The time it takes for light to travel back to earth

euser1984

Someone was telling me that light takes about an hour or so to travel to earth from Saturn.

Does this mean if I am looking at it through a telescope, that I'm seeing something that happened an hour or so ago?

Thanks.

cruizer101

Yeah, its kinda crazy when you think about, looking into the past.

Some of the stars we see might not exist anymore because they are so far away the light that is currently reaching us may be from ages ago when they still did exist.

Even the light from the sun is about 8 minutes old when it reaches us

euser1984

So that means that if we were far enough away from earth with a massive telescope where the light has travelled to we could see dinosaurs?

murphyme2010

In principle, yes.
You would need a VERY big telescope though to see T-Rex:)

There are objects which have been gravitationally lensed in such a way that we see two images of the object but the light path for the two images is different and thus we see the same object at two different times.

Hijpo

If it takes the suns rays 84 minutes to reach Saturn, then another 10 minutes for the light from saturn to reach earth, does that mean we see saturn an hour and a half after the light hits it?

Nerro

euser1984 wrote: »

So that means that if we were far enough away from earth with a massive telescope where the light has travelled to we could see dinosaurs?

Not quite, unless you can actually travel faster than speed of light
The light from time of the dinosaurs already left earth and its millions of light years away already ,since nothing can travel faster than light you will never catch it.

euser1984

Is einsteins space time related to this in anyway? Or theories about black holes?

euser1984

Nerro wrote: »

Not quite, unless you can actually travel faster than speed of light
The light from time of the dinosaurs already left earth and its millions of light years away already ,since nothing can travel faster than light you will never catch it.

Maybe there is somebody over there already though, like part of our mind :eek:

RebelButtMunch

Hijpo wrote: »

If it takes the suns rays 84 minutes to reach Saturn, then another 10 minutes for the light from saturn to reach earth, does that mean we see saturn an hour and a half after the light hits it?

Nope. You see saturn ten mins ago. Unlesd someone on saturn had a mirror pointed at the sun. Then you would see the sun in the mirror as it was 94 mins ago

Capt'n Midnight

Back in 1676
http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html

The first real measurement of the speed of light came about half a century later, in 1676, by a Danish astronomer, Ole Römer, working at the Paris Observatory. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. Actually, Römer found, for several months the eclipses lagged more and more behind the expected time, but then they began to pick up again. In September 1676,he correctly predicted that an eclipse on November 9 would be 10 minutes behind schedule. This was indeed the case, to the surprise of his skeptical colleagues at the Royal Observatory in Paris. Two weeks later, he told them what was happening: as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io (actually reflected sunlight, of course) took time to reach the earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth’s orbit compared with the point of closest approach. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter.

ps200306

euser1984 wrote: »

Someone was telling me that light takes about an hour or so to travel to earth from Saturn.

Does this mean if I am looking at it through a telescope, that I'm seeing something that happened an hour or so ago?

Thanks.

This leads to the conclusion that everything you see is in your past. Your outstretched hand is about two nanoseconds in your past. From O'Connell bridge, the light on the top of the Spire is a microsecond ago. The sound of Big Ben on the BBC News by radio from London is one and a half milliseconds ago. The signal from the Sky TV satellite is a tenth of a second ago, and the moon is a second and a half ago.

Things get complicated when you start moving. If you go from O'Connell Bridge to the dining hall in Trinity College, the Spire is now two microseconds ago instead of one. Suppose someone claps their hands on the top of the Spire and you witness this from O'Connell Bridge. That clap is still a microsecond in the future of someone at the dining hall. Let's suppose you yourself travel at the speed of light from O'Connell bridge to the dining hall, arriving there after one microsecond. From O'Connell bridge you just saw the clapper's hands coming together. At the dining hall, one microsecond later, the same image is just arriving. So, on your journey, you would see the clapper frozen in time. If you travelled just a bit slower than light, the image would have overtaken you slightly, so you'd see the clapper moving in extreme slow motion rather than frozen (and making a nonsense of the phrase "run like the clappers" ). The slower you move, the less the slow-motion effect, but there's always a bit of it -- even if you just move your hand away from your face, it moves very slightly in slow motion relative to your head.

That's what was happening with the Jupiter satellite measurement by Römer, mentioned above. As Jupiter moved away from us, the satellite was moving in slow motion and getting further and further behind the expected eclipse time.

jane82

Does it mean that the planets we are looking at as possible earth replacements may be completely different now to the light we are seeing?
Could there be a great wall of China there but we.wont see it for another few hundred years?

shruikan2553

jane82 wrote: »

Does it mean that the planets we are looking at as possible earth replacements may be completely different now to the light we are seeing?
Could there be a great wall of China there but we.wont see it for another few hundred years?

