Different Slices of Now Observable from Earth?

[Vanadium90]

Different "Slices of Now" Observable from Earth?

Brian Greene's excellent series continued last week on PBS with "The Illusion of Time" in which he showed with graphics how different "nows" occur depending on the motion of an observer. Using the example of a space alien 10 billion light years from earth, its "now" depends on whether the alien is stationary,moving toward, or moving away from the earth. If the alien had a powerful telescope, it can control its "now" by hopping on a bicycle and moving away from the Earth to look 200 years into our past, remain stationary and see our present, or move toward the Earth and see our future.(Remember the alien is looking at things 10 billion years old, it is not seeing things that didn't already happen on earth). Although the velocity of the alien is slow, the great distance causes an even small change in the angle of a slice of now to produce an effect of centuries in time of its "now".

My question is why don't we see this effect on Earth when we look at distant galaxies?

The observers on Earth are not all moving at the same velocity and this should produce the effect.
How so?
Example:Suppose a galaxy is 10 million light years from Earth on the same plane as the Earth's equator. A telescope on the equator is moving 1000 miles per hour (rotational speed of the Earth at the equator) toward the galaxy.
A telescope at 45 degrees N. Latitude and the same longitude is moving at 700 miles per hour toward the same galaxy(1000 mph x cos 45) for a difference of 300 miles per hour.
Calculation:
Using the formula in Fabric of the Cosmos p.540, the difference in "nows" due to the motion of an observer is :velocity x distance/c^2, where c is the speed of light.
In my example the distance is 10 million light years or 6 x 10^19 miles. The speed of light is 186,000 miles per second.
For velocity, actually a difference in velocity, (1000 mph-700 mph) = 300 miles per hour or 0.083 miles per second.
In other words the "slice of now" for the telescope on the equator is different(ahead in time) than the" slice of now" for the telescope at 45 degrees N. Latitude.
The result I get is: 0.083miles/sec x 6 x 10^19 miles/ (186,000)^2.= 4.4 years.
This answer does not seem correct: A telescope on the equator would see events(e.g. the start of a supernova) 4.4 years before the telescope at 45 degrees N. Latitude if the motion of the telescopes is toward the galaxy!
I know the motion of the Earth is more complicated than rotation on axis(to extend the analogy of Brian Greene,it would be similar to the difference of "nows" of two bugs walking with different speeds on the head of the cycling alien), but shouldn't the difference in velocities of the two scopes produce different "nows"? Could somebody help me with this?

 

[Janus]

The difference in "nows" does not effect what the telescope sees, it effects when each telescope decides when the light left and thus what time it is "now" at the point the light left.

For example. let's say that the telescopes have are moving relative to each other and both are looking at the same distant clock. At the exact moment that the two telescopes are next to each other, they read what the clock says. The same light enters both telescopes and so they both read the same time on the clock. They then can each calculate how much time has passed for that light to reach them and how much time has ticked away on the clock since then. They will arrive at different answers. What time it is "now" at the clock is different for them. It really is a different time "now" at the clock for each telescope.

 

[PAllen]

This is all assuming you accept that it is meaningful to talk about distant 'now'. Clearly, you have no idea about even the current existence of something you are seeing 10 million year old image of.

I prefer to avoid any concept of distant now, and focus,even for arguments sake, on past simultaneity. This is less mysterious, and also shows the limitation of the point of view. You say that the two telescopes disagree on how long ago, for them, the light was emitted: 10 million +/- 4 years. Much less significant that way (in fact, no chance that +/- 4 years is statistically significant; such distances are not known to adequate precision). Further, even that is true only if you assume the telescopes had constant relative motion for 10 million years; otherwise the answer would be different.

The alternative point of view (which I hold) is that distant simultaneity is somewhere between completely meaningless or subject to arbitrary convention.

 

[PatrickPowers]

Brian Greene's excellent series continued last week on PBS with "The Illusion of Time" in which he showed with graphics how different "nows" occur depending on the motion of an observer. Using the example of a space alien 10 billion light years from earth, its "now" depends on whether the alien is stationary,moving toward, or moving away from the earth. If the alien had a powerful telescope, it can control its "now" by hopping on a bicycle and moving away from the Earth to look 200 years into our past, remain stationary and see our present, or move toward the Earth and see our future.(Remember the alien is looking at things 10 billion years old, it is not seeing things that didn't already happen on earth). Although the velocity of the alien is slow, the great distance causes an even small change in the angle of a slice of now to produce an effect of centuries in time of its "now".

My question is why don't we see this effect on Earth when we look at distant galaxies?

The observers on Earth are not all moving at the same velocity and this should produce the effect.
How so?
Example:Suppose a galaxy is 10 million light years from Earth on the same plane as the Earth's equator. A telescope on the equator is moving 1000 miles per hour (rotational speed of the Earth at the equator) toward the galaxy.
A telescope at 45 degrees N. Latitude and the same longitude is moving at 700 miles per hour toward the same galaxy(1000 mph x cos 45) for a difference of 300 miles per hour.
Calculation:
Using the formula in Fabric of the Cosmos p.540, the difference in "nows" due to the motion of an observer is :velocity x distance/c^2, where c is the speed of light.
In my example the distance is 10 million light years or 6 x 10^19 miles. The speed of light is 186,000 miles per second.
For velocity, actually a difference in velocity, (1000 mph-700 mph) = 300 miles per hour or 0.083 miles per second.
In other words the "slice of now" for the telescope on the equator is different(ahead in time) than the" slice of now" for the telescope at 45 degrees N. Latitude.
The result I get is: 0.083miles/sec x 6 x 10^19 miles/ (186,000)^2.= 4.4 years.
This answer does not seem correct: A telescope on the equator would see events(e.g. the start of a supernova) 4.4 years before the telescope at 45 degrees N. Latitude if the motion of the telescopes is toward the galaxy!
I know the motion of the Earth is more complicated than rotation on axis(to extend the analogy of Brian Greene,it would be similar to the difference of "nows" of two bugs walking with different speeds on the head of the cycling alien), but shouldn't the difference in velocities of the two scopes produce different "nows"? Could somebody help me with this?

