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Dean View Drop Down
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Direct Link To This Post Posted: July 25 2013 at 01:28
Ermm any further discourse on this will be nothing more than us contradicting each other, I'm not going to agree to differ because you have admitted you're not conversant with the fictional science used in SF, so I'm simply stopping at this point.
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Direct Link To This Post Posted: July 25 2013 at 03:51
The predicted future "collision" between the Milky Way and Andromeda


"While the Andromeda Galaxy contains about 1 trillion (1012stars and the Milky Way contains about 300 billion (3×1011), the chance of even two stars colliding is negligible because of the huge distances between the stars"

More generally


"Colliding galaxies are common in galaxy evolution. Due to the extremely tenuous distribution of matter in galaxies, these are not collisions in the normal sense of the word, but rather gravitational interaction"




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Direct Link To This Post Posted: July 25 2013 at 03:58
Originally posted by Equality 7-2521 Equality 7-2521 wrote:

By definition, if you're getting to point B from point A they lie in the same lightcone.
OK in that sense I agree, but you can't 'hit any matter in the way through the wormhole' since by definition the distance between the two mouths of the wormhole is zero. There is no space in between the two mouths so where there's no space there can't be any matter.
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Direct Link To This Post Posted: July 25 2013 at 04:18
And btw, dark matter is supposed to not interact with ordinary baryonic matter except gravitationally, so hitting a dark matter particle would not be even noticed (most likely we are constantly hitting dark matter particles and we do not notice). Even some normal baryonic matter like neutrinos (which admittedly are supposed to have a really tiny but anyway non-zero mass) can be crossed through without any effect since they basically do not interact with our constituent particles. Trillions of neutrinos cross through the 'solid' Earth every second and they do not 'hit' anything, not because they do not encounter any other particles along their path or 'manage to avoid the collision', sure they do encounter other particles along their path, but they simply don't care about each other.

Edited by Gerinski - July 25 2013 at 04:54
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Direct Link To This Post Posted: July 25 2013 at 06:39
Originally posted by Gerinski Gerinski wrote:

Originally posted by Equality 7-2521 Equality 7-2521 wrote:

By definition, if you're getting to point B from point A they lie in the same lightcone.
OK in that sense I agree, but you can't 'hit any matter in the way through the wormhole' since by definition the distance between the two mouths of the wormhole is zero. There is no space in between the two mouths so where there's no space there can't be any matter.


This is not true.
"One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. "
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Direct Link To This Post Posted: July 25 2013 at 06:54
Originally posted by Equality 7-2521 Equality 7-2521 wrote:

Originally posted by Gerinski Gerinski wrote:

Originally posted by Equality 7-2521 Equality 7-2521 wrote:

By definition, if you're getting to point B from point A they lie in the same lightcone.
OK in that sense I agree, but you can't 'hit any matter in the way through the wormhole' since by definition the distance between the two mouths of the wormhole is zero. There is no space in between the two mouths so where there's no space there can't be any matter.


This is not true.
Thanks for such an elaborated reply.
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Direct Link To This Post Posted: July 25 2013 at 07:29
It's my lunch break, so I'll try to be brief...
Originally posted by Gerinski Gerinski wrote:

The predicted future "collision" between the Milky Way and Andromeda


"While the Andromeda Galaxy contains about 1 trillion (1012stars and the Milky Way contains about 300 billion (3×1011), the chance of even two stars colliding is negligible because of the huge distances between the stars"

More generally


"Colliding galaxies are common in galaxy evolution. Due to the extremely tenuous distribution of matter in galaxies, these are not collisions in the normal sense of the word, but rather gravitational interaction"




Okay, we will continue to contradict each other.
 
There is a very low probability that any one specific star would collide with any of the 1 trillion stars of the approaching galaxy, however there is a very high probability¹ that any one of the 300 billion stars would collide with any one of the 1 trillion stars of the approaching galaxy. Why is this high? Because stellar collisions are not rare events on a cosmological timescale. In our Milky Way galaxy stellar collisions occur once every 10,000 years. Got that? In a "static" galaxy where stars are separated by light-years, a star will collide with another star at some time in a 10,000 year window. This is because the density of the galaxy is not constant and the velocities and trajectories of the stars within it are not uniform. Even Sol and Proxima Centauri, which are located in a low density part of the outer spiral arm (the Orion Arm) that is orbiting the galactic centre at 200 km/s, are not moving at exactly the same velocity on exactly the same trajectory. In the time it takes Andromeda to collide with the Milky way there would have been 400,000 stellar collisions within our galaxy alone - it is safe to assume a greater number will have occurred in the Andromeda galaxy. So introduce another 1 trillion stars into that same volume of space and the number and frequency of those collisions will increase - it cannot decrease, so therefore there is a high probability that one of those stellar collisions will be between stars from each galaxy, I expect that the probability is so close to 1 that it is an inevitability. Also, even though (as I said) stars are unable to take evasive action to avoid a collision, gravitational attraction means they are inclined to do the opposite - if they get close enough to collide, they will collide.
 