Yup, there could also be a planet somewhere that is seeing us as celts.

john the one

So if I could build a periscope that goes to Saturn and back, I could look at myself actually looking at myself in the past and future?

ps200306

john the one wrote: »

So if I could build a periscope that goes to Saturn and back, I could look at myself actually looking at myself in the past and future?

No, you can only look into the past, never into the future.

john the one

Yeah I was only trying to be a smart arse. Has to be a way of seeing into the future

ps200306

euser1984 wrote: »

Is einsteins space time related to this in anyway? Or theories about black holes?

Yes, the effects being described concerning motion are the "time dilation" aspect of Einstein's theory of Special Relativity. When Einstein formulated General Relativity, he realised that gravity causes similar effects. So time slows down near the surface of the earth compared to far away from it. The stronger the gravity, the stronger the effect, so someone clapping their hands at the boundary of a black hole would be seen by a further away observer as frozen in time, just like the effect I mentioned in the Spire example above where travelling away at the speed of light would result in a similar observation.

ps200306

john the one wrote: »

Yeah I was only trying to be a smart arse. Has to be a way of seeing into the future

We're all time travellers into the future. Just wait a while and you'll get there. :pac:
Can't see it in advance though, so can't help with next year's Cheltenham.

john the one

ps200306 wrote: »

We're all time travellers into the future. Just wait a while and you'll get there. :pac:
Can't see it in advance though, so can't help with next year's Cheltenham.

Hang on....what if I traveled for 50 years into space and back, at extremely high velocities, when I get back to earth, more that 50 years will have passed so technically I will come back in the future from my perspective.

dimcoin

euser1984 wrote: »

Someone was telling me that light takes about an hour or so to travel to earth from Saturn.

Does this mean if I am looking at it through a telescope, that I'm seeing something that happened an hour or so ago?

Thanks.

Think about this, when mars rovers send data back to earth, or a command is sent from earth to the rovers...it takes minutes each way for the data to move across the space between earth and mars.

ps200306

john the one wrote: »

Hang on....what if I traveled for 50 years into space and back, at extremely high velocities, when I get back to earth, more that 50 years will have passed so technically I will come back in the future from my perspective.

Not as such. if you bring a clock with you and keep an eye on it, you'll see that it just goes at the normal constant rate. So you're just travelling into the future at the same rate as you always do. When you get back, you'll find that everyone else has aged faster than you, though. So you'll be fifty years older but your pals will be older again.

Capt'n Midnight

john the one wrote: »

So if I could build a periscope that goes to Saturn and back, I could look at myself actually looking at myself in the past and future?

If you had a big enough mirror you could see yourself in the past. Except you need a good telescope because the photons spread out.


If you are 30cm from a mirror it will take light one nanosecond each way. So you see yourself 2ns ago. But your vision works at about 40ms and cognation may be over several seconds so not really practical.

PAL TV uses delay lines. Light (and sound) are encoded on a signal. At the receiver every second line is converted from electrical to sound and then put in a delay line that takes 64us (or whatever ) to travel across a quartz crystal and is then converted back to electrical signal. Weird stuff when you consider the technology available in early colour TV's.

Tordelback

This of course is the basis of one of the best bits of Flann O'Brien's The Third Policeman, where the philospher de Selby uses the "appreciable and calculable interval of time between the throwing by a man of a glance at his own face in a mirror and the registration of the reflected image in his eye" to view his younger self by employing two mirrors and a telescope. He observes himself (in increasingly tiny reflections) right back to the age of 12, after which point his efforts to go further back are frustrated by ‘the curvature of the earth and the limitations of the telescope.’

O'Brien was a true genius.

Hijpo

RebelButtMunch wrote: »

Nope. You see saturn ten mins ago. Unlesd someone on saturn had a mirror pointed at the sun. Then you would see the sun in the mirror as it was 94 mins ago

Ah, i figured we see saturn because the suns light illuminates it.

ps200306

Hijpo wrote: »

Ah, i figured we see saturn because the suns light illuminates it.

Yes but you are seeing light scattered off the surface of Saturn, not a coherent image of the sun. Otherwise you could say you are seeing the core of the sun from 100,000 years ago when you look at it, 'cos that's how long it takes photons on average to reach the surface after being bounced around inside after they are produced.

bombs away

What I cant understand is that the most distant galaxies we can see are 13 billion light years away give or take. So the light that we see is from just after the big bang right? But just after the big bang the observable universe was alot smaller than it is now. And the earth didn't even exist back then. So how come the light has taken over 13 billion years to reach us even though when it was sent intially the diameter of the total universe was alot smaller and this planet was'nt even around.

