My first cool zoo image

A world of galactic hurt. I think I’ll mark that as “merger“.



Check one for kfs1!

Notice the nice dark dust lanes? That’s what I study… but fake ones, as in [IMG][/IMG]

spiral ,clockwise, non-colliding galaxies (the right could be elliptic im not sure)
Do i get a cookie now?

Here’s something I don’t understand, Lutorm… Galaxies at don’t appear to have any kind of visual doppler effect. When you’re looking at a galaxy at 45 degrees, I would have thought that the tens or hundreds of thousands of light years it takes for light to travel from the far side before reaching the photons that comprise the near-side image would lead to some kind of distortion or doppler type effect?

I would think if there’d be any doppler effect it would not be between the far side and near side of a Galaxy, but between the right side and left side of a Galaxy – between the side that is moving toward us the viewer and the side that is moving away from us the viewer.

You’re right kfs, there’s a difference in the time at which you’re seeing the close and far side, but it’s not big enough to be noticeable. A galaxy is about 10,000 light years across, but the rotation period is about 100 million years, so the difference is only 0.01% of the rotation period. Things generally happen so slowly on astronomical scales that the speed of light is effectively infinite. There are a few situations where this is not true, but they have to do with things orbiting black holes or even the moons of Jupiter (an old experiment to measure the speed of light). Galaxies are much too big and move much too slowly.

Mangydog, you CAN of course see the Doppler effect in the light coming from the approaching and receding side of the galaxy. That’s how galaxy rotation speeds are measured.

the speeds of the moons of jupiter are dependant upon the accuracy of the clocks on earth and the abililty of two observers on earth to correlate their clocks to determine their differences in viewing the arrival of (the photons of) a moon of jupiter into their telescopes.

The doppler effect of a galaxy’s rotation does NOT make the speed of light “infinite”. Infinity is a MUCH MUCH MUCH higher rate than 186,000 miles per second (approx) of light (reduced by the refractive index of the space through which the light travels, i.e. the more clear (less mass) the faster light is (i.e. less affected by mass)).

“Matter tells space how to bend, and space tells matter how to move”

Speed is a time and space measurement: miles per *hour*. kilometers per second.

Galaxies rotate at a determinable speed, and that speed is ALWAYS displaying a “Doppler” effect of matter moving away (red-shifted) and matter moving toward (blue shifted).

The average of moving “away” from earth and moving “toward” earth (earth’s position is ‘arbitrary’ in the sense of not being the center of anything) equals the average speed of the rotation of the galaxy.

IF the speed of light were ‘infinite’ (as an approximation) than ALL STARS light would be infinitely fast, and therefore the entire night sky would be evenly filled with starlight and therefore appear as if a daylit sky.

Infinity is a remarkably large number. So is true ‘random’.

Point: If an infinitely large, infinitely random universe existed: anything that could happen, would happen…an infinite number of times. (Everett’s ‘Many Worlds’ theorem, is not disproved (or falsifiable), so far).

Now…your question KFS1 of ’45 degrees’ (presumes viewing angle from earth) and NOT showing Doppler effect is … wrong…mistaken…misspoken.

The doppler effect appears for any object moving in relation to the observer (e.g. trains moving towards or away from you) and viewing angle does NOT negate this effect.



Ramp and I had been talking about this and were drawing parallels. If you hold up a plate and turn it to a similar angle to say Andromeda’s angle relative to us, the light from the far side and the near side read your eye nearly simultaneously and there is relatively little time or gravity to affect them. But in the case of the galaxy, the light starting out from the distant edge – it would seem – takes that much significantly longer that, combined with rotation and gravity etc, you’d figure there’d be some distortion of it – as mangy says, around either down the center or – as we were thinking – along the center.

Yes, doppler is the wrong term, I have no idea what the term would be… Distortion. More than just the simple effect of perspective. Some kind of spoking or something.

