Questions about Physics, Biology and perspective

[bennyboy]

Okay I'm sitting here trying to use my imagination and reason to draw inferences about the universe.  They are purely speculative, but I hope a couple of our in-house scientists would be willing to shed some light on them-- am I just pulling stupid stuff out of my ass?


1)  Red shift
In astrophysics, red shift is taken as a measurement of relative motion-- i.e. moving away.  However, I'm curious about whether light can undergo a kind of Lorentz transformation.  Now, I know there's only one speed of light, in any frame of reference, BUT it seems to me that if something is scaling in size relatively due to its speed, then the relative wavelegth of light emitted from it would be similarly transformed.  In other words, you could get a red or blue shift based on relative velocity of the emitting object without regard to direction.  So does a red shift really prove that the universe is "expanding," i.e. that everything is moving away from each other, or could it be that WE are moving relatively slowly compared to the things we are observing?

2)  Doppler effect pt. 2
So if something was moving at near the speed of light toward us, I assume it would be blue-shifted.  But how much of the blue shift would come from its velocity, rather than its direction?  Does the train-passing-by description of the Doppler effect really work for light?

[brewer]

Good question. I've had the same thought. Haven't got a clue to the answer.

[Angrboda]

Okay I'm sitting here trying to use my imagination and reason to draw inferences about the universe.  They are purely speculative, but I hope a couple of our in-house scientists would be willing to shed some light on them-- am I just pulling stupid stuff out of my ass?


1)  Red shift
In astrophysics, red shift is taken as a measurement of relative motion-- i.e. moving away.  However, I'm curious about whether light can undergo a kind of Lorentz transformation.  Now, I know there's only one speed of light, in any frame of reference, BUT it seems to me that if something is scaling in size relatively due to its speed, then the relative wavelegth of light emitted from it would be similarly transformed.  In other words, you could get a red or blue shift based on relative velocity of the emitting object without regard to direction.  So does a red shift really prove that the universe is "expanding," i.e. that everything is moving away from each other, or could it be that WE are moving relatively slowly compared to the things we are observing?

From what I understand, it's not simply that other stars appear red shifted, but that the further away from us a star is, the greater the red shift and by implication the greater velocity. This holds true no matter which direction we look. This would only occur if the universe was expanding in all directions.

[Anomalocaris]

Okay I'm sitting here trying to use my imagination and reason to draw inferences about the universe.  They are purely speculative, but I hope a couple of our in-house scientists would be willing to shed some light on them-- am I just pulling stupid stuff out of my ass?


1)  Red shift
In astrophysics, red shift is taken as a measurement of relative motion-- i.e. moving away.  However, I'm curious about whether light can undergo a kind of Lorentz transformation.  Now, I know there's only one speed of light, in any frame of reference, BUT it seems to me that if something is scaling in size relatively due to its speed, then the relative wavelegth of light emitted from it would be similarly transformed.  In other words, you could get a red or blue shift based on relative velocity of the emitting object without regard to direction.  So does a red shift really prove that the universe is "expanding," i.e. that everything is moving away from each other, or could it be that WE are moving relatively slowly compared to the things we are observing?

2)  Doppler effect pt. 2
So if something was moving at near the speed of light toward us, I assume it would be blue-shifted.  But how much of the blue shift would come from its velocity, rather than its direction?  Does the train-passing-by description of the Doppler effect really work for light?

I am not sure what you mean by somethings "scaling in size Relatively due to its speed"  If the thing is the universe, then I believe the wave length of Light traveling in the universe would expand, or red shift, or scale, if you will, in proportion to the expansion of the universe, irrespective of which direction the light, or the emitter of the light,  is going in the universe.    It is also a separate effect from which direction the light, or the emitter of light, is going with respect to an observer. If an object is moving towards the observer, but it is sufficiently far from the observer such that in the time it take for light to reach the observer, the universe would have expanded appreciably, then the color of the light the observer would see is a combination of two effects, blue shifting because the emitter was moving towards the observer, red shifting because the wave length scaled with the size of the universe, and the size of the universe increase between when the light was emitted, and when it was observed.

If the light of something moving near us is blue shifted, then the blue shift would be a function of Both velocity of the emitting body and the direction it is moving.  But because it is near us, light from it need not travel a long time to reach us, and the universe would not have expanded much in the time it takes for the light to travel to us, therefore little of its observed wave length would be due to light wave length scaling with the size of the universe.

[bennyboy]

From what I understand, it's not simply that other stars appear red shifted, but that the further away from us a star is, the greater the red shift and by implication the greater velocity.  This holds true no matter which direction we look.  This would only occur if the universe was expanding in all directions.

