Big bang question. Any physics major here?

[Mat]

Ooo, the Twin Paradox.

 

[mervyn]

It's a bit more complex than that (obviously). In terms of the mountain analogy it's the other way around: that you travel faster the further away from the source of gravity you are, so a clock at the top of a mountain will go faster than a clock at sea level. But the difference is so slight you wouldn't notice it: the difference is something like 10 nanoseconds a day different per km above sea level. So we're in atomic clock territory to measure that kind of difference. So if there was a hypothetical clock at the altitude of the summit of Everest (8,800metre above sea level) since the formation of the Earth 4.5 billion years ago, that clock would right now be just over 30 minutes faster than another hypothetical clock at sea level.

However, if you gave an astronaut in orbit above the Earth's atmosphere (such as those on the International Space Station) a clock, what you see there is that their clock will go slower than the hypothetical one at sea level: this is because the astronaut's relative velocity to the clock at sea level is greater, and greater velocity = slower time (for them, being in effect perpetual "free-fall" around the Earth, velocity is a bigger factor than gravity). So you could have a hypothetical scenario of three astronauts with a clock: one is launched into space to the ISS, one climbs a mountain to watch the launch, and the third stays at Cape Canaveral. The one on the mountain ages quicker than the one at the space centre in Florida, but they both age quicker than the one up in the ISS.

There is a practical effect from this. I read recently that the clock on satellites conveying satnav info have to be constantly (albeit minutely) adjusted to take account of the time difference on the earth’s surface.

 

[Ave IT]

There is a practical effect from this. I read recently that the clock on satellites conveying satnav info have to be constantly (albeit minutely) adjusted to take account of the time difference on the earth’s surface.

Oh yes indeed, (this is kind of my field). That is an important factor and the whole GPS system would fall into heap if the 'time dilation' effects were not taken into account - so you need to know the difference in gravity experienced between the satellites and us on the Earth and the relative speed differences. This 'time dilation' may not seem big - it's the order of hundreds picoseconds per second that passes on Earth - but enough to cock-up the SatNav system that relies on the precise differences in the time of signals that arrive from the various satellites that are in view of the receiver .... which is how it is able to calculate where you must be located to see those differences.

This stuff about 'time dilation' still blows my mind to be honest even after all these years. It's real, undeniable and very observable but defies our innate human experience and conscious model of how things must be. At the extremes the effects are really, really spooky. The Hubble & JWST can now take photographs of super-heated stellar material falling into a black hole that has been accelerated to close to light speed before it disappears into the void forever. The truly bizarre thing is that although this material itself disappears into the black-hole in an infinitesimally small time (for the material) we see it forever on the rim of the black hole frozen still because our clocks are moving so very much faster than its time is passing. Think of that in reverse a moment. If you were unlucky enough to be heading towards that black-hole you could look back at Earth and in the micro-seconds before you disappear you'd see the entire History of Earth flash before you in an instant...... How mad (but real) is that!!

 

[MickyD]

If people would like to get in to emotive discussions about god/creationsim they are welcome to do so on other forums away from Pasoti. This thread or forum isn't the place for those types of discussions.

I've just exceeded the speed of light and travelled back in time, Superman stylee, to read this. Can't disagree - I was one of those guilty of turning it into something of an Opinions thread.

Now, if only I had a way to confess my guilt and seek absolution...

 

[Andy S]

I've just exceeded the speed of light and travelled back in time, Superman stylee, to read this. Can't disagree - I was one of those guilty of turning it into something of an Opinions thread.

Now, if only I had a way to confess my guilt and seek absolution...

You could always make a contribution to the Admin's Christmas Party...rum is good!

 

[Andy S]

One for the Boffins, about SatNav.

During our summer months (October to end of April) we sit in the hot tub of an evening and watch the satellites going over. We're pretty lucky in that we have very little light pollution.

Am I right in thinking that the satellites that provide the SatNav service are all in geostationary orbits? If so, at what altitude do they operate and, do they rotate?

Reason being, every now and again (not a regular occurrence) there will be a very quick flash in the sky. I've always thought that it would be the reflection of the sun off the surface that was spinning. Please bear in mind that I am old!

If anyone can answer, much obliged.

 

[mervyn]

One for the Boffins, about SatNav.

During our summer months (October to end of April) we sit in the hot tub of an evening and watch the satellites going over. We're pretty lucky in that we have very little light pollution.

Am I right in thinking that the satellites that provide the SatNav service are all in geostationary orbits? If so, at what altitude do they operate and, do they rotate?

