Is Krauss looking at this right?

[tar]

Have you seen this talk from Lawrence Krauss? It's just slightly over an hour, and discusses this concept quite a bit.

 

 

 

In this video, Krauss, having the desire to be the first to know how the universe ends, describes what the universe will look like to a Milky Way civilization many billions of years in the future. He says that since the universe will be expanding at an accelerated rate, eventually everything outside the Milkyway will be out of sight. I disagree on general princple. It will just look a whole lot redder. Like the radio waves we view as the cosmic background radiation.

 

And although this might mean they are not "visible", being of such long wavelength as to be out of a humans visible light spectrum, the waves will certainly not be invisible to science, as he projects.

 

Regards, TAR2

 

Plus he ignores the records that all the previous civilizations might have left, for those "hundreds of billion of years in the future" scientists.

 

For instance, what if his video was passed down from parent to child, for a zillion generations?

 

P.S. I grabbed Inows post off the Philosophy thread, "How did everything really begin?".

Edited August 12, 2012 by tar

[dimreepr]

If universal expansion is accelerating, then the observable universe is constantly shrinking. The source of light, relative to us, is travelling away faster than the light it's emitting.

Edited August 12, 2012 by dimreepr

[tar]

If universal expansion is accelerating, then the observable universe is constantly shrinking. The source of light, relative to us, is travelling away faster than the light it's emitting.

 

dimreepr,

 

Well that is certainly understandable. But you are basically talking about "new" light being emitted from galaxy Z, right now. I am talking about the light that is already on its way from galaxy Z. This light, that is in transit, does not have a way to stop being on its way. It seems to me we should not be expecting galaxies to blink out of existence, but to become slower and slower, and dimmer and dimmer, at announcing their presence.

 

After all if we "see" a galaxy today, we are seeing it at a huge distance, as it "looked" local to it, a long time ago. The actual galaxy does not look like that now, local to it, it has evolved in the mean time. Stars have formed, stars have gone super nova, civilizations perhaps have emerged, and the distance between our galaxy and galaxy Z has grown in the mean time.

 

I am not sure that I understand the basis on which we consider galaxy Z currently existing within the observable universe, and what process we would see it go through as it "left" the observable universe...given the next trillion years to watch it. How could it EVER become invisible to us?

 

Where and how we see galaxy Z is not where and how it currently is. And we are not in possession of any actual way to ever know, at the same time, what is happening in Galaxy Z and what is happening in the Milky Way.

 

But I have a philosophical/conceptual problem in understanding what someone means by "the universe is currently expanding at rate Q". And I don't follow how one could determine the total number of particles in the observable universe without being able to judge as well, how many particles are leaving the observable universe every year. It seems particles would have to be leaving, for there to be a Krauss time, when the only particles left, are in the milky way.

 

And I know of no observation that would best be described as "seeing" a particle leave. What is outside our observable universe, must have "already" left. Everything else must still be visible, and we can look back, in all directions, to the time when the universe became transparent, and we can see every single particle, there is to see. Even if we see them as background microwave radiation. They never "blink out".

 

Regards, TAR2

 

Thought experiment:

 

A cesium clock with its digital readout facing us is launched away from the center of the MilkyWay at relativistic speeds. It is given a power source and guidance system that will keep it moving away from the center of the Galaxy at this constant speed. We have devised a way for the readout to be powerful enough to be seen from the distance the clock will obtain from the center of the Milky Way in 50 billion years of said travel. At some point the expansion of the universe will be a significant addition to its distance from us, so that it is receding from us, faster than the speed of light.

 

What time is it, at the clock in 50 billion years? What time do we read on the clock? (here on Earth, in 50 billion years).

Will the clock ever blink out of view? When and why?

 

It occurred to me that ALL radiation we witness is "leftover" radiation from an "earlier" universe. And quite close to "none" of the radiation we witness now is being emitted now. Even the stuff we see happening a foot away happened picoseconds earlier. That puts any observer quite unable to observe anything that is happening now, till later. And puts a rather high significance on the manner in which the whole universe announces its presence to us, all at once, now. Seems inappropriate to think that there could be a different kind of "all at once", than this one here and now that we have.

Edited August 12, 2012 by tar

[dimreepr]

Well that is certainly understandable. But you are basically talking about "new" light being emitted from galaxy Z, right now. I am talking about the light that is already on its way from galaxy Z. This light, that is in transit, does not have a way to stop being on its way. It seems to me we should not be expecting galaxies to blink out of existence, but to become slower and slower, and dimmer and dimmer, at announcing their presence.

 

After all if we "see" a galaxy today, we are seeing it at a huge distance, as it "looked" local to it, a long time ago. The actual galaxy does not look like that now, local to it, it has evolved in the mean time. Stars have formed, stars have gone super nova, civilizations perhaps have emerged, and the distance between our galaxy and galaxy Z has grown in the mean time.

 

I am not sure that I understand the basis on which we consider galaxy Z currently existing within the observable universe, and what process we would see it go through as it "left" the observable universe...given the next trillion years to watch it. How could it EVER become invisible to us?

 

Where and how we see galaxy Z is not where and how it currently is. And we are not in possession of any actual way to ever know, at the same time, what is happening in Galaxy Z and what is happening in the Milky Way.

 

But I have a philosophical/conceptual problem in understanding what someone means by "the universe is currently expanding at rate Q". And I don't follow how one could determine the total number of particles in the observable universe without being able to judge as well, how many particles are leaving the observable universe every year. It seems particles would have to be leaving, for there to be a Krauss time, when the only particles left, are in the milky way.

