andrew s

2 minutes ago, saac said:

I wouldn't be so kind  

Jim 

Oh you engineer. Remember we would not be here but for quantum mechanics.  Classical atoms are unstable.

Regards Andrew 😌

2 hours ago, iantaylor2uk said:

I'm pretty sure people have fired single photons at double slits to show that photons only interfere with themselves (rather than other photons) to create the well known diffraction pattern associated with the wave like behavior of photons.

Indeed they have. They are dilute beams so that on average only one photon goes through at a time. The beam is not a single photon state in the sense I intended. 

Regards Andrew 

PS It might be worth adding for those less familiar with the topic that a single photon doesn't make an interference pattern.  It just makes a localised "hit". You have to pass a large number of single photon throught for the pattern to appear in line with the quantum perdition. Or better still prediction . Thanks @MalcolmP.

25 minutes ago, Michael Kieth Adams said:

Why do they not break down to particle and antiparticle?    There must be some process that limits or prevents this

They have to be energetic enough to produce pair production. Then they can cause a star to collapse into a super nova see here

Regards Andrew 

6 hours ago, saac said:

The Crookes Radiometer is a device with an interesting history of what it purported to demonstrate regarding light and exchange of momentum.  Turns out like that other physics toy curio, the drinking bird, the explanation owes more to heat transfer.  It's an interesting device to generate discussion on the nature of light anyway. 

https://en.wikipedia.org/wiki/Crookes_radiometer

 

Jim

That was my incorrect post 😊

Solar sails are the only macroscopic example I can come up with.

Regards Andrew 

6 hours ago, Michael Kieth Adams said:

 an artifact of an electric wave and a magnetic one at right angles to each other?

In essence that is exactly what it is. 

However, the connection between light and normal matter is demonstrated by pair production where a high energy gamma ray can produce an electron positron pair. 

Similarly,  a particle and its anti-particle can annihilate to photons.

Regards Andrew 

Incorrect post removed

This gives the experimental rest mass as less tha 10^-48 kg.

The photon is the most enigmatic of quantum particles.  

From a quantum field theory perspective it is a localised excitation of the field. However,  its location is impossible to measure, most states of the field don't have any meaningful number of photons and it's very difficult to create single photon states!

I could go on.

Regards Andrew 

1 hour ago, JeremyS said:

Indoor plumbing not reached your part of France, Olly? 🤔

😊

My thoughts exactly. The other possibility is, at 70, he may have forgotten where it was or was reverting back to the days when he made toy binoculars from used loo rolls.

😊

Regards Andrew 

1 hour ago, SteveBz said:

I'm using them as a stepping stone into spectroscopy. I've joined a group of six or eight like minded folks to try to track a group after rapidly varying Be stars for longer periods of time than we might otherwise do.

I'm using a lowspec 3 and just putting the final touches to a star'ex.

Kind regards,

Steve.

Sounds a good interesting project.  Best of luck with it. 

This used to be a major topic on the ARAS forum but it seems to have disappeared.  Maybe @robin_astro know what happened. 

Regards Andrew 

The web site has gone. I found a link in a BAA post I made about it.

What's your interest in them?

Regards Andrew 

1 hour ago, SteveBz said:

No, it's even better. But it was @skybadger

Sorry 😞.  My mistake. 

I discovered a couple of candidate Be stars during a spectroscopic search but the Web site with them on seems to have gone missing. 

Regards Andrew 

Try here . Regards Andrew 

Looks like the same link @SteveBz gave. 

Always good to seeing someone take the path less travelled.  Regards Andrew 

Simple answer is we don't know enough about dark matter to know what happens if and when it collides with itself or normal matter. As @robin_astro pointed out one idea is that it is it's own anti particle in which case we would expect it to give off its energy as radiation - presumably electromagnetic. 

There are lots of good ideas but no experimental results to sort them out.

Regards Andrew 

1 minute ago, Mark2022 said:

Less exposure on the moon would have had Jupiter and the stars all but disappear though.

Another layer ? I still like it though. Regards Andrew 

I rather like that. A bit less exposure on the moon, so it showed some detail, and it would have been even better. 

Regards Andrew 

Give me general relativity over existential philosophy any day. 😉

Regards Andrew 

48 minutes ago, Michael Kieth Adams said:

That’s what I thought.  Dark matter clumps which means it has to lose momentum and should mean that it gives something up.  Have Imissunderstood something?     Mike

It does not need to "lose" momentum to "clump" on galactic scales. Dark matter, via the gravitational interaction, just converts its initial momentum into orbital momentum just as ordinary matter would.

It's only when normal matter gets close enough for friction to come into play e.g. star, planet formation and accretion disks that energy and momentum are lost via EM radiation. 

Regards Andrew 

59 minutes ago, ollypenrice said:

In wafting, sing-song voice, 'I can't hear you, I can't hear you!'  However, you are welcome to add diff spikes to any of my images. May I suggest carving them into your screen with a pointed stick???

Now look what you made me do!

thanks Andrew 

Sorry @JeremyS but it was a Takahashi Sky 90. Just totally  underwhelming. 

Regards Andrew 

There are no laws in art only opinions.

Diffraction spikes rule ok. 😊

Regards Andrew 

The idea that dark matter is in a halo around a spiral galaxy is misleading.  Its distribution is similar to the of normal matter see here . As an example for NGC 3198.

The bottom figure shows the density of normal matter (b) and darkmatter (h).

So they have an additive gravitational effect holding the galaxy together while flattening the rotation curve.

Regards Andrew 

1 minute ago, billhinge said:

Yes I appreciate that, point is that there are still many mysteries to solve, we don't know everything and we may even have got some assumptions wrong? (not picking on anything specific)

 

Indeed, but I am always amazed by what we do know 😊

Regards Andrew 

4 minutes ago, billhinge said:

In radioactive decay the negative mass square is a consequence of conservation of momentum and energy of the resultant positron and neutrino (since thats its 'job') not a speed measurement.  It isn't even an unusual result, there are some people who take this literally but most state that it is systematic error 'somewhere' since its 'obvious' that it cant be negative and there it must be positive. I make no claim for either viewpoint, my beef is that if there is systematic error then someone should identify where. The most successful standard model says neutrino mass = 0 and therefore must travel at c (but they can't since they oscillate). 

 

I know that. I was pointing out an alternative approach based on speed measurement. One way to avoid systematic error, which are very difficult to identify,  is to use very different techniques. 

As you say they oscillate so they should have some mass. There are a  number of unexplained anomalies within the standard model of particle physics,  as there are within the LCDM and come to that any theory you care to mention.

They are still currently the best we have. It's not for lack of effort by theoretical physicist but  no one seems to have made any significant advances in a long time.

Regards Andrew 

An indirect way to approach the mass of a neutrino is to measure its speed. This gives a speed difference from that of light of 2x 10^-9 from the supernova 1987A. Whatever the mass it's very small.

When trying to measure a value you not only have to worry about statistical variations but also systematic errors. The latter are very hard to eliminate. 

As an example this is the history of the speed of light measurements

There looks to be a systematic difference between the early and late results.

Regards Andrew