Post by: guest39538 on 12/06/2016 07:36:12
Post by: chris on 12/06/2016 09:58:17
For our own star (the Sun), we cannot see back much more than about 8 minutes, which is the time it takes light to reach us on Earth owing to the finite speed of light (300,000 km/s). But for more distant stellar objects, the light has been travelling for a considerable time - in some cases billions of years. This means that the view we are receiving now relates to how that object looked billions of years ago when the light first issued from its surface.
The most pertinent case in point is that of a supernova, when a star explodes at the end of its life. We may see that supernova appear abruptly here on Earth where previously there was just a pinprick of light in the sky (the former star). But by the time we see that explosion the star is already gone.
Post by: Colin2B on 12/06/2016 10:15:14
If you are angling towards your theory that sight is instantaneous, this thread will be moved.
As Chris says, what we see of the sun is the light which left it 8 mins ago and is now striking our eyes and the earth around us.
Post by: evan_au on 12/06/2016 13:21:05
Quote from: theBox
How do we see the sun in its past if we have a clear line of sight?A "line of sight" is a geometrical construct that shows the path that light would take from a source to your eye (if there is nothing opaque in the way).
Since there is transparent, almost-empty space between us and the Sun, there is a clear line-of-sight between any sunny location on the Earth and the Sun.
This geometrical path identifies the path of the light, but not the elapsed time.
The elapsed time is provided by Einsteins' General Relativity, which states that light always travels at c, when measured in your frame of reference.
In a moderately weak gravitational field (like the Solar System), you can estimate this time by taking the distance (about 150x106 km) and dividing by the speed of light in a vacuum (close to 300,000 km/s), which gives an elapsed time of 500 seconds, or 8 minutes and 20 seconds.
So we always see the Sun as it was about 8 minutes in the past. For the avoidance of doubt, that means 8 minutes in our past, and 8 minutes in the past of the Sun.
The calculations of General Relativity would be a bit more complex near a black hole - and in fact, you would get multiple answers, corresponding to a set of "line of sight" paths which take multiple orbits around the black hole on the way to Earth.
Post by: Colin2B on 12/06/2016 13:26:34
For the avoidance of doubt, that means 8 minutes in our past, and 8 minutes in the past of the Sun.Im glad you added that, I might have been confused [;)]
Post by: guest39538 on 12/06/2016 16:44:11
Quote from: theBox
How do we see the sun in its past if we have a clear line of sight?
Quote from: Evan
A "line of sight" is a geometrical construct that shows the path that light would take from a source to your eye (if there is nothing opaque in the way).
Since there is transparent, almost-empty space between us and the Sun, there is a clear line-of-sight between any sunny location on the Earth and the Sun.
This geometrical path identifies the path of the light, but not the elapsed time.
So you admit there is a clear line of sight, ok , good that's a start!
Now please consider the clear line of sight, I can see and you can see the entire distance between you and the Sun, you can see the surface of the sun through the clear line of sight and because the fact that space is transparent and almost visually ''empty'' between us and the Sun.
Let us do this scenario, you are (a) and the sun is (b)
we will represent the clear line of sight as
Xcls=
ok?
You admit between (a) and (b) there is ''nothing'' to see, it is visually ''empty'' space.
so we can say ?
Xcls=
Now if we say that 0=unknown length, how much of the length between
do we see? Do we see the entirety of the length right upto the Sun's surface?
[ Invalid Attachment ]
''Since there is transparent, almost-empty space between us and the Sun, there is a clear line-of-sight between any sunny location on the Earth and the Sun.''
Post by: Alan McDougall on 12/06/2016 21:27:45
Due to the limiting fundamental of the speed of light c, it is impossible to actually observe anything or event in our own relative present, no matter where we look, we are always seeing things as they existed in the past.
Alan
Post by: evan_au on 13/06/2016 00:07:56
Quote from: TheBox
Let us do this scenario, you are (a) and the sun is (b)Reminder: Light travels from the source (Sun) to detector (your eye). The direction is (b) to (a).
Your mathematical notation implies the Greek idea that something travels from your eye to the Sun.
Quote
so we can say ? Xcls=a0bNo. This notation suggests that there is an undefined point 0, and the light from the Sun travels to point 0, and then travels from there to your eye.