 

And I know of no observation that would best be described as "seeing" a particle leave. What is outside our observable universe, must have "already" left. Everything else must still be visible, and we can look back, in all directions, to the time when the universe became transparent, and we can see every single particle, there is to see. Even if we see them as background microwave radiation. They never "blink out".

 

 

It’s not about distance other than its relation the rate of acceleration, the universe will do so ad infinitum.

 

The light from these galaxies is constantly being updated and red shifted as the relativistic speed approaches c but once c is exceeded the updated light wouldn’t be able to catch us up however far the light wave is stretched.

I’m not sure the rate of expansion has, as of yet, exceeded c, perhaps one of the expert members could enlighten us.

 

 

Thought experiment:

 

A cesium clock with its digital readout facing us is launched away from the center of the MilkyWay at relativistic speeds. It is given a power source and guidance system that will keep it moving away from the center of the Galaxy at this constant speed. We have devised a way for the readout to be powerful enough to be seen from the distance the clock will obtain from the center of the Milky Way in 50 billion years of said travel. At some point the expansion of the universe will be a significant addition to its distance from us, so that it is receding from us, faster than the speed of light.

 

What time is it, at the clock in 50 billion years? What time do we read on the clock? (here on Earth, in 50 million years).

Will the clock ever blink out of view? When and why?

 

 

When you say relativistic speeds I assume you mean c, in which case, it will never be out of range so will never disappear, as the relativistic speed will never exceed c. However, if we take your thought experiment a stage further and place said clock on a planet in a galaxy outside our local group, at some point the relativistic speed will exceed c, at which time it would blink out of view.

 

I have no idea how this observation would manifest. I’m afraid my math isn’t up to calculating the time dilation involved. Just to clarify Krauss didn’t say 50 billion years he said 50 times the current age of the universe.

 

 

[tar]

dimreepr,

 

The light from these galaxies is constantly being updated and red shifted as the relativistic speed approaches c but once c is exceeded the updated light wouldnt be able to catch us up however far the light wave is stretched.

 

I have seen this before, but do not know what it means. If by "updated' light, you mean the next pulse eminating from the distant atom, that is in a star in Galaxy X that has just now sent out the previous pulse, I can imagine what you mean, that this pulse's chances, do not look very good, of ever reaching the Earth. But the previous pulse that happened just now, DOES have a chance of reaching Earth in whatever amount of Billions of years it will take to make the transit, and it will arrive at the Earth in whatever stretched and weakened state it arrives and that "next" pulse is lined up immediately behind and will be arriving here at the speed of light.

 

The pulse we saw just now, from Galaxy X, had a follower, and I see no reason to believe that there should come a time when we see a pulse from Galaxy X that does NOT have a follower.

 

And we can make the 50Billion 500Billion, I still have the same concern. I don't know how we are going suggest the blinkage out occurs on "this end".

 

 

Regards, TAR2

Edited August 12, 2012 by tar

[michel123456]

If Krauss is right, it is a good indication that he is wrong.

[tar]

If Krauss is right, it is a good indication that he is wrong.

 

Is that a yes or a no?

 

 

When you say relativistic speeds I assume you mean c, in which case, it will never be out of range so will never disappear, as the relativistic speed will never exceed c. However, if we take your thought experiment a stage further and place said clock on a planet in a galaxy outside our local group, at some point the relativistic speed will exceed c, at which time it would blink out of view.

 

 

dimreepr,

 

By relativistic speed I mean close enough to C for relativistic effects to be significant.

 

But I think you are wrong in your determination of when the galaxy will "blink out". When the galaxy reaches a recessional velocity greater than C, it is, at that moment many of billions of light years distant. We will not witness that moment until the light eminated from the galaxy reaches Earth many billions of years later. It certainly can't "blink out" here, at the same moment it's recessional velocity exceeds C. If there is a blink out witnessed here, it would have to be an event that occurred billions of years ago. And billions of years ago, the observable universe was a whole lot smaller than it is now, and not expanding as fast as it is now. And not so likely to have galaxies receeding at C plus velocities from each other.

 

Remember, we are just imagining what the universe must be doing now. What we see, is what it did before, and the deeper we look, the longer ago the event was.

 

We can't actually see those way distant events 'til they get here, way later.

 

Regards, TAR2

Edited August 16, 2012 by tar

[JMJones0424]

NOTE: The portion of the video pertinent to this discussion begins around

and lasts about two minutes.

Perhaps "blink out" is an inappropriate description.

 

I am unsure of the following, and would like to know if this is an accurate representation of what we'd expect to observe (admittedly over billions of years) or where I've gone wrong.

 

 

Assuming expansion continues, then in a finite amount of time, a galaxy that is now near the edge of our observable universe will eventually move outside of our observable universe. If you imagine a stream of photons emitted from that galaxy to us in a perfect vacuum, then every photon we receive is slightly more redshifted than the last, and it takes a slightly longer time for each successive photon to reach our telescopes. At some point, the time between photons will approach infinity, and by that time, the wavelength will have gotten so large that we can't feasibly detect the incoming photons anymore. So the far away galaxies won't necessarily disappear, but rather slowly fade away. I think this is similar to what we'd expect to observe from an object falling past the event horizon of a BH.

