Edge of the Universe

[SpeedFreek]

So what about the objects we have seen, that were moving away from us faster than light at the time those objects emitted that light? Everything from around redshift z~1.6 upwards, to z=1090 (all light emitted between 9 and 13.7 billion years ago)?

Are those objects not part of our observable universe?

Light emitted from any object that is actually moving away from us at greater than c will never reach us, so the object will not be observable by us. There are some situations where an object appears to be moving at greater than c relative to us, but that is an illusion. Bottom line, any object, the light from which we can see at any given moment, is part of our Observable Universe at that given moment. (as the name implies)

http://www.universetoday.com/2008/04/22 ... -of-light/

Well, that article is quite correct, and we will never see any light from galaxies that are receding faster than light. The reason for this is that the rate of expansion of the universe is accelerating. As the article says, most of the galaxies will eventually have receded beyond our cosmological light horizon. The article is talking in the context of viewing future events in an accelerating universe.

But we have seen galaxies that were receding faster than light. Their light was emitted long ago in the past, whilst the rate of expansion was decelerating. The rate of expansion was decelerating for over 7 billion years, and during this time all that light I talked about earlier (from everything we see today with a redshift of over z=1.4) made its way into our Hubble sphere. But now the rate of expansion is accelerating, so the light from galaxies that are receding at apparently superluminal recession speeds will never find itself in a region of space that is receding from here at less than the speed of light.

Again, the galaxies we see at redshift z~1.4 were receding at the speed of light 9 billion years ago - they were on the edge of our Hubble sphere. Galaxies at redshift z=7 were receding at multiples of the speed of light nearly 13 billion years ago - they were a long way beyond our Hubble sphere. The particle horizon - the "emission" region for the CMBR photons we currently detect, at z=1089, was receding from here at over 50 times the speed of light at recombination, when those photons could first move freely through a transparent universe, 13.7 billion years ago. The surface of last scattering, as it is called, was over 50 times the Hubble distance away.

 

[mental_avenger]

Well, that article is quite correct, and we will never see any light from galaxies that are receding faster than light.

Ok, we agree.

The reason for this is that the rate of expansion of the universe is accelerating.

No, the reason for this is that if a source of light is traveling away from us at greater than c, then the light cannot travel fast enough to overcome the velocity of the source of that light.

 

[SpeedFreek]

No, the reason for this is that if a source of light is traveling away from us at greater than c, then the light cannot travel fast enough to overcome the velocity of the source of that light.

Unfortunately, that is only (approximately) correct in a universe where the rate of expansion has never been decerating but is constant, or accelerating. The picture is confusing when considering a rate of expansion that was initially very fast but decelerated quickly and continued to decelerate for a long time, before levelling out and starting to accelerate quite recently.

And even more unfortunately, it is not actually correct at all. There are two horizons involved here - the Hubble horizon, where a source of light recedes at c, and the cosmological event horizon, which is more distant, and is the actual horizon from beyond which we can see no light. This is complicated, but is important if the rate of expansion is accelerating. The event horizon is the distance beyond the Hubble horizon that an object can emit light that will make it to the Hubble horizon before the accelerating expansion stops it! This horizon is currently 16 billion light-years away, and we will never see light here that was emitted right now, more than 16 billion light-years away, but we will eventually see light that is emitted at 15 billion ly right now, outside of our Hubble sphere.

A lot of popular articles simplify the picture and refer just to the Hubble horizon, but if you think about it, as an object is moving away from us at c on the Hubble horizon, light is passing that object in the other direction and so, by definition, will be inside of our Hubble horizon. Light emitted on the Hubble horizon is immediately inside it! The real event horizon is beyond the Hubble distance, and a really very simplified way to think of it is that it is the distance where light itself is receding from us more than c, rather than simply an object that emits light receding at c. If light is receding from us at less than c, it might eventually reach us, as it is moving towards us just a little bit faster than that. It all depends on how the rate of expansion changes, and the distances involved.

But simply put, if the rate of expansion remains constant, then the distance to those horizons remains constant, and objects constantly pass beyond those horizons. Any light emitted just beyond the event horizon will be moving towards us at c, as the region of space it is in recedes from the Hubble horizon at a little more than c, so it cannot make any headway and will never reach us. Any light emitted just inside that distance will be in space that is receding from our hubble horizon at less than c, so it will pass that horizon and eventually reach us.

