The Illusion of Anti-Gravity (aka a Superior Mirage)

[jedishrfu]

TL;DR Summary

Ship of photographed floating in the air due to local weather conditions,

https://www.theguardian.com/science/2021/mar/05/ship-hovering-above-sea-cornwall-optical-illusion

 

[DrStupid]

The image is quite impressive but the explanation in the link seems to be incomplete. Where does the gap between ship and water come from?

 

[etotheipi]

Reference:
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/mirage.html

 

[anorlunda]

@etotheipi gave the correct explanatory image. But this experiment with a sugar solution in a glass shows the same physics in a more intuitive way. Note the multiple images, some upside down.

 

[DrStupid]

View attachment 279214

Reference:
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/mirage.html

That's what I mean. What stops the effect at the waterline of the ship? With the explanation in the link I would expect something like this:

That fits better to the image posted by @anorlunda. There is a gap between the refracted images below and the reflected image above. But a reflected ship would appear upside down.

 

[russ_watters]

That's what I mean. What stops the effect at the waterline of the ship?

I think it's coincidence. The ship happens to be at the right spot.

 

[DrStupid]

I think it's coincidence.

That's not what I mean. Why does it stop at all?

 

[Ibix]

That's not what I mean. Why does it stop at all?

I presume what we're seeing below the ship is a reflected image of the sky higher up - note that the sky below the ship is lighter than that immediately above it, similar to the higher sky. Perhaps the camera is above one thermal boundary?

Not completely satisfied with that argument.

 

[Ibix]

Here's a way that would work:

(edit: refracted ray corrected - thanks to DrStupid in #11)
A layer of warm air (grey) lies just above the sea, extending part way to the ship. Having a slightly lower refractive index than the air above it, there's a potential for an observer above the layer, such as one on land (green), to see a total internal reflection of the sky in a narrow range of angles (delimited by the black rays), with sea at lower angles (slightly refracted as per the grey ray) and the undistorted ship/sky above it. The diagram is obviously Euclidean, but on the curved Earth if the upper black ray were to pass just over the horizon you would see ship but no sea, with sky below it, and sea below that - as in the picture in the OP.

Warm air trapped below cold seems quite unusual, but presumably that's why this is a fairly rare phenomenon. Perhaps it could be warm sea air and then a cold breeze starts blowing off shore over a cliff and trapping a pocket of the warm air? Cornwall is largely granite, so has some fairly abrupt cliffs.

 

[anorlunda]

That's not what I mean. Why does it stop at all?

That is not a 3D object in the sky, it is a projection. Light from the projection is directional.

So in the ship case, those other multiple images are invisible because their light is not aimed in the direction of your eyes. If the photographer climbed a tall ladder, he may have been able to photograph a different version of that mirage image.

I've seen many mirages over water. About 2/3 of them appear disconnected from the horizon, and the other 1/3 are connected. Perhaps 10% of the time, multiple images of the object are visible. Where there are multiple mirage images seen, odd numbered ones are upside down, and even numbered ones right side up. So the ship in that picture was the 2nd (or perhaps 4th) of multiple mirage images. All the other images were aimed elsewhere, and thus invisible to the camera.

There's an old story that predates the Internet about a village in Africa that appeared one day in the sky over Washington DC. It sounds like a myth, but it illustrates the possibilities.

Submariners are aware of analogous sonar effects. Gradients in ocean temperature and salinity can cause sounds to travel along curved paths, and to allow them to travel much further than they normally would. It is also somewhat analogous to how light travels around curves in fiber optics. The index of refraction is different in the core and the cladding of the fiber, and that difference is what provides the necessary nonlinearity.

 

[DrStupid]

A layer of warm air (grey) lies just above the sea, extending part way to the ship. Having a slightly lower refractive index than the air above it, there's a potential for an observer above the layer, such as one on land (green), to see a total internal reflection of the sky in a narrow range of angles (delimited by the black rays)

That might be possibe. But would't that be inferior mirage?

, with sea at lower angles (slightly refracted as per the grey ray) and the undistorted ship/sky above it.

The refraction in your image doesn't seem to be correct.

 

[Ibix]

That might be possibe. But would't that be inferior mirage?

I guess. So either that's not the explanation (although it appears to fit the observations) or the Grauniad has something wrong.

The refraction in your image doesn't seem to be correct.

It looked right refracted through my wine glass. Now corrected, thanks.

 

[anorlunda]

I can't resist one more addition. This is close to my favorite topic. See my Insights Article, https://www.physicsforums.com/insights/rainbows-not-vampires/

In the following picture, ask yourself why the sky inside the rainbow annulus is so much brighter than the sky outside the annulus.

The answer is that sunlight reflected and refracted from raindrops inside the annulus is directed back toward the camera, while sunlight reflected and refracted from raindrops outside the annulus is directed elsewhere.

(This is my favorite photo because that is me on my boat in the center of the picture.)