 

Krauss seems to claim as much at

by saying that evidence of the CMBR will "redshift away"

[StringJunky]

NOTE: The portion of the video pertinent to this discussion begins around

and lasts about two minutes.

Perhaps "blink out" is an inappropriate description.

 

I am unsure of the following, and would like to know if this is an accurate representation of what we'd expect to observe (admittedly over billions of years) or where I've gone wrong.

 

 

Assuming expansion continues, then in a finite amount of time, a galaxy that is now near the edge of our observable universe will eventually move outside of our observable universe. If you imagine a stream of photons emitted from that galaxy to us in a perfect vacuum, then every photon we receive is slightly more redshifted than the last, and it takes a slightly longer time for each successive photon to reach our telescopes. At some point, the time between photons will approach infinity, and by that time, the wavelength will have gotten so large that we can't feasibly detect the incoming photons anymore. So the far away galaxies won't necessarily disappear, but rather slowly fade away. I think this is similar to what we'd expect to observe from an object falling past the event horizon of a BH.

 

Krauss seems to claim as much at

by saying that evidence of the CMBR will "redshift away"

 

Here's a link to a paper he did with Scherrer that talks about this. I've linked to it in another recent thread so apologies if you've already read it.

[dimreepr]

Having read the paper I think the statement that’s most pertinent to this discussion is

 

 

“While objects will not be observed to cross the event horizon, light from them will be exponentially redshifted, so that within a time frame comparable to the longest lived main sequence stars all objects outside of our local cluster will truly become invisible.”

 

Krauss & Scherrer

 

 

As has been suggested my use of the term “blink out” is wrong, that however doesn’t mean the galaxies won’t disappear for a future observer.

 

 

 

 

[tar]

Here's a link to a paper he did with Scherrer that talks about this. I've linked to it in another recent thread so apologies if you've already read it.

 

StringJunky,

 

Well thankyou, that Essay certainly removed my "blinking out" concerns.

 

However, I am left with an inablility to consider the difference between the universe, the observable universe, and the universe we witness. I am not sure which ones are to be considered as happening now. And which ones a formula or recipe is referring to.

 

And I am having a hard time with "dropping out of the Hubble flow". What forces are so overpowering to strings of Galaxies, that are so ineffective against our local group?

 

And if the obseverable universe is currently here, and that which is outside our horizon is likewise currently here, but unobservable, then, when (if) we are an island "universe", it would be required that other similar islands are also currently existing in the universe, having similar characteristics of island nature with very little evidence of everything else. But these other islands must still exist in the universe, and such a situation, being the case, would in no way signal the end of the universe, but merely describe how it will look, to ANY island observer, when it gets to that state.

 

Several possible ways remain to allow for scientists in a local cluster civilization of sentient beings to determine the nature of the entire universe, even that which is out of sight and radiowave detection. The constant density of the dark energy field could turn out to provide a media upon which the rest of the universe is painted. Bearing in mind, all that we have discovered in the last hundred years, the potential knowledge of the universe attainable by a civilization or other sentient being that emerges in the meantime, in 100 billion years, is quite significant. Such a being, could easily have devised a method to detect photons with wavelengths in the billions of ly long range, and therefore NOT consider the source of such radiation "outside" the observable universe. But consider it the natural state of the universe. Much like we do now. Krauss' description of the fate of the universe, even if true, does not take it, all the way, to the end.

 

Regards, TAR2

 

Nor force a situation where the beginning of space and time is unreachable in thought and experiment.

[tar]

And the universe has come up with surprises before. Look at peanut butter cups. What field theory predicts THAT?

 

If the universe and the hubble flow created peanut butter cups, in only 13billion years, I would be surprised myself, if it just sat around for the next 100 billion just twittling it's opposable thumbs.

[md65536]

Such a being, could easily have devised a method to detect photons with wavelengths in the billions of ly long range, and therefore NOT consider the source of such radiation "outside" the observable universe.

 

No, I think that's a misunderstanding of this.

 

I also don't understand but I'll try to reason through this; I might be wrong.

 

 

Suppose a distant object is so far away and that all of the space between us and it is expanding, such that it is being separated from us faster than c. Then photons leaving that object now will never reach us, not after infinite time (assuming the expansion continues). There is nothing to see at any wavelength.

 

But consider a closer object that is visible. The space between us and it is expanding, and because there is always more intervening space, which itself will also be expanding, so the object will appear to be accelerating away from us. Thus the apparent speed of an object will be proportional to its distance. It will appear red-shifted because it is moving away from us, and it will get redder and redder because it is accelerating.

 

As it appears to approach a speed of c, its red-shifted light will approach a frequency of 0. Yes, any speed infinitesimally less than c will produce a frequency infinitesimally greater than 0 and is theoretically detectable. But once the separation speed reaches (and exceeds) c, there is no longer anything to detect, even theoretically.

 

 

So visible objects that are receding at speeds less than c are expected to recede faster, and redder, until they disappear at a separation speed of c and a color with zero energy. Any objects that have reached that speed are theoretically undetectable.

 

 

I guess the misconception might be that the frequency would approach 0 as the separation speed approaches infinity, but it hits 0 at c, while the separation speed keeps increasing beyond that.