If the rate of expansion accelerates, then the horizons get closer to us. As the rate accelerates, the event horizon closes in, overtaking light that is heading towards us, putting that light forever out of our view. The rate at which all the distant galaxies pass out of view accelerates, rather than remaining constant, as it would with constant expansion.

But in a decelerating universe, the horizons move away from us, and the faster the deceleration, the faster those horizons recede. In the early universe the rate of expansion was incredibly fast, but was also decelerating very fast too, thus the event horizon was receding and overtaking the light emitted from galaxies that were outside of that horizon when they emitted that light. In a universe that has decelerated, like ours did for 7 billion years or so, it is possible to see light that was emitted from objects that were beyond that horizon when they emitted that light.

The rate of expansion has only been accelerating for the past 5 billion years or so, and the rate of expansion today is nothing like as fast as it was in the early universe. For a long time, the event horizon was receding, letting the light from galaxies with apparently superluminal recession speeds proceed into our Hubble Sphere. The edge of the Hubble sphere has a comoving distance of 14 billion ly and the event horizon has a comoving distance of 16 billion ly, whereas the particle horizon, the most distant (in time) place from which we have received photons, has a comoving distance of over 46 billion ly.

You can find out more about everything I have said above in the following paper:

Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe

We use standard general relativity to illustrate and clarify several common misconceptions about the expansion of the Universe. To show the abundance of these misconceptions we cite numerous misleading, or easily misinterpreted, statements in the literature. In the context of the new standard Lambda-CDM cosmology we point out confusions regarding the particle horizon, the event horizon, the ``observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests. Attempts to restrict recession velocities to less than the speed of light require a special relativistic interpretation of cosmological redshifts. We analyze apparent magnitudes of supernovae and observationally rule out the special relativistic Doppler interpretation of cosmological redshifts at a confidence level of 23 sigma.

Finally, here is a rough little animation that might help (or might not!) It shows the recession of the particle horizon at the top, a high redshift galaxy in the middle and a galaxy that was receding at c at the bottom. Our point of view is on the right hand side, and the little white dashes represent the photons on our light-cone (the history of the light we currently see). The top and middle photons are initially outside of our Hubble horizon until they pass the galaxy that is receding at c, when it emits its photon.

Have fun!

 

[mental_avenger]

That entire scenario appears to be based upon the assumption that “space” is somehow expanding, rather than matter expanding into space. That is an assumption that was created in order to prevent some observations from invalidating Einstein’s theory. If matter is expanding into space, then Einstein must have gotten something wrong. We certainly cannot have that.

Matter expanding into a static infinite space makes perfect sense, and also fits observations. IMO it is erroneous to think that nothing can end up traveling at faster than c relative to anything else. A simple model of a universe where everything is expanding away from everything else shows how relative velocities greater than c are inevitable. Let’s start with only four objects in a straight line, all moving away from each other. B moves away from A at velocity x. C moves away from B at velocity x. D moves away from C at velocity x. Therefore, C moves away from A at velocity 2x simply due to it’s relative distance from A. D moves away from A at 3x. IOW, the further apart two objects are, the faster their relative velocities will be. With enough particles, and enough time, relative velocities greater than c are inevitable, even though the relative velocities between the nearest two particles is still only x.

 

[SpeedFreek]

That entire scenario appears to be based upon the assumption that “space” is somehow expanding, rather than matter expanding into space. That is an assumption that was created in order to prevent some observations from invalidating Einstein’s theory. If matter is expanding into space, then Einstein must have gotten something wrong. We certainly cannot have that.

From that paper:

We use standard general relativity to illustrate and clarify several common misconceptions about the expansion of the Universe.....In the context of the new standard Lambda-CDM cosmology we point out confusions....

Do you know what Lambda represents?

 

[ramparts]

That entire scenario appears to be based upon the assumption that “space” is somehow expanding, rather than matter expanding into space. That is an assumption that was created in order to prevent some observations from invalidating Einstein’s theory. If matter is expanding into space, then Einstein must have gotten something wrong. We certainly cannot have that.