This is not a wild guess based upon assumed densities (little stars far apart) but a reasonable prediction based upon what is happening now.
 
[¹if you write your name on a piece of paper and put it into a hat with nine other names then the probability that in a single draw your name will be picked is 0.1, however, the probability that someone's name will be picked it exactly 1]
 
And this is just taking stellar collisions into account. On smaller scales celestial collisions of non-stellar objects is even more common.
 
 
Also. The current explanations assume that the galaxies will mesh perfectly (because stars are small and the distance between them is great), but there is nothing to suggest that would be the case.
 
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Direct Link To This Post Posted: July 25 2013 at 07:32
EDIT: Dean jumped me. This is @Gerinski

You keep saying by definition for things that have nothing to do with definition. It's a bit tricky to talk about distance directly in a wormhole (or ever in some sense). It's probably better to think about the time it takes to transverse one. This varies depending on the exotic matter distribution, but it could take anywhere from an hour to a few hundred days (with these numbers being independent of our non-wormholian measurements of distance). With this in mind, it makes little sense to say the distance between openings is zero. 

Edited by Equality 7-2521 - July 25 2013 at 07:33
"One had to be a Newton to notice that the moon is falling, when everyone sees that it doesn't fall. "
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Direct Link To This Post Posted: July 25 2013 at 07:50
Again Confused ...
I never said that celestial bodies will never eventually collide. Gravitational attraction is likely to cause a lighter body to eventually fall into a heavier body. But we have been orbiting the Sun for quite a while and we did not yet fall into it. That takes a little while.

I said that if two galaxies 'collide', head to head collisions between their constituent stars are extremely unlikely, they will just pass through each other and gravitational effects will alter their structure, but body-to-body collisions will still be extremely unlikely, and I guess that the articles I provided support this claim.
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Direct Link To This Post Posted: July 25 2013 at 07:57
Originally posted by Gerinski Gerinski wrote:

It is clear that dark matter can't be 'invisible, non-light-radiating normal matter', that possibility was outruled long ago.
At any rate, for what we now know, there is nothing bigger than an helium atom which could likely get in the way of any intergalactic ship in normal circumstances (unless the extremely improbable fact of hitting a bigger solid body). Dark matter seems to exist, but on the scale of conceivable space travel it is probably neglectable. We have launched the Voyager probes to the edge of the Solar system and without any corrections for dark matter they are still behaving as we think they should. Dark matter has only influence on very large structures, most probably our first space travels would be within regions not affected at all by dark matter anyway?
Originally posted by Gerinski Gerinski wrote:

And btw, dark matter is supposed to not interact with ordinary baryonic matter except gravitationally, so hitting a dark matter particle would not be even noticed (most likely we are constantly hitting dark matter particles and we do not notice). Even some normal baryonic matter like neutrinos (which admittedly are supposed to have a really tiny but anyway non-zero mass) can be crossed through without any effect since they basically do not interact with our constituent particles. Trillions of neutrinos cross through the 'solid' Earth every second and they do not 'hit' anything, not because they do not encounter any other particles along their path or 'manage to avoid the collision', sure they do encounter other particles along their path, but they simply don't care about each other.
You are over thinking.
 
I never said dark matter or dark energy would affect intergalactic travel, I said the reagion between galaxies was not empty, however I did imply that since the effects of dark matter and dark energy are gravitational there would be something would be affected.That said, I could care less whether dark matter affects intergalactic travel or not because there is nothing to suggest that exotic matter is the only substance to be found between galaxies.
 
Dark matter is a hyperthetical substance used as an explanation for the total mass of the universe and it is predicted that it is found in the space between galaxies. This prediction does not exclude the possibility that the space between galaxies also contains other matter. What was ruled out was the quantity of normal matter between galaxies was insufficient to account for the total mass of the Universe, not that it didn't exist at all.
 