Probably not making a huge amount of sense with that but I can't get my head around it. The only way I can think of it happening is if the universe itself is expanding at a faster speed than the light within it. Is this what is happening?

euser1984

This is gone over my head now but I will say that my understanding of this thus far is: We constantly see the past through our eyes and the further we move away the more behind we can get. Just like watching someone hit a hammer from the distance - the sound we hear happened in the past.

When we look through a telescope and observe planets we see the past also. If there was alien life on Saturn and we had contact with them, they could ask us if 'said' made a mistake ten minutes ago and isn't admitting it and vice versa; we could say wait another five minutes until we can see it. The further we go distance wise, the more back in time we can go....

Capt'n Midnight

Nothing moves faster than light in a vacuum but right after the big bang the universe was full of expanding stuff, there still wasn't a vacuum.

So stuff could move faster than the speed of light in the stuff.

bombs away

whaaaaa

Mind blown

COYVB

Capt'n Midnight wrote: »

Nothing moves faster than light in a vacuum but right after the big bang the universe was full of expanding stuff, there still wasn't a vacuum.

So stuff could move faster than the speed of light in the stuff.

Also, if you have an area (the universe of stuff that came about after the big bang) expanding at just the speed of light in all directions, the speed that two opposite sides of the universe are traveling apart would be faster than the speed of light (double? or would it increase as the distance between the points gets larger?)

ps200306

Capt'n Midnight wrote: »

Nothing moves faster than light in a vacuum but right after the big bang the universe was full of expanding stuff, there still wasn't a vacuum.

So stuff could move faster than the speed of light in the stuff.

That's not quite right. The speed of light in a vacuum is the fastest anything can go, light or anything else. In a non-vacuum, the maximum speed is lower, e.g. light travels slower through air (99.97% of the vacuum speed) or water (75%) or glass (60%) or diamond (40%).

However, the "stuff" that was expanding after the Big Bang (and still is today) is space ... and there's no restriction on how fast that can expand even though there's a limit on how fast anything can travel through space. So different parts of space, along with the material embedded in it, were carried away from each other faster than light could travel between them in the early universe.
`

bombs away wrote: »

What I cant understand is that the most distant galaxies we can see are 13 billion light years away give or take. So the light that we see is from just after the big bang right? But just after the big bang the observable universe was alot smaller than it is now. And the earth didn't even exist back then. So how come the light has taken over 13 billion years to reach us even though when it was sent intially the diameter of the total universe was alot smaller and this planet was'nt even around.

Probably not making a huge amount of sense with that but I can't get my head around it. The only way I can think of it happening is if the universe itself is expanding at a faster speed than the light within it. Is this what is happening?

Yes, and this leaves us with an odd situation. In the extremely early universe, there was hot matter uniformly distributed, and light was produced everywhere throughout the universe. It couldn't get very far because it interacted with matter which also filled the universe, so the light was scattered in random directions. But after about 300,000 years, the matter in the universe had expanded and cooled sufficiently to become transparent, and the light was set free to travel in randomly oriented straight lines throughout the universe. It was -- and still is -- everywhere: we're embedded in it and it comes to us from all directions in the sky. This is the so-called cosmic microwave background radiation. Then there is the light from stars and galaxies that formed much later, when the expanding space had carried clumps of matter far apart. We see the light from these arriving from different epochs in the past depending on distance.

As you say, the universe was smaller then, and we do indeed see galaxies in a less evolved state than galaxies near us, and also tidally distorted by gravity because the galaxies were closer together and interacted more.

COYVB wrote: »

Also, if you have an area (the universe of stuff that came about after the big bang) expanding at just the speed of light in all directions, the speed that two opposite sides of the universe are traveling apart would be faster than the speed of light (double? or would it increase as the distance between the points gets larger?)

That's right -- it's possible for parts of the universe to be increasing in separation faster than the speed of light. However, although we sometimes talk about the recession speed, it's actually inaccurate to say that anything is travelling faster than light -- it's space that's expanding. The analogy is sometimes given of raisins in rising bread, or dots on a balloon that is being inflated: the raisins (or dots) are being carried away from each other by the space between them expanding, not because of any proper motion of their own. The more space in between, the faster they are carried apart. When light travels to us across long reaches of space, the expansion of space affects the light as it travels. It has the effect of stretching the light wave throughout it's journey, so that when it arrives to us its wavelength is longer (which, for light, means redder), a phenomenon called the cosmological red shift. The red shift of the galaxies is one of the primary pieces of evidence for the Big Bang.