1

Just Chat! / Re: It's too quiet: I'm seeking a recommendation for a thread to read.

« on: 11/07/2022 16:55:14 »

Ditto with my airsource heat pump.

I'm relatively pachydermous but along with my colleague and mentor the rhinoceros, I object to being labelled as a philosopher. That's an insult to a physicist! Scientists ask interesting questions in the hope of getting answers. Philosophers tell you that you don't understand the question.

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Chemistry / Re: Why does austenitic stainless steel fail to burn?

« on: 10/07/2022 20:05:45 »

It's an interesting question and I don't know the answer.
The melting ranges of stainless- and mild- steel pretty much overlap.

Nickel is easier to melt than iron, and harder to oxidise, but the melting point of the oxide is lower than for iron.
Chromium is harder to melt than iron and it is easier to oxidise than iron, but the melting point of the oxide is higher.

(with some hand waving added because the oxides aren't well defined and they will be mixtures anyway)

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Chemistry / Re: Why does austenitic stainless steel fail to burn?

« on: 10/07/2022 13:48:33 »

Plasma cutter works on all grades of stainless steels, using the available oxygen in the compressed air, and the electric arc, to cut it with ease. Lot faster than the grinder, and cheaper to run than the gas axe as well, just needs a lot of compressed air, and power for the arc. even works on other metals, including things like titanium and aluminium, and even copper, though the cut and finish can vary, it depends a lot on flow rates, cutting speeds and arc power.  Laser cutting simply provides the energy input using a laser of some form, generally IR to provide the energy to melt the material, and the air jet does the rest.

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That CAN'T be true! / Re: What causes motion sickness?

« on: 09/07/2022 13:46:45 »

What other explanation could there be for such dogged unwillingness to accept sound scientific argument together with error repetition ad infinitum?

https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect

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Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 08/07/2022 15:15:45 »

My malfunctioning engineering brain viewed the two frames as being sufficiently similar as to consider them equivalent.

As @Halc points out, Penrose is taking a presentist view and a Newtonian one at that. I think he is trying to show how the man in the street might view it by assuming the 2 participants share a common present - and in everyday life it is a good approximation.
One thing he mentions is often misunderstood by some visitors here. They assume relativity is the result of the time taken for light to travel from event to observer rather than that effect being an add on. He explains that information about the Andromedian fleet can only be accessed at the speed of light, so neither of the parties on earth can have knowledge of what is happening, on their present time slice, in Andromeda.

Edit: whoops, collision with ES. Different points however

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Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 08/07/2022 15:11:53 »

Hi.

I'm glad you're getting better @paul cotter .

A higher speed of one participant would give a greater slope to your green line.

   Yes, that's absolutely correct.   However, that's not where the emphasis is usually placed.
The emphasis is usually put on the massive distance between Earth and the Andromeda galaxy.   As such even a tiny slope on the green line results in a huge difference in deciding which event is happening now in Andromeda.
 
  So, the common development of the Andromeda paradox is that even if you did something like just turn or shake your head in different directions,  that slow movement is important when magnified over huge distances.   It's an effect of special relativity that doesn't require space rockets and travel at speeds close to that of light etc.

Best Wishes.

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Just Chat! / Re: The coronation of "eternal student", why was i not invited?

« on: 06/07/2022 12:10:49 »

Hi.

   I'm not annoyed.    The last post was meant to be humorous it just evidently didn't work well, sorry.
I should probably be catching up on all the housework I haven't done while I was ill,  instead of posting too much for the next few days.   Meanwhile, I'm glad you're still OK but I should imagine you will need to keep taking it easy for a few more days.

Best Wishes.

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Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 05/07/2022 16:31:28 »

Hi.
    No worries @paul cotter ,  rest up and get well soon.
Best Wishes.

9

Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 05/07/2022 16:02:05 »

Halc, I would argue that the two participants have contradictory positions: one says the decision has not been made yet and the other says the fleet is in transit already.

Yes,l those statements are contradictory, but neither of them says those things. Both statements presume absolute simultaneity by their omission of frame references, and it can easily be shown that neither postulate of special relativity can be true if the premise of absolute simultaneity (an intuitive assumption) is taken.

Part of the confusion is the statement of something not measured: 'The Andromedons are considering whether to attack' for instance is not something that anybody has measured. It is simply an abstract statement, a description of the scenario in question by the narrator so to speak. No observer can measure this, certainly not A and B. Likewise, if Andromedons are somehow considering an attack on Earth, it isn't against humans since there's no way they could have yet measured a species yet to have evolved. We are no more fact to an Andromedon obesrver than they are fact to us.

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Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 05/07/2022 15:00:42 »

Hi again.

    Well look, here's the thing from first principles...