- If point 0 is on the line of sight, why bother mentioning it? It is already included in the path ab.
- If point 0 is not on the line of sight, then light will take longer than 8 minutes to reach your eye (not shorter, as you seem to think).
- How do you propose to divert light to 0 when it is on its way from the Sun to your eye? Smoke and mirrors? We are talking about empty space!
So I suggest that we cannot usefully say Xcls=a0b, when we are talking about empty space.
Quote
Now if we say that 0=unknown length how much of the length between (a and b) do we see?The light we see travels the full distance from the Sun to Earth, which is:
- 152,100,000 km at its farthest (Aphelion on July 4th), and
- 147,095,000 km at its closest (Perihelion in January)
- Minus (up to) 695,700 km for the radius of the Sun (and 6,378 for the radius of the Earth)
But we don't actually see anything originating from this path, because the path itself is transparent, and does not add any visible light to what we see of the Sun's surface.
Quote
Do we see the entirety of the length right up to the Sun's surface?We can verify that there is a transparent path for the entire distance between the Sun and our eye.
In theory, we could tell if an opaque object crosses the path between the Sun and our eye; in practice, a potentially devastating meteorite 10km across would be totally lost in the bright glare of the Sun. It is much easier to spot the reflected glow of a dark meteorite against the blackness of space than it is to spot the dark side of a meteorite against the glare of the Sun.
Quote
Mr Box, If you are angling towards your theory that sight is instantaneous...You don't need to go into space to see that light takes a finite time to go places, and that we are always seeing things as they were in the past.
Computerized Stock Market traders know that the bid which reaches the destination first wins. So they want to minimize the delay before their bids reach the stock exchange.
Much of the world's telecommunications passes through optical fibers, where the speed of light is about 5μs/km (instead of about 3.3μs/km in a vacuum or air).
So to cut out these microseconds, the traders want their computers adjacent to the Stock Market computers, so they see each stock market transaction as it was, as soon as possible after it happens.
The "Great Circle Distance (http://www.gcmap.com/dist?P=LHR-JFK&DU=km&DM=&SG=&SU=mph)" between London and New York is about 5600km (this is a geodesic on the surface of the Earth). That means that an observer in New York is always observing the London Stock Exchange as it was at least 28ms in the past (5600km x 5us/km).
There are various projects underway around the world to reduce the elapsed time between different stock markets, so traders can play them off against each other.
By laying a new fiber between London and New York whose path is just 500km shorter, they can shave 2.5ms off the elapsed time. So traders will see events a shorter time in the past, and users of the new fiber can beat traders using older fibers.
The big money says that the speed of light is finite, and every time you see something distant using electromagnetic radiation, you are seeing that event as it was some time in the past.
Unlike Stock Market computers, your brain takes about 300ms to process an optical event. So your conscious mind is always seeing the world as it was about 300ms in the past (the amygdala has slightly less processing time, so you can react to visual danger signs before you consciously see them).
Post by: Alan McDougall on 13/06/2016 01:16:16
I am somewhat baffled as to why evan_au has to go into all that detail, full of mathematical equations and distances from the sun at its furthest and nearest to earth, in its orbit etc, etc "to answer a rather simply question"
In a clear vacuum you do not ever see a beam of light, one only observes the point where it strikes it target or impinges on it.
When we see a beam of light we are actually seeing the light from the source, reflecting off dust particles in the atmosphere.
Light travels in a straight line in a vacuum, so why ask what happens if we have a clear line of sight, the question would be redundant if the moon were between the earth and the sun etc?
We know for a fact that c is finite and the consequences can by dire if this is not taken into consideration, such as an exact compensation for it both for c and for relativity, which have to be set with extreme precision, on our GP satellites, or they would almost immediately give the wrong position for ground based observers hoping to know their exact location on planet earth.
Alan
Post by: guest39538 on 13/06/2016 07:05:32
I am somewhat baffled as to why evan_au has to go into all that detail, full of mathematical equations and distances from the sun at its furthest and nearest to earth, in its orbit etc, etc "to answer a rather simply question"
There isn't just us reading this post so Evan explains so all people who read know what we are talking about, but yes it is a simple question with a simple answer.