Matter expanding into a static infinite space makes perfect sense, and also fits observations. IMO it is erroneous to think that nothing can end up traveling at faster than c relative to anything else. A simple model of a universe where everything is expanding away from everything else shows how relative velocities greater than c are inevitable. Let’s start with only four objects in a straight line, all moving away from each other. B moves away from A at velocity x. C moves away from B at velocity x. D moves away from C at velocity x. Therefore, C moves away from A at velocity 2x simply due to it’s relative distance from A. D moves away from A at 3x. IOW, the further apart two objects are, the faster their relative velocities will be. With enough particles, and enough time, relative velocities greater than c are inevitable, even though the relative velocities between the nearest two particles is still only x.

I think you're tying yourself up in knots. Speeds faster than the speed of light are impossible for very good reasons. Read up on special relativity (Wikipedia should do, and some sites set up by scientists for laymen), I don't think this space is sufficient to explain the arguments, but read up on special relativity and violations of causality. There are some pretty big issues if objects can travel faster than c. But! These causality issues are resolved if it's not actually matter expanding, but the space carrying it.

My recommendation: rather than getting indignant and assuming that your opinions are right and a few decades of scientific research are all wrong, perhaps it would be wise to voice your questions as such (rather than fact) and do some more learning on the subject. It is very clear you have a lot to learn. Cosmology is a fascinating field of research, and by assuming you already know the answers, you shut yourself out of some wonderful knowledge.

 

[mental_avenger]

I think you're tying yourself up in knots. Speeds faster than the speed of light are impossible for very good reasons.

All of which are tied inexorably to the teachings of the great god, TOR. Did you happen to read the simple model I posted? Although rather primitive in concept, and only showing 2D, it shows that objects can travel faster than c relative to each other without violating even Special Relativity which says that objects cannot be accelerated to the speed of light. The model shows objects at +c only relative to each other, not relative to their starting point.

Read up on special relativity (Wikipedia should do, and some sites set up by scientists for laymen), I don't think this space is sufficient to explain the arguments, but read up on special relativity and violations of causality.

Actually I have studied the subject(s) quite extensively.

There are some pretty big issues if objects can travel faster than c. But! These causality issues are resolved if it's not actually matter expanding, but the space carrying it.

A resolution created to deal with observed phenomena which would otherwise violate TOR.

My recommendation: rather than getting indignant and assuming that your opinions are right and a few decades of scientific research are all wrong, perhaps it would be wise to voice your questions as such (rather than fact) and do some more learning on the subject.

And who the hell are you to preach to me? I might have more books in my personal library on Relativity, Cosmology, Space, and related subjects than you have even read. Just because I suggest alternative explanations for observed phenomena does not mean I haven’t studied the relevant conventional wisdom on the subject.

As for voicing my opinions, does the internet slang term “IMO” seem familiar? It should, I use it enough. How about qualifiers I have used such as ”my observations appear to agree with”?, “probably”, “possible”, “usually”, “not necessarily”, “appears to be based upon”, etc.? Actually, it is you, not I, who are voicing your opinions as if they were indisputable fact.

It is very clear you have a lot to learn. Cosmology is a fascinating field of research, and by assuming you already know the answers, you shut yourself out of some wonderful knowledge.[/

It is very clear that you have a LOT to learn about discussion. And it is clear that you have shut yourself off from the possibility of some wonderful alternative explanations.

 

[dangineer]

Mental Avenger, in your model above, you discussed what would happen only taking into account Newtonian mechanics. I believe this is why you were accused of not understanding physics. Taking this same scenario and applying Lorenz transformations to the relative motions of all the objects shows that no object travels faster than c relative to any other object, including the point of origin (in this case the point of origin would be treated as another observer).

I'm sure since you have studied Special Relativity in some depth you have looked at this situation before. I'm just curious as to why you don't think it applies. A lot of people have gone around claiming that Einstein was wrong. This may very well be true, so I would certainly like to see why. Perhaps you can show us how specifically Einstein's theory is invalidated and show us another approach that accounts for everything relativity accounts for, as well as the things that it doesn't account for.

Also, the expanding universe hypothesis is actually validated by Einsten's theory as it coincides with predictions made by General Relativity. Special Relativity alone doesn't account, and even contradicts, many of the observations of the distant universe, but General Relativity, on the other hand, does a very good job of explaining what we observe.