We cannot see what is in the space between galaxies, not because it is invisible, or because it isn't there, or because it is not reflective but because it does not emit radiation (eg light) and there is no localised radiation source to reflect radiation from (to detect reflected light the source has to be local - it is not that it is not reflective, it is that it is not reflecting). The Earth, or Venus or any of the other solar bodies do not reflect starlight sufficiently to be detected at stellar distances - if we had no Sun then Venus (for example) would reflect nothing and so would be invisible and essentially undetectable from space. As it is we can see Venus as a very bright point of light at dawn and dusk, but if it was floating in the region of space midway between here and Proxima Centauri we would never see it - now transport it to the region of space between The Milky Way and Andromeda... at that distance even a lump or rock as big as Venus would not be big enough to be detected - we simply would not know it was there. No one can say there is no normal matter in the space between galaxies.
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Direct Link To This Post Posted: July 25 2013 at 08:01
Originally posted by Gerinski Gerinski wrote:

Again Confused ...
I never said that celestial bodies will never eventually collide. Gravitational attraction is likely to cause a lighter body to eventually fall into a heavier body. But we have been orbiting the Sun for quite a while and we did not yet fall into it. That takes a little while.

I said that if two galaxies 'collide', head to head collisions between their constituent stars are extremely unlikely, they will just pass through each other and gravitational effects will alter their structure, but body-to-body collisions will still be extremely unlikely, and I guess that the articles I provided support this claim.
Wikipedia is not a supportive article - I have given extensive explanation as to  why I believe those articles are not the definitive answer. All you have done is contradicted me.
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Direct Link To This Post Posted: July 25 2013 at 08:16
Originally posted by Dean Dean wrote:

Originally posted by Gerinski Gerinski wrote:

Again Confused ...
I never said that celestial bodies will never eventually collide. Gravitational attraction is likely to cause a lighter body to eventually fall into a heavier body. But we have been orbiting the Sun for quite a while and we did not yet fall into it. That takes a little while.

I said that if two galaxies 'collide', head to head collisions between their constituent stars are extremely unlikely, they will just pass through each other and gravitational effects will alter their structure, but body-to-body collisions will still be extremely unlikely, and I guess that the articles I provided support this claim.
Wikipedia is not a supportive article - I have given extensive explanation as to  why I believe those articles are not the definitive answer. All you have done is contradicted me.
I did not learn that by Wiki but by several books I have read, several of them before Wiki existed, I just resorted to Wiki as an easy way to point these subjects to anyone interested in them. I could quote several other books by respected scientists saying the same, so don't take Wiki as my source.
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Direct Link To This Post Posted: July 25 2013 at 08:34
Originally posted by Dean Dean wrote:

 
 
We cannot see what is in the space between galaxies, not because it is invisible, or because it isn't there, or because it is not reflective but because it does not emit radiation (eg light) and there is no localised radiation source to reflect radiation from (to detect reflected light the source has to be local - it is not that it is not reflective, it is that it is not reflecting). The Earth, or Venus or any of the other solar bodies do not reflect starlight sufficiently to be detected at stellar distances - if we had no Sun then Venus (for example) would reflect nothing and so would be invisible and essentially undetectable from space. As it is we can see Venus as a very bright point of light at dawn and dusk, but if it was floating in the region of space midway between here and Proxima Centauri we would never see it - now transport it to the region of space between The Milky Way and Andromeda... at that distance even a lump or rock as big as Venus would not be big enough to be detected - we simply would not know it was there. No one can say there is no normal matter in the space between galaxies.
Astronomers and Cosmologists are not so naive as to neglect that there must be invisible bodies made of baryonic matter floating around that we can't see.
Dark matter considerations arise more from the Big Bang models, which can not explain how so much ordinary matter as it seems it would be needed to produce the gravitational effects we observe could have been produced by the Big Bang, as well as why does that 'dark matter' not interact with ordinary matter neither electomagnetically, nor via weak force or via strong force, apparently it only interacts gravitationally. That's what makes it 'dark matter', not the fact that it does not emit or reflect light.
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Direct Link To This Post Posted: July 25 2013 at 09:00
Originally posted by Gerinski Gerinski wrote:

Originally posted by Dean Dean wrote:

 
 