Post by: Alan McDougall on 13/06/2016 07:12:03
I am somewhat baffled as to why evan_au has to go into all that detail, full of mathematical equations and distances from the sun at its furthest and nearest to earth, in its orbit etc, etc "to answer a rather simply question"
There isn't just us reading this post so Evan explains so all people who read know what we are talking about, but yes it is a simple question with a simple answer.
That is completely illogical and unacceptable, his long drawn out full of unnecessary equations and exact distances and statistics does, "just the opposite confuse people" rather than enlighten them to the ease of understanding one of the most basic questions in all of physics . My motto "Keep it harp and simple"
It is also logical that he should have kept it simple, precise and readable, especially in light of the fact that many guests to the forum might like to actually read and comprehend answers to basic question on a visit to the forum and join, without fear of being rejected as ignorant.
Post by: evan_au on 13/06/2016 07:46:19
Quote from: Alan McDougall
I am somewhat baffled as to why evan_au has to go into all that detailI am sorry that you don't like my tutorial on beams of light in space. I agree that your explanation is much more succinct.
But my purpose was to clarify the details of vague claims made by TheBox, so they (hopefully) would not keep repeating.
Post by: Alan McDougall on 13/06/2016 08:17:33
Quote from: Alan McDougallI am somewhat baffled as to why evan_au has to go into all that detailI am sorry that you don't like my tutorial on beams of light in space. I agree that your explanation is much more succinct.
But my purpose was to clarify the details of vague claims made by TheBox, so they (hopefully) would not keep repeating.
I accept your explanation, although your answer was very detailed it was correct and most of the TheBox posts make no sense to me!
Post by: guest39538 on 13/06/2016 11:05:15
Post by: Colin2B on 13/06/2016 14:26:09
It is very interesting how the mods are editing the thread and deleting my questions that puts their answers in trouble.Not so. You were told at the beginning that this part of the forum is not a platform for new theories.
Evan has taken trouble to explain in great detail in various posts how sight works.
You see the sun when the light which has travelled from it strikes your eye, that light has taken 8mins to get from the sun hence we see its past.
Any other explanation is a new theory and should be posted elsewhere.
.......most of the TheBox posts make no sense to me!No, but they can make really good teaching material by pointing out the errors in his thinking. Schoolchildren laugh at his ideas, but just as a comic keeps a straightman, the inverse is often useful.
Post by: guest39538 on 13/06/2016 16:21:26
You see the sun when the light which has travelled from it strikes your eye, that light has taken 8mins to get from the sun hence we see its past.
Any other explanation is a new theory and should be posted elsewhere.
Ok, let me just ask questions then and not mention any ideas I have.
''You see the sun when the light which has travelled from it strikes your eye''
So how do I see the light in space that is not striking my eye and allowing me to observe a distance between me and the sun?
Post by: Alan McDougall on 13/06/2016 16:49:22
You see the sun when the light which has travelled from it strikes your eye, that light has taken 8mins to get from the sun hence we see its past.
Any other explanation is a new theory and should be posted elsewhere.
Ok, let me just ask questions then and not mention any ideas I have or you will only insult anyway with your Dunning and Kruger effect, you are not even a scientist and have no qualification to even give answers yet you mock me?
I would like to know who you think you ar with your high and mighty attitude since becoming a mod?
''You see the sun when the light which has travelled from it strikes your eye''
So how do I see the light in space that is not striking my eye and allowing me to observe a distance between me and the sun?
Because my ignorant friend, you can only see it when the photons reaches the retina of your eyes and impinges of it enough for you silly brain to register the fact. You cannot see a light beam coming at you, like you would observe a railway train coming toward you!
Post by: guest39538 on 13/06/2016 17:00:37
Quote from: mcdougall
Because my ignorant friend, you can only see it when the photons reaches the retina of your eyes and impinges of it enough for you silly brain to register the fact. You cannot see a light beam coming at you, like you would observe a railway train coming toward you!
Clearly you are answering a question I did not ask.
I have not mentioned beams in the question and you have not answered the question.
The question was -
So how do I see the light in space that is not striking my eye and allowing me to observe a distance between me and the sun?