 

[nsaspook]

https://www.skybrary.aero/index.php/Fata_Morgana

 

[anorlunda]

Thanks @nsaspook . You expanded my vocabulary. People do use fata morgana for cases like that ship. But definition I looked up for Fata Morgana says

A Fata Morgana (Italian: [ˈfaːta morˈɡaːna]) is a complex form of superior mirage that is seen in a narrow band right above the horizon. It is an Italian term named after the Arthurian sorceress Morgan le Fay, from a belief that these mirages, often seen in the Strait of Messina, were fairy castles in the air or false land created by her witchcraft to lure sailors to their deaths.

Fata Morgana mirages significantly distort the object or objects on which they are based, often such that the object is completely unrecognizable.

That high distortion seems to best fit this image (which is not a waterfall, not a cliff)

 

[nsaspook]

Thanks @nsaspook . You expanded my vocabulary. People do use fata morgana for cases like that ship. But definition I looked up for Fata Morgana saysThat high distortion seems to best fit this image (which is not a waterfall, not a cliff)
View attachment 279245

Sailors have called them that for ages. Sometimes the images are distorted but other times they are not. I've never heard that high distortion was a requirement for Fata Morgana mirage.

The phenomenon, known as Fata Morgana, creates a mirage when the sun heats up the atmosphere above the land or oceans, which creates a gradient of temperatures

https://www.dailymail.co.uk/news/article-9329233/Plane-sailing-Cruise-ships-appear-floating-air.html

 

[nsaspook]

Sailors have called them that for ages. Sometimes the images are distorted but other times they are not. I've never heard that high distortion was a requirement for Fata Morgana mirage.
View attachment 279246
View attachment 279247https://www.dailymail.co.uk/news/article-9329233/Plane-sailing-Cruise-ships-appear-floating-air.html

The more I look at this picture the more it seems to be just a false horizon instead of a real Fata Morgana mirage.

 

[A.T.]

Submariners are aware of analogous sonar effects. Gradients in ocean temperature and salinity can cause sounds to travel along curved paths, and to allow them to travel much further than they normally would.

It also allows subs to hide from surface sonar, below a temperature jump.

 

[A.T.]

The image is quite impressive but the explanation in the link seems to be incomplete. Where does the gap between ship and water come from?

Maybe it's not a superior mirage of the ship, but an inferior mirage of the sky:
https://www.animations.physics.unsw.edu.au/jw/light/mirages-green-flash-sky-colours.htm

 

[sophiecentaur]

What stops the effect at the waterline of the ship?

That's not what I mean. Why does it stop at all?

My suggestion about this is that colour and luminance of the illuminated sea surface in that particular picture is very similar to the illuminance and colour of the sky. It's not only the ship image that is observed but the image of a wide band of distant sea surface, over the horizon. Both ship and distant sea are imaged above the real horizon

Also you shouldn't expect any 'horizon' that's actually along the waterline of the ship. A ship that's 'this side' of the horizon doesn't have a waterline at the level of the horizon so you wouldn't actually expect any extension of the waterline.

I can fool myself into seeing a darker band above and below the ship and the temptation is to think of that as low cloud. But it may not actually be. The refraction can be displacing light from notionally above the horizon to arrive from a higher altitude. There will be hundreds of such pictures that don't actually work as well as the one in the newspaper. Also, you can never be sure that the dreaded Photoshop has not played some part in the process.

 

[A.T.]

My suggestion about this is that colour and luminance of the illuminated sea surface in that particular picture is very similar to the illuminance and colour of the sky.

Well, if that's the case, then we don't need any mirage of the ship to explain the picture. It's simply sea of the same color as the sky, that we see under the ship.

Now you just have to explain why the color of the sea suddenly changes from dark to sky color. And to me this looks like what is shown in post #19.

 

[sophiecentaur]

Now you just have to explain why the color of the sea suddenly changes from dark to sky color. And to me this looks like what is shown in post #19.

The 'sudden change' will be because the direct view of the sea below the real horizon is unadulterated by that optical effect. Also, the examples in post #19 give an image inversion (a reflection phenomenon). The image in the OP shows a non-inverted ship so reflection doesn't explain it. The effect must involve refraction. So you would need to explain how your (often applicable) mirage explanation fits the facts here.

Looking in detail at that image, I have to be a bit suspicious. For the ship to appear that large (ignore the use of a long lens - the horizon 'height' is due to the same focal length) it can't be very far away so the photo must be from a low position, which would give a horizon distance of much less than the ship distance (5km for a camera height of 2m or 4.4 km for a height of 1.5m). A ship of height 20m would be 20km the other side of 'our' horizon. So, if it's below the horizon to start with, it would need to be about 24km away. A ship 250m long would subtend about 5° and image implies it's lifted by a bit less - say 2°. As for the appearance of the illuminated sea, 'beyond the horizon', the effect on a distributed object would be to produce a distributed image (no simple ray trace for the sky). You would expect the ship image to be covered with the same diffuse sky image and that would just reduce contrast (which you can see).