Reason being, every now and again (not a regular occurrence) there will be a very quick flash in the sky. I've always thought that it would be the reflection of the sun off the surface that was spinning. Please bear in mind that I am old!

If anyone can answer, much obliged.

It’s watching the satellites passing over that blows my mind. I watch a satellite take, say, exactly 12 seconds to pass horizon to horizon. However the time taken aboard the satellite is, say, 11.9 seconds, (I know the real difference is minuscule). So, someone sitting inside that satellite is experiencing slower time even as I’m looking at them. I now have a grasp of the reason, but I still find it mind boggling.

 

[MickyD]

One for the Boffins, about SatNav.

During our summer months (October to end of April) we sit in the hot tub of an evening and watch the satellites going over. We're pretty lucky in that we have very little light pollution.

Am I right in thinking that the satellites that provide the SatNav service are all in geostationary orbits? If so, at what altitude do they operate and, do they rotate?

Reason being, every now and again (not a regular occurrence) there will be a very quick flash in the sky. I've always thought that it would be the reflection of the sun off the surface that was spinning. Please bear in mind that I am old!

If anyone can answer, much obliged.

I'll think about the rum!

Yes, satnav satellites, like many other communication-type satellites, are in geostationary orbit 22,000-odd miles (35,000-odd km) above the Earth's surface.

But there are so many satellites in low-Earth orbit these days (LEO, the non-geostationary kind) that it's quite common to see glints of sunlight off them - and sometimes you can even watch them with the naked eye crossing the sky if the lighting conditions are right, which by the sound of it you already do while in the hot tub! But anyway, I'm pretty sure that geostationary satellites are way, way too far away to be seen with the naked eye, never mind reflections.

Elon Musk's LEO Starlink mini-satellites, on the other hand, are in the process of pretty much destroying gound-based astronomy, thanks to the huge numbers of them already up there in great long chains, with many thousands yet to launch.

Here's what they can look like on a time exposure to capture the stars' movement in the sky. That lattice-like pattern to the top right is a train of Starlink satellites.



And here's a train of them, isolated.

 

[Andy S]

I'll think about the rum!

Yes, satnav satellites, like many other communication-type satellites, are in geostationary orbit 22,000-odd miles (35,000-odd km) above the Earth's surface.

But there are so many satellites in low-Earth orbit these days (LEO, the non-geostationary kind) that it's quite common to see glints of sunlight off them - and sometimes you can even watch them with the naked eye crossing the sky if the lighting conditions are right, which by the sound of it you already do while in the hot tub! But anyway, I'm pretty sure that geostationary satellites are way, way too far away to be seen with the naked eye, never mind reflections.

Elon Musk's LEO Starlink mini-satellites, on the other hand, are in the process of pretty much destroying gound-based astronomy, thaks to the huge numbers of them already up there in great long chains, with many thousands yet to launch.

Here's what they can look like on a time time exposure to capture the stars' movement in the sky. That lattice-like pattern to the top right is a train of Starlink satellites.

View attachment 16101

And here's a train of them, isolated.

View attachment 16102

Thanks Micky.

Starlink is a regular visitor to our skies in the summer. Our record is 56 in one "train". "Normal" satellites we can see anything up to 20 a night, sometimes only 1 or 2. ISS is also a regular.

I love anything to do with space...the final frontier...and desperately hope that Betelgeuse has it's hairy fit in my lifetime.

 

[MickyD]

Thanks Micky.

Starlink is a regular visitor to our skies in the summer. Our record is 56 in one "train". "Normal" satellites we can see anything up to 20 a night, sometimes only 1 or 2. ISS is also a regular.

I love anything to do with space...the final frontier...and desperately hope that Betelgeuse has it's hairy fit in my lifetime.

Wow, that's fantastic. When I was a lot younger I lived in the mountains of North Wales, then on the edge of Dartmoor, and we had amazing night skies. Now I live in Brighton and we can't see much of anything.

Yes, come on, Betelgeuse, get on with it! Get Lauren Boebert to give you a hand!

 

[Ave IT]

I'll think about the rum!

Yes, satnav satellites, like many other communication-type satellites, are in geostationary orbit 22,000-odd miles (35,000-odd km) above the Earth's surface.

But there are so many satellites in low-Earth orbit these days (LEO, the non-geostationary kind) that it's quite common to see glints of sunlight off them - and sometimes you can even watch them with the naked eye crossing the sky if the lighting conditions are right, which by the sound of it you already do while in the hot tub! But anyway, I'm pretty sure that geostationary satellites are way, way too far away to be seen with the naked eye, never mind reflections.

