andrew s

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 

30 minutes ago, iantaylor2uk said:

Another issue is that it is well known that fractals appear almost everywhere in the natural world (see for example Mandelbrot's book). These usually arise due to scale invariance and simple rules. However, such curves are usually not differentiable, so standard methods of analysis (calculus) are not easily applied to them. If space-time is in fact fractal, then a lot of the maths being done may need to be revised.

The interplay between mathematics and theories physics is fascinating.  Sometimes mathematics has led other times physics. Several attempts at using new maths have been tried as by Penrose and Twistor Theory. 

Personally,  I don't think we are lacking in ideas and effort by theoretical physicists or in mathematics.  Rather, we don't have any new observations to motivate particular lines of enquiry. 

The LHC has failed to find convincing evidence for physics beyond the standard model.  Cosmology has dark energy  and matter but efforts to go further refining what they are other than numbers in LCDM lack motivation. 

I hope the JWST can find some new observations to light the way.

It seems we have reached a point where we have probed most of the scales and energy ranges experimentally available to us on Earth so we are left with looking up at the sky for guidance.

Regards Andrew 

In all these discussions it's important to remember that modern physical theories are just mathematical models. 

The best theories give valid predictions within their range of applicability. All theories have limits.

If you want more look to philosophy or religion. 

Regards Andrew 

40 minutes ago, iantaylor2uk said:

I don't think you have answered the key question of whether particles are "in" space-time or some how "sit on top of" space-time.

I've seen the explanation in terms of field before but this simply begs the question as to which is the more fundamental, space-time, or fields?

In QM space-time is the geometry on which the fields are described.  Particles are elements of our models that describe what we observe. What they are in reality or if they exist as in Plato's forms is anyone's guess. 

This is the paper on gamma rays mentioned. 

Regards Andrew 

Firtly, no theory is ever "right" just more or less accurate at prediction. No one knows what reality is .

In quantum mechanics particles are localised excitations in quantum fields which are continuous.

While some measured values are indeed found to be discrete they in reality are not a single precise value but have a range. For example , spectral lines width depends on the life time of the states involved.

It maybe that a better set of theories of reality based on discrete components exist, but we don't have one yet.

Regards Andrew 

PS Several attempts have been made to create space time etc. from basic discrete elements but have not been successful.  

59 minutes ago, iantaylor2uk said:

I'm suggesting that if mathematical models of physical processes predict infinities then it is the model that is wrong, even if it may give a good description of the physics.

A model in my view is right or wrong in how well it predicts the observation we can make, not one's  personal philosophical preferences. 

Virtuality all of our physical model rely on the continum. Space and time are continuous in SR, GR and QED not to mention all classical physics. 

1 minute ago, iantaylor2uk said:

when we have leant over the last 100 years or so that physics is essentially digital and not continuous at the smallest scales.

Can you give any examples? It's not true in quantum physics as many mistakenly think. Even spectral lines are broad to a greater or lesser degree.

Time of flight companions of light of different frequencies from distant events showed none of the expected differences expected from quantized space.

We can agree to differ 😉 

Regards Andrew 

20 minutes ago, iantaylor2uk said:

I've been thinking a bit about these various videos, and the potential links to physics. 

I think a lot of problems arise when we think of infinity. 

I would suggest that infinity is a purely mathematical concept, and in fact never exists in real physical systems.

There are a lot of particles in the universe, but I don't believe there are an infinite number. There are space time theories out there (loop quantum gravity for example) where there is a minimum size for space-time - crucially it can't get infinitesimally small. Some people may like string theory, but as I understand string theory just assumes space-time is there so is no help in understanding space-time.

I know there are some cosmologists out there that think the universe is infinite - this is simply untestable, so you could argue it is not really a valid theory (since theories need to be testable, in my view).

It would be interesting, I think, to further explore what the implications are if we decide, in principle, that infinities never exist in real physical systems. 

Interesting, you dismiss the current standard cosmology as untestable as it includes an infinite flat space but not loop quantum gravity or string theory(ies) which don't have any testable predictions!

Personally I have no idea if reality includes infinities or not but that does not stop very good mathematical models of reality (our physical theories) including them!

Regards Andrew

Obviously, the subject matter is of interest giving both a chance to learn and contribute. 

However, with some long past exceptions (one when I was reported for abuse) it is the civility of the discussions, the good humour (and bad jokes) that keep me coming back. 

This is in stark contrast to say the RSPB community which I recently joined.One where I had expected the same friendly behaviour as at their sites. However, some of the replies just make me wince and reluctant to post.

Regards Andrew 

I think I will ask my bank for an increasing overdraft of 1,2,3 thousands pounds per day and then claim they must pay me £1000/12.

Wish me luck.

Regards Andrew 

I think Groucho Marx had it about right on clubs 🤔  Regards Andrew 

This will probably never be settled but just to spice it up what should we call kit licking, tripod sniffing,  kit collecting, Takahashi obsessives like @JeremyS ?

Regards Andrew 

For me a stargazer romantically engages with the heavens while an astronomer intellectually engages with it. I see no reason why you can't be both, possibly at the same time.

Regards Andrew