It occured to me that the effect would probably be lost in seeing the image at a human scale in a 1024×768 image, but it still seems to make sense that there should be some kind of visual artefact to the appearance of a galaxy that hints at its massive gravity and comparatively slow rotation.

Another explanation of why you don’t we considered was that you’re perceiving billions of point-sources seperated by lightyears as a single, whole, mass, and that might conceal any such effect the way your hand appears whole. But, again I guess we were expecting that when you scaled up the subject being viewed, effects of negligible influence at the scale of a plate might have some visible artefact — a plate isn’t going to show a lensing effect to the human eye, for instance.

Joker – for an object rotating around its center, the angle of viewing does affect doppler. If view a bicycle wheel perpendicular to its rotation, you see no doppler effect but as you begin to turn it to a non-perpendicular angle the image of the spokes begins to aquire a doppler effect (although probably too small for the human eye to see).

Of course, if you turn it edge on, the wheel itself obscures any possibility of seeing the effect ;)

A galaxy doesn’t rotate nearly fast enough to have any noticeable velocity induced distortion effects. The Sun takes 250 million years to complete one orbit around the galaxy. A galaxy is on the order of tens of thousands of light years across, do the math.

I’m not sure what to make of your comments Joker, they have no common thread going through them. I never said that the speed of light was infinite, I said that for the purpose of calculating image distortions of astronomical objects due to finite light travel time, the speed of light could just as well be infinite.

And Roemer’s measurement of the speed of light by using Jupiter’s moons only requires an accurate clock, see:

That’s completely different from considering the distances BETWEEN galaxies, which are large and the speed of light most definitely not effectively infinite on those scales.

Kfs, I think the problem is that galaxies are not perfectly smooth, symmetrical entities. If they were, you *might* be able to see some distortion in the spiral structure arising from the finite light travel time. Same goes for the gravitational bending of light, it happens but does not bend the light enough to be noticeable. Galaxies DO distort the images of background sources noticeably and it is used to measure the mass of the galaxy, but you need to average over hundreds of galaxies to get a measurable signal.

They also, occasionally, distort the image of a background galaxy that is *exactly* aligned with the line of sight noticeably, for example

First: Gravity is an effect of mass bending space.

It *always* has time, because no matter how small you slice time, gravity permeates all space. Do not separate yourself from the universe. You must constantly obey every physical law you try to grok. YOU gravitate. It just washed out by the earth.

If I put you and Ramp alone (no other dark or normal matter) in the universe, and separated you by ANY random distance…you would *gravitationally* drift towards each other slowly gaining velocity until your skeletons meet each other in some far future. :)

The electro-magnetic force is what you feel when you touch with your fingers…its the *only* thing that keeps your hand from passing through the keyboard and table. It is VERY powerful at short distances, (actually electroweak force melding electricy, magnetism and the particle level weak force.

Gravity is VERY weak. You defeat gravity every time you pick up a book from a table, or your cat from the floor. Gravity builds up by mass, and the earth is a very large bunch of mass compared to you. That is why you are pulled downward, as if to a point at the very center of the earth, because all the mass of the earth is bending the space around you (think 3D plz).

Gravity is caused by this mass (let’s say earth) bending space so that your shortest distance is always moving in a straight line, but that straight line is curved by the presence of the earth’s mass. (the moon “rotates” around the earth – but it is taking the shortest possible straight route around the earth (the straightest line) in space…but space is bent around the earth’s mass.

SO: gravity does not “gravitate”, it simply is GEOGRAPHY. There is NO TIME COMPONENT.

DOPPLER: KFS1 to Joker: “for an object rotating around its center, the angle of viewing does affect doppler.”

The angle changes the Doppler *effect* to YOU (relativity), but does not negate the doppler effect (as I said). The doppler effect is observed for sound waves from a train approaching you (and then moving away from you). You don’t need “rotation”.