Edited August 24, 2012 by md65536

[tar]

md65536,

 

Well, I am almost with you on the photon leaving that object now, in its theorectical inability to ever make up an ever increasing gap...except I have tried the ole ant on the rubberband thing often in my mind, and it seems to me, that still the ant will make progress enough to finally get to the other end, because the faster than ant recession speed of the pulled end of the band where it started, is not the same recession speed of the point on the rubberband where it is currently walking. Though the pulled end will continue to accelerate way past ant speed, if you fill the rubberband with ants walking toward the stationary end and allow a new ant to start the trip on the pulled end, right behind the previous ant, there is continually an increasing number of ants in transit, as the gap between the ants falling off the stationary arrival point increases. That is, there is always the next ant to arrive, who does not care about the faster than ant recession speed of the distant end, but cares only about the recession speed of the point in the band where its walking.

 

There is an ample supply of next ants that can complete the trip, regardless of whether the one that just stepped on the band, ever gets here.

 

Maybe.

 

And scientifically, I would expect that since by this time, the expansion of the universe is causing an object somewhere to be receeding from us at a faster than C speed, we should still expect in the future to receive all the photons inbetween in increasing intervals. What is the longest wavelength radio wave we currently recieve? What is the longest radiowave it is theoretically possible to receive? Does not seem to me, that there is a possible way for anything that ever was in sight, to get out of sight.

 

And of course, we can't see what the object is doing now. We never could. We have always seen what it did before.

 

Regards, TAR2

Edited August 24, 2012 by tar

[JMJones0424]

tar- please watch the two minutes starting at 50:50 and read the paper linked by StringJunky again.

 

Your ant analogy is incomplete as it does not include the increasing wavelength and decreasing intensity of the photons observed by a far future observer.

 

Not sure about the largest wavelength we can detect. I know we use VLF for communication to submarines, but the wavelength is so long (Freq=c/wavelength) that the transmission is inefficient due to antennas that are less than one wavelength long. 3kHz=100km. I'm pretty sure that we would be unable to detect anything from space at this low of a frequency, as our ionosphere would block it, but I suppose an array of satellites could conceivably detect it. Krauss's paper linked by StringJunky gives the plasma frequency of the interstellar medium of 1kHz, or 300km wavelength. Any wavelength larger than this would practically be unobservable. However, once the wavelength gets far larger, and surpasses even the width of the galaxy, then there's no hope of even theoretically detecting it.

[md65536]

Though the pulled end will continue to accelerate way past ant speed, if you fill the rubberband with ants walking toward the stationary end and allow a new ant to start the trip on the pulled end, right behind the previous ant, there is continually an increasing number of ants in transit, as the gap between the ants falling off the stationary arrival point increases. That is, there is always the next ant to arrive, who does not care about the faster than ant recession speed of the distant end, but cares only about the recession speed of the point in the band where its walking.

 

There is an ample supply of next ants that can complete the trip, regardless of whether the one that just stepped on the band, ever gets here.

 

 

Perhaps, but... it's not just the end of the rubber band that's accelerating away. It's the stretching across the entire rubber band that's accelerating. With the right conditions you should be able to have a situation where one ant never makes it, even though the ant immediately in front of it does, even if the ants are as crowded as possible.

 

Suppose that every 1m of a rubber band stretches to 1.1m every 1 second, and that an ant will have traveled .1 m along that stretched rubber band after 1 second. If you start with a 1m band, after 1 second, the band is 1.1m but the the ant is still 1m away from its destination. After 2 seconds the band is 1.21m but the ant is still 1m away from its destination. Repeat forever...

 

This apparently "stuck" ant happens in this case when the ant leaves the start of the rubber band at the moment that the start of the band is moving away from the destination at the same speed that the ant moves. The main thing is that I've defined the acceleration in terms of how fast a given length expands over time, and kept that as a fixed rate. So each "stretched" meter of rubber continues to stretch at the same rate, however there are ever more meters there that are stretching.

 

In the case of photons you wouldn't consider one "stuck", perhaps rather you'd say that the distance the photon must travel is expanding at a rate that is as fast as the rate at which the photon can cover that distance.

 

---

Edit: I thought about it some more and I think perhaps you're right.

 

Yes, if you fill up the rubber band with an infinite amount of ants you should continually be able to have ants stepping off the far end. At the same time, as per the above, you can have an ant "stuck" at a fixed distance from the far end, and all the ants behind it getting farther from the destination end.

 

A red shift would be accompanied by an apparent slowing of the observed object's clocks.

If the object were infinitely bright and you received an infinite density of photons from it, then you'd see it red-shift to a frequency of 0 while its clock appears to slow to a halt. You'd still have trouble seeing a finitely bright object because the photons that approach 0Hz approach zero energy.

 

So I guess it would fade to nothing over infinite time, however the image that you see of the object would approach its "last" visible state as it reaches its horizon.

 

So it's true that you won't be able to see the object in any of its states that occur after it passes a recession speed of c, but it's also true that you should be able to see the object in a state before that, forever or until you are no longer able to detect the increasingly redder and lower energy light from it.

Edited August 25, 2012 by md65536

[tar]

tar- please watch the two minutes starting at 50:50 and read the paper linked by StringJunky again.

 

Your ant analogy is incomplete as it does not include the increasing wavelength and decreasing intensity of the photons observed by a far future observer.

 

Not sure about the largest wavelength we can detect. I know we use VLF for communication to submarines, but the wavelength is so long (Freq=c/wavelength) that the transmission is inefficient due to antennas that are less than one wavelength long. 3kHz=100km. I'm pretty sure that we would be unable to detect anything from space at this low of a frequency, as our ionosphere would block it, but I suppose an array of satellites could conceivably detect it. Krauss's paper linked by StringJunky gives the plasma frequency of the interstellar medium of 1kHz, or 300km wavelength. Any wavelength larger than this would practically be unobservable. However, once the wavelength gets far larger, and surpasses even the width of the galaxy, then there's no hope of even theoretically detecting it.

