffraction pattern after being dispersed by the transmission grating. I don't understand why you are still disputing that). 

 According to the measurements, multiplying two independent locally measurable physical properties of electromagnetic radiation, frequency and wavelength gives a constant value, v which  is independent of the orientation.  (The experiment could be made more robust by actually measuring both wavelength and frequency at both ends of the leg rather than assuming the frequency is unchanged. Although this would need two clocks, importantly there is no requirement for them to be synchronised as in the time of flight measurement, just for them to run at the same rate which would be the case for two clocks at rest relative to each other as here, according to special relativity ) 

You offer QED as an explanation as to why this value v may not be the velocity of light and which might be constant even in the case of a universe where the speed of light is different dependent on the direction of travel of the light beam but I have only seen generalisations so far. (The quantum effect of the observer on the measurement and the properties of the grating are spurious here. I don't understand why you would introduce these except to muddy the waters)

I fully expect there is some reason it would give the same result even in a universe where the speed of light is directional  but as an experimental physicist I have the luxury of saying "here are my measurements, explain them 🙂"

Cheers

Robin

8 hours ago, billhinge said:

I don't say the speed of light 'isn't' isotropic so why do you assert I'm a believer of  alternative universes and physics? Proof and belief in assumption/axioms are subtly different

Sorry if I implied this, it was not my intention.  I invited you to demonstrate how a universe where the speed of light depends on the direction of measurement could be compatible with the results of my experiment which appears to suggest that either the speed of light is not direction dependent or as Andrew proposed, space is anisotropic (so the distance between two  objects stationary relative to each other depends on the direction of measurement).  Either viewpoint may be valid as Andrew proposed but personally I would then apply Occam's razor and chose  the simplest solution which is space is isotropic and the speed of light is not direction dependent)

The arguments put forward in the videos in the thread to support the impossibility of measuring c using one way time of flight measurements do not involve quantum mechanics. (Once we introduce quantum effects all bets are off 😉) They revolve around the impossibility of two observers agreeing on a common time, (which is correct and a consequence of relativity) and hence assert that any experiment to measure the speed of light must involve a round trip leading to the conclusion that the speed in each direction cannot be independently determined.  There is an important difference however between my experiment and one and two way time of flight measurements. My method uses one clock (The frequency of the light) and a unidirectional light beam (lamp, transmission diffraction grating, camera rigidly mounted in line, a fixed distance apart)

You can see the apparatus I use (an astronomical grism spectrograph with a built in calibration lamp) here 

https://www.shelyak.com/wp-content/uploads/Alpy600Demetra-510x320.png

The question I pose is why would this measure the same wavelength of the light independent of the direction it is pointing in if the speed of light is direction dependent ?

Cheers

Robin

5 hours ago, billhinge said:

I can (and most others do) measure the square mass of a neutrino to be a negative number but we choose to say the error on the calculation is just enough to to make it a small positive number and hence not an imaginary mass when square roots are taken. This was my final year grad project study on radioactive decay many years ago, neutrino mass being an easily calculated by product based on conservation of momentum. But it wasn't a direct neutrino mass calculation.

That is a "straw man" argument, irrelevant to the discussion here. What you are talking about there is experimental uncertainty. It is in principal possible to measure the mass of the neutrino (and possibly will done be at some point in the future) The assertion made at the top of this thread was that it was impossible in principle to determine if the speed of light was the same independent of direction since it could only be measured in a round trip.  I offered a way that it could in be measured one way (in principle and in practise to any degree of precision one chose)

Cheers

Robin

4 hours ago, billhinge said:

you are missing the point, sure you can calculate a value for c from other formula such as Maxwells eqns 

No I am not. The original hypothesis at the top of the thread  was that it was impossible to measure the speed of light in any one direction. My method (looking for any change in the diffraction pattern produced by a beam of photons sent in different directions) just uses a clock (the frequency of the photon generated by a transition of an electron between two energy levels in an atom),  a yardstick and geometry.  The only formula is v=lamda*f   which identical to that used to calculate the velocity in the return trip experiment (v = 2d/t) except in my experiment the beam is only sent one way. If the velocity of light was direction dependent the wavelength (as measured in the diffraction pattern) would be different. It is not, therefore the conclusion is that either the speed of light is independent of direction,  or  space is not isotropic such that the measurement of distance is in some way directional, (and in some peculiar way, since the diffraction pattern is produced orthogonal to the direction of the light beam)  You are welcome to formulate such an alternative universe and propose how it may be tested.

Cheers

Robin

53 minutes ago, billhinge said:

You always need a two way trip

No you dont. See my suggestion here (and subsequent discussion) which only relies on space being isotropic.

and continued here

Cheers

Robin

There is also a 9 year old supernova SN2013ej still visible in the JWST image of M74 

https://www.wis-tns.org/object/2013ej

https://www.wis-tns.org/astronotes/astronote/2022-147

Robin

It could be anything eg a foreground object.  First step would be to compare it with archive images though it might be tough to find ones deep enough.  Is there a fits version of the image with coordinates (WCS) so we know where exactly it is ?

This is the possible supernova they claim to have found comparing with archival Hubble images

https://www.wis-tns.org/object/2022owj

https://www.wis-tns.org/astronotes/astronote/2022-145

but at mag 25 in the IR it is unlikely anyone will get a spectrum to confirm it

Cheers

Robin

On 22/07/2022 at 09:13, gilesco said:

"Cows, small or far away?"