We cannot see what is in the space between galaxies, not because it is invisible, or because it isn't there, or because it is not reflective but because it does not emit radiation (eg light) and there is no localised radiation source to reflect radiation from (to detect reflected light the source has to be local - it is not that it is not reflective, it is that it is not reflecting). The Earth, or Venus or any of the other solar bodies do not reflect starlight sufficiently to be detected at stellar distances - if we had no Sun then Venus (for example) would reflect nothing and so would be invisible and essentially undetectable from space. As it is we can see Venus as a very bright point of light at dawn and dusk, but if it was floating in the region of space midway between here and Proxima Centauri we would never see it - now transport it to the region of space between The Milky Way and Andromeda... at that distance even a lump or rock as big as Venus would not be big enough to be detected - we simply would not know it was there. No one can say there is no normal matter in the space between galaxies.
Astronomers and Cosmologists are not so naive as to neglect that there must be invisible bodies made of baryonic matter floating around that we can't see.
Dark matter considerations arise more from the Big Bang models, which can not explain how so much ordinary matter as it seems it would be needed to produce the gravitational effects we observe could have been produced by the Big Bang, as well as why does that 'dark matter' not interact with ordinary matter neither electomagnetically, nor via weak force or via strong force, apparently it only interacts gravitationally. That's what makes it 'dark matter', not the fact that it does not emit or reflect light.
So? I'm not disputing any of that. The premis was you could fly at superluminal speeds between any two points in space and not hit anything. You cited the emptiness of intergalactic space, which I disputed, you cited the low density of galaxies, which I disputed. the how where or whyfore of dark matter is irrelevant. I never claimed that dark matter was so called because it does not emit or refect light, I simple state that all matter in the space between galaxies neither emits nor reflects light.
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Direct Link To This Post Posted: July 25 2013 at 09:14
No probs, at some point I understood that you were arguing that intergalactic space is not as empty as I was saying because dark matter or dark energy lies there.
Misunderstanding cleared.
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Direct Link To This Post Posted: July 26 2013 at 13:55
Another interesting fact which I'm afraid is not often properly reflected in Sci-Fi movies is this:
If you travel at a significant fraction of the speed of light (but still below it), the colour of the things you see will change. Looking ahead everything you see will turn more blue-ish and looking back everything will turn more reddish (due to Doppler effect).

A related curious but more intriguing fact is that looking ahead everything will look as in fast-forward motion, while looking back everything will look in slow-motion, the effect being more severe the faster you travel.
To understand that, consider the following example:
Say that we are travelling to Sirius which is roughly 9 light years away from Earth, and that we travel at 0.1 c (c being the speed of light). The travel will take 90 years.

Because of the finite light speed, the Sirius we see today from Earth is the Sirius of 9 years ago. However the Sirius we would see on arrival would obviously be the Sirius of 90 years from now. This means that in an elapsed time for us of 90 years we would see actually 99 years of change process. 99 years of visual history would be seen in 90 years time. So we would actually see it as a movie played in fast-forward motion, sped up by 10%. We would for example see the star moving 10% faster than what currently appears from Earth.

The fact that travelling at significantly enough speed, things behind you will look slowed down and things in front you will look sped-up is a scientifically accepted truth. The funny fact is, if we extrapolate this towards the speed of light, for the "looking back to where you come from" case we can get sensible answers: if the faster we travel the more slowed-down things will appear, in the ultimate case of travelling at the speed of light they will appear as frozen in time (and consequently we can even hypothesize than in the (in principle impossible) case of travelling faster than light, if we looked back to where we come from, we would see things happening "backwards in time").

However, for what we might see ahead of us we don't have similar sensible answers. The faster we travel the more sped-up will things appear to happen, that is scientifically sound, but what does the ultimate limit of sped-up mean when we would reach the light speed? (let alone faster). "Infinitely sped-up", what does that mean? would we see the final fate of the universe? Do photons see the end of the universe?


Edited by Gerinski - July 26 2013 at 14:00
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Direct Link To This Post Posted: July 26 2013 at 14:26
Since photons don't have eyes I doubt they would see anything.

It doesn't even make sense to talk about seeing when you're at the speed of light. You don't have a meaningful reference frame.

But anyway, the technical term for this is Terrell Rotation. Which people can feel free to look up.
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Direct Link To This Post Posted: July 26 2013 at 14:47
Originally posted by Equality 7-2521 Equality 7-2521 wrote:

Since photons don't have eyes I doubt they would see anything.

It doesn't even make sense to talk about seeing when you're at the speed of light. You don't have a meaningful reference frame.

But anyway, the technical term for this is Terrell Rotation. Which people can feel free to look up.
The extreme hypothetizations regarding what we might see at light speed were just an added amusing bonus thought experiment, the point was the scientifically sound facts that travelling fast, things ahead will look more blue-ish and in fast-motion, and the opposite when looking back where you come from. Nothing more and nothing less.

And btw, photons do travel through 3-D space at a certain speed, eyes exist, photons reach eyes, photons reach moving eyes... I don't see what is unscientific about wondering how would eyes moving at c perceive photons, even if we know that physical eyes can not travel at c. Schrodinger's Cat was neither a physical cat.

