andrew s

35 minutes ago, scarp15 said:

Very nice have one of those bolt cases (which are not as easy to come by nowadays I believe), which I've used to securely transport a 31mm Nagler. Quite agree with you concerning UK bicycle parts manufacturer Hope. Have a set of their wheels and hub on one of my bikes and as a convert to Flat pedals, two bikes now have fitted Hope Flat pedals, quality of engineering and design is superb. 

... and have the noisiest rear hubs going. I free wheel to warn pedestrians (on a cycle path) of my approach.  

Regards Andrew 

Just seen on the BBC Web site that gene therapy has allowed a child who was born deaf to hear without aids. Her treatment started just before she was one and 6 mth later can hear sounds as soft as a wisper and is starting to kalk.

Just brilliant. 

Regards 

I have just sold my last astronomical telescope so it's very quiet here.

Regards Andrew 

I used to joke that physics is just mathematics with boundary conditions.

It's perhaps more than that. Physics is applying mathematics to an end while for mathematician it is an end in itself. 

It would, however, be hard to tell some theoretical physicists from some applied mathematician (beard length perhaps?). 

If it works physicists are content to use it even if it is not subject to a rigorous proof. As Godel proved there may be accurate theorems that can't be proved! 

Regards Andrew 

2 hours ago, JeremyS said:

Likewise remiss regarding Bernard Lovell.

 

Strange the small gap between saint and sinner. Had not Sputnik 1 been launched when it was he might have gone to jail for overspending public money 🤔 

Regards Andrew 

The interplay of mathematics and physics is fascinating.  At times maths has led e.g. Riemman geometry required for general relativity and others where physics led e.g. the Dirac delta function eventually made respectable by mathematicians with via the theory of distributions.

Regards Andrew 

1 hour ago, vlaiv said:

Important thing to note is that this model does not have equations like we are used to.

It can only be solved by numerical methods / via computer because it is set of differential equations without nice analytical solution.

Observational data is fed into computer program and best fit is produced. This best fit yields certain functions and numerical values for constants that we have as "solution" to the model.

 

Actually,  that's the norm in physics. Even the 3 body problem in Newtonian physics has to no analytical solutions. It is also true in quantum mechanics,  fluid mechanics and solid state physics etc. etc.

Regards Andrew 

I can recommend "An Introduction to Modern Cosmology" by Andrew Liddle. It should provide the background you need.

Regards Andrew 

You pays your money and takes your choice.

Regards Andrew 

I have not studied any of the AI noise reduction methods but your proposal, at face value, makes sense. 

All image processing loses information.  However, it allows us to modify an image so we can visually see the "features " in the image we want to bring out.

At a practical level it would be easy to do both (AI then bin or bin then AI) and compare the result.

Regards Andrew 

3 hours ago, vlaiv said:

more sensitive when paired with said optics is:

area_of_pixel * QE_at_wavelength

In another words - if you have camera with 3.8um that has 50% of QE at Ha line and you have camera with 2.4um with 85% QE at Ha - first will be faster if you consider using them with the same scope because

3.8 * 3.8 * 50%  = 7.22

2.4 * 2.4 * 85% = 4.896

7.22 > 4.896

Yes that's true per pixel but for the same total effective area of sensor, the one with the higher QE will detect more photons. 

Which will give the cleaner image will depend on read noise,  telegraph noise and other sensor parameters.

Regards Andrew 

1 hour ago, vlaiv said:

I always wondered how large this effect is - particularly on galactic scales.

For example - when computing Hubble's law - do we have to take into account relative difference between galaxy masses? Origin galaxy and MW?

When light leaves origin galaxy it will be red shifted, but then when it "falls into" MW it will be blue shifted - difference between those two will be some percent of total red shift - but how large is the effect?

See here the sun's gravitational [removed word] is approximately equivalent to 633 m/s receeding. Regards Andrew 

Removed word was shift !

On 20/04/2024 at 11:07, ollypenrice said:

I realize that the Doppler redshift is a product of velocity, not acceleration, but my point is that, to have a velocity, an object must at some point have been accelerated.