JMJones0424,

 

I watched again a few times, did not read the paper again. Krauss' point, that I object to, on basic principle is that he said that at some point a distant galaxy would be receeding from us, at greater than C, and then said "it would disappear".

 

At the point at which a distant galaxy reaches a recessional speed from the Milky Way that is greater than C, it is also true that that galaxy is many billions of light years away, which means there is at least as many billions of lightyears supply of photons, that will continue to reach us, from the long existence that Galaxy had, while it was NOT receding from us faster than C. So whatever is the case, it is not the case that the Galaxy will disappear WHEN it exceeds a C recessional speed.

 

And I am still unsure as to why we don't consider the photons on their way, as existing in and of themselves. That is, that as a photon propagates its way toward Earth, it is no longer subject to the recessional speed of its source, but is only hampered in its journey, by the rather mild (in relationship to its own speed) expansion of the space through which it propagates. With this thought in mind, the photon is continually entering, existing in, and emerging again into an area of space, that is receding from us, due to expansion, at a lesser rate, than the area of space it was traveling through the moment before.

 

For an ant to get "stuck", the rubberband would have to be expanding locally, faster than ant speed. This is not the case with our observed situation. We can see stuff very far away with relatively minimal reddening. The space between is expanding at no where near © ant speed. No ant, will ever be stuck. We are not pulling the distant end fast enough for that.

 

Which leads me to believe, that not only is it the case that a Galaxy does not disappear at the moment it exceeds C recessional speed, but that there may be a case for its photons to continue to enter areas of space that have not yet exceeded C recessional speed. An thusly leaves a possibility that they will eventually arrive here.

 

Regards, TAR2

Edited August 27, 2012 by tar

[md65536]

For an ant to get "stuck", the rubberband would have to be expanding locally, faster than ant speed.

 

The ant doesn't get stuck locally. It appears stuck relative to its destination. It keeps moving along the (local) rubber band at the same rate, but the entire length of rubber band in front of it keeps expanding at the same rate that it is moving along it.

 

Which leads me to believe, that not only is it the case that a Galaxy does not disappear at the moment it exceeds C recessional speed, but that there may be a case for its photons to continue to enter areas of space that have not yet exceeded C recessional speed. An thusly leaves a possibility that they will eventually arrive here.

Yes, the receding galaxy really does disappear in ITS moment that it exceeds a recessional speed of c.

However, with the extreme apparent time dilation that comes with extreme red-shifting, that galactic moment will appear to pass very very slowly from Earth.

 

 

Suppose that an object is about to reach a recession rate of c relative to Earth. (I'm not sure how it would know because, to it, Earth would also be redshifted to nothingness, but say it has calculated the exact moment.) Suppose this object wants to signal the Earth, and it sends a huge blast of say 2^1,000,000 photons in the final second before hitting c.

 

Then after some many billions (or trillions or higher??? i dunno) of years, Earth receives the first of these photons, and say it detects them though they're terribly redshifted. Earth will receive the rest of the photons over the rest of time, at ever-decreasing rates. For the sake of argument, say that each year it receives half of the remaining photons. Well after a million years, it is receiving only one photon per year.

 

Yes, it can theoretically still receive photons from the object (and know that it existed) essentially forever, but those photons are all from the object before it reached a recession rate of c. You may say that the rate of expansion of intervening space is "mild" when less than c, but if the space between the object and Earth is expanding at a rate of c, then the photon will always have a continuously expanding space left to go no matter how many light years it travels (according to an observer on Earth).

 

Edit: I think I see a source of confusion...

If we think of the object as moving through space at near c, and that an emitted photon has a fixed distance to travel to Earth, then certainly the photon will make it. However, it's not a fixed distance, because the space between it and Earth are expanding at a rate of near c. In fact I think it's best to consider Earth and the distant object not to be moving at all, only that the space between them is expanding at a great rate. This is like saying "Now that you understand special relativity, ignore it! because this is a very different complication than the rules of SR in flat space."

 

The problems with detecting the object (as an image of it in a much much younger state), include:

- It is red-shifted to nothingness. The photons carry very little energy, and I guess require a very big antenna.

- The image is time-dilated to apparent stillness. Supposing that the rate of photon release from the object is constant, the rate you receive them is ever diminishing. It appears dimmer.

- The object appears to be moving away at nearly c, and appears smaller and smaller ie. you receive an ever-decreasing fraction of its light.

 

The object would become practically impossible to detect before it became theoretically impossible, but at some point the energy from it would be less than say random noise, or perhaps less than from vacuum energy or something?, in which case the object really would be theoretically impossible to detect.

Edited August 27, 2012 by md65536

[JMJones0424]

Krauss' point, that I object to, on basic principle is that he said that at some point a distant galaxy would be receeding from us, at greater than C, and then said "it would disappear".

I agree. Claiming that distant galaxies will at some time in the future "disappear" is a poor wording of the situation. I think that you need to take his statement in this video, which is geared towards the layman, with a grain of salt. The paper is more precise in spelling out his assertion.