    Start with a person called Paul on planet earth.   For ease of notation and calculation we will set up an inertial reference frame S  with co-ordinates (space, time) = (x,t) .  We'll only need one spatial dimension (just choose a Cartesian frame so that planet earth and the Andromeda galaxy lie along the x-axis).  We'll have Paul stationary in the frame S and, as usual, we'll put him at the origin.
    To keep it all simple, we'll have the Andromeda galaxy at a distance of  3 million light years from earth and we'll have it remain stationary in the frame S.     (In reality it's about 2.5 million light years away and you probably know it's on a collision course - none of that's necessary for the Andromeda Paradox, we just need it at a large distance).

    Let's draw a spacetime diagram now and put some important worldlines on it.


Andromeda1.png (12.8 kB . 1304x767 - viewed 1534 times)

   Since Paul and the Andromeda Galaxy are stationary in this frame, their worldlines run parallel to the t axis.   Paul's is precisely along the t-axis.

    Now the general idea is that there are some Aliens living in the Andromeda galaxy and they will decide to invade earth.    Let's mark some important events on the spacetime diagram.     Event (x_A, t_m )   =   where and when the Andromedans have a meeting to consider invading Earth.       Event  (x_A , t_L ) =  where and when the Andromedans launch the invasion fleet.
   Paul considers events lying along the x-axis to be events occurring at time t = 0,  or  "right now".   So, for Paul,  the Andromedans are having a meeting to decide if they will invade.


Andromeda2.png (12.89 kB . 1304x767 - viewed 1535 times)

   Now Paul has a friend,  let's call them Bicycle Brenda,  who is riding a bicycle past Paul at a low speed and just happens to be travelling toward the Andromeda Galaxy.   So Let's consider how Brenda sees the universe next...
   This is just standard stuff in Special Relativity,  Brenda sees Paul looking slightly thinner than if he was at rest,  this is length contraction  etc.   However, since the speed of Brenda relative to Paul is so small, it's not like this really matters or is significant.   Anyway, the easiest way to determine how Brenda sees the world is to construct another inertial frame around Brenda where she is the one at rest at the origin.   We'll set up a second frame S'  in which Brenda is at rest and at the origin.   We'll synchronise the times t and t' = 0 when Paul and Brenda are precisely at the same place (I'll say just passing each other but for the Mathematics it will fine if Brenda has ridden right through Paul at the event t=t'=0 and x=x'=0).   Anyway,  the frames   S with co-ordinates (x,t)   and    S'  with co-ordinates (x',t') are just Lorentz boosts of each other as usual,   with a tiny off-set velocity v = bicycle speed = 2 mph.

    We need the Lorentz transformations to see how the events in S are mapped to events in S',  we'll just focus on the time coordinate t'

t'  =   
   You can check this equation yourself, see https://en.wikipedia.org/wiki/Lorentz_transformation , for example.  Spend a moment and remind yourself how this equation can be used.    Given an event with values (x,t) specified in the frame S,   this equation determines the t' value that would be assigned to the corresponding event in frame S'.    We could determine the corresponding x' value with another equation - but we won't need that for this example. 

    Brenda considers all events in the universe to be happening "right now"       if and only     they have t' coordinate = 0 in the frame S'.    They can have any  x' co-ordinate (that just tells her how far away the events are in space),  just so long as  t' =0  (that means they are happening at t'=0 or right now, where "now" means when Brenda passed Paul and said "Hi!"  or  "Sorry I just ran you over" etc).
    We want to mark all the events on our spacetime diagram that are happening right now for Brenda.  However, our spacetime diagram only has an axis marked up for the frame S.
    Set t'=0  in the above Lorentz transformation equation and determine the set of all events (described with co-ordinates  x,t in S) that are happening "right now" for Brenda,  that's what we will mark on our diagram.

   From the Lorentz transformation equation we obtain:
0 =  t'   =      
   ≠ 0    ,  so divide ,   re-arrange and obtain 

  t' = 0        <=>        t = 

On the R.H.S.   we have the relationship  between  t and x  (the co-ordinates in S  that describe events happening now throughout the universe for Brenda.
    v =  small positive value =  bicycle speed in S and it's in the +ve x direction.
    c²  =  a massive number.

v/c²   is therefore a tiny positive number.      t =    (v/c²) . x   describes a straight line on the diagram that is almost parallel to the x-axis.   However the key is that it is NOT quite parallel to the x-axis, it is angled upward a tiny bit.   Moreover the Andromeda galaxy is a long way along the x-axis, so let's  put this line of constant  t'  on our spacetime diagram and see what happens.....


Andromeda3.png (16.34 kB . 1304x767 - viewed 1537 times)

   The Green line shows all the events throughout the universe that are happening now (t'=0) for Brenda.
Hopefully you can see that, as far as Brenda is concerned, when she passed Paul and said "Hi!"  the invasion from Andromeda has been launched.   Meanwhile, for Paul the meeting to decide whether to invade Earth or not hasn't finished yet.