I've been thinking along these same lines. It makes sense, but surely we would have heard of this effect. It would be possible to view a supernova several times. We can't, so that's not the way the U works. It is an experimental fact that moving toward a source doesn't seem to give you information any sooner than it does to a close neighbor who is at rest.

The whole "now slice" thing is just a calculated number, so IMO it gives no support at all to the idea that the future already exists. Maybe yes, maybe no.

 

[Vanadium90]

Thank you PatrickPowers. I have been wondering about this since I read Fabric of the Cosmos two years ago.
The way I understand the alien in Brian Greene's PBS Nova series is that the alien is on a planet 10 billion light years from earth. Our planet and his planet are not moving relative to one another. Ten billion years from today the alien sets up a telescope and looks at the Earth and sees a giant digital clock display: NOV 16, 2011. His companion alien,also looking through a telescope, riding away from Earth on a bicycle at a speed of 5 miles/hour looks at the same clock and tells him that he is wrong. The clock reads NOV 15, 1931 for the alien on a bicycle.
No requirement is necessary about constant relative motion to achieve their 10 billion light year separation.
And I don't see any difference between aliens on bicycles and telescopes at different latitudes.

 

[PAllen]

Thank you PatrickPowers. I have been wondering about this since I read Fabric of the Cosmos two years ago.
The way I understand the alien in Brian Greene's PBS Nova series is that the alien is on a planet 10 billion light years from earth. Our planet and his planet are not moving relative to one another. Ten billion years from today the alien sets up a telescope and looks at the Earth and sees a giant digital clock display: NOV 16, 2011. His companion alien,also looking through a telescope, riding away from Earth on a bicycle at a speed of 5 miles/hour looks at the same clock and tells him that he is wrong. The clock reads NOV 15, 1931 for the alien on a bicycle.
No requirement is necessary about constant relative motion to achieve their 10 billion light year separation.
And I don't see any difference between aliens on bicycles and telescopes at different latitudes.

No, you misunderstand Greene's claim. They both see the same reading on the clock, but the issue is what time of their's do they think the image comes from. What is on the image is irrelevant for this. Using a certain set of conventions and distance measurement, they come to different conclusions about when they think the identical image they both see was actually sent.

Their past history does matters or not depending on assumptions about how measurements were made, in particular, how distance was measured (that is the only basis for knowing how long ago to interpret the signal came from). However, there are conventions that don't depend on past history (e.g. a distance based on luminosity and red shift). So, for simplicity, forget about the past history issue.

Having come to different conclusions about when the image is from, they come to different conclusions about what time they think the clock would read 'now', assuming it still exists and nothing has changed in 10 billion years.

 

[Vanadium90]

No, you misunderstand Greene's claim. They both see the same reading on the clock, but the issue is what time of their's do they think the image comes from. What is on the image is irrelevant for this. Using a certain set of conventions and distance measurement, they come to different conclusions about when they think the identical image they both see was actually sent.

The following is from the transcript from the Nova series The Illusion of Time:

BRIAN GREENE: To get a feel for the bizarre effect this can have, imagine an alien, here, in a galaxy 10-billion light years from Earth, and way over there, on Earth, the guy at the gas station. Now, if the two are sitting still, not moving in relation to one other, their clocks tick off time at the same rate, and so they share the same now slices, which cut straight across the loaf. But watch what happens if the alien hops on his bike and rides directly away from Earth.

Since motion slows the passage time, their clocks will no longer tick off time at the same rate. And if their clocks no longer agree, their now slices will no longer agree either.

The alien's now slice cuts across the loaf differently. It's angled towards the past. Since the alien is biking at a leisurely pace, his slice is angled to the past by only a miniscule amount. But across such a vast distance, that tiny angle results in a huge difference in time. So what the alien would find on his angled now slice—he considers as happening right now, on Earth—no longer includes our friend at the gas station, or even 40 years earlier when our friend was a baby.

Amazingly, the alien's now slice has swept back through more than 200 years of Earth history and now includes events we consider part of the distant past, like Beethoven finishing his 5th Symphony: 1804 to 1808.

DAVID KAISER: Even at a relatively slow speed we can have, actually, tremendous disagreements on our labeling of "now," what happens at the same time, if we're spread out far enough in space.

BRIAN GREENE: And if that's not strange enough, the direction you move makes a difference, too. Watch what happens when the alien turns around and bikes toward Earth. The alien's new "now slice" is angled to…toward the future, and so it includes events that won't happen on Earth for 200 years: perhaps our friend's great-great-great granddaughter teleporting from Paris to New York.


From this transcript it is clear to me that both aliens do NOT see the same reading on the clock( in this case the stationary alien sees the man at the gas station, the moving alien sees Beethoven.)

 

[DrGreg]

From this transcript it is clear to me that both aliens do NOT see the same reading on the clock( in this case the stationary alien sees the man at the gas station, the moving alien sees Beethoven.)

From another thread...