 

At the point at which a distant galaxy reaches a recessional speed from the Milky Way that is greater than C, it is also true that that galaxy is many billions of light years away, which means there is at least as many billions of lightyears supply of photons, that will continue to reach us, from the long existence that Galaxy had, while it was NOT receding from us faster than C. So whatever is the case, it is not the case that the Galaxy will disappear WHEN it exceeds a C recessional speed.

Agreed.

 

And I am still unsure as to why we don't consider the photons on their way, as existing in and of themselves. That is, that as a photon propagates its way toward Earth, it is no longer subject to the recessional speed of its source, but is only hampered in its journey, by the rather mild (in relationship to its own speed) expansion of the space through which it propagates. With this thought in mind, the photon is continually entering, existing in, and emerging again into an area of space, that is receding from us, due to expansion, at a lesser rate, than the area of space it was traveling through the moment before.

Yes, agreed.

 

For an ant to get "stuck", the rubberband would have to be expanding locally, faster than ant speed. This is not the case with our observed situation. We can see stuff very far away with relatively minimal reddening. The space between is expanding at no where near © ant speed. No ant, will ever be stuck. We are not pulling the distant end fast enough for that.

Again, I would like to point out the limitations of your ant analogy. Not all ants on the rubberband are the same from a far future observer's perspective. At some time far in the future, the rate at which those ants reach their endpoint in our galaxy will be so low that far future observers will not receive them in the intensity required for those observers to notice that ants are headed their way. Even if they could, with some future technology, detect the incoming "ants" there is a point in the even farther distant future that the wavelength of those "ants" will be so large that no antenna, even one the size of the galaxy, would be able to detect them. This is the point that Krauss is making, and his point is more precisely spelled out in the paper. What you are objecting to is a description in a talk geared towards the layman and as such, the talk is not specifically correct, although his intended point (as I understand it) is correct.

[tar]

OK, I suppose the paper and the theory are right. And the laymen audience is an excuse.

 

But still I am not clear on the WHEN aspect of these occurences. I am not sure that a presentist and eternalist viewpoint can be held similtaneously. It seems to me, that one must see the one, and imagine the other, and know how the two are linked.

 

The one ant arriving with the next one due in a little longer period of time than the interval between the previous and the one, raises an interesting question in my mind, considering the wave/particle duality of light, which puts another crimp in the ant analogy. The ants are only particle, arriving all at once. Where with light, the question of WHEN it is arriving, might be a significant one, especially when considering a photon with a wavelength of 300,000 km. Does the photon arrive at the beginning of the second, the midpoint, the end, or what? A photon with a wavelength 400,000lys long, arrives WHEN? Is it potentially receivable at any time within that 400,000 years, at which point the entire wavefunction would collapse? Would the electric and magnetic fields along the whole length of the wave collapse instantly and similtaneously? In the eternalist, godlike sense of at the same time? As in our penpal across the galaxy whose reply always comes 800,000 years after our query...sorry, just overran my brains ability to shift between the eternal view and the present view. Can't complete my question.

 

Hard enough to comprehend light as a particle, if you also have to concieve of it, as having a "length"...geez...I've lost my bearings. OK, forget the ants. They don't have enough of the required properties.

 

But consider this. When we "view" a distant galaxy, we do it over time anyway. I read once that a telescope might collect one photon every few hours from a very dim and distant galaxy, and we record and accumulate these photons, and build an image, that is then presented as a color coded image we "just" took a picture of. As if that is the way we see it, all at once. In that regard, an image of a distant galaxy in a magizine, is NOT how we really see it, anyway. It is already dim enough, to be considered "invisible", if you don't allow for any interpolation, imagination, and compiling of information.

 

Considering that scientists 100,000,000,000 years in the future will be quite capable of noticing things that we have not even imagined yet, and considering they will undoubtably have the ability to record things and pass the information on, and "build" images as we do, I do not think they could possibly be less informed about the universe, including its beginnings, than Krauss is now. In fact, it seems more reasonable to expect the opposite of what Krauss proposes, and imagine a future scientist looking with pity upon the poor early 21st century physicist on Earth who was struggling to explain third stage latent effermosity as "dark matter".

 

Regards, TAR2

 

Any 4 growth zorknoid would see how silly those early galaxy scientists were.

 

Krauss knows how the universe ends? I don't think so, its only just begun.

Edited August 28, 2012 by tar

[md65536]

I was hoping someone more knowledgeable would reply but instead you get this!

The one ant arriving with the next one due in a little longer period of time than the interval between the previous and the one, raises an interesting question in my mind, considering the wave/particle duality of light, which puts another crimp in the ant analogy. The ants are only particle, arriving all at once. Where with light, the question of WHEN it is arriving, might be a significant one, especially when considering a photon with a wavelength of 300,000 km. Does the photon arrive at the beginning of the second, the midpoint, the end, or what? A photon with a wavelength 400,000lys long, arrives WHEN? Is it potentially receivable at any time within that 400,000 years, at which point the entire wavefunction would collapse? Would the electric and magnetic fields along the whole length of the wave collapse instantly and similtaneously? In the eternalist, godlike sense of at the same time? As in our penpal across the galaxy whose reply always comes 800,000 years after our query...sorry, just overran my brains ability to shift between the eternal view and the present view. Can't complete my question.

 

I think the ant analogy is fine, and it can be tweaked to match the details of cosmological inflation. I don't see any details that are more confusing than useful.