 

[mental_avenger]

Mental Avenger, in your model above, you discussed what would happen only taking into account Newtonian mechanics. I believe this is why you were accused of not understanding physics. Taking this same scenario and applying Lorenz [sic] transformations to the relative motions of all the objects shows that no object travels faster than c relative to any other object, including the point of origin (in this case the point of origin would be treated as another observer).

I am sure you understand that the Lorentz transformation is used to explain direct observations within the context of special relativity. Therefore, I am sure that you can understand that bringing up Lorentz transformations amounts to circular reasoning in this instance. Ignoring that for a moment, there is another observation I’d like to make here, and that is that the Lorentz tranformation and special relativity both deal with what one observer can observe relative to another observer, NOT what is actually happening. Due to several factors, what we are allowed to observe necessarily appears distorted by relative motion.

Let me put it another way. Attempting to view events from a relativistic point of view and then trying to figure out what is really going on is a lot like translating something into another language and then back again. It is a wasted step and can lead to confusion.

I'm sure since you have studied Special Relativity in some depth you have looked at this situation before.

Se the above.

A lot of people have gone around claiming that Einstein was wrong. This may very well be true, so I would certainly like to see why. Perhaps you can show us how specifically Einstein's theory is invalidated and show us another approach that accounts for everything relativity accounts for, as well as the things that it doesn't account for.

And there’s the rub. Einstein’s theory isn’t just one theory, as no one part of it stands alone. It requires all the postulates and exceptions in order to be valid. I can (and have) explained several observed phenomena in a manner which does not depend upon TOR being the correct explanation. But remember, just because observations appear to fit a theory does not necessarily mean the premise of the theory is correct. It only means that the observations and results coincide.

Also, the expanding universe hypothesis is actually validated by Einsten's theory as it coincides with predictions made by General Relativity.

The fact that it coincides is not proof that the premise is correct. The fact that Einstein’s calculations seem to “predict” later observations does not mean that the premise is correct.

Special Relativity alone doesn't account, and even contradicts, many of the observations of the distant universe, but General Relativity, on the other hand, does a very good job of explaining what we observe.

Yes, my point exactly. If you add enough exceptions and conditions, almost any theory can account for almost anything, eventually.

 

[dangineer]

"the Lorentz tranformation and special relativity both deal with what one observer can observe relative to another observer, NOT what is actually happening."

This must be where the disconnect lies between your ideas and most of the physics community's. It makes complete sense that Relativity describes what we observe, and not necessarily what is really going on. However, this is the difficulty in developing a scientific theory: we only know what we observe, and if something else is going on that we cannot observe, there's no way we'll ever know.

According to Einstein's description of GR and SR, there is no "actual reality." Everything that happens is relative to everything else. There's no absolute reference frame that everything can be measured from with no variance from observer to observer (this was, on the other hand, the premise of Newton's description of the universe). Now this is, of course, Einstein's interpretation of his own theory.

What it sounds like you are trying to explain to me above is that you have proposed elements of an alternative theory that also coincides with what we observe, and yet doesn't need GR and SR. However, you didn't mention anything that directly invalidates Relativity. I was wondering if you had any evidence that does this. I'll also try to sort through all your posts to take a look at your ideas more closely.

 

[ramparts]

And it is clear that you have shut yourself off from the possibility of some wonderful alternative explanations.

Absolutely not! I have shut myself off, however, from the possibility of finding wonderful alternative explanations from non-experts on internet forums. Just a matter of saving time. If there is a good idea, it needs to be formulated rigorously and written up, and I'll consider it when it does. Otherwise, if I spent time looking at all the "alternative explanations" propagating throughout the interwebs, when would I ever get to sleep?

 

[ArcCentral]

And it is clear that you have shut yourself off from the possibility of some wonderful alternative explanations.

Absolutely not! I have shut myself off, however, from the possibility of finding wonderful alternative explanations from non-experts on internet forums. Just a matter of saving time. If there is a good idea, it needs to be formulated rigorously and written up, and I'll consider it when it does. Otherwise, if I spent time looking at all the "alternative explanations" propagating throughout the interwebs, when would I ever get to sleep?

Shut off maybe, but certainly willing to spend all your valuable time conversing about alternative explainations you completely disagree with, thereby getting no sleep apparently. :lol: I've heard this broken record before, and it would seem to still be on the turntable, running at a universal constant of 45 revolutions per minute. One might think there is something to learn from this, but it never appears to take hold. Somehow ya just can't get enough when there is an addiction involved.

