light shouldn't be able to travel far

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light shouldn't be able to travel far
« on: May 09, 2016, 02:25:44 PM »
When light travels it expands in all directions, like a balloon inflating.
So the power of the light decreases greatly as it travels
Official source here:
https://van.physics.illinois.edu/qa/listing.php?id=21368
 Read the third paragraph, he talks about light expanding and getting weaker.

It seems unlikely that light would be able to travel billions of miles without getting too weak to detect.

The scientist says lights power decreases by the distance travelled squared,is that an exponential decrease?

The same thing applies to any wave.
You need a huge transmitter just to get a radio signal to broadcast over several miles.

Just thinking about our sun, we are really close to our sun compared to a distant Star but our sun can only heat us up to about 120 degree Fahrenheit which isn't much compared to how hot it is really.
« Last Edit: May 09, 2016, 02:33:50 PM by Ex-Globe »
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Re: light shouldn't be able to travel far
« Reply #1 on: May 09, 2016, 03:44:57 PM »
Welcome to the Flat Earth Society forum, Ex-Globe!

When light travels it expands in all directions, like a balloon inflating.
So the power of the light decreases greatly as it travels
The power per unit area decreases due to spreading, yes. The total power does not decrease due to spreading alone, but does by absorption; "empty space" contains little that will absorb light and similar electromagnetic radiation.

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Official source here:
https://van.physics.illinois.edu/qa/listing.php?id=21368
 Read the third paragraph, he talks about light expanding and getting weaker.

It seems unlikely that light would be able to travel billions of miles without getting too weak to detect.
It depends on its intrinsic brightness (how strong it is to start with).

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The scientist says lights power decreases by the distance travelled squared,is that an exponential decrease?

No. That is in inverse square decrease; it is of the form 1/distance2.

An exponential decrease with distance would be of the form 1/(some factor)(some other factor) X distance.

That's why it takes larger telescopes to see dimmer objects. A telescope will collect four times as many photons as one with half the aperture, so, in principle, it takes a telescope twice as large in diameter to see a point source twice as far away as brightly, all else being equal (intrinsic brightness, absorption, etc.).

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The same thing applies to any wave.
You need a huge transmitter just to get a radio signal to broadcast over several miles.

Not true. It's possible to communicate around the globe using milliwatts of transmitted power, sometimes a lot less.

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Just thinking about our sun, we are really close to our sun compared to a distant Star but our sun can only heat us up to about 120 degree Fahrenheit which isn't much compared to how hot it is really.

The Earth just intercepts a tiny fraction of the solar radiation; the cross section of the Earth is about 50,000,000 square miles (50 million). A sphere the radius of the Earth's orbit is 109,000,000,000,000,000 square miles (109 quadrillion) surface area, so only about .045 millionths of one percent of solar radiation is intercepted by the Earth. Of that, about half the energy reaching the top of the atmosphere is reflected or absorbed and re-radiated back into space by the atmosphere.
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Re: light shouldn't be able to travel far
« Reply #2 on: May 09, 2016, 04:59:02 PM »
Thanks do you know of anyone who has calculated the minimum size the nearest Star has to be,to be visible?
It would be interesting to see how big the furthest visible star has to be,to be visible with the naked eye.there is no need to include telescopes in this as the eye can see stars that are billions of miles or light years(?) Away.
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sokarul

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Re: light shouldn't be able to travel far
« Reply #3 on: May 09, 2016, 05:32:06 PM »
Thanks do you know of anyone who has calculated the minimum size the nearest Star has to be,to be visible?
It would be interesting to see how big the furthest visible star has to be,to be visible with the naked eye.there is no need to include telescopes in this as the eye can see stars that are billions of miles or light years(?) Away.
It's not about size, it's about intensity.  Your eye needs about 5 photons to see a star.
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Re: light shouldn't be able to travel far
« Reply #4 on: May 09, 2016, 05:35:12 PM »
Ah, so a very distant Star could be young and therefore very bright
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sokarul

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Re: light shouldn't be able to travel far
« Reply #5 on: May 09, 2016, 06:31:42 PM »
Ah, so a very distant Star could be young and therefore very bright
I don't think size is an indication of a star's age.
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Re: light shouldn't be able to travel far
« Reply #6 on: May 09, 2016, 07:10:33 PM »
OK so there is a visible star 8200 light years away

http://physics.stackexchange.com/questions/45759/what-is-the-farthest-away-star-visible-to-the-naked-eye

Has anyone done any calculations to see how far away our sun would be visible to?
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rabinoz

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Re: light shouldn't be able to travel far
« Reply #7 on: May 09, 2016, 08:00:30 PM »
The scientist says lights power decreases by the distance travelled squared,is that an exponential decrease?