Doppler is an effect of waves (sound or light or ANY type of wave) that has a limited speed, and the source of that wave is moving in relation to you (there’s relativity again). As in Zen, there is no difference if the train is moving on the tracks, or if are moving by the train. To YOU watching the train, the doppler effect occurs either way (in the exact same outcome of ‘seeing’ or ‘listening’ by you).

The spectra of the visible light that is leaving two different sides of a galaxy are far apart and UNRELATED. They (those two different streams of photons) don’t know that they are dopplered, because *they* are not – they are doppler’ed when observed by YOU. They are separated by light years of space and even were emitted at far different times, depending upon your viewing angle.


You don’t need velocity, gravity can do it, as the light is slowed down trying to ‘escape’ out of a gravity well.

But I think you mean that the velocity of a rotating galaxy is not high enough to be near ‘light speed’ and thereby cause velocity-related ‘distortion’.

But the doppler effect occurs at human speeds (trains, sirens, galaxy spectra viewed on earthly telescopes), and galaxies are rotating at very much faster than what we perceive as normal here on earth’s surface.

Thus: Doppler is an “effect” and not a property of the universe, itself.


btw – if you really want to be freaked out with ordinary physics; google up “Spooky Action At A Distance” (Einstein’s failure to disprove parts of quantum physics actually predicted the impossible…or is it?).

keyser: Ok – that is probably the factor I was missing. In the 100k light years or so a photo takes to reach the leading edge, I’m guessing that the resulting “pixel” for its point of origin hasn’t changed at all.

Joker – you seem to be operating on keywords only. I draw a comparison between a photon crossing a plate (diameter, 20cm, mass 4×10^-21) and a galaxy (diameter 200kly, mass 7.1×10^11) and suggest that in the case of photons traversing the plate “there is relatively little time or gravity to affect them”. Note that I said “time or gravity”.

Are you suggesting that:
a) Two photons departing at two different points 200,000 light years apart along the angle of viewing will not be hindered by time and arrive at the same moment?
b) that the gravity of an individual object has no bearing on how much it bends photons, that the andromeda galaxy pulls on a photon 1mm from it as much as a plate does?

Re-reading my post and then re-reading yours, that continues to appear to be the implication.

Again, in the case of the plate, the amount of time elapsing between two photons departing near and far edges of the plate is infinitesimal.

Secondly, doppler is a result of the motion of an object relative to you shifting frequency of waves of pressure or light, etc, emanating from or reflected by the object

If you turn a spoked wheel perpendicular to you, so you are facing it diamer-on, and rotate the wheel, you experience no doppler effect. The wheel is neither rotating nor receding.

If, however, you begin to turn the wheel to an angle, pretty much one half of the spokes are now moving towards you while pretty much the rest are moving away from you.

If you continue turning it until it is edge on, any doppler effect of light waves will be invisible to you, obscured by the rim of the wheel.

Think of it this way. Consider a point on the surface of a star in Andromeda. This star is near the outer rim of Andromeda so visually it is below the galactic center. The point is about to emit a photon which will subsequently arrive at earth along with others to form an image of Andromeda on the back of your retina. Ignore the wave function for now.

Since the origins are many lightyears apart but the photons will arrive in a space no more than a few cm large, they must all have different trajectories – if these photons all travelled to earth along the same, flat, plane, only one at most could reach the CCD.

If you drew a simple line-of-perspective to each of the points from which these photons were emitted, would any of these lines accurately reflect the initial angle at which the photon departed? I believe the answer is no. The photon from the nearest edge would presumably be closest, but the photons from the center and rear of the galaxy would have been bent onto the course that finally directed them at the CCD, with photons departing nearest the center of the galaxy experiecing the strongest gravity and thus the most bending.

A little knowledge is a dangerous thing…

Joker, you need to think about which effects are *relevant* to the question kfsone asked… For determining whether there is visible distortion to the appearances of galaxies, nothing of this matter. What matter is the timescale of light travel across the galaxy in relation to the timescale for the galaxy to change its appearance (in this case, rotate).