Elon Musk's LEO Starlink mini-satellites, on the other hand, are in the process of pretty much destroying gound-based astronomy, thaks to the huge numbers of them already up there in great long chains, with many thousands yet to launch.

Here's what they can look like on a time time exposure to capture the stars' movement in the sky. That lattice-like pattern to the top right is a train of Starlink satellites.

View attachment 16101

And here's a train of them, isolated.

View attachment 16102

Great photos showing starlink sats .... (bloody Musk is out of control but don't want to provoke an 'Opinions' debate ).

Bring a pedant have to point out the GPS sats are not geostationary. They orbit Earth twice a day at altitude of roughly 12,000 miles so you wouldnt see them. They are arranged in 6 different planes which are equally spaced & tilted with respect to each other and the sats orbiting in each plane equally spaced. This means they pass over the same areas repeatedly so a few ground stations on Earth can upload correction data continually to mitigate effects of atmospherics and ageing of the satellite to maintain accuracy (or if desired delibrately scramble it).

By the way your phone or any device you own does NOT communicate with the sats -it is just a receiver of the signals. The idea is that each satellite keeps sending a repeated stream of data part of which identifies itself and the time of transmission. As long as your receiver can 'see' at least 4 sats (preferably more) then by a process of 'triangulation' it can calculte exactly where on the Earth's surface it must be. Time of arrival of the signals though is key - hence knowing all the 'time dilation' effects.

The EU satellite system is known as Galileo, the Russian is Glonass and Chinese is Beidou which all use similar operating methods but at different orbital rates and altitudes then there are localised systems over India and Australia & New Zealand. The UK has proposed launching its own system following Brexit...... but best not go there or I won't be able to resist a rant.

 

[Ave IT]

I love anything to do with space...the final frontier...and desperately hope that Betelgeuse has it's hairy fit in my lifetime.

Hopefully, Betelgeuse doesn't do its thing in your lifetime Andy!! - rather hoping it already did it's thing during the lifetime of your distant ancestors 640-odd years ago.

 

[GreenThing]

As part of my radio and radar training I had to cover parabolic navigational aids, which was a predecessor to GPS and works on a similar principal. Decca was the British one while Loran was the American version.

If you take two transmitters which transmit at the same time and sit exactly inbetween them, you will receive the signals at the same time and if you move off the centre line but stay equal distance, you will plot a straight line. If you then sit closer to one transmitter and plot your position, then move off centre to a place where the time difference stays constant, you will plot a curved parabolic line.

Radio beacons were placed around the coast and transmit synchronous signals for a receiver to pick up. The parabolic lines for the different time distances are drawn on a navigation chart. So to find your position, you tune your Decca receiver to receive signals from the nearest transmitters, the time difference will be shown on 3 coloured dials. Go to your Decca chart and find the parabolic line that matches the number for each dial (the lines are coloured differently to match the dials on the receiver) and where they intersect, that’s your position.

The system has been switched off as GPS has taken over, but there was a Decca station on the coast south of Kingsbridge.

 

[MickyD]

Great photos showing starlink sats .... (bloody Musk is out of control but don't want to provoke an 'Opinions' debate ).

Bring a pedant have to point out the GPS sats are not geostationary. They orbit Earth twice a day at altitude of roughly 12,000 miles so you wouldnt see them. They are arranged in 6 different planes which are equally spaced & tilted with respect to each other and the sats orbiting in each plane equally spaced. This means they pass over the same areas repeatedly so a few ground stations on Earth can upload correction data continually to mitigate effects of atmospherics and ageing of the satellite to maintain accuracy (or if desired delibrately scramble it).

By the way your phone or any device you own does NOT communicate with the sats -it is just a receiver of the signals. The idea is that each satellite keeps sending a repeated stream of data part of which identifies itself and the time of transmission. As long as your receiver can 'see' at least 4 sats (preferably more) then by a process of 'triangulation' it can calculte exactly where on the Earth's surface it must be. Time of arrival of the signals though is key - hence knowing all the 'time dilation' effects.

The EU satellite system is known as Galileo, the Russian is Glonass and Chinese is Beidou which all use similar operating methods but at different orbital rates and altitudes then there are localised systems over India and Australia & New Zealand. The UK has proposed launching its own system following Brexit...... but best not go there or I won't be able to resist a rant.

Not pedantic at all - I stand corrected. I'd forgotten that about the satnav orbits and didn't look it up.

 

[TimHarris]

Special relativity doesn’t explain when I’m in a work meeting with so little gravity to it why time slows down.