Scientific 'things you've always wanted to know' thread
Discussion
mattnunn said:
A few questions
1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
1. you might have heard about this http://en.wikipedia.org/wiki/Laser_cooling using laser beams from different directions to effect a reduction in the motion of atomic in a gas, and the energy is reduced simplistically you can consider it to be cooled. speed of light doesn't come into it really.1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
2. a perfect vacuum is just that "perfect" - if it isn't then it is necessary to take account of the additional matter
3. one might suggest it it maths, but experiments to date confirm it - lets see what the neutrino experiments come to... audience is out, but principle of least astonishment suggests something unique in experiment and not universal.
Now if you aren't mathematically minded - skip some (all) of maths here http://en.wikipedia.org/wiki/Maxwell's_equations
one of the most beautiful things to come from maxwells formalisation of electricity and magnetism is that c falls out naturally as being proportional to other fundamental constants mu nought and epsilon nought - in a vacuum.... and when we consider electromagnetic waves (aka light) passing through matter c is revised by the inclusion of mu_r and epsilon_r the 'relative" parameters for the matter in question; in effect light slows down as it passes through matter and this is the basis of many of the optical phenomena you'll be familiar with - http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/... and these relative parameters are intimately related to the physical materials properties of the matter in question.... in an insulator at lower frequencies (audio, rf and low GHz)n we might consider the permittivity, epsilon_r, as a characteristic parameter (this would be used in calculating practical things such as capacitance in capacitors) and at much higher frequencies (light) it become less appropriate and we then use terms such a the refractive index (n & k)....
physprof said:
mattnunn said:
A few questions
1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
1. you might have heard about this http://en.wikipedia.org/wiki/Laser_cooling using laser beams from different directions to effect a reduction in the motion of atomic in a gas, and the energy is reduced simplistically you can consider it to be cooled. speed of light doesn't come into it really.1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
2. a perfect vacuum is just that "perfect" - if it isn't then it is necessary to take account of the additional matter
3. one might suggest it it maths, but experiments to date confirm it - lets see what the neutrino experiments come to... audience is out, but principle of least astonishment suggests something unique in experiment and not universal.
Now if you aren't mathematically minded - skip some (all) of maths here http://en.wikipedia.org/wiki/Maxwell's_equations
one of the most beautiful things to come from maxwells formalisation of electricity and magnetism is that c falls out naturally as being proportional to other fundamental constants mu nought and epsilon nought - in a vacuum.... and when we consider electromagnetic waves (aka light) passing through matter c is revised by the inclusion of mu_r and epsilon_r the 'relative" parameters for the matter in question; in effect light slows down as it passes through matter and this is the basis of many of the optical phenomena you'll be familiar with - http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/... and these relative parameters are intimately related to the physical materials properties of the matter in question.... in an insulator at lower frequencies (audio, rf and low GHz)n we might consider the permittivity, epsilon_r, as a characteristic parameter (this would be used in calculating practical things such as capacitance in capacitors) and at much higher frequencies (light) it become less appropriate and we then use terms such a the refractive index (n & k)....
mattnunn said:
physprof said:
mattnunn said:
A few questions
1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
1. you might have heard about this http://en.wikipedia.org/wiki/Laser_cooling using laser beams from different directions to effect a reduction in the motion of atomic in a gas, and the energy is reduced simplistically you can consider it to be cooled. speed of light doesn't come into it really.1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
2. a perfect vacuum is just that "perfect" - if it isn't then it is necessary to take account of the additional matter
3. one might suggest it it maths, but experiments to date confirm it - lets see what the neutrino experiments come to... audience is out, but principle of least astonishment suggests something unique in experiment and not universal.