Right. You're describing the Doppler effect. But that's why I'm asking about Lorentz transformations: specifically, are there cases in which something happening in one frame of reference will affect our perception of everything relative to us?

[bennyboy]

I am not sure what you mean by somethings "scaling in size Relatively due to its speed"  If the thing is the universe, then I believe the wave length of Light traveling in the universe would expand, or red shift, or scale, if you will, in proportion to the expansion of the universe, irrespective of which direction the light, or the emitter of the light,  is going in the universe.    It is also a separate effect from which direction the light, or the emitter of light, is going with respect to an observer.    If an object is moving towards the observer, but it is sufficiently far from the observer such that in the time it take for light to reach the observer, the universe would have expanded appreciably, then the color of the light the observer would see is a combination of two effects, blue shifting because the emitter was moving towards the observer, red shifting because the wave length scaled with the size of the universe, and the size of the universe increase between when the light was emitted, and when it was observed.

If the light of something moving near us is blue shifted, then the blue shift would be a function of Both velocity of the emitting body and the direction it is moving.  But because it is near us, light from it need not travel a long time to reach us, and the universe would not have expanded much in the time it takes for the light to travel to us, therefore little of its observed wave length would be due to light wave length scaling with the size of the universe.

That brings me to another question, actually.

If the universe is expanding, then how does this affect the value of "c" over time? Do those kilometers themselves stretch out, meaning that light will still take the same time to travel, or does the distance the light have to travel actually increase while it's "in flight"? I suppose it must be the latter, since light emitted 14 billion years ago has taken a long time to reach us on Earth, and at the Big Bang everything was packed into an infinitesimal volume.

The other thing is-- if space is expanding uniformly, wouldn't the light transmitter, the light itself, and the receiving mechanism all be expanding identically, meaning that you'd "sense" the light as being the same frequency as when it left the emitter? Why would there be any sense of relative change at all?

I suppose that means that the red shift isn't a transformation but really is only a Doppler effect. But would that mean that objects in the far reaches of the universe are actually traveling away from us at greater than the speed of light, or will it be kind of like an inverse Black Hole, where all very far objects appear to flatten into a tremendous sphere at massive distance? Why isn't there a "wall" of super-low frequency light crushing us right now?

[Alex K]

Okay I'm sitting here trying to use my imagination and reason to draw inferences about the universe.  They are purely speculative, but I hope a couple of our in-house scientists would be willing to shed some light on them-- am I just pulling stupid stuff out of my ass?


1)  Red shift
In astrophysics, red shift is taken as a measurement of relative motion-- i.e. moving away.  However, I'm curious about whether light can undergo a kind of Lorentz transformation.  Now, I know there's only one speed of light, in any frame of reference, BUT it seems to me that if something is scaling in size relatively due to its speed, then the relative wavelegth of light emitted from it would be similarly transformed.  In other words, you could get a red or blue shift based on relative velocity of the emitting object without regard to direction.  So does a red shift really prove that the universe is "expanding," i.e. that everything is moving away from each other, or could it be that WE are moving relatively slowly compared to the things we are observing?

2)  Doppler effect pt. 2
So if something was moving at near the speed of light toward us, I assume it would be blue-shifted.  But how much of the blue shift would come from its velocity, rather than its direction?  Does the train-passing-by description of the Doppler effect really work for light?

Did someone leave the physics bat signal on again? Oh, a question!

Ok, we have to disentangle two different things here which are often confused

- cosmological redshift
- doppler effect

For all we know, the cosmological redshift which you see when observing far away galaxies is not sensibly described as a Doppler effect. What happens here is that the waves are sent out from the source, and then, as space expands during the travel time of the light wave, the wave gets stretched by the same factor and accordingly acquires a lower frequency. This is different from doppler shift in a very important way: With doppler shift, what counts is the velocity of the source at the instant the light gets emitted, and the velocity of the receiver at the instant it gets received. Cosmological red shift shows the sum of all expansion between those two moments in time. The two only yield the same result in some special cases, in the real world, they don't match up and only the wave-stretching red shift description is correct.

Now to your second point about doppler shift. Yes indeed, there is a Lorentz Transformation for light. You can either apply it to the places of the wave crests and throughs, or to the momentum of the photon, and both give you exactly the same result. One could say that this Lorentz transformation already encompasses the phenomenon "Doppler shift", because the LT provides the complete description how anything will look differently to an observer moving relative to my frame.