If you check the quotation..., the word "see" was never used. This isn't about what someone would see with their eyes, it's about what someone "considers as happening right now". The alien would have to wait 10 billion years to actually see the events on Earth that are being considered, and would then conclude the events being viewed happened 10 billion years earlier.

 

[Zentrails]

The alternative point of view (which I hold) is that distant simultaneity is somewhere between completely meaningless or subject to arbitrary convention.

I wouldn't call that an alternative point of view, seeing how that is pretty much what Einstein had to say about the concept of "simultaneity."

"Now" and "the present" are simply abstract human constructs that exist only in our imaginations.

 

[chaszz]

Originally Posted by PAllen:
"The alternative point of view (which I hold) is that distant simultaneity is somewhere between completely meaningless or subject to arbitrary convention."

Zentrails:
I wouldn't call that an alternative point of view, seeing how that is pretty much what Einstein had to say about the concept of "simultaneity."

"Now" and "the present" are simply abstract human constructs that exist only in our imaginations.

I hope I'm not being inappropriate in bringing something from the quantum world in here, but I think it is useful in this discussion of what "now" is or isn't. Particles which become entangled and then are separated by large distances have been shown to communicate with each other many thousands of times faster than the speed of light. Say an entangled particle was removed from its mate by a slower-than-light spaceship traveling centuries to another star system. Then the descendants of the original scientists cause the earth-bound particle to move in a certain way. The distant particle is found, by the descendants of those who traveled with it, to move the same way. When it reacts to a movement by its earth-bound mate would it not be acting in a "now" shared by both particles?

Einstein believed this entanglement could not exist but it was later proven that it can. Does this affect his contention that "now" has little meaning in widely separated spaces? Or rather is this an example of the often-cited conundrum that quantum mechanics and relativity cannot be reconciled with each other?

 

[PatrickPowers]

The following is from the transcript from the Nova series The Illusion of Time:

BRIAN GREENE: To get a feel for the bizarre effect this can have, imagine an alien, here, in a galaxy 10-billion light years from Earth, and way over there, on Earth, the guy at the gas station. Now, if the two are sitting still, not moving in relation to one other, their clocks tick off time at the same rate, and so they share the same now slices, which cut straight across the loaf. But watch what happens if the alien hops on his bike and rides directly away from Earth.

Since motion slows the passage time, their clocks will no longer tick off time at the same rate. And if their clocks no longer agree, their now slices will no longer agree either.

The alien's now slice cuts across the loaf differently. It's angled towards the past. Since the alien is biking at a leisurely pace, his slice is angled to the past by only a miniscule amount. But across such a vast distance, that tiny angle results in a huge difference in time. So what the alien would find on his angled now slice—he considers as happening right now, on Earth—no longer includes our friend at the gas station, or even 40 years earlier when our friend was a baby.

Amazingly, the alien's now slice has swept back through more than 200 years of Earth history and now includes events we consider part of the distant past, like Beethoven finishing his 5th Symphony: 1804 to 1808.

DAVID KAISER: Even at a relatively slow speed we can have, actually, tremendous disagreements on our labeling of "now," what happens at the same time, if we're spread out far enough in space.

BRIAN GREENE: And if that's not strange enough, the direction you move makes a difference, too. Watch what happens when the alien turns around and bikes toward Earth. The alien's new "now slice" is angled to…toward the future, and so it includes events that won't happen on Earth for 200 years: perhaps our friend's great-great-great granddaughter teleporting from Paris to New York.


From this transcript it is clear to me that both aliens do NOT see the same reading on the clock( in this case the stationary alien sees the man at the gas station, the moving alien sees Beethoven.)

No no no. If they each have a telescope they will both see almost exactly the same moment 10 billion years ago no matter how fast or slow they are moving.

This "now slice" thing is entirely imaginary with no physical meaning, as far as we know.

 

[PatrickPowers]

I hope I'm not being inappropriate in bringing something from the quantum world in here, but I think it is useful in this discussion of what "now" is or isn't. Particles which become entangled and then are separated by large distances have been shown to communicate with each other many thousands of times faster than the speed of light. Say an entangled particle was removed from its mate by a slower-than-light spaceship traveling centuries to another star system. Then the descendants of the original scientists cause the earth-bound particle to move in a certain way. The distant particle is found, by the descendants of those who traveled with it, to move the same way. When it reacts to a movement by its earth-bound mate would it not be acting in a "now" shared by both particles?

Einstein believed this entanglement could not exist but it was later proven that it can. Does this affect his contention that "now" has little meaning in widely separated spaces? Or rather is this an example of the often-cited conundrum that quantum mechanics and relativity cannot be reconciled with each other?

As for the entangled particles and their state, that's a mystery. There are other threads about that. It is hard to propose how it works because there is very little experimental evidence to go on, other than that it occurs.

As to Einstein and "now", General Relativity shows that the concepts of time and distance are quite slippery and the simple universal now idea of the past doesn't work. If there is a "now" then it lies outside of almost all of the physics we know.

As for QM and GR, I dunno.

 

[Zentrails]

As for the entangled particles and their state, that's a mystery. There are other threads about that. It is hard to propose how it works because there is very little experimental evidence to go on, other than that it occurs.

As to Einstein and "now", General Relativity shows that the concepts of time and distance are quite slippery and the simple universal now idea of the past doesn't work. If there is a "now" then it lies outside of almost all of the physics we know.

As for QM and GR, I dunno.

Exactly, entanglement is a great mystery and there is no such thing as "now" in physics.
Even the Greeks understood that.
That's one of the reason Newton/Liebniz expanded on Greek ideas to invent calculus.