The simple fact that you need to ask "is that an exponential decrease?" means you have no idea what you are talking about!
No an "inverse square law" is simply  an "inverse square law".

Do a far easy calculations.

The sun's power density on the earth's surface is roughly 1,000 W/m2 and it is about 149x106 km away.
Alpha Centauri (closest bright star) has a luminosity about 1.52 times the sun and is about 41.3x1012 km away, so applying the inverse square law puts the power density from Alpha Centauri at around 2.0 nW/m2, very easily visible.
(Hope I haven't made a massive error!)

Actually 2.0 nW/m2 is a "massive" signal when we start looking at large optical and radio telescopes!

Of course we can see much fainter stars than this - the human eye is pretty sensitive!



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Pezevenk

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Re: light shouldn't be able to travel far
« Reply #8 on: May 10, 2016, 06:40:49 AM »
Thanks do you know of anyone who has calculated the minimum size the nearest Star has to be,to be visible?
It would be interesting to see how big the furthest visible star has to be,to be visible with the naked eye.there is no need to include telescopes in this as the eye can see stars that are billions of miles or light years(?) Away.

That would depend on the brightness of the star and the sensitivity of the instrument. One of the most distant objects visible to the naked eye is the Andromeda galaxy, which is about 2.5 million light years away, and the most distant individual star visible with the naked eye is V762 Cas, which is about 16300 light years away. The nearest star is 4.24 light years away, but it's all about the brightness of the star, and not the size. I'm gonna do the math and tell you later.
« Last Edit: May 10, 2016, 06:43:50 AM by Definitely Not Official »
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Re: light shouldn't be able to travel far
« Reply #9 on: May 10, 2016, 09:30:11 AM »
Of course we can see much fainter stars than this - the human eye is pretty sensitive!
If I remember correctly, one of the most sensitively-seeing people in the world can recognize a person from about five miles away (no source, I saw it on Imgur), and even in good conditions there's still some adverse atmospheric effects. If humans can see the light reflected from a person at that distance and sometimes even recognize them, a thousands-of-miles-across ball of nuclear fire will easily be seen across even a huge distance when there's negligible material to absorb and deflect the light.

With outer space being a vacuum and light not being absorbed by anything, the light maintains virtually all of its original intensity--the star only has to be close enough for it to take up angular space in our vision for us to see it. You'll notice the "bigger" looking stars tend to be much brighter; only those that "appear smaller" tend to look dim, their light comes from too weak a source at their distance to occupy as many receptors in our vision.
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Pezevenk

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Re: light shouldn't be able to travel far
« Reply #10 on: May 10, 2016, 12:37:11 PM »
After doing the math, I found that under optimal conditions, you could theoretically see a star that is 4.24 light years away (the distance to our closest star) that is about 0.0057 times as bright as the sun is. That means that if you were viewing both the sun and that star from the same distance, the star would look 0.0057 times as faint as the sun. It's a bit more than it sounds though. That means that you should be able to see subdwarfs and mayyybe the very brightest of white and red dwarfs from that distance. Proxima centauri (the nearest star to Earth), which is a red dwarf, isn't nearly bright enough to be seen with the naked eye. I'm not sure why you asked that question, but I hope this answers it.
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Jadyyn

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Re: light shouldn't be able to travel far
« Reply #11 on: May 10, 2016, 02:54:09 PM »
Actually, the farthest thing you can see in a dark sky is the Andromeda Galaxy as a smudge. That is millions of light years away (lots of stars in it). (http://space-facts.com/andromeda/)
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Pezevenk

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Re: light shouldn't be able to travel far
« Reply #12 on: May 11, 2016, 12:17:40 AM »
Actually, the farthest thing you can see in a dark sky is the Andromeda Galaxy as a smudge. That is millions of light years away (lots of stars in it). (http://space-facts.com/andromeda/)

That's what I said:
"One of the most distant objects visible to the naked eye is the Andromeda galaxy, which is about 2.5 million light years away"

But apparently it's not. Some people can see the M33 and particularly the M81 galaxies. But that would be extremely hard to see. The farthest thing I've ever seen is indeed the Andromeda galaxy.
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