The physical details of doppler-shifted light and sound are a bit different : If you walk towards an ambulance or vice versa, the speed of sound changes relative to you or the ambulance, because the sound propagation is tied to the air. When you walk towards an ambulance, you will simply pass more wave cycles per time because you pass faster by the fixed wave train moving in the air. With Light, though, the speed relative to the observer never changes, so this is indeed different, but one can look at it from a similar perspective:

Imagine you observe someone walk towards a light source that is resting for you. You will think, ok, from my perspective, this person passes more wave cycles per time, because she is approaching the light source. Naively, they should observe a higher frequency by that amount you count. Then you remember Einstein and that the moving person's clock will run differently from yours. You adjust for that and find, that your naive estimate was a bit low - the "slowing" of time for the moving observer will add to the observed blue shift. However, for speeds lower than the speed of light, your initial naive estimate as seen from the outside - which is identical to what you would get for sound if the air is resting for you - will be very accurate. Only when the moving observer approaches the light source near the speed of light from your perspective, the time dilation effect will dominate, and the blue shift will be much stronger than the naive estimate you get by counting how many waves per second she passes on your clock. When this distinction becomes important, one talks of a "relativistic doppler effect", because the naive analogy to sound etc. doesn't work any more.

[BrianSoddingBoru4]

Dammit. I was about to use my vast knowledge of physics to give EXACTLY the answer Alex just gave.

Boru

[Alex K]

That brings me to another question, actually.

If the universe is expanding, then how does this affect the value of "c" over time?  Do those kilometers themselves stretch out, meaning that light will still take the same time to travel, or does the distance the light have to travel actually increase while it's "in flight"?  I suppose it must be the latter, since light emitted 14 billion years ago has taken a long time to reach us on Earth, and at the Big Bang everything was packed into an infinitesimal volume.

I'm not 100% sure I understand how you mean it. Maybe it would be important to clearly define what we mean by speed of light. The best is to define it locally: You have a defined short ruler and a clock, both are calibrated by using some atomic process for example. You send a light signal along your ruler and measure the time, and divide. It is important that your apparatus is small enough that space is not noticeably bent along the ruler, and does not expand noticeably while the experiment is running. This is what is meant by local - you make the experiment arbitrarily small and quick to eliminate all curvature effects and get as a result the speed of light at that point.

This local speed of light apparently remains, for all we know, constant during cosmic expansion and is the same everywhere, whether you are orbiting a black hole or are in free space. It certainly does so in Einstein's description via General Relativity. What can happen though is that the distance the light travels in a year will be more than a light year once the light has arrived, because the distances it has already travelled will get stretched by cosmic expansion. This is why the radius of the visible universe is given as 46 billion light years, even though the light was only travelling for 13.8 billion years: The distance between us and the source has been stretched in the meantime by space itself expanding.

The other thing is-- if space is expanding uniformly, wouldn't the light transmitter, the light itself, and the receiving mechanism all be expanding identically, meaning that you'd "sense" the light as being the same frequency as when it left the emitter?  Why would there be any sense of relative change at all?

This is not a trivial question at all. The answer is well known, but it is a kind of subtle argument. I always knew that the size of matter etc. isn't affected by cosmic expansion, but it was only after I got my degree, when preparing for a public lecture, that I thought, wait a minute, do I understand this? and did the -admittedly short- calculation myself. When you look at the maths, you notice the following: If you use the appropriate definitions, the strength of the forces such as the strong force, weak force and electromagnetism as well as the masses of particles do not change with the expansion of space. It is those forces and masses which define the sizes of atoms and atomic bonds, and hence the sizes of stuff. If the atoms in matter weren't bound together by an unchanging electromagnetic force and also gravity, they would parttake in cosmic expansion. In fact, all matter does experience a minuscule tug from cosmic expansion trying to inflate it. But this force is so tiny that it only becomes important compared to electric and gravitational attraction at scales far beyond our galaxy.

I suppose that means that the red shift isn't a transformation but really is only a Doppler effect.  But would that mean that objects in the far reaches of the universe are actually traveling away from us at greater than the speed of light, or will it be kind of like an inverse Black Hole, where all very far objects appear to flatten into a tremendous sphere at massive distance?  Why isn't there a "wall" of super-low frequency light crushing us right now?

see my last post. The cosmic expansion is not sensibly described as stuff moving away at speed x.

[bennyboy]

Great stuff, Alex!

You've very effectively answered most of my questions, but now I will for sure want to ask some more. . . but after I process for a couple days and try to imagine the implications of what you've said.