wave length, relativity and doppler.

[gauchecritic]

As you may have guessed I've just started reading Hawking's 'A Brief History of Time' and being a hard sci-fi fan I'm quite open to esoteric concepts. When Hawking states if this... then this follows, I'll accept it as given until the light dawns and I understand how and here's the but;

I'm on about chapter three, he's explained Newton, Ptolemy, Copernicus, Keppler etc and is now moving onto relativity. I understand, in the limited way I'm able, about there being no absolutes in space/time except the speed of light and that all events and measurements must be relative to each other. I was quite delighted to find that I'd actually heard of the Michelson/Morley experiments without actually knowing about the detail.

Now we're onto gravity, light and wavelengths. Here's the first part:

Gravity affects the energy of light. When light emerges from a gravity well it loses energy and its wavelength grows.
Standing at the top of a mountain and observing events below, due to the relative positions of observers and the effects of gravity on light, the events at ground level appear to be slower.
OK. I get that and it seems that caesium clocks and towers and measuring events does actually prove that time slows physically. ie the clock at the top of the tower actually runs more quickly. Then there's the Twins Paradox etc.

Now comes the second part:

The Doppler effect. Motor racing demonstrates this very well. eeeeeeeeeyowwwwwwwwww
The wavelength of sound reduces as it moves towards and elongates as it moves away. In terms of light the colours actually shift. Blue shifted as they move towards and red shifted as they recede.

Stars with a blue cast are approaching, those with a red cast are receding

That's fine. I understand that but...

he goes on to say that when measuring the distance of stars we can 1. assume that the wavelengths received are those distributed and 2. ignore the effects of gravity as per the caesium clocks thing.

so in one case we're measuring the position and motion of stars using the absolute of light speed as a gauge and in the other we're using position and non motion as a measure of time with light being a constant.

What I can't reconcile is how wavelength can determine relative speed and at the same time measure relative time.

It seems we're measuring exactly the same results (wavelength) and determining two completely different events.

 

[dr_mabeuse]

You mean he's saying we can ignore the effect of gravity on the wavelength of light?

 

[trysail]

The Doppler effect. Motor racing demonstrates this very well. eeeeeeeeeyowwwwwwwwww

guache,
Excellent visual illustration of the Doppler! Nicely done!

The phenomena of the red shift ( and its other half— the blue shift ) is sublimely, superbly cool, as is the whole concept of relativity, curved space and time.

I have to admit it, though, toward the end of A Brief History of Time, Hawking lost me. Thus, as much as I'd like to help out, I'm afraid it's a bit of a lost cause. Maybe one of these days, I'll re-tackle Hawking but it ain't gonna happen this afternoon. Good luck with finding the answer to your question.

 

[gauchecritic]

You mean he's saying we can ignore the effect of gravity on the wavelength of light?

When measuring the position of stars but it becomes the largest effect when measuring time.

 

[Weird Harold]

What I can't reconcile is how wavelength can determine relative speed and at the same time measure relative time.

It seems we're measuring exactly the same results (wavelength) and determining two completely different events.

I don't think you can determine both at the same time. You can however, caluclulate one and then calculate the other and switch back and forth fast enough to gain the appearance of simultaneity (sp).

 

[dr_mabeuse]

When measuring the position of stars but it becomes the largest effect when measuring time.

Gravity would affect relativistic time, I think, in that things we observed near the event horizon of a black hole, say, would seem to be slowed down to an observer external to that gravitational frame of reference. Is that what you mean?

Otherwise Harold's right. Speed is distance divided by time. You can't determine distance and time at the same time.

 

[Sir Victor]

A key concept of relativity is that it is all about the frame of reference. Doppler happens because the source of the sound is moving relative to the observer's frame of reference. The actual wavelength doesn't change, but rather the observed wavelength does when observed from another reference frame. You'd measure the normal wavelength if you measured it from the source's moving reference frame.

I wasn't completely clear about your question but thought this might help.

Its been a while since I read the book.

 

[gauchecritic]

Thank you both. so we're getting into quantum theory yes?

Because of the tool we can measure one or the other.

Does this mean that we need to know more about gravity?

 

[gauchecritic]

Thank you Sir, I understand that the wavelength is affected by reception, so (to clear it in my mind) do we know what happens to the bits in between the peaks of wave length (are the troughs deeper?) for us to be able to differentiate between gravity effects and speed effects?

(given that I kind of understand that acceleration gives the effect of gravity)

ETA it would be unusual if anyone understood any question that I ask here in the AH.

Edited again to add: because of my usual awkward/amorphous phrasing.

 

[Sir Victor]

Yes what they're talking about sounds something like the Heisenberg uncertainty principle from quantum, however perhaps they can comment as I don't think quantum applies to the question that you posed:
http://en.wikipedia.org/wiki/Uncertainty_principle

Regarding your new question, I'll have to think about it for a bit, not sure of the answer. Interesting food for thought!