 

[ArcCentral]

I have my own ideas as an alternative to the expansion of space, and the possibilities of an edge to existence, but have very little desire to discuss them in a caustic environment. Mental_avenger apparently has some too, without peer revue? Who would have thought? :lol:

I'd be more interested in some mental_avenger details, but it looks as though it's gonna be a war of words, rather than an exchange of ideas. This is unfortuanate to no end from past experience of reading countless threads. Odds are that mental_avenger is speaking blather, but thats OK, because {{News Flash!}} it's just a conversation.

 

[ramparts]

Shut off maybe, but certainly willing to spend all your valuable time conversing about alternative explainations you completely disagree with, thereby getting no sleep apparently. :lol: I've heard this broken record before, and it would seem to still be on the turntable, running at a universal constant of 45 revolutions per minute. One might think there is something to learn from this, but it never appears to take hold. Somehow ya just can't get enough when there is an addiction involved.

Perhaps you're right I'm here to teach what I can (the relatively little I can), to learn from others, and most importantly, to always be thinking about the subjects. Maybe, though, one's "alternate theory" is best left be, so long as it's not stopping anyone else from going out and learning real physics. People, it seems, can be very attached to their theories, and perhaps it's best not to attack that.

And once again, the thread has drifted significantly

 

[mental_avenger]

I have my own ideas as an alternative to the expansion of space, and the possibilities of an edge to existence, but have very little desire to discuss them in a caustic environment. Mental_avenger apparently has some too, without peer revue? Who would have thought? :lol:

I'd be more interested in some mental_avenger details, but it looks as though it's gonna be a war of words, rather than an exchange of ideas. This is unfortuanate to no end from past experience of reading countless threads. Odds are that mental_avenger is speaking blather, but thats OK, because {{News Flash!}} it's just a conversation.

As I have often noted, no one on an internet forum “wins” any debate. IMO it is basically an exchange of ideas. Unfortunately you are correct about the often caustic environment, but then I have been posting in that caustic environment for about 7 years and have developed somewhat of a immunity to it. And considering the venue, my posts are “peer reviewed” all the time.

If you really are interested in details, I can share some of them. These are not opinions harvested from books or web sites, but rather my own conclusions or musings after reading books or web sites. IMO there are a variety of reasonable, though sometimes mutually exclusive explanations for observed phenomena. I think that many differences of opinion arise from our inability to construct models that accurately represent phenomena we cannot see by using familiar analogies. For example, we have fairly common models of the structures of atoms and molecules, and yet the actual structure may be dramatically different than our models. We may simply have no analogy in the macro world that can accurately represent what exists in the micro world.

 

[ArcCentral]

I like to think of this stuff as fun, regardless of the mistakes I may or may not make in my musings of new theories, and most definitely offer the same courtesy to those on a similar path. Thusly when someone has an idea I consider to be wrong everyway but loose, I'll generally humor them with enough rope to hang themselves, rather than organize a posse to hang em high.

 

[ArcCentral]

If you really are interested in details, I can share some of them. These are not opinions harvested from books or web sites, but rather my own conclusions or musings after reading books or web sites. IMO there are a variety of reasonable, though sometimes mutually exclusive explanations for observed phenomena. I think that many differences of opinion arise from our inability to construct models that accurately represent phenomena we cannot see by using familiar analogies. For example, we have fairly common models of the structures of atoms and molecules, and yet the actual structure may be dramatically different than our models. We may simply have no analogy in the macro world that can accurately represent what exists in the micro world.

I get the impression from some of your other postings, that there is a center to the Big Bang. Is this a correct assumption? Also, you seem to imply that matter is moving through space, as opposed to space expanding. Is this also a correct assumption?

 

[mental_avenger]

I like to think of this stuff as fun, regardless of the mistakes I may or may not make in my musings of new theories, and most definitely offer the same courtesy to those on a similar path. Thusly when someone has an idea I consider to be wrong everyway but loose, I'll generally humor them with enough rope to hang themselves, rather than organize a posse to hang em high.

It’s all great entertainment. But more than that, it is always a learning experience. Not only do we learn from what others post, we also learn by researching other sources before posting comments. (at least I do)

I get the impression from some of your other postings, that there is a center to the Big Bang. Is this a correct assumption?