You have dx, dy, dz, & dt;
not "space" at a "particular time" = "the present" although that is the way beginning physics is often taught even at the college level.
That's a good thing.
It's one of the many paradoxes that makes physics endlessly fascinating.

Some of the Greek philosophers had an amazingly good intuitive grasp of physics/chemistry.
I wonder if at least some of that was passed down to them by the Minoans?

 

[chaszz]

...Some of the Greek philosophers had an amazingly good intuitive grasp of physics/chemistry. I wonder if at least some of that was passed down to them by the Minoans?

The ancient Greeks invented democracy, drama, lyric poetry, the syllogism, philosophy, history as a literary form, geometry, a fundamental steam engine (Hero's engine), naturalistic sculpture and painting, among other things. They accomplished most of these things without much if any influence from other civilizations. Is there any indication or evidence that the Minoans influenced them in natural science?

 

[Q-reeus]

No no no. If they each have a telescope they will both see almost exactly the same moment 10 billion years ago no matter how fast or slow they are moving...

[STRIKE]Correct, but to add somewhat to PAllen's remarks in #3 that hit the nail on the head here: "Further, even that is true only if you assume the telescopes had constant relative motion for 10 million years; otherwise the answer would be different." Actually it would be 10 billion years, but that was no doubt just a typo[/STRIKE].[Oops - my mistake there; a different scenario - confused by OP going from Greene's 10 billion ly to a 10 million ly scenario. The latter is purely a relative time dilation matter] Anyway, Greene's gee whiz example is no more profound than saying that signals move at a finite speed. The 'magical' difference of ~ 200 years of history perceived by the bicycling alien amounts to no more than, after 10 billion years of steady riding in a straight line (that would be magical), he/she/it has moved 200 light years away from a reference 'stationary' alien. So naturally the signals received differ by 200 years (with some tiny frequency redshift thrown in). Profound? Amazing? I think not.

And it's not limited to light. A far more achievable analogue would be 'hearing the past' differently. There's someone chopping wood in the forest. Light being so much faster than sound, a distant observer/listener notes a certain delay between sighting axe-fall and hearing the 'chop'. Someone else in an aircraft moving away/toward the axeman, but at the sane instant location as the stationary observer, will receive the 'chop' sound later/sooner - purely on the basis of how further/closer to the source at the time the 'chop' sonic hits. No need to think in terms of sonic 'light cones' or 'world lines' to get that, although to jazz it all up and sell a book, that sort of embellishment would help.

 

[chaszz]

[STRIKE]Correct, but to add somewhat to PAllen's remarks in #3 that hit the nail on the head here: "Further, even that is true only if you assume the telescopes had constant relative motion for 10 million years; otherwise the answer would be different." Actually it would be 10 billion years, but that was no doubt just a typo[/STRIKE].[Oops - my mistake there; a different scenario - confused by OP going from Greene's 10 billion ly to a 10 million ly scenario. The latter is purely a relative time dilation matter] Anyway, Greene's gee whiz example is no more profound than saying that signals move at a finite speed. The 'magical' difference of ~ 200 years of history perceived by the bicycling alien amounts to no more than, after 10 billion years of steady riding in a straight line (that would be magical), he/she/it has moved 200 light years away from a reference 'stationary' alien. So naturally the signals received differ by 200 years (with some tiny frequency redshift thrown in). Profound? Amazing? I think not.

And it's not limited to light. A far more achievable analogue would be 'hearing the past' differently. There's someone chopping wood in the forest. Light being so much faster than sound, a distant observer/listener notes a certain delay between sighting axe-fall and hearing the 'chop'. Someone else in an aircraft moving away/toward the axeman, but at the sane instant location as the stationary observer, will receive the 'chop' sound later/sooner - purely on the basis of how further/closer to the source at the time the 'chop' sonic hits. No need to think in terms of sonic 'light cones' or 'world lines' to get that, although to jazz it all up and sell a book, that sort of embellishment would help.

The TV program did not mention an observer. Therefore the reception of information at the other end billions of years later is not part of the issue. The issue was the simultaneity of the bicycling alien's frame of reference with the frame of reference of the Earth human. Not taking any position on this, or on the meaning or non-meaning of simultaneity in widely separated frames, but just pointing out that bringing in an observer and a time-lag complicates the issue above and beyond what was said on the program.

 

[Q-reeus]

The TV program did not mention an observer. Therefore the reception of information at the other end billions of years later is not part of the issue. The issue was the simultaneity of the bicycling alien's frame of reference with the frame of reference of the Earth human. Not taking any position on this, or on the meaning or non-meaning of simultaneity in widely separated frames, but just pointing out that bringing in an observer and a time-lag complicates the issue above and beyond what was said on the program.

Must admit to not having watched the TV program - just what was presented in #1 basically - which contained:

"Using the example of a space alien 10 billion light years from earth, its "now" depends on whether the alien is stationary,moving toward, or moving away from the earth. If the alien had a powerful telescope, it can control its "now" by hopping on a bicycle and moving away from the Earth to look 200 years into our past, remain stationary and see our present, or move toward the Earth and see our future.(Remember the alien is looking at things 10 billion years old, it is not seeing things that didn't already happen on earth)."