I thought your intro was very clearly stated, just that the question was a bit unclear, but that is part of understanding - finding the right questions to ask.

Are you aware that most of these theories are flawed? We don't have a unified model for the Universe, rather Relativity for some problems and Quantum for others. This is much of what the book is about.
We don't fully understand time, why is it effected by gravity and motion, even what is the essence of time? It feels like we understand it since we experience it everyday but what is it from a theoretical standpoint?
Light - we have particle and wave theories for solving different types of problems there is no unified theory that works universally - last I checked, therefore both theories are flawed.

 

[Huckleman2000]

As I recall, measuring "red-shift" (the Doppler effect) is done by analyzing the spectrum of a light-source. Each element has a distinct spectrographic signature, i.e., it absorbs or emits light at certain wavelengths. For example, suppose helium gas emits orange and blue light at wavelengths exactly 103 units apart. The spectrum of a star moving towards us would have those helium bands at 103 units apart, but with wavelengths shortened to yellow and violet. A star moving away from us would have the bands shifted more towards red and green, but still 103 units apart.

Overall, the universe (time-space) is expanding, which means that everything seems to be moving away from everything else, and that the farther away something is, the faster it appears to be moving. I'm not sure exactly, but I seem to remember some Constant value related to this. Thus, with the Constant, red-shift is a proxy for both speed and distance.

Does that sound right?

 

[cantdog]

As I recall, measuring "red-shift" (the Doppler effect) is done by analyzing the spectrum of a light-source. Each element has a distinct spectrographic signature, i.e., it absorbs or emits light at certain wavelengths. For example, suppose helium gas emits orange and blue light at wavelengths exactly 103 units apart. The spectrum of a star moving towards us would have those helium bands at 103 units apart, but with wavelengths shortened to yellow and violet. A star moving away from us would have the bands shifted more towards red and green, but still 103 units apart.

Overall, the universe (time-space) is expanding, which means that everything seems to be moving away from everything else, and that the farther away something is, the faster it appears to be moving. I'm not sure exactly, but I seem to remember some Constant value related to this. Thus, with the Constant, red-shift is a proxy for both speed and distance.

Does that sound right?

The constant was thrown out. That was Einstein's fudge to keep the universe stable. (It's not stable, but expanding, and relativity predicts that. But everyone thought it was stable, so to bring the stability back, Einstein threw in his constant. Now that we accept its expansion, we have tossed the cosmological constant back out.) But that isn't gauche's question.

He says if gravity effects time as well as energy and matter, why doesn't it throw off the calculation of how much the universe is expanding? Right?

 

[gauchecritic]

I think Huck has answered a part of what I can't quite grasp but also;

if gravity effects time as well as energy and matter, why doesn't it throw off the calculation of how much the universe is expanding?

is a part of the question but not the whole.

It seems that, like the 'ether' and 'phlogiston' there's something that's missing from the whole thing. Which is possibly why they can't unify the theories.

I get relativity (or accept it anyway) and thanks to all who answered I get the doppler as it refers to light but I still can't quite get my head round how gravity affects light in the same way that motion does and how you can ignore one when measuring the other.

For example, the caesium clock thing. I'm assuming that the clock works by counting decay particles and that gravity affects the speed of the particles, so the further away from the gravity well that caesium is, the closer it gets to its 'natural' decay rate.

All that this tells me is that a body tied to gravity is affected by gravity

 

[trysail]

The infinite:

http://farm2.static.flickr.com/1100/1477228934_fb14126509_o.jpg

 

[3113]

Dark Matter, Dark Energy

It seems that, like the 'ether' and 'phlogiston' there's something that's missing from the whole thing. Which is possibly why they can't unify the theories.

Well, there's a couple of things. First, hell yes there's stuff that's missing. And no, we don't know how that stuff affect other stuff because we haven't yet found that stuff. Like Dark Matter which is, briefly: "...hypothetical matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter."

And then there's Dark Energy: "Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space." AND Dark Energy "...tends to increase the rate of expansion of the universe. Dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate."

There's this whole percentage of stuff that we can't see, haven't found, and yet it apparently is there in some way or other because it's affecting the calculations. The space between stuff we can actually see and find out there in the universe is not, it seems, as empty and full of nothing as we thought.

Second, you're assuming that it all can be unified. Einstein wanted it all unified. He didn't like Quantum Physics. But it seems now that things follow different laws when they go very small than they do when they're very big. Which means there is no unified laws that affect everything in exactly the same way. Out there with the planets and you're playing one game. Head down to the particle level and it's a whole other game. So if you're trying to apply the rules of one game to the other level, it won't work.

I may be telling you stuff you already know there, but you're not being very clear in what is confusing you, or what you do and don't understand. Is this in any way on the right track?

 

[shereads]

When I read that book, I tried so hard to grasp the concepts that my head hurt. It was a good hurt, though. Sudden glimpses of brilliance can be hard on the eyes, but it's wonderful to know they're there.

I especially like Hawking's comment on the possibility of time travel: If it's possible, it's already happening - and has always been happening.