Now if you aren't mathematically minded - skip some (all) of maths here http://en.wikipedia.org/wiki/Maxwell's_equations
one of the most beautiful things to come from maxwells formalisation of electricity and magnetism is that c falls out naturally as being proportional to other fundamental constants mu nought and epsilon nought - in a vacuum.... and when we consider electromagnetic waves (aka light) passing through matter c is revised by the inclusion of mu_r and epsilon_r the 'relative" parameters for the matter in question; in effect light slows down as it passes through matter and this is the basis of many of the optical phenomena you'll be familiar with - http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/... and these relative parameters are intimately related to the physical materials properties of the matter in question.... in an insulator at lower frequencies (audio, rf and low GHz)n we might consider the permittivity, epsilon_r, as a characteristic parameter (this would be used in calculating practical things such as capacitance in capacitors) and at much higher frequencies (light) it become less appropriate and we then use terms such a the refractive index (n & k)....
physprof said:
mattnunn said:
physprof said:
mattnunn said:
A few questions
1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
1. you might have heard about this http://en.wikipedia.org/wiki/Laser_cooling using laser beams from different directions to effect a reduction in the motion of atomic in a gas, and the energy is reduced simplistically you can consider it to be cooled. speed of light doesn't come into it really.1) i heard they did an experiment at manchester uni using a very very cold gas they could slow and almost stop proton, so is the speed of light in space not variable to temperature?
2) I find it hard to grasp space being a perfect vacuum, i.e devoid of matter, it's obviously full of stuff, anti matter whatever, sowhy do we say C is speed of light in a vacuum?
3) isn't the reality that C is just a handy constant for the maths but in reality it's not constant?
2. a perfect vacuum is just that "perfect" - if it isn't then it is necessary to take account of the additional matter
3. one might suggest it it maths, but experiments to date confirm it - lets see what the neutrino experiments come to... audience is out, but principle of least astonishment suggests something unique in experiment and not universal.
Now if you aren't mathematically minded - skip some (all) of maths here http://en.wikipedia.org/wiki/Maxwell's_equations
one of the most beautiful things to come from maxwells formalisation of electricity and magnetism is that c falls out naturally as being proportional to other fundamental constants mu nought and epsilon nought - in a vacuum.... and when we consider electromagnetic waves (aka light) passing through matter c is revised by the inclusion of mu_r and epsilon_r the 'relative" parameters for the matter in question; in effect light slows down as it passes through matter and this is the basis of many of the optical phenomena you'll be familiar with - http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/... and these relative parameters are intimately related to the physical materials properties of the matter in question.... in an insulator at lower frequencies (audio, rf and low GHz)n we might consider the permittivity, epsilon_r, as a characteristic parameter (this would be used in calculating practical things such as capacitance in capacitors) and at much higher frequencies (light) it become less appropriate and we then use terms such a the refractive index (n & k)....
In the same way I know at an constant 70mph I could travel the 210 miles from London to Manchester in (let's say) 3 hours, if the motorways were a metephorical vacumm (i.e no other cars), but if my proporgation up the M6 is, as it undoubtly will be, I can no longer rely on the 70mph constant. I'm an RF engineer, I understand how waves propogate in free space and around our planet, what bugs me is that do we really know that when you get outside our solar system a few hundered miles (or more) that there isn't just a thick blanket of goo through which everything proporgates really slowly so the stuff on the other side appears when measured with electromagnetic waves to be further away than it is?
Right I'm waffling, the question, could something be very many measured light years away i.e as measured by a radio telescope, but in actual miles be much closer than the number of light years would suggest?
mattnunn said:
Thanks for that Proffesor, I am still hung up on this though, I remember 18 years ago when I did physics A level being hung up on the same thing... I knew E = mc2 was a bad thing to mention because it only confuses me further, what I really want to know is that when people talk about distance in the universe in terms of light years etc and we measure distance as a function of time it takes for light to travel in space we assume a perfect vacum, but do we know it's not a perfect vacumm in space - right?