The pictures in post #16 are far more believable with the detail on the ships implying they are close and the camera height (note the swans) not more than 1m. You can see an horizon that's about the same distance as the ships, the 'lift' is less and you could even interpret the pale band below the ships in terms of reflections of the cloud layer and not a real horizon at all. The ships may well be following a narrow channel so the effect is the same for all of them.

Would the OP image possibly appear as large and sharp as any of the proposed explanations would allow? I spent two or three years on the UK National Coastwatch and, through good binoculars, a ship that distant wouldn't have looked that good. The horizon from our Obs post was about 12km. Local viz conditions may have been different so I my experience may be misleading me.

PS Fata Morgana rules! Very Pirates of the Caribbean.

 

[A.T.]

Also, the examples in post #19 give an image inversion

What are you talking about? The primary clear images of the truck and the island in post #19 are not inverted, because they are seen directly. It's only the sky beneath them that is a "reflection", and on it there might be a faint "reflection" of the object itself.

 

[sophiecentaur]

What are you talking about?

I'm talking about the case where there is no reflection (I can't see one in the OP) For your scenario, if the sky forms a virtual image below the ship then why is there no inverted image there ? The waterlines are all straight (all example images) but the truck roof is very evident.
There will be several alternative temperature gradients and we need to pick the right one to account for the OP. It is difficult and we need to remember the long distances and the small angles involved. The diagrams all exaggerate the vertical scale. If there is warm air over cold then the curvature of the rays will be downward and, if there is cool air over warm, the rays will be bent upwards. For warm air in a pocket on a road, there can be internal reflection. If there is cold air on the ground, you will only get refraction. Your examples assume warm air below cold air. One problem I have with the cold over warm situation is that the apparent horizon is very straight and your explanation would need very uniform air layers.

If the ships are all beyond the horizon then light from the ships and the sky behind them must be bend downwards (up and over the horizon) so they must appear in the sky above the horizon direction.

I don't think it's at all clear, when we get down to it.

 

[A.T.]

The more I look at this picture the more it seems to be just a false horizon instead of a real Fata Morgana mirage.

I'm also tending towards that. It definitely doesn't look like a superior mirage from beyond the real horizon. Here a quite extensive analysis of the picture and the possible effects:

 

[sophiecentaur]

That guy put enough work into it to get a good explanations - well found @A.T.
He shows how it can't be a reflection effect - although he doesn't hint at the term TIR, which is a fair hand waving shorthand.
It's very common to get light and dark stripes due to the clouds. You just need the 'right' sort of cloud pattern plus a bit of judicious 'levels and curves' adjustment to get the sky and that patch of sea looking similar.

 

[sophiecentaur]

That guy put enough work into it

The "looming' he refers to would 'raise' the horizon, as well as the ship. When I next have a theodolite with me, I must try to see if this actually happens to a significant degree.

I am also relieved that my observations and comments about the unnaturally clear images of ships seem to be justified. Images of ships which are tens of km away are seldom as sharp and coloured and, to be below the real horizon, that's the sort of distance involved.

 

[nsaspook]

The "looming' he refers to would 'raise' the horizon, as well as the ship. When I next have a theodolite with me, I must try to see if this actually happens to a significant degree.

I am also relieved that my observations and comments about the unnaturally clear images of ships seem to be justified. Images of ships which are tens of km away are seldom as sharp and coloured and, to be below the real horizon, that's the sort of distance involved.

Very often the Fata Morgana (as a general term) images we saw were changing in shape, clarity and position due to several visual factors affecting the image. Looming and Mirage effects all happening at the same time. Fairly clear images did happen during the changing sequences but major distortions happened too.

 

[sophiecentaur]

Fairly clear images did happen during the changing sequences but major distortions happened too.

That's the sort of thing I'd expect if the image is formed by irregular atmosphere. The images - particularly the original one, are very clear (even several ships in a row, later on) and they all have to look of 'the original' and nothing mucked about with by the air. An almost direct view from not very far away.

 

[hutchphd]

Isn't this the same effect that some folks think sunk HMS Titanic? The watch was scanning ahead to the horizon for icebergs ...not for icebergs in the sky. I think it was warm water (Gulf Stream) and cold air. Maybe I've got this screwed up but I think it is still puzzling why there was insufficient warning from the watch.

 

[DrStupid]

I think it is still puzzling why there was insufficient warning from the watch.

I think it's not that puzzling with the stars as the only source of light.

 

[hutchphd]

Dark adapted from parapet on a crystal clear night with low swell I would think one should be able to see an iceberg ahead using only starlight, absent these aberrations. I am interested in the opinion of the several good sailors amongst us.

 

[ardnog]

Isn't this the same effect that some folks think sunk HMS Titanic?

HMS is reserved for warships of the Royal Navy.

I'd need to check Titanic but it would have been SS (steamship) or RMS (Royal Mail Ship).

Regarding the photo, it looks like the British have resorted to innovative tactics to beat the post-Brexit trade tarriffs.