As far as the doppler effect goes, if there’s no line of sight velocity, no doppler effect. Yes, there is in principle a relativistic doppler effect arising from the time dilation of a rapidly moving source even if it’s moving perpendicular to you, but that is utterly unobservable.

Kfs, I didn’t understand your example about Andromeda. To any observable accuracy, the paths of photons coming from different parts of it are straight lines. The geometry is not advantageous for bending them. The photons coming from the center of the galaxy experience no bending at all, because they are climbing radially out of the gravitational field. The ones coming from the farthest edges bend slightly inward, but because the bending happens close to their point of origin, it causes little net distortion to the appearance of the galaxy.

The effect of time travel across the galaxy will be that because you see the closest part 0.01% of a revolution later than the farthest part, the spiral pattern will appear to be about 0.01%*360 = .036 degrees more advanced on the close side than on the back side. Given that the spiral arms are so fuzzy and irregular, that’s not going to be noticeable.

Remember I’m talking about a galaxy that is at an angle to us, so the photon is travelling at ~45 degrees relative to the local galactic plane – thus the gravitational pull is likely to be at an angle.

Illustration absolutely not to scale ;)

Recalling the inverse square law, it seems to me that the top photon would be pulled towards the bottom of the image although from the outside it is relatively distant from the mass acting on it.

The central photon is angled away from the direction of travel so the central photon is going to remain in the downward pulling part of the galaxy (relative to the direction of travel)

Lutorm wrote:
The effect of time travel across the galaxy will be that because you see the closest part 0.01% of a revolution later than the farthest part, the spiral pattern will appear to be about 0.01%*360 = .036 degrees more advanced on the close side than on the back side. Given that the spiral arms are so fuzzy and irregular, that’s not going to be noticeable.

Yeah, I thought they rotated an order of magnitude (at least) faster, and I think I mentioned that I thought the effect would be lost in the merging of so many point sources into a seeming single source — since none of the sources are resolved, each pixel is actually likely to be the combination of several sources. Presumably the exposure length neccessary also further obliterates any trace of it?

@LUTORM: “drink deep the drafts of that Pierian spring; for a little knowledge is but a dangerous thing”. Good point.

@KFS and LUTORM: As to ‘keyword’ type response, I plead ‘guilty’ upon re-reading. Not a physicist by training but a rapturous dilettante by reading. (second meaning of dilettante, if you please, otherwise its perjorative: “2. a lover of an art or science, esp. of a fine art.”)

doppler effect requires multiple waves arriving to the observer.

@KFS: You are changing from doppler effect, to the change in trajectory of a photon caused by the gravitational mass of the galaxy that emits it. To be precise (and avoiding keyword-ism) LUTORM has demonstrated that the change in trajectory of an emitted photon caused by galactic mass is negligible. The photon in your example KFS has been emitted by a sun, whose local gravitational mass is greater (locally) than the galactic mass spread out across the entire galactic plane (possibly a galactic halo when we include ‘dark matter’ because it appears to be a part of galactic gravity).

Just a pedantic point on the example of a (presumably) spiral galaxy seen face on: The individual stars are moving both toward and away, and exihibit doppler effects that way, and the example of a rotating wheel with every particle remaining in place is not relevant.

Summary of points (anti-keywordist):

1) Gravity is not a part of the doppler effect
2) Infinity is such a large number, it is rarely correctly used in approximations (i.e. infinity is infinitely far from the largest number you can imagine (even ‘googleplex’ :P).
3) doppler effect does not require rotation
4) photons leaving different edges of a galaxy were emitted at different times when seen by any ‘observer’ not stationary with respect to the movement of that galaxy

Hopefully that ties in what I was randomly putting into longer words above. But, since its not mathematical, it depends upon imprecise words and will not settle anything with a mathematician/physicist using precisely defined symbols. double :P

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