In the same way I know at an constant 70mph I could travel the 210 miles from London to Manchester in (let's say) 3 hours, if the motorways were a metephorical vacumm (i.e no other cars), but if my proporgation up the M6 is, as it undoubtly will be, I can no longer rely on the 70mph constant. I'm an RF engineer, I understand how waves propogate in free space and around our planet, what bugs me is that do we really know that when you get outside our solar system a few hundered miles (or more) that there isn't just a thick blanket of goo through which everything proporgates really slowly so the stuff on the other side appears when measured with electromagnetic waves to be further away than it is?
Right I'm waffling, the question, could something be very many measured light years away i.e as measured by a radio telescope, but in actual miles be much closer than the number of light years would suggest?
Its actually relatively easy to create a vacuum of around 1e-10 mbar (with some stainless steel kit, some pumps etc and a budget of maybe £50-100K) in the laboratory (http://en.wikipedia.org/wiki/Ultra-high_vacuum )In the same way I know at an constant 70mph I could travel the 210 miles from London to Manchester in (let's say) 3 hours, if the motorways were a metephorical vacumm (i.e no other cars), but if my proporgation up the M6 is, as it undoubtly will be, I can no longer rely on the 70mph constant. I'm an RF engineer, I understand how waves propogate in free space and around our planet, what bugs me is that do we really know that when you get outside our solar system a few hundered miles (or more) that there isn't just a thick blanket of goo through which everything proporgates really slowly so the stuff on the other side appears when measured with electromagnetic waves to be further away than it is?
Right I'm waffling, the question, could something be very many measured light years away i.e as measured by a radio telescope, but in actual miles be much closer than the number of light years would suggest?
recall we live in an environment of around 1000 mbar, pressure in space is around 1e-11 to 1e-12 mbar ... so it is relatively easy to undertake a speed of light experiment in a "vacuum".
see some math here ... http://www.virginia.edu/ep/SurfaceScience/class2.h... and half way down you'll see an argument for how much (little) stuff is in a vacuum.... "UHV is typically considered to be below 10-9 Torr. Even at the best vacuum normally used in surface science experiments, ~10-11 Torr, there are still plenty of molecules in the gas phase, about 300 hundred thousand per cm3. In interstellar space, the density of molecules can be as low as a few per cm3, corresponding to about 10-16 Torr." BTW 760 Torr ~ 1 atmosphere ~ 1000 mbar
compared to low vacuum, far less soft or solid matter, there is not a lot in vacuum.
soda said:
Quick one from me, although beer has been consumed 
The moon has a strong effect on the oceans in the form of tides. Does it have any effect on the atmosphere though, and if not, why not?
Slight changes in atmospheric pressure I believe. However the Moon has a massive effect on one of my cats. The moon has a strong effect on the oceans in the form of tides. Does it have any effect on the atmosphere though, and if not, why not?
dickymint said:
soda said:
Quick one from me, although beer has been consumed
The moon has a strong effect on the oceans in the form of tides. Does it have any effect on the atmosphere though, and if not, why not?
Slight changes in atmospheric pressure I believe. However the Moon has a massive effect on one of my cats. The moon has a strong effect on the oceans in the form of tides. Does it have any effect on the atmosphere though, and if not, why not?
soda said:
dickymint said:
soda said:
Quick one from me, although beer has been consumed
The moon has a strong effect on the oceans in the form of tides. Does it have any effect on the atmosphere though, and if not, why not?
Slight changes in atmospheric pressure I believe. However the Moon has a massive effect on one of my cats. The moon has a strong effect on the oceans in the form of tides. Does it have any effect on the atmosphere though, and if not, why not?
I have asked this before but cannot remember the answer.
Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
Getragdogleg said:
I have asked this before but cannot remember the answer.
Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
It's a combination of Refraction and Perspective.Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
annodomini2 said:
Getragdogleg said:
I have asked this before but cannot remember the answer.
Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
It's a combination of Refraction and Perspective.Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
k off big torch but HOW ? With pitchers for the simple man.
Getragdogleg said:
I have asked this before but cannot remember the answer.
Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
Light doesn't travel in straight parallel lines.Why when the sun shines through clouds does it create shafts of light that seem to converge on a point close to the back of the cloud ?