Time being infinite in both directions, anything that can happen will happen - and has happened, and is happening right now ..Which makes me think that those infinite monkeys with their infinite typewriters will not only compose the complete works of Shakespeare, but finished it just now and are starting again.

Edited to add: I hope this is pertinent to your discussion. I honestly have no idea. :emoticon:

 

[dr_mabeuse]

There are several string theories which provide Grand Unified Theories (or GUTs) of everything, but these can neither be proven nor disproven, so there's no way to tell whether they're true or not, and they require the assumption of the existence of as many as eleven dimensions to make them work, which kind of ruins their elegance.

Lord Rayleigh said it best though, when he said that it's very likely that the universe is not only stranger than we imagine, but stranger than we can imagine.

 

[3113]

Which makes me think that those infinite monkeys with their infinite typewriters will not only compose the complete works of Shakespeare, but finished it just now and are starting again.

You trained those monkeys...didn't you?

 

[DrHappy]

he goes on to say that when measuring the distance of stars we can 1. assume that the wavelengths received are those distributed and 2. ignore the effects of gravity as per the caesium clocks thing

It's been a while since I've read the book, but I THINK that he's stating here that in a typical real life red shift measurement that an astronomer may encounter, the effect of gravity will be much smaller than the effect of velocity. Therefore, you can ignore the effects of gravity and still obtain an accurate answer.

 

[gauchecritic]

It's been a while since I've read the book, but I THINK that he's stating here that in a typical real life red shift measurement that an astronomer may encounter, the effect of gravity will be much smaller than the effect of velocity. Therefore, you can ignore the effects of gravity and still obtain an accurate answer.

But the thing that bugs me is that gravity affects light. Velocity affects light. Acceleration acts like gravity and so can be presumed (I hope) to have both gravity and velocity affects on light.

The only mention of acceleration (so far) is that only light can travel at light speed because it is massless and doesn't require infinite energy to attain lightspeed.

I may be wondering a bit here. (or do I mean wandering?)

 

[neonlyte]

It's a bit like the cat in the box, does it exist before you open the box and look at it? Gravity and time, - like electricity and fate - have unseen yet pre-determined effects... usually. The gravity well fucks the theory by providing an explanation for an anomaly. The easiest way to see visual simulation of a gravity well is from an aeroplane.

From the ground, the plane in the sky flies a straight line. From inside the plane, an passenger feels the plane flying a straight line and believes the plane is flying a straight line if he/she looks out of the window. The plane is physically flying an unseen and unseeable curved line along an altitude parallel to the surface of the earth. Seen from a plane flying above, on a very clear day you would see the lower plane break over the horizon, as it passes beneath you, it is very evidently not flying a straight line but is skewed to its direction of travel by the influence of the jet stream. In other words seen from below the plane flies a straight line, seen from above the same plane is flying a curved line in certainly 2 and possibly 3 dimensions. Theoretically 4 dimensions since the juxtaposition of the two observers - one above and one below - see the same object at precisely the same two different times as the lower observer is subject to a higher gravity field than the observer in the plane flying above.

 

[Sir Victor]

But the thing that bugs me is that gravity affects light. Velocity affects light. Acceleration acts like gravity and so can be presumed (I hope) to have both gravity and velocity affects on light.

The only mention of acceleration (so far) is that only light can travel at light speed because it is massless and doesn't require infinite energy to attain lightspeed.

I may be wondering a bit here. (or do I mean wandering?)

I think that acceleration acting like gravity is a simulation type effect, it does not have the same properties as real gravity. Acceleration works in a centrifuge or flight simulator, but it only provides a similar simulation type effect as I understand it.

 

[shereads]

You trained those monkeys...didn't you?

If I did, I am. And I will.

 

[shereads]

He loses me completely when he talks about gravity affecting time. As if time is a thing, like potting soil or cornflakes.

My head hurts again.

 

[DrHappy]

But the thing that bugs me is that gravity affects light. Velocity affects light. Acceleration acts like gravity and so can be presumed (I hope) to have both gravity and velocity affects on light.

Hmmm... I'm not sure what you've read and haven't read.... but maybe this is what you are looking for. Forgive me if I am stating the obvious or if I am incomprehensible.

According to general relativity, gravity isn't a force. Gravity acts by warping space time. Objects don't fall to earth. Earth actually shrinks the space between the earth and the object. In order to keep from falling, you have to continuously accelerate away from the center of the earth. When you "stand" on the earth, you are actually accelerating away from it at 9.8 meters/sec. The force on your feet is the same as if you were not on earth, but were in a rocket accelerating at 9.8 meters/sec/sec. If you jump out of a plane, you will "fall" at 9.8 meters/sec/sec. This is how fast space is shrinking between you and the earth.

Imagine you are sitting far away from the earth. Someone on the earth is shining a flashlight at you. One way of looking at the redshift phenomenon via gravity is that the light experiences the redshift because it experiences acceleration away from you while inside the gravity field.