 

Wavelength is not the same as "length of a photon." A photon is a point particle (no width or radius). Anywhere a photon is measured, it is measured as a point. Any effect of a single photon on anything else is consistent with the photon behaving as a point particle (otherwise it would constitute a measurement of the photon as something other than a point particle).

 

Regardless of wavelength, it will be measured at a point and a time that is consistent with c. I guess the "phase" of a photon might be anywhere along the wavelength... but a photon of light with a wavelength of 400,000 LYs arrives 1 second after traveling 1 LS, or 100 B years after travelling 100 B LYs.

 

Considering that scientists 100,000,000,000 years in the future will be quite capable of noticing things that we have not even imagined yet, and considering they will undoubtably have the ability to record things and pass the information on, and "build" images as we do, I do not think they could possibly be less informed about the universe, including its beginnings, than Krauss is now. In fact, it seems more reasonable to expect the opposite of what Krauss proposes, and imagine a future scientist looking with pity upon the poor early 21st century physicist on Earth who was struggling to explain third stage latent effermosity as "dark matter".

 

Krauss said he was talking about some distant-future civilization that is not connected with us. It's so far in the future that presumably our civilization and all of its knowledge would be destroyed. He's speaking of that hypothetical situation. Then, even if they become exceedingly advanced, they still would not be able to detect evidence of some of what we can see now.

 

It's interesting to see how the expression of a belief turns into a barrier to understanding. Like, "That's what GR says, but I still believe there must be some way that it's wrong." There's less desire to understand things we don't believe in.

 

Yes, our scientific understanding will change and there's a lot we don't know. It's not the case that we expect far-away objects to become unobservable because we can't imagine any way for them to be visible --- it's that to the best of our knowledge and a few assumptions we can prove that they will be invisible. If Krauss is wrong, it would take more than just incomplete knowledge on our part. It would require that what we know is fundamentally wrong.

Edited August 31, 2012 by md65536

[tar]

I was hoping someone more knowledgeable would reply but instead you get this!

 

 

I think the ant analogy is fine, and it can be tweaked to match the details of cosmological inflation. I don't see any details that are more confusing than useful.

 

Wavelength is not the same as "length of a photon." A photon is a point particle (no width or radius). Anywhere a photon is measured, it is measured as a point. Any effect of a single photon on anything else is consistent with the photon behaving as a point particle (otherwise it would constitute a measurement of the photon as something other than a point particle).

 

Regardless of wavelength, it will be measured at a point and a time that is consistent with c. I guess the "phase" of a photon might be anywhere along the wavelength... but a photon of light with a wavelength of 400,000 LYs arrives 1 second after traveling 1 LS, or 100 B years after travelling 100 B LYs.

 

 

 

Krauss said he was talking about some distant-future civilization that is not connected with us. It's so far in the future that presumably our civilization and all of its knowledge would be destroyed. He's speaking of that hypothetical situation. Then, even if they become exceedingly advanced, they still would not be able to detect evidence of some of what we can see now.

 

It's interesting to see how the expression of a belief turns into a barrier to understanding. Like, "That's what GR says, but I still believe there must be some way that it's wrong." There's less desire to understand things we don't believe in.

 

Yes, our scientific understanding will change and there's a lot we don't know. It's not the case that we expect far-away objects to become unobservable because we can't imagine any way for them to be visible --- it's that to the best of our knowledge and a few assumptions we can prove that they will be invisible. If Krauss is wrong, it would take more than just incomplete knowledge on our part. It would require that what we know is fundamentally wrong.

 

md65536,

 

I do not think we are fundamentally wrong. Just that we have to be careful about taking a conceptual logical certainty about what has "to be" the case, here and now...later...as something that IS the case from some, one, overall perspective, now.

 

The problem with the ant analogy, is we can freeze the whole line of ants in time, at some "moment", where we can envision the ant about to step off the band, "at the same time", as the ant just climbing aboard, on the pulled end.

 

In what actual way is it possible or instructive to freeze both ends of the rubberband that we have mentally stretched from here to the distant galaxy, and consider both ends and all points between as being viewable, or comprehensible, as existing NOW? It has to be a god's eye view we take. It has to be a "mental" construction, an image, of the thing, that we know has to be true, because it adds back correctly, in retrospect, later, so we know it really had to have been the case in the first place.

 

But you cannot have your cake, and eat it to. If a godlike perspective is an actual thing, then god exists. It is not fair for Krauss to laugh at believers in a greater reality, at the same time as he claims ownership of knowledge of a greater reality. An understanding of "the way it is" from a perspective so great, that it does not require you and me, or even our civilization, or any record of our civilization, to conceptualize.

 

So in comes the "point" particle, that has to now exist, has to be "stretched" to exist all at once, simultaneously, at both ends of our galaxy, if it is to have a wavelength of 400,000lys. This is not like an ant.

 

Regards, TAR2

[md65536]

The problem with the ant analogy, is we can freeze the whole line of ants in time, at some "moment", where we can envision the ant about to step off the band, "at the same time", as the ant just climbing aboard, on the pulled end.

I don't see that as a terrible problem. Why can't you imagine doing the same thing for photons? Or on the other hand, how to do imagine practically freezing say a lightyears-long line of ants? Either it's a problem of imagination, and "freezing" photons or ants mentally are both possible, or it's a problem of practicality, and neither are possible.

 

Any analogy by definition will be dissimilar in some ways, so it can be picked apart by being too literal. Certainly it is possible to imagine a photon being transmitted at the same time as another one is received. Doppler effects (redshifting) complicate things, for example if the distant object is emitting a photon every second, we won't receive them at a rate of one per second. It's the same with the ants... they will also be Doppler shifted.