The sun is a very long way away so my head tells me the shafts of light should not fan out like they do but be parallel to each other.
Get a pieces of card, cut a hole in it, shine a torch through the hole onto a wall (you'll need to turn the lights of. The illumination on the wall is considerably large than the hole you cut, right? The further away from the hole you are, the bigger the illumination.
Further more, as light is a wave and a particle this happens too...
http://en.wikipedia.org/wiki/Double-slit_experimen...
That explains why the light through a hole is cone shaped, but it does not explain why lots of holes make a pattern that seems to originate from a central light source closer than the sun.
I cannot explain it but In my head the 10 or so shafts I saw this evening looked like they all came from a central light source, the light source is the sun and that is millions of miles away yet if you drew lines along the shafts they would converge very close to the back of the cloud.
I cannot explain it but In my head the 10 or so shafts I saw this evening looked like they all came from a central light source, the light source is the sun and that is millions of miles away yet if you drew lines along the shafts they would converge very close to the back of the cloud.
Jandywa said:
The gravitational pull of the sun on the earth is greater than that of the moon. So why is it the moon that affects the oceans and not the sun?
The sun does affect the tides - it forms about a third of the total effect. The moon affects the tides more obviously because it is closer and orbiting us. As it proceeds around its orbit, its position relative to the sun changes. When the moon is in roughly in line with the sun, on the very inside or outside of its orbit then the tides are at their highest (new moon and full moon), called spring tides. When the moon is half way between, the tides are at their minimum and are called neap tides.Since orbits are elliptical and not circular, the moon earth and sun are all moving slightly closer and further away from each other. This also has an effect on the tide height. Spring tides occur roughly every 2 weeks, and the orbit of the earth around the sun causes the height of the spring tides to slowly increase and decrease throughout the year. The very highest tides are when the moon and sun are aligned, and when the earth is closest to both the moon and the sun.
Gravitational effect of the sun is 46% of the moon so as TS says roughly one third of the total effect.
But the tides happen on both sides of the earth slightly unequal which is why there is a variation between tide heights within a day. Centrifugal force in a reduced gravity situation is what causes the other side and gives the 2 tides within 25 hours. (One orbit of the moon).
And the earlier post mentioning tide heights is confusing. The tidal bulge is 'only' between 4 and 30 inches dependant on where and when you are but we see tidal heights of upto 30 feet in places as it is effectively like a wave going round the world and when it meets continental shelves and land it gets chanelled and piles up.
All gets a bit complicated but dependant on geography you can have one, two, four or no tides within a day.
Also the earth is not round just to mess with heads some more.
But the tides happen on both sides of the earth slightly unequal which is why there is a variation between tide heights within a day. Centrifugal force in a reduced gravity situation is what causes the other side and gives the 2 tides within 25 hours. (One orbit of the moon).
And the earlier post mentioning tide heights is confusing. The tidal bulge is 'only' between 4 and 30 inches dependant on where and when you are but we see tidal heights of upto 30 feet in places as it is effectively like a wave going round the world and when it meets continental shelves and land it gets chanelled and piles up.
All gets a bit complicated but dependant on geography you can have one, two, four or no tides within a day.
Also the earth is not round just to mess with heads some more.
Getragdogleg said:
That explains why the light through a hole is cone shaped, but it does not explain why lots of holes make a pattern that seems to originate from a central light source closer than the sun.
I cannot explain it but In my head the 10 or so shafts I saw this evening looked like they all came from a central light source, the light source is the sun and that is millions of miles away yet if you drew lines along the shafts they would converge very close to the back of the cloud.
I cannot explain it but In my head the 10 or so shafts I saw this evening looked like they all came from a central light source, the light source is the sun and that is millions of miles away yet if you drew lines along the shafts they would converge very close to the back of the cloud.
for the purpose of this discussion why not consider the source of the light to be the hole in the cloud?
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