That direction of motion is the key, straight away is telling me this has nothing to do with time dilation (or is quite separate from it), but rather simply a signal delay effect owing to accumulated distance. How else to sensibly interpret the dependence on direction of relative motion? Have never read anything by Greene, but understand he is a highly regarded physicist as well as popularizer of physics. Just seemed here though he was being somewhat misleading as to what's really going on. But I stand ready to be corrected.
[I suppose one could argue SR doppler shift is what is implied - receding motion leads to redshift = 'slower observed time-rate' and vice versa for advancing motion. But this is still just an inferrential affair where the cyclist 'projects' that rate into the distant past/future in order to arrive at a ~ 200 yr differential. Otherwise, the meaninglessness of 'distant simultaneity' has been answered earlier by others here.]

 

[Zentrails]

The ancient Greeks invented democracy, drama, lyric poetry, the syllogism, philosophy, history as a literary form, geometry, a fundamental steam engine (Hero's engine), naturalistic sculpture and painting, among other things. They accomplished most of these things without much if any influence from other civilizations. Is there any indication or evidence that the Minoans influenced them in natural science?

You left out a lot of stuff, especially Archimedes' and Lucretius' (a Roman who wrote about many ideas he credited to the Greeks) contributions.

There's certainly no doubt that the Greeks "thought" about these things, but usually didn't actually do anything practical with that knowledge, which is very puzzling. Very little written documents still exist from that period, only later Roman copies, making it very tough to figure them out.

There's no doubt also that the Greek civilization started AFTER the Minoan civilization died out.
There's no doubt that the Minoan civilization was every bit as advanced as the Greeks - they controlled most of the coastal regions of the Mediterranean and had at least some contact with the Old Kingdom of Egypt.

What's in doubt is what happened during and after the Thera volcanic explosion:
https://en.wikipedia.org/wiki/Minoan_eruption

Nobody knows where the Greeks came from and why they were so advanced in their thinking, but it is highly probable that at least some Greeks were survivors of the dying Minoan civilization. There is also limited and controversial evidence that their earliest written language was based at least partly on Minoan writing.

As excavations on Santorini continue, I think some of these questions will probably be answered.

Some people believe that the very nature of consciousness (among educated people) itself changed fundamentally at around this period of time.

https://en.wikipedia.org/wiki/Bicameralism_(psychology)

When we try to talk about "now" we are also clearly talking about the nature of our own consciousness.

 

[Zentrails]

Must admit to not having watched the TV program - just what was presented in #1 basically - which contained:

That direction of motion is the key, straight away is telling me this has nothing to do with time dilation (or is quite separate from it), but rather simply a signal delay effect owing to accumulated distance. How else to sensibly interpret the dependence on direction of relative motion? Have never read anything by Greene, but understand he is a highly regarded physicist as well as popularizer of physics. Just seemed here though he was being somewhat misleading as to what's really going on. But I stand ready to be corrected.
[I suppose one could argue SR doppler shift is what is implied - receding motion leads to redshift = 'slower observed time-rate' and vice versa for advancing motion. But this is still just an inferrential affair where the cyclist 'projects' that rate into the distant past/future in order to arrive at a ~ 200 yr differential. Otherwise, the meaninglessness of 'distant simultaneity' has been answered earlier by others here.]

"SR doppler shifts" are due to very simple Euclidean geometry.
"GR doppler shifts" are due to not so simple non-Euclidean geometry.

Neither have anything whatsoever to do with "time dilation."

 

[Q-reeus]

"SR doppler shifts" are due to very simple Euclidean geometry.
"GR doppler shifts" are due to not so simple non-Euclidean geometry.

Neither have anything whatsoever to do with "time dilation."

What makes you think that? In particular, how do you explain GR 'Doppler' other than as relative time dilation. Are you of the 'tired light' school of thought - energy is 'drained' from an escaping photon? While it could be considered a matter of taste as to whether one interprets it as the clock (oscillator) running slower 'down there' vs energy loss in climbing a gravitational potential well, redshift of continuous monochromatic radiation can only properly be reconciled with differing clock rates. You can test that out with your own gedanken experiment. As for SR case, transverse Doppler shift of an orbiting body (gravity excluded here) just happens to coincide with lower clock rate (time dilation) - a coincidence? Hardly.

 

[Vanadium90]

Dr. Greg. Although Brian Greene does not use the word "see", I inferred it since the same slice of now means the alien would see the event(Beethoven or the gas station) if it had a large enough telescope. After all it is 10 billion light years away. Seeing this distance is just a technicality.

So in my example let's make it something the alien can see that is closer:A galaxy 10 million light years away.
Also instead of Beethoven, let's make the event a start of a supernova easily seen with a small telescope.
Since 10 million light years is 1000 times closer and since this time effect is linear with distance, the alien cycling toward the galaxy should see the start of the supernova 200 years/1000 or 0.2 years before the stationary alien. So if a supernova exploded in the galaxy exactly 10 million years ago,the stationary alien would see it 10 million years later.
The alien cycling toward the galaxy would see it 9,999,999.8 years later. The cycling alien would see it sooner.

 

[Zentrails]

What makes you think that? In particular, how do you explain GR 'Doppler' other than as relative time dilation. Are you of the 'tired light' school of thought - energy is 'drained' from an escaping photon? While it could be considered a matter of taste as to whether one interprets it as the clock (oscillator) running slower 'down there' vs energy loss in climbing a gravitational potential well, redshift of continuous monochromatic radiation can only properly be reconciled with differing clock rates. You can test that out with your own gedanken experiment. As for SR case, transverse Doppler shift of an orbiting body (gravity excluded here) just happens to coincide with lower clock rate (time dilation) - a coincidence? Hardly.

