can we see it #2 ?

[michel123456]

The photon will be redshifted, (...)

 

That's enough to me.

[Spyman]

No, it is not.

Ooh Yes, the two statements you made are wrong according to current models of cosmic expansion.

An object receding from us via cosmic expansion at FTL would not be visible, as the distance the photon has to travel would increase faster that it does the traveling, and it would never reach us.

False.

A photon that is emitted from an object receding FTL will never reach us. It is true that some of the light reaching us now is from objects that are currently receding FTL, but were not when that light was emitted.

False.

 

 

The truth is that we currently are observing objects that were receding from us FTL due to cosmic expansion, when the light we see now were emitted.

 

 

 

The early universe is different, because the rate of expansion was decreasing (rapidly). In the intervening time, the Hubble sphere grew faster than the universe and overtook it.

 

When the rate is constant or accelerating (as it is now and will continue to be AFAWK), it remains true that objects receding FTL will never be observed.

No, the rate of the expansion is accelerating right now but we STILL continue to observe objects that are receding from us FTL even though the expansion is accelerating and not decreasing.

 

 

What I should say is, anything emitted after the first ~ 5 billion years from an object receding FTL will never reach us.

The fact that our possible descendents won't be able to observe light emitted right now from very distant objects, in a far far distant future due to an accelerated expansion is NOT an valid argument against us seeing objects today that were receding from us FTL in a distant past when they emitted that light from a relative close distance.

 

Sure, if the expansion of the Universe continues to accelerate as it does then we will never be able to observe light emitted outside of the Hubble sphere right now, but we will still be able to continue to observe objects receding from us FTL in plenty of Billions of years to come.

 

And then after some ~ 2 Trillions of years objects outside of the Local Supercluster will get redshifted so much that they will not be detectable any more.

Edited June 14, 2010 by Spyman
Spelling

[michel123456]

Never begin a sentence with "The truth is..."

 

I could agree with the following, as it looks to be a (unbelievable) fact:

"we are currently observing objects that were receding from us FTL when the light we see now were emitted."

[Sisyphus]

Sure, if the expansion of the Universe continues to accelerate as it does then we will never be able to observe light emitted outside of the Hubble sphere right now,

 

This is what I'm saying.

 

So, to rewind:

 

Could we observe an object moving away from us FTL?

 

Invalid scenario.

 

Can we currently observe any objects that are currently receding due to cosmic expansion FTL?

 

Yes.

 

Can we currently observe any objects that were receding due to cosmic expansion FTL when the light we're seeing was emitted?

 

Yes. (But not light that was emitted after the first 5 billion years or so of the universe.)

 

Will we observe light that is emitted now from any object receding due to cosmic expansion FTL?

 

No. (Unless cosmic expansion stops accelerating.)

 

Do we all agree?

[Spyman]

Never begin a sentence with "The truth is..."

I also said:

according to current models of cosmic expansion.

 

----------

 

Do we all agree?

Ok.

[michel123456]

Almost.

The "due to cosmic expansion" part is not necessary: it is an explanation.

You don't have to insert an explanation when describing an observation.

 

If you want to stick strictly to observation you have to say:

 

"Can we currently observe any* objects that are currently moving away FTL?"

 

Yes.

 

* with the reservation: the word "any" means "any object placed upon the surface of our past light-cone". All other hypothetical objects, if existing, are not observable.

[Sisyphus]

Almost.

The "due to cosmic expansion" part is not necessary: it is an explanation.

You don't have to insert an explanation when describing an observation.

 

If you want to stick strictly to observation you have to say:

 

"Can we currently observe any* objects that are currently moving away FTL?"

 

Yes.

 

* with the reservation: the word "any" means "any object placed upon the surface of our past light-cone". All other hypothetical objects, if existing, are not observable.

 

They're not moving FTL relative to us, though, they are receding. We most definitely do not observe them moving FTL.

 

The observations, if you want to get technical, are patterns of different shiny spots in the sky. Cosmic expansion scenarios are the only explanations that fit the observations and successfully predict others.

[michel123456]

In your post #20, you agreed with my post #19.

[Sisyphus]

In your post #20, you agreed with my post #19.

 

Huh? Post #20 is you.

[michel123456]

Sorry.

In your post #19, you agreed with my post #18.

(for some mysterious reason, posts numbers disappear when I reply to thread)

[Sisyphus]

I agreed that photons move at C, and said that objects moving faster than C and emitting photons make little physical sense. What is your point?

[michel123456]

IIRC Relativity does not forbid FTL objects. What Relativity states is that SOL is impassable (that it cannot be passed). SOL is an obstacle, a barrier, that does not mean that on the other side of the barrier, nothing exists.

 

It is commonly believed that if FTL objects were to exist, they would be invisible, because photons would go the wrong way.

My point is that, as far as I can understand, when the axiom of Relativity states that C is constant, such hypothetical FTL objects must be visible. There is no way for photons to "go back" (see my post #18) because C is constant.