Anyway, I hope that will help to see the gist of the argument for the Andromeda paradox.
As @Halc mentioned earlier,   you need to be carefull not to say,  Brenda "sees" the invasion has been launched.    There's no way light has reached Brenda's eyes yet, she isn't going to "see" the ships coming.   It's only that the launch is actually happening now for Brenda even though there is no practical way she could know.  So, for example, she couldn't tell Paul that it was coming.... unless she has some  extra-sensory perception and says "you know, it's funny but every time I ride my bike in that direction I just feel like some invasion has been launched".  In which case Paul would reply, "have you tried changing to a lower gear?  You might be getting yourself a little out of breath and dizzy".

Best Wishes.

11

Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 05/07/2022 12:52:43 »

Hi.

    Hope you are well.   This is is obviously one of the most interesting posts available today.  I have a strong suspicion that @Halc is about to write a post since the forum is reporting him as viewing this post at the moment.    He obviously has a head start over me, so I'm just going to wait and see what gets done.
      For the short term, I'm just going to glance through some YT videos that might exist and see if I can recommend one.

Best Wishes.

12

Physics, Astronomy & Cosmology / Re: Why can't i understand the andromeda paradox?

« on: 05/07/2022 12:50:06 »

The 'paradox' is simply an extreme illustration of the relativity of simultaneity, which is fundamental to special relativity but something one might find contrary to  one's everyday life intuitions.

Two observers

Careful about this wording. Nobody is observing anything in the Andromeda scenario. It is just a reference to two different inertial frames which differ by perhaps a walking pace.

Two observers speculate on a distant(spatially+temporally) event

A pair of spatially distant events. Since one each distant event is simultaneous with the respective observer event, neither is temporally distant.

with contradictory opinions

The 'opinions' are just statements of simultaneity relative to mildly different frames, and as such are not contradictory, which is why this isn't an actual paradox.  The scenario does not involve empirical measurements of any kind.

A could be correct with B incorrect or B could be correct with A incorrect.

They're both correct. Neither says anything that contradicts the other.

There will always be uncertainty about future events-what am I missing?

There is no mention of a future event here. Both A&B are making statements about a current event, something that cannot be measured since one can only measure something in the past if it has any spatial separation from you.

So relative to frame A, the Andromedons have not yet decided to make this attack, and relative to frame B, the Andromedons are currently in flight with their battle fleet. Neither statement contradicts the other.

This only seems paradoxical since one's intuitions tend to assume absolute simultaneity where simultaneity is not frame dependent, and thus the above statement reduces to: The Andromedons  have not yet decided to make this attack and the Andromedons are currently in flight with their battle fleet. That statement is contradictory, but under SR, it is not even wrong since it lacks frame references.

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Just Chat! / Re: The coronation of "eternal student", why was i not invited?

« on: 05/07/2022 11:09:04 »

Yes, folks, merit and effort are recognised even under a hereditary monarchy.

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Just Chat! / Re: What is the value of spam?

« on: 05/07/2022 11:05:43 »

Years ago I swore never to buy anything whose television adverts belittle men. That immediately reduced the impact of TV advertising by 50%, and everything else seems to be aimed at women who want to stink, children who want to take distorted photographs, or people who would rather wait for a delivery than open a can of beans.

15

Just Chat! / Re: The coronation of "eternal student", why was i not invited?

« on: 05/07/2022 11:00:38 »

I just noticed this morning that "eternal student" has been crowned with the regal title "naked science forum king", a most auspicious honour that he(she) did not desire.

I think he views it more as a suspicious honour

Regardless of that, why did I not receive an invitation to the coronation ceremony and the subsequent food and drink debauch?

The peasantry are not usually invited to these events. It’s only people like Alan who are sufficiently skilled at débauching - it takes years of dedicated practice

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Just Chat! / Re: The coronation of "eternal student", why was i not invited?

« on: 05/07/2022 10:53:01 »

Suspected republican.

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Just Chat! / Re: What is the value of spam?

« on: 05/07/2022 00:11:36 »

I wonder how effective online advertising actually will be in the future, at present it is apparently effective, but will we begin to become desensitised to it

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Just Chat! / Re: What is the value of spam?

« on: 04/07/2022 22:25:05 »

….but these are proper ads, would these be classed as spam?

The rules here expressly forbid unsolicited advertising, so yes we view them as spam
Many of the ads we remove are links to porn, prostitution and gambling sites. The objective of the poster is to get people to click and so up the search rating of the site, as well as drawing in the susceptible.

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Just Chat! / Re: What is the value of spam?

« on: 04/07/2022 18:56:01 »

A great deal to people who profit from it.

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Just Chat! / Re: What is the value of spam?

« on: 04/07/2022 18:02:20 »

It has fed armies and unfortunate schoolchildren for a few generations. Its principal value has been to make people appreciate good food, when the spam runs out, and it can be used as bait for various inedible fish.