Here's what WIKI says:
"When using special relativity's relativistic Doppler relationships to calculate the change in energy and frequency (assuming no complicating route-dependent effects such as those caused by the frame-dragging of rotating black holes), then the Gravitational redshift and blueshift frequency ratios are the inverse of each other, suggesting that the "seen" frequency-change corresponds to the actual difference in underlying clockrate. Route-dependence due to frame-dragging may come into play, which would invalidate this idea and complicate the process of determining globally agreed differences in underlying clock rate.

While gravitational redshift refers to what is seen, gravitational time dilation refers to what is deduced to be "really" happening once observational effects are taken into account."

There are "coincidences" to be found all though out nature that mean nothing.
Explain to me how "time dilation" can give you a blue shift.

 

[Q-reeus]

Here's what WIKI says:
"When using special relativity's relativistic Doppler relationships to calculate the change in energy and frequency (assuming no complicating route-dependent effects such as those caused by the frame-dragging of rotating black holes), then the Gravitational redshift and blueshift frequency ratios are the inverse of each other, suggesting that the "seen" frequency-change corresponds to the actual difference in underlying clockrate...
While gravitational redshift refers to what is seen, gravitational time dilation refers to what is deduced to be "really" happening once observational effects are taken into account."

There are "coincidences" to be found all though out nature that mean nothing.
Explain to me how "time dilation" can give you a blue shift.

I don't get your problem here - as it says in the bit you quoted from Wiki: "...Gravitational redshift and blueshift frequency ratios are the inverse of each other, suggesting that the "seen" frequency-change corresponds to the actual difference in underlying clockrate."
It's just a matter of relative rates. The observer further out inteprets the clock further in as time dilated (running slow = redshifted). Conversely, an observer further in interprets the clock further out as time contracted (running fast = blue-shifted). Substitute light source = oscillator for 'clock', and there it all is.

 

[Naty1]

A complementary perspective on a portion of post #22

Once it became accepted that light is an electromagnetic wave, it was clear that the frequency of light should not change from place to place, since waves from a source with a fixed frequency keep the same frequency everywhere. One way around this conclusion would be if time itself was altered—if clocks at different points had different rates. This was precisely Einstein's conclusion in 1911.

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

 

[DrGreg]

So if a supernova exploded in the galaxy exactly 10 million years ago,the stationary alien would see it 10 million years later.
The alien cycling toward the galaxy would see it 9,999,999.8 years later.

Correct.

The cycling alien would see it sooner.

Incorrect. Both aliens see the explosion at exactly the same time, but they disagree when it happened.

 

[Zentrails]

I don't get your problem here - as it says in the bit you quoted from Wiki: "...Gravitational redshift and blueshift frequency ratios are the inverse of each other, suggesting that the "seen" frequency-change corresponds to the actual difference in underlying clockrate."
It's just a matter of relative rates. The observer further out inteprets the clock further in as time dilated (running slow = redshifted). Conversely, an observer further in interprets the clock further out as time contracted (running fast = blue-shifted). Substitute light source = oscillator for 'clock', and there it all is.

Sorry, the problem is the use of the word "Doppler."
There are three types of true "Doppler" effects, Universe expansion source-observer velocity differentials, ordinary source-observer velocity differentials, and bending of EM by gravitational space-time distortions.

There is indeed a gravitational redshift, which is seen in things like white dwarfs.
It is certainly due to large distortions in space-time of some sort.

That is NOT a Doppler shift. That's my point.

But there is indeed a Doppler shift that can be caused by gravity, too.
That famous radio wave experiment showed that pulses take longer and longer to reflect back when the beam travels closer and closer to the sun.
Since the speed of light is constant in a vacuum, no matter what, that can only mean one thing - the path is longer, i.e. an apparent velocity differential.

That time delay causes a Doppler shift. The delay is longer than would be predicted by GR geometry alone, but that's irrelevant, it's still a Doppler shift caused by geometry. It is exceedingly tiny but not non-existent. That's because it appears as if there is relative movement, even though there isn't - the source and observer don't intrinsically "know" that the beam has passed through a portion of distorted space-time.

In fact, Einstein's first prediction of how much the sun would bend star light was off by a factor of two. His geometry was off. His geometry changed until if fit the facts. He threw in the "cosmological constant" for the same reason, by his own admission.

Gravitational lensing also causes a Doppler shift.
It's exceedingly tiny as well, but it's there.
Neither one of these two examples of gravitational Doppler shifts can possibly be blue-shifting because neither case can result in a shorter apparent path.

Einstein's thought experiments were interesting, but there haven't been too many more since GR that have resulted in advanced theories.
In fact, Einstein spent a lot of the rest of his life trying to think up experiments that disproved QM.

What does any of this have to do with "slices of now"?
A lot, IMO. What is the minimum possible Doppler shift?
Plank time? If so, that's your "slice of now."
But what if you add time dilation, then what?

Yes, I am being an ash. LOL
Heck, I could have brought up the quasar red-shift problem to become a double ash.

 

[Zentrails]

Correct.Incorrect. Both aliens see the explosion at exactly the same time, but they disagree when it happened.

Ah, yes, the uncertainty principle. LOL

 

[pervect]

No, it has nothing to do with the uncertanity principle

Let us imagine we have two different aliens, one with a huge telescope who is stationary, another with a huge telescope on a bicycle who is moving.

Furthermore, we'll assume that the stationary alien and the moving alien are very very very close to one another.

Then there are three events here: EVENTS are points on a space-time diagram.

1) The event at which the light from the explosion is emitted
2) The event at which the light is received by the stationary alien
3) The event at which the light is received by the moving alien.