If this is not English enough for you maybe I will define just for you,
Do I see light in space that as not entered my eye?
Post by: chiralSPO on 13/06/2016 20:11:07
Do I see light in space that as not entered my eye?
no
Post by: Alan McDougall on 13/06/2016 21:31:31
Quote from: mcdougallBecause my ignorant friend, you can only see it when the photons reaches the retina of your eyes and impinges of it enough for you silly brain to register the fact. You cannot see a light beam coming at you, like you would observe a railway train coming toward you!
Clearly you are answering a question I did not ask.
I have not mentioned beams in the question and you have not answered the question.
The question was -
So how do I see the light in space that is not striking my eye and allowing me to observe a distance between me and the sun?
If this is not English enough for you maybe I will define just for you,
Do I see light in space that as not entered my eye?
It is as clear as mud!
Post by: guest39538 on 13/06/2016 22:09:36
Are you saying it is not light in space and it is dark?
Do I see light in space that as not entered my eye?
no
Then why can I see distance ?
Post by: Alan McDougall on 13/06/2016 23:29:31
Are you saying it is not light in space and it is dark?
Do I see light in space that as not entered my eye?
no
Then why can I see distance ?
It has become obvious that you are looking for a pseudo-science metaphysical answer to your question , maybe you think the eye can reaching out into space and picking up the image of the sun, long before any proton reached the earth and then follow it back and it this way observe distance.
If this is the case then it remains nonsense Period!
Post by: Colin2B on 14/06/2016 00:11:41
Are you saying it is not light in space and it is dark?
Do I see light in space that as not entered my eye?
no
No, he means exactly what he said.
You do not see light that has not entered your eye.
Then why can I see distance ?Look up stereoscopic vision
... you are not even a scientist and have no qualification to even give answersAll the moderators on this site are fully qualified to answer questions. I don't usually bother to mention my qualifications, but as you ask, I have been studying science, particularly physics, since secondary school and have numerous certificates and qualifications. The last science qualification I gained was awarded by one of the UK's top universities. In my career I have given lectures to international conferences organised by recognised professional organisations and have been invited to be a visiting lecturer, on post grad courses, to a number of universities.
We don't mock you it is you who do the mocking. You mock the great scientists who studied light from Ibn Al-Haytham through Galileo and Newton to those who use the science to bring you the technology you use. You also mock yourself because you are capable of understanding but choose not to.
We treat you no differently to others on this forum. When you signed up you agreed to abide by the acceptable usage policy.
Post by: jeffreyH on 14/06/2016 03:09:05
Ok, let me just ask questions then and not mention any ideas I have or you will only insult anyway with your Dunning and Kruger effect, you are not even a scientist and have no qualification to even give answers yet you mock me?
You see the sun when the light which has travelled from it strikes your eye, that light has taken 8mins to get from the sun hence we see its past.
Any other explanation is a new theory and should be posted elsewhere.
I would like to know who you think you ar with your high and mighty attitude since becoming a mod?
''You see the sun when the light which has travelled from it strikes your eye''
So how do I see the light in space that is not striking my eye and allowing me to observe a distance between me and the sun?
I have always been pretty straight with you, listened and even helped you clarify some of your assertions. Now that you have resorted to insults I will be much less inclined to do so in the future.
Post by: guest39538 on 14/06/2016 07:09:19
Quote from: dougal
It has become obvious that you are looking for a pseudo-science metaphysical answer to your question , maybe you think the eye can reaching out into space and picking up the image of the sun, long before any proton reached the earth and then follow it back and it this way observe distance.
What I know, not think , is that space is transparent and I can see distance and light through the entirety of the distance between objects.
I can see it is light less than a foot away from me, light that is obviously not in my eyes.
Post by: Colin2B on 14/06/2016 09:25:44
I can see it is light less than a foot away from me, light that is obviously not in my eyes.That light may be a foot away, but it still has to travel from the bulb or object, enter your eye, hit your retina, and send the image to your brain. That is the process of seeing, it is well understood and there is plenty of information on the web if you don't believe us.
Why not go and ask on some other fora, there are lots of them, I'm sure they would be happy to explain.
Did you look up stereoscopic vision - ie distance perception?