As I said, we can make an educated guess as to what we might "see" if travelling at c or even faster looking back to where we were coming from, the question becomes why can't we make a similarly educated guess for what we would "see" in the direction of our motion.


Edited by Gerinski - July 26 2013 at 16:06
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Direct Link To This Post Posted: July 26 2013 at 18:01
Gah. The speed of light is a limit.
 
Light is an electromagnetic wave (EM wave), it behaves like a wave and it behaves like a particle, but it is still an electromagnetic wave so when you Doppler shift it it behaves like a wave and does all the things a wave can do. Doppler shifting at approaching light-speeds causes the EM wave to shift out of the visible portion of the spectrum - whether you have eyes or not, you cannot see it once that happens. The energy that a photon has is proportional to the frequecy of the EM wave - gamma rays have lots of energy, radio waves have not so much. [edit: my train of thought stopped here.. what I intended to say was as you Doppler shift the lightwaves the energy of the photon is increased or decreased depending on the direction of the shift]
 
 
 
 
As you approach the speed of light the light reflected off objects behind you will be stretched from optical wavelengths down into non-visible wavelengths. You will see nothing, but you could still detect them with suitable detectors - travelling at 99.9999% the speed of light you would be able receive 'light' from an object you are moving away from on a domestic radio receiver. The Doppler formula is very simple:
 
 
f = \left ( \frac {c + v_\text{r}}{c} \right ) f_0
 
c \; is the velocity of waves in the medium;
v_\text{r} \, is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source (and negative in the other direction);
If v_\text{r} \, =c \;  then the formula will yield f=2fo for light coming from the object you are moving towards and f=0 for objects you are moving away from. So if you were travelling at precisely the speed of light then the light reflected off objects behind you will be travelling at the same speed and will never reach you, light coming from objects in front of you will be compressed and shift up into the ultraviolet spectrum. Looking back all you would see would be total darkness (and here I mean absolute darkness - a black so black it would scare the crap out of an Goth) because it is not just the optical region of the spectrum that has been shifted to zero, all EM frequencies (radio, radar, microwaves, infrared, visible light, ultarviolet, x-rays, gamma ray) have been shifted to zero. However, looking forward you will see something because in that direction all the infrared frequencies will have been shifted into the optical region of the spectrum, you will see the heat profiles of objects. [This is an effect that many text books fail to mention].
 
So hyperthetically, if it were possible to travel faster than light what would happen would be that the lower EM frequecies would continue to be doppler shifted up the spectrum - visible light would shift through the ultraviolet into x and gamma rays (which is not a good thing for the observer) and lower frequencies such as heat and radio waves would be shifted into the visible region. We would see the Universe as a radio telescope sees it.
 
What we would not see would be the future, no matter how fast we travel we can only see things that have happened. We can only see a photon of EM radiation after it has left the source - the faster we go the sooner we see it but we will never see it before it has happened:
 
at v_\text{r} \, =0 we see it at "1t"; at v_\text{r} \, =c \; we see it at "0.5t"; at v_\text{r} \, =2c \;  we see it at "0.33t"; at v_\text{r} \, =4c \;  we see it at "0.2t" ...
 
(where t is the time it takes the light to reach us if we were stationary)
 
...if you plotted that on a graph you would see that it is not a linear relationship, it approaches zero but never reaches it and it certainly never (ever) goes negative. Even if we were travelling at 1 million times the speed of light we would see events on Sirius 8.6µS after they occured, except we would "see" visible light from Sirius as X-Rays and what we would see as visible light would have left Sirius as radio waves.
 
/edit: Sorry, that last section is poorly explained. This is not the time relative to when it occured, but relative to when we would have seen it if we were stationary. Think of it as a race towards each other: If we start 9 meters apart and you walk towards me at 3.24 km/h while I stand still you will reach me in 10 seconds. If we repeat the excercise but we now walk towards each other at 3.24km/h we will meet in the middle in 5 seconds. If we do that again but I run at 12.96 km/h we will meet in 2 seconds and 1.8m from your starting point (and 7.2m from mine).


Edited by Dean - July 27 2013 at 04:37
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Direct Link To This Post Posted: July 26 2013 at 18:08
Also (incidentally) looking back to where you came from while travelling faster than the speed of light you would still see nothing, you would not see events from the past because you are travelling faster than the lightwaves that carry that information, you would be overtaking those photons, so they would be moving away from your eyes, not towards them.
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