It might be helpful to draw a space time diagram, and put the events on it.

There are also four non-zero coordinates of interest. While events are points on the space-time diagram, coordinates are labels that are used to describe the points. The moving alien and the stationary alien each have a coordinate system. But the coordinate systems are different.

The four coordinates of interest are

t1_moving and d1_moving, which are the time and distance coordinates (labels) that the moving observer assigns to event #1, the explosion.

t1_stationary and d1_stationary, which are the time and distance coordinates (labels) that the stationary observer assigns to event #1, the explosion.

Event #1 is the same event, but the labels assigned to it are different for the two observers.

Event's #2 and #3 are different events, but we'll assume that the aliens are "on top of one another" so that they can be considered to be essentially the same event.
Then we can say that event #2 and event #3 both have coordinates of t2_moving = t2_stationary = t3_moving = t3_stationary = 0. Similarly d2_moving = d2_starionary = d3_moving = d3_stationary = 0. So that's about 8 zero coordinates .

The coordinate's or labels that are given to event #1 are considerably different, for the moving observer and the stationary observer. Note that the event of seeing the light (event #2) has coordinates of (0,0) for both the moving and stationary observer. The coordinates assigned to the occurence of event #1 are not to be confused with the actual process of the reception of the light signal, but require some evaluation to turn the raw sensory data (seeing) into a consistent description of the universe (the coordinates assigned to the event).

What is being said , however, doesn't have anything to do with seeing, and bringing seeing into the picture is confusing things.

What is being said is that each alien, the moving and the stationary alien, have a concept of "now".

When an event occurs "now", you do not see it immediately, you see it after a lightspeed delay. It's wrong to think of "now" as the events that you SEE at the same time. "now" is the set of events that you assign as occurring "at the same time".

The point is that the set of events on the space-time diagram, which can be drawn as a line on the space-time diagram, that represents "now", is a different set of events ( a different line on the space-time diagram) for the moving alien and the stationary alien.

 

[Q-reeus]

Sorry, the problem is the use of the word "Doppler."
There are three types of true "Doppler" effects, Universe expansion source-observer velocity differentials, ordinary source-observer velocity differentials, and bending of EM by gravitational space-time distortions.
There is indeed a gravitational redshift, which is seen in things like white dwarfs.
It is certainly due to large distortions in space-time of some sort.
That is NOT a Doppler shift. That's my point.

OK have no objection on that. Your mentioning of cosmic expansion is relevant for an accurate picture of how it would work out in the real universe: http://en.wikipedia.org/wiki/Metric_expansion_of_space, but we are just looking at distance and speed as factors here (in OP's query), or gravity as it has entered the discussion.

But there is indeed a Doppler shift that can be caused by gravity, too.
That famous radio wave experiment showed that pulses take longer and longer to reflect back when the beam travels closer and closer to the sun.
Since the speed of light is constant in a vacuum, no matter what, that can only mean one thing - the path is longer, i.e. an apparent velocity differential.

Light speed is only constant in vacuo as locally measured. Choosing SC's (standard Schwarzschild coordinates), the coordinate speed of light is less than the proper (locally measured) value c by a factor (1-GM/r) in the radial direction, and by a factor (1-GM/r)^1/2 in the tangent direction to a central mass. The split between distance contraction and time dilation as contributions to reduced c is considered by many as somewhat arbitrary. Using ISC's (Isotropic SC's), coordinate c is less than proper c by a different, isotropic factor, and the split between length contraction and time dilation will also be taken as different. In both cases though, overall signal delay is due to both gravitationally effected distance and time, not just distance.

That time delay causes a Doppler shift. The delay is longer than would be predicted by GR geometry alone, but that's irrelevant, it's still a Doppler shift caused by geometry. It is exceedingly tiny but not non-existent. That's because it appears as if there is relative movement, even though there isn't - the source and observer don't intrinsically "know" that the beam has passed through a portion of distorted space-time.

There is both angular deflection and time delay, but no appreciable 'Doppler shift' (frequency change) provided light source and observation points are both far from the source of gravity. Only a significant difference in gravitational potential between source and receiver will give redshift/blueshift.

Gravitational lensing also causes a Doppler shift.
It's exceedingly tiny as well, but it's there.
Neither one of these two examples of gravitational Doppler shifts can possibly be blue-shifting because neither case can result in a shorter apparent path.

Covered by previous comment - Shapiro type time delay does not imply frequency change.

 

[Q-reeus]

Originally Posted by Vanadium90:
"So if a supernova exploded in the galaxy exactly 10 million years ago,the stationary alien would see it 10 million years later.
The alien cycling toward the galaxy would see it 9,999,999.8 years later."

Correct.
Originally Posted by Vanadium90: "The cycling alien would see it sooner."

Incorrect. Both aliens see the explosion at exactly the same time, but they disagree when it happened.

The assumption here being that stationary and cycling aliens are coincident when the light arrives. Is that any different to my alternate 'physical' explanation in #17, where the cycling alien observes a motional doppler shift, infers a slower time rate for events, and projects that rate for 10 million years in order to arrive at a different 'now' than the stationary alien? I try to give a handle on how the sense of a different now is arrived at by the actual observers. In #1, two different effects are presented. The first - Greene's scenario, implies a dominant ~ v/c factor (for v<<c) at work (motional Doppler -> spatial/temporal separation 'over time'), the latter one implies a far smaller ~ v²/c factor (transverse Doppler = time dilation) http://en.wikipedia.org/wiki/Relativistic_Doppler_effect. Both are generally always present, but the former is overwhelmingly greater for v << c. Disagree?