All velocities are relative. You only have a velocity with respect to something else. Change the "something else" and you (may) change the velocity.  Being earth bound we tend to only think of a singular velocity relative to the ground.

Regards Andrew 

19 minutes ago, Brian O said:

where can we start any measurements and what form of energy existed?

Currently,  the earliest data we can get is from the CMB. We may potentially be able to get information from before that from primordial gravitational waves but don't hold your breath. 

The infered initial ratio of H to He & Li also give information on the first nuclear synthesis and hence the conditions at that time.

Regards Andrew 

Just now, iantaylor2uk said:

The only way time is measured in relativity is by using photons - think of special relativity where Einstein uses mirrors and photons to explain the change of length with speed. 

If there aren't any photons, there is no way of measuring time, and so I doubt that special and general relativity apply in an era before photons. 

In GR spacetime is just geometry with a well defined metric. Add an equation of state of the contents and you're away.

To provide experimental data to test such a mathematical model you don't just need photons you need to build both clocks and measuring sticks and an intelligence to use them. 

Once you are reasonably happy with the models predictions v your observations you can extrapolate to the time before clocks (or photons) as time like space is just a parameter in the model. The tricky bit is having an equation of state not the geometry. 

Regards Andrew 

The speed of light isn't really (or just) the speed of light! It's is a shorthand for the upper speed limit to the propagation of information in a spacetime governed by GR. That's partly why we need inflation to explain the amazing uniformity of the CMB as otherwise it could not be in the near perfect thermal equilibrium we observe. Without it, regions of space would get too far apart to "communicate" about their temperatures. Inflation stops them from getting out of equilibrium as the expansion is too fast to allow it.

Regards Andrew 

2 hours ago, robin_astro said:

What caused inflation is an open subject but why specifically is "faster than light inflation" during the inflationary period seen as a problem any more than it is now with the current rate of expansion which, depending on the coordinate system used, also implies "faster than light velocities". 

According to the description in the link posted by Andrew, inflation took place under conditions where "normal", (though extreme) physics holds.  Isn't the question of "faster than light velocities" at both early and late times therefore resolved within the framework of general relativity as Zermelo points out without the need of new physics?

Robin

You are Zermelo are quite right.

In the curved space time of GR comparing velocities of separated objects is generally impossible. However,  locally the speed of light is always measured to be the same i.e. c.

To see the issue of comparing velocities. Take two cars on opposite side of the equator heading north at a velocity v. Initially,  their relative velocity is zero. But, by the time they get to the pole they are heading for a collision at relative velocity 2v.

Regards Andrew 

11 minutes ago, Brian O said:

Can anyone help me out here, please?

This may help you. Regards Andrew 

Time has many faces. We have our experience of it passing, our psychological time, and its rate can slow and speed-up with our moods. 

In physics time is a parameter in a space-time geometry that we find effectively underpins our best theories. 

There is no requirement for them to be the same.

Regards Andrew 

The simple answer is we don't know. There are several theories covering the proposed period of inflation. 

It is still an active area of research but unless we get a quantum theory of gravity it is all very speculative. 

As @saac points out all the forces were unified. Particles only condensed out after inflation so our normal ideas of entanglement etc. don't apply.

Regards Andrew 

Just now, Ratlet said:

Wouldn't that be great?  Presumably if your head isn't causing a reflection, then you wouldn't interfere with the light path.  You could replace the secondary with an eyepiece.  You could do visual with a RASA at F2!

 

Yes but becoming a Vampire is a high price to pay 🧛‍♂️🦇 Regards Andrew 

2 hours ago, JeremyS said:

It’s not just the diffraction spikes you have to worry about.

Just because you don't have a reflection in a mirror @JeremyS is no reason to dislike Newts 😅 

Regards Andrew 

Platesolving should not depend on camera orientation.  Not sure what software you are using so can't comment on settings. Regards Andrew 

I have the Swarovski CL 8x25 a perfect pocket bino. Ideal for hikes and casual through the window observing. Not tried the Zeiss.

Regards Andrew 

We were out when the postman tried to deliver 3 parcels today. One required a signature. 

Later today he returned to try again as he had seen my wife jogging so new we were about.

Can't better that.

Regards Andrew