Bizarrely in our universe this is only true out to about 1.5 redshift. Beyond that all cows look about the same size or even larger 🙂

https://arxiv.org/abs/astro-ph/9905116v4

Section 6 "Angular Diameter Distance"

10 minutes ago, robin_astro said:

Very true. They have no concept of time, a bit like me these days 😀

In which case substitute neutrino for photon in the above 🙂

3 hours ago, andrew s said:

photons don't have a reference frame. If they did they would be a rest in it? 😊

Very true. They have no concept of time, a bit like me these days 😀

26 minutes ago, robin_astro said:

In  who's reference frame ? not the photons 😉

 

 

....and any case, since the  (apparent) magnitude is a measurement of the photon flux at the earth, the brightness did peak at ~mag 12 in August 2013 😉 

3 hours ago, StuartT said:

...although probably a bit earlier back in time than that, in all fairness.. 😉

In  who's reference frame ? not the photons 😉

On 19/07/2022 at 08:00, IB20 said:

Some nice images of Messier 74 yesterday from JWST

They have spotted in that image what is left of SN 2013ej , a supernova which reached a bright mag 12 back in August 2013

https://www.wis-tns.org/astronotes/astronote/2022-147

Robin

Now there's an interesting challenge, to observe an occultation of a star by the ISS. I wonder how narrow the path would be ?  I did catch a Jupiter transit quite a while back though when the data to be able to predict  this sort of thing were not so widely available (and the ISS was quite a bit smaller)

http://www.threehillsobservatory.co.uk/astro/astro2_image_67.htm

Cheers

Robin

15 hours ago, Mandy D said:

Thankfully things have changed.

Amen to that, though there is still some way to go in Physics. When I started my degree in 1969 there were just three women in my year (~5%). That has improved to ~25% now I understand

Cheers

Robin

21 minutes ago, robin_astro said:

The "recession velocity"  depends on the observer (relativity) and the cosmological model so you can't really talk about a recession velocity.

If you want to dig further down this rabbit hole then you could visit  NED Wright's cosmology FAQ 

https://www.astro.ucla.edu/~wright/cosmology_faq.html

Cheers

Robin

11 minutes ago, robin_astro said:

The Hydrogen Lyman alpha line, at 121.6 nm in the UV at rest is commonly used to measure objects at high redshifts. This will therefore be at 7x121.6 nm or 0.85um so at the short wavelength end of the  JWST spectrum

Extending this to more familiar wavelengths, at redshift 6 H alpha is at 7x656.3nm or 4.6um (JWST can see down to 28.3 um so theoretically  it could see H alpha at 42 redshift or 60 million years after the big bang !)

Robin

14 hours ago, ollypenrice said:

It seems to me that the diagram in James' original post is confusing. Why does the point of focus lie so far inside the tube?

It doesn't in most "real" refractors (well it might do if you rack the focuser fully out.)  If it did  prime focus astrophotography would not be possible. 

Cheers

Robin

The "recession velocity"  depends on the observer (relativity) and the cosmological model so you can't really talk about a recession velocity. Using the currently accepted parameters for the universe, objects from 1 billion years after the big bang will show a cosmological redshift ~6.  You can use Ned Wrights cosmology calculator to play with these figures.

https://www.astro.ucla.edu/~wright/CosmoCalc.html

The Hydrogen Lyman alpha line, at 121.6 nm in the UV at rest is commonly used to measure objects at high redshifts. This will therefore be at 7x121.6 nm or 0.85um so at the short wavelength end of the  JWST spectrum (it is even in the passband of amateur CMOS/CCD detectors.)   Here for example are some of my spectra showing Lyman alpha of objects at 4.5 redshift, shifted in to the red regiom of the visible spectrum (~1.3 billion years after the big bang)

https://britastro.org/observations/observation.php?id=20210411_134753_85f4b3ebf4faaefe 

Cheers

Robin

They are the same thing. You remove the eyepiece and focus the moon on the card. For objects at infinity (like the moon) the plane where the image is in focus is the principle focus. (rack the focuser all the way in so the focal plane is outside the tube)

Cheers

Robin

8 minutes ago, robin_astro said:

 (Photo electric photometry (PEP) perhaps, though I know nothing about it)

If that is the case then PEP photometrists are pretty rare animals these days. The AAVSO PEP section would probably be your best bet for advice

https://www.aavso.org/aavso-photoelectric-photometry-pep-section

Cheers

Robin

What sort of photometry are you doing that requires just keeping the star within a 1-2 arcmin box ? (Photo electric photometry (PEP) perhaps, though I know nothing about it).  Most photometry these days is done by measuring from images where the star image has to kept be moderately tight and round eg within say a few arcsec at most during the exposure.

(Perhaps best moved to the variable stars subforum ?)

Cheers

Robin

9 minutes ago, robin_astro said:

Here is H beta in Vega for example

Contrast that with H Beta in supergiant star Rigel for example. Although similar temperature to Vega and more massive, it is much larger  so the surface gravity (which reduces as the square of the radius) is much lower so the lines (which are produced in the photosphere at the surface of the star) are less pressure broadened)

5 hours ago, Graham Beamson said:

Here are my Altair, Deneb and Vega spectra

The FWHM looks about right. The resolution of the ALPY600 is ~12A and the lines in giant stars like Deneb are narrow enough to estimate the resolution but the Balmer lines of main sequence stars like Altair/Vega are intrinsically wider than this so don't represent the true resolution of the spectrograph. Here is H beta in Vega for example