 

[Zentrails]

Ah, yes, the uncertainty principle. LOL

That was a feeble attempt at a joke.

Here's an interesting use of Doppler shifts involving gravity:
http://dawn.jpl.nasa.gov/DawnClassrooms/3_instrument/pdfs/Gravity_Measurement_TG.pdf

 

[Zentrails]

OK have no objection on that.

Covered by previous comment - Shapiro type time delay does not imply frequency change.

Yes, of course, but it can cause an apparent velocity differential under the right conditions which gives a Doppler shift.
I could propose a thought experiment to show that, but it's getting a little too much off topic, probably.
It is under conditions where the source and the observer are at exactly the same gravity potential.
I think I was wrong about gravitational lensing possibly causing a Doppler shift, I'm still thinking that one out.

 

[Q-reeus]

Yes, of course, but it can cause an apparent velocity differential under the right conditions which gives a Doppler shift.
I could propose a thought experiment to show that, but it's getting a little too much off topic, probably.
It is under conditions where the source and the observer are at exactly the same gravity potential.

Ah - you may be right about a 'blip' type Doppler shift if this is referring to a dynamical aspect, where one is bouncing radar off say an orbiting planet as it passes behind the sun, or where a massive object passes across the line of sight between 'fixed' source and receiver. That should give a very small and brief kind of 'FM modulation' that symmetrically cancels as per a full sinusoidal cycle (modulation cycle that is - not one cycle of emitted radiation!). It would not exist though if source, receiver, and gravitating mass are in a fixed relation. So was that your idea?

 

[Zentrails]

Ah - you may be right about a 'blip' type Doppler shift if this is referring to a dynamical aspect, where one is bouncing radar off say an orbiting planet as it passes behind the sun, or where a massive object passes across the line of sight between 'fixed' source and receiver. That should give a very small and brief kind of 'FM modulation' that symmetrically cancels as per a full sinusoidal cycle (modulation cycle that is - not one cycle of emitted radiation!). It would not exist though if source, receiver, and gravitating mass are in a fixed relation. So was that your idea?

OK, no one is yelling at me for being off topic so here goes:
Tell me if you can spot any logical errors in my thought experiment.

You have two spaceships traveling far apart form each other, but at the same velocity/direction.

They each use sophisticated celestial navigation to remain exactly the same distance from each other at all times (pretending they are in "flat" space at all times during the experiment).

Ship One continuously (no pulses) broadcasts an ordinary, spherically radiating microwave signal set at an extremely precise fixed frequency as monochromatic as possible.

Ship Two (the observer) receives the signal and measures the frequency with extreme precision.

They both pass by the sun perfectly tangentially, with the sun in between, so eventually the signal is blocked.

As the ships approach the sun, the apparent distance between the ships should increase at a rate which is mostly (but not a simple calculation as it is in "flat" space) proportional to the tangential velocity of the two ships with respect to the sun because the signal now has to pass through a region of space where the signal follows geodesic lines.

The two ships have to be far enough apart at all times from the sun to be in "flat" space and at exactly the same respective gravitation potential, to eliminate any "ordinary" gravitational red-shift.

If that rate of change of apparent distance is reasonably constant, there should be a Doppler shift in the received signal which remains about the same until the sun blocks the signal.

As they regain the signal after passing the sun, the Doppler shift should still be there exactly the same as before they passed the sun.

Once the two ships get far enough away for the sun to be in "flat" space again, the Doppler shift disappears.

The key is that the ships HAVE to use celestial navigation at all times to remain the exact same distance from each other, ignoring GR space-time distortion that the radar signal passes through.

If the apparent distance change includes an apparent acceleration, then the Doppler shift should also change as the ships approach and depart the area near the sun.

The experiment then can be repeated, with Ship Two now merely reflecting the signal back to Ship One. Now, Ship One is both the source and observer. The observed Doppler shift should be exactly twice as large, no matter if there is just an apparent constant velocity OR acceleration.

Of course, the Doppler shift will be tiny, probably unmeasurable even by the finest interferometer techniques available presently. I'm also assuming that any possible gravitational lensing will be far lower in intensity than the apparent Doppler effect and that the observer ship can completely eliminate the effect of any microwave signals coming from the Sun during the experiment. That could be the fly in the ointment.

As far as relating this experiment to the thread topic, I think it is safe to assume that both ships would experience the "same now" as an abstract construct, while their instruments are simultaneously telling them otherwise.

ha, ha, get it? simultaneously telling them they are not in the "same now." LOL

 

[Q-reeus]

They both pass by the sun perfectly tangentially, with the sun in between, so eventually the signal is blocked.

This is equivalent to my second guess. If the radar beam is very narrow it will be blocked, otherwise diffraction ensures some 'gets by'.

If that rate of change of apparent distance is reasonably constant, there should be a Doppler shift in the received signal which remains about the same until the sun blocks the signal.

To the extent rate of change is constant, yes. But I would expect a rather peaky affair.

As they regain the signal after passing the sun, the Doppler shift should still be there exactly the same as before they passed the sun.

Not quite - sign of Doppler shift will reverse owing to reversed sense of rate of change of time delay. That's what was meant by 'full sinusoidal modulation cycle' - cancels overall. You will have to do your own searching on this, but in general it should be covered in theory of electro-optic phase modulation - in particular the transient modulation of the refractive index of an optical fiber element passing monochromatic light. Fascinating to consider the fine details. but yes, this is getting off topic.