[swansont]

[math]\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/math]

 

is imaginary when v > c, which implies an imaginary mass, which as far as we know is unphysical. Which takes us back to "Once you hypothesize a physically impossible situation, you can't draw any valid conclusions."

[Sisyphus]

It is commonly believed that if FTL objects were to exist, they would be invisible, because photons would go the wrong way.

 

Who believes that?

[Johanluus]

"It is commonly believed that if FTL objects were to exist, they would be invisible, because photons would go the wrong way".

 

Would it not be more correct to say that " light goes in all directions at c at the instant the photons leave the FTL moving object"

So when we absorb the photon some time later dt , the object has moved further than dtc.

[michel123456]

Who believes that?

Is this a misunderstanding?

And the photons that reach you would have infinite energy if moving towards you, and negative energy if moving away.

---

Merged post follows:

Consecutive posts merged

"It is commonly believed that if FTL objects were to exist, they would be invisible, because photons would go the wrong way".

 

Would it not be more correct to say that " light goes in all directions at c at the instant the photons leave the FTL moving object"

So when we absorb the photon some time later dt , the object has moved further than dtc.

 

Yes.

 

The last part of your sentence means light would be redshifted.

---

Merged post follows:

Consecutive posts merged

[math]\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/math]

 

is imaginary when v > c, which implies an imaginary mass, which as far as we know is unphysical. Which takes us back to "Once you hypothesize a physically impossible situation, you can't draw any valid conclusions."

 

Discussion will go out of tracks. I don't want to argue whether imaginary results have some physical meaning, or whether this mathematical factor is ad hoc. There are arguments to provide.

 

My point is: we are actually observing galaxies going away from us at FTL velocities, and they are redshifted. Because our Theory forbids the fact*, we have another explanation which is "space itself expands", which IMO is not an easy explanation. The peculiar part is that this explanation arises from the same theory that forbids FTL objects to exist. The circle is closed and everybody is happy. We are observing FTL objects, but no, we are not, we are observing SlowerTL objects & space expands FTL.

IMO it is contortionism.

It is much simpler to accept directly what we are observing. It is consistent with the fundamental axiom of Relativity, that SOL is constant for any FOR.

 

* I think it does not forbid the fact. "In the language of special relativity, a tachyon is a particle with space-like four-momentum and imaginary proper time. A tachyon is constrained to the space-like portion of the energy-momentum graph. Therefore, it cannot slow down to subluminal speeds." from wiki http://en.wikipedia.org/wiki/Tachyon

Edited June 19, 2010 by michel123456
Consecutive posts merged.

[Spyman]

My point is: we are actually observing galaxies going away from us at FTL velocities, and they are redshifted.

Distant objects are though to be receding from us FTL by calculating their velocities using a formula for cosmological redshift on the redshift we observe. If you don't think space is expanding and that the objects are actually speeding away from us through space FTL then you will have to use formulas for relativistic doppler to calculate their speed with the observed redshift, which will result in a speed value that is lower than c.

 

If you don't accept that the models for expanding space is true then you can't use their math as proof of objects moving FTL either.

 

Redshifts are attributable to three different physical effects. The first discovered was the Doppler effect, familiar in the changes in the apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles; an observed redshift due to the Doppler effect occurs whenever a light source moves away from an observer. Cosmological redshift is seen due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase of their distance from Earth. Finally, gravitational redshifts are a relativistic effect observed in electromagnetic radiation moving out of gravitational fields.

http://en.wikipedia.org/wiki/Redshift

 

 

It is much simpler to accept directly what we are observing.

Slightly offtopic but if we assume that cosmological expansion is false and that all distant objects we can observe actually are moving away from us through space, then we can conclude that the Universe seems to have a center and amazingly the Earth is placed in the core very close to it.

 

Similar I can go outside, look up on the sky and conclude that by simple direct observation the Sun is orbiting Earth.

 

In physical cosmology, the Copernican principle, named after Nicolaus Copernicus, states that the Earth is not in a central, specially favored position. More recently, the principle has been generalized to the relativistic concept that humans are not privileged observers of the universe.

 

Measurements of the effects of the cosmic microwave background radiation in the dynamics of distant astrophysical systems in 2000 proved the Copernican principle on a cosmological scale. The radiation that pervades the universe was demonstrably warmer at earlier times. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the universe is experiencing a metric expansion.

http://en.wikipedia.org/wiki/Copernican_principle

[Sisyphus]

Is this a misunderstanding?

 

Yes. I was saying that the relativistic redshift from objects moving faster than light is nonsensical. For objects moving towards you, the blueshift (and hence the energy of the photons) approaches infinity as the relative velocity of the object approaches infinity. For an object moving away, the redshift approaches infinity, and the energy of the photons approaches zero. For FTL objects, the results are simply imaginary.

 

My point is: we are actually observing galaxies going away from us at FTL velocities, and they are redshifted. Because our Theory forbids the fact*, we have another explanation which is "space itself expands", which IMO is not an easy explanation.