 

 

It is not fair for Krauss to laugh at believers in a greater reality, at the same time as he claims ownership of knowledge of a greater reality.

 

I think it is consistent, because in both cases it's about accepting what the evidence tells you, and not accepting ideas for which there's no evidence. Ironically the argument is that in the very distant future there may eventually be no evidence of a universe, and anyone who believes in only what the evidence says, will be wrong.

 

 

So in comes the "point" particle, that has to now exist, has to be "stretched" to exist all at once, simultaneously, at both ends of our galaxy, if it is to have a wavelength of 400,000lys. This is not like an ant.

No, the photon isn't stretched. It's the same with other types of waves... if you change the wavelength of ocean waves by changing the water depth, it doesn't stretch the water molecules. If you have corks bobbing on the surface it doesn't change the distance between corks (only the distance between wave crests). A photon is a particle, not the entire wave. If both wave-like and particle-like properties could be completely expressed in the same thing (a photon) there would be no wave-particle duality of light.

 

 

You can add a wave-like property to the ants... Suppose the rubber band is twisted so the ants spiral around as they walk. Sometimes they're on top of the band, and sometimes they're hanging upside down. If it's 400,000 LY between points where they're at the top of the rubber band, you wouldn't say that the ant was stretched to 400,000 LY. Or if the band is vibrating, or if the ant is breathing once per second or whatever... the wavelength is not the length of the ant.

[tar]

md65536,

 

But, the problem is, that the universe has this speed of light, limit, that it actually has no way of exceeding.

 

Conceptually, we break this law, when we freeze two distant ends into the same "moment". Not wrong, per se, to do this, but it requires a clear understanding of what is being conceived of as actual and what is being conceived of as conceptual.

 

Take the understanding that "in reality" the universe is currently a honeycomb like structure of strings of galaxies, demarking immense voids between, the whole structure expanding, and being 13.7billion years old.

This is not what we "see". It's what we figure it to be, based on what we actually see. In actuality, the further we look, the further away, the further back into the history of the universe, we peer, the further away from the actual condition or state of the "current" universe, we witness.

 

If the voids were expanding and compressing the stars and galaxies into the strings between, similar to a handful of suds, with the voids being air and the galaxies being the soap and water, locally, at the position of a particular soap molecule, any number of apparent observations could exist that would lead one to believe the universe must be in this or that state of expansion or contraction, or flow this way or that. These generalizations however, would not have to hold for every other current molecule of soap. There could be big bubbles and little, surface tension merging and splitting bubbles, stretching and compressions of many varied sorts, occurring, simultaneously, when viewed from a god like view, not bound by the speed of light, and able to conceive of the whole handful of suds at once.

 

As it stands though, we do not actually see what Alpha Centurie is doing currently. We see what it was doing last year, and we will see what it is doing now, next year. For something on the other side of the Milky Way, we see what it was doing 400,000 years ago, and we will see what it is currently doing in 400,000 years. How much of a pretense is it, to claim to know what the universe is doing now, and what it will be doing in 100,000,000,000 years, or what indeed, information a sentient being, existing in the future, might have to work with?

 

Regards, TAR2

Edited September 2, 2012 by tar

[md65536]

But, the problem is, that the universe has this speed of light, limit, that it actually has no way of exceeding.

 

Conceptually, we break this law, when we freeze two distant ends into the same "moment". Not wrong, per se, to do this, but it requires a clear understanding of what is being conceived of as actual and what is being conceived of as conceptual.

Yes, c must be taken into account... none of this would be the same without it. There would probably be no concept of "objects are unobservable beyond an event horizon" without a speed limit.

 

No law is broken, unless you're speaking of some "godlike perspective", but I never do (unless speculating), and I don't even know what it is (is it a perspective that can take on an arbitrary simultaneity, or is it a universal simultaneity??? If the latter, I don't think it has any connection to physical reality that would help in understanding GR and inflation).

 

Due to relativity of simultaneity, if you consider a "moment" across a distance, you are implying a specific frame of reference, because the events of that moment won't in general be simultaneous in other frames of reference.

 

As it stands though, we do not actually see what Alpha Centurie is doing currently. We see what it was doing last year, and we will see what it is doing now, next year. For something on the other side of the Milky Way, we see what it was doing 400,000 years ago, and we will see what it is currently doing in 400,000 years. How much of a pretense is it, to claim to know what the universe is doing now, and what it will be doing in 100,000,000,000 years, or what indeed, information a sentient being, existing in the future, might have to work with?

I don't think that "now" always means the same thing in cosmology (it may be observations made "now", or it may be a prediction based on such observations, depending).

 

The value of physics lies in its predictive power. Scientists have evidence of stuff going all the way back to just after the big bang, and they can extrapolate. Theories are evaluated based on their accuracy and precision, and we end up with degrees of confidence in various predictions. I don't think Krauss or anyone else is more confident in the ideas discussed in this thread, than say that the laws of gravity won't change in the foreseeable future. And even though the laws of gravity have extremely reliable predictive power, it's not completely certain that they'll never change. I don't see Krauss or any physicist speaking of absolute certainty in anything (I'm sure some do, but I'd call that a belief), and I wouldn't call it a pretense to make physical predictions based on GR and inflation, or the laws of gravity for that matter.