Well, almost by definition, if all the matter in the known Universe came from a singularity, and all matter expanded outward from that point, that would be the “center”. However, from our current perspective, that would be essentially irrelevant. That point would now be so far away as to be beyond the edge of the observable universe. Everything should appear to be moving away from everything else from anywhere within the observable universe.

Also, you seem to imply that matter is moving through space, as opposed to space expanding. Is this also a correct assumption?

That is a correct assumption. It is part of the concept of infinite space.

One difference you may note in my viewpoints is the concept of simultaneity. I fully understand the concept of relativity when applied to two events occurring separated in space. But the generally accepted view really only addresses what we can observe. IMO, two events can take place simultaneously even when separated in space, and that will occur whether or not we are able to observe the events, or our relative position if we do observe them. Another way of looking at it would be an event such as an exploding star. (assuming for the sake of this example that the explosion is a single instantaneous event) Such an explosion could take place at this exact moment 1000 light years away. We wouldn’t see the light from that explosion for 1000 years, but that does not mean it did not take place at this exact moment. I understand that is an absolute that relativity denies. All the same, IMO it is an absolute that is independent of observation.

Granted, when the problem is approached from the perspective of requiring data from observers, then relativity becomes a factor, but only in the comparing of the data. The absolute still exists.

 

[SpeedFreek]

For anyone who doesn't know about it, the easiest way to see how simultaneity works is the classic "train passing the platform" example.

Bob is sitting in the middle of a train carriage, and above his head is a light bulb. It emits a burst of light. When the light from that bulb reaches each end of the carriage, it causes light bulbs at each end of the carriage emit a burst of light. Bob sees that the lights at each end of the carriage flash at the same time (he has eyes in the back of his head!). A simple scenario.

But the train is passing through a station when this all happens, and Tom, who is standing on the platform, happens to be right in line with that carriage. In fact, he is right in line with the centre bulb as it flashes. But the carriage is moving in relation to him, and because of this, his view of the scene is different.

From Toms point of view, the bulb at the trailing end of the carriage flashes before the bulb at the front of the carriage.

Now Tom knows what the speed of light is, so he does the calculations to subtract the time it took for the light from the bulbs at the ends of the carriage to reach him. After doing the math, he finds that the bulb at the trailing end still flashed before the bulb at the front!

So now he calculates the light travel time from the bulb at the centre of the train to the bulb at each end. He knows that light travels at c, and that the train was moving relative to him. Once the flash of light was emitted from the centre of the train, the light propagates at c relative to Tom, but not relative to the train, from Toms point of view! As the train was moving across his view, the light actually reached the bulb at the back of the train before it reached the front.

So to Bob, the bulbs at the ends of the carriage lit up simultaneously, but Tom they did not. But it is not about light-travel time, it is about the constancy of the speed of light. It is not that the light from simultaneous events took different amounts of time to reach different observers, it is that the speed of light is the same to all observers, and therefore the distance between events is not absolute.

MA, it seems that you accept all this, and yet still think there is some sort of absolute simultaneity that is separate from what different observers see. So, in that absolute sense, did the bulbs at the ends of the carriage turn on at the same time, or not?

 

[mental_avenger]

MA, it seems that you accept all this, and yet still think there is some sort of absolute simultaneity that is separate from what different observers see. So, in that absolute sense, did the bulbs at the ends of the carriage turn on at the same time, or not?

Thank you for using that example, one of my favorites, and one of Einstein’s first IIRC. That example illustrates my point very well. While the bulbs turned on at the same time, different observers saw the results from different frames of reference. You could add a third observer zooming by in an airplane and get different observations, but that would not effect what the bulbs actually do. Again, the difference is what is actually happening vs what we can observe, depending upon our frame of reference.

 

[SpeedFreek]

While the bulbs turned on at the same time, different observers saw the results from different frames of reference.

The bulbs only turned on at the same time in the frame of reference of the carriage. Observers not in that frame of reference calculate different results. It is not what the observers see, it is what they calculate after taking all relativistic effects into account, that matters. The fact that the bulbs turned on at the same time is only a fact to those within the carriage.