 

Spyman's last post explains why this is incorrect. We do not directly observe galaxies moving away from us at FTL velocities. We observe redshift. This redshift interpreted as relativistic doppler effects yields velocities slower than C. However, calculated taking into account cosmic expansion, you can get FTL results. If you deny cosmic expansion, then there's no way to interpret what you are seeing as FTL.

[michel123456]

(...) If you don't accept that the models for expanding space is true then you can't use their math as proof of objects moving FTL either.(...)

True.

 

Slightly offtopic but if we assume that cosmological expansion is false and that all distant objects we can observe actually are moving away from us through space, then we can conclude that the Universe seems to have a center and amazingly the Earth is placed in the core very close to it. (...)

 

Offtopic: there are other available explanations that don't put necesseraly the Earth at the centre. We discussed that in another thread where acceleration was to only needed input in order to get the same resulting expansion.

 

Besides (really offtopic), I am maybe the only one here to believe that we are not only "nowhere" in space, but also "nowhere" in time, something that the Copernican principle does not take into consideration (I call that the Extended Copernican Principle).

[Spyman]

Offtopic: there are other available explanations that don't put necesseraly the Earth at the centre.

No, if distant objects are moving through space then we can measure their trajectories and find out if their paths coincide. If all objects paths coincide at one single point then that would seem to be a center of the Universe. By current observation, that all distant objects are receding from us in all directions of the skye, we can conclude that the center is very close to Earth.

 

But there is no center of the Universe inside space according to possible explanations of our observations.

 

Your "other available explanations" are not valid accepted models by the scientific community.

 

 

We discussed that in another thread where acceleration was to only needed input in order to get the same resulting expansion.

If you want to make a new model and put your own trust in it then thats fine, but if you want others to belive in it, then it has to be better than the existing one, either by simpler and easier math that gives the same results that the existing one or it must give results that better fits observation. In the latter case you also need to find a way to distinguish which model that is correct and then prove it by predictions and observations.

 

IMHO, your accelerating model seems to be misapplied math and are either not able to give predictions on its own or does result with redshifts that does NOT fit observation.

 

I tried to understand and point out flaws but you didn't want to listen so I finally gave up on the dead horse.

Edited June 22, 2010 by Spyman
Spelling

[michel123456]

I tried to understand and point out flaws but you didn't want to listen so I finally gave up on the dead horse.

 

I know you always try to understand and I really appreciate your interest. I always listen, but I am stubborn.

Never mind, end of offtopics.

Back to the first question:

"Can we see it ?"

 

Here an extract from another post of yours in another thread:

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be moving away from each other at a speed greater than the speed of light (meaning that one cannot be observed from the other).

(emphasis mine)

 

That is exactly the point I want to discuss. The "meaning that one cannot be observed from the other" is IMO completely misplaced.

[Johanluus]

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be moving away from each other at a speed greater than the speed of light (meaning that one cannot be observed from the other).

 

There is a very fundamental difference in the two statments highlighted above when they are isolated , which leads to confusion for many people including myself.

 

The first implies that the OBJECT has a momentum relative to a force and energy, measured by both observers.

 

The second first implies that the OBJECT is being "moved" by the expansion of space , rather like two boats on the ocean drifiting apart due to ocean currents.

 

Not quite the momentum energy required in the former statment.Where a reference inhertial frame is required. Hence special relativity.

[Spyman]

That is exactly the point I want to discuss. The "meaning that one cannot be observed from the other" is IMO completely misplaced.

Your personal opinion of cosmological phenomenas can't be debated, you need to put forth some scientific arguments...

[michel123456]

Your personal opinion of cosmological phenomenas can't be debated, you need to put forth some scientific arguments...

 

 

Bis repetita placent.

 

Post # 18

 

"Well, from the beginning of this thread, I am speaking about "objects moving away from one another" and not about "the amount of space in between them increasing", following this simple reasoning:

 

Lets take an object moving away (not receding) at speed v < C.

 

This objects emits light that we can see.

The naive concept says that the photons that will reach us must have speed C-v= F. (1)

But Relativity shows it is completely wrong. Photons are reaching us at C. Always.

 

Now, when we take another hypothetical object moving away at v > C, we are used to make the same substraction, putting C-v= F (2) and obtaining negative F, meaning that the photons will go away from us instead of reaching us.

 

IMO it is exactly the same error, because equation (1) is the same erroneous with equation (2). We simply cannot make the naive substraction, it is wrong. In any case, the photons will reach us at C." (text mine, emphasis mine)

 

And the hypothetical FTL object is observable. and redshifted.

 

Because if we assume the contrary, I would like you to explain me how it would be possible to have photons influenced by the relative speed of the (hypothetical) object.

Edited June 23, 2010 by michel123456

[swansont]

You've already indicated (post 41) you don't want to discuss this in terms of relativity. How then shall we proceed?