You could add a third observer zooming by in an airplane and get different observations, but that would not effect what the bulbs actually do. Again, the difference is what is actually happening vs what we can observe, depending upon our frame of reference.

And we can calculate what is happened, independently of what we observe, by knowing how our observations are subject to relativistic effects. After we do this, we find that what is actually happening is not absolute. The bulbs did not flash at the same time in any absolute sense, only in a sense local to that frame of reference.

Even if we invoke the infamous "gods-eye view", where we are taking the frame of reference of an impossible something that can "instantly" flit between different reference frames, we find that there are no events separated by space that are simultaneous from all frames.

 

[dangineer]

"It is not what the observers see, it is what they calculate after taking all relativistic effects into account, that matters."

Actually, SpeedFreek, the observers in this case don't have to take into account relativistic effects. The relativistic effects are what they actually observe. Then when you apply relativity to the situation, it makes sense of the whole mess.

Mental Avenger, I see that you do accept relativity, but still believe that something happens "behind the scenes" that we cannot observe.

I'm still a little confused about the example you gave with the superluminal recession. You said that we are observing what's actually happening - objects are in fact receding from us faster than light. However, then you say that relativity does explain what we observe (or at least that's what I think you're saying). According to relativity, we wouldn't see things moving away from us faster than light, unless space was expanding (if you assume space is expanding, General Relativity predicts exactly what we see). So why doesn't relativity apply in this instance, but does apply in other instances?

 

[darkmatter4brains]

IN SR, the first thing they taught us was that the observer is NOT a person - its an infinite lattice work of rods and clocks. The clocks are all synchronized based on a light pulse that is sent out from the center of the lattice work. This takes the whole "what did the observer see" out of the picture. At that point you know there are no effects due to the delay of a photon actually reaching your eye, or somerthing going on behind the scenes. Simultaneity just goes out the window, period.

 

[mental_avenger]

The bulbs only turned on at the same time in the frame of reference of the carriage. Observers not in that frame of reference calculate different results. It is not what the observers see, it is what they calculate after taking all relativistic effects into account, that matters.

Again you bring it back to what can be observed from a certain frame of reference that is in motion relative to another frame of reference. But the apparent difference is only an illusion created by the relative motion of the frames. Events take place independently of the frames of reference from which they are observed.

Let’s modify the train thought experiment slightly to eliminate the illusion. The train, the lights, Bob, and Tom are all still where we left them. However, instead of watching the lights come on, Bob and Tom each have an electronic comparator which measures any difference between the turning on of the lights. Each comparator is connected to the two lights with equal length wires. Of course Tom’s wires are long and flexible in order to remain attached as the train moves past. When the lights turn on, both Bob’s comparator and Tom’s comparator see the lights turn on at the same time. Amazing. Bob is in one frame of reference and Tom is in another, and yet they both measure the event as occurring simultaneously.

The fact that the bulbs turned on at the same time is only a fact to those within the carriage.

I disagree. The fact that the bulbs turned on at the same time is and will always be a fact to those two bulbs. Any differences are only observational differences.

The bulbs did not flash at the same time in any absolute sense, only in a sense local to that frame of reference.

That is the only sense that matters when determining what actually happens. Anything different is an observational difference, whether that observation is made by people, instruments, or simply a non occupied point in space.

 

[mental_avenger]

Mental Avenger, I see that you do accept relativity, but still believe that something happens "behind the scenes" that we cannot observe.

No, that isn’t it. It doesn’t happen behind the scenes, it happens in plain sight. And we can observe it. However, different frames of reference moving relative to each other will perceive the events differently.

Perhaps the difference here is that I choose to consider events in the absolute sense and work with them in the absolute sense, then calculate how they will appear to different relativistic frames. Others may choose to consider events relativistically and work with them relativistically, then calculate them to a given reference frame.

I'm still a little confused about the example you gave with the superluminal recession. You said that we are observing what's actually happening - objects are in fact receding from us faster than light. However, then you say that relativity does explain what we observe (or at least that's what I think you're saying). According to relativity, we wouldn't see things moving away from us faster than light, unless space was expanding (if you assume space is expanding, General Relativity predicts exactly what we see). So why doesn't relativity apply in this instance, but does apply in other instances?

Perhaps you misunderstood, or I explained poorly. If objects are moving away from us faster than c, then we cannot see them and the light from them will never reach us.