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Different Slices of Now Observable from Earth?

  1. Nov 14, 2011 #1
    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.
    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?
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  3. Nov 14, 2011 #2


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    Re: Different "Slices of Now" Observable from Earth?

    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.
  4. Nov 14, 2011 #3


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    Re: Different "Slices of Now" Observable from Earth?

    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.
  5. Nov 15, 2011 #4
    Re: Different "Slices of Now" Observable from Earth?

    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.
  6. Nov 16, 2011 #5
    Re: Different "Slices of Now" Observable from Earth?

    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.
  7. Nov 16, 2011 #6


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    Re: Different "Slices of Now" Observable from Earth?

    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.
    Last edited: Nov 16, 2011
  8. Nov 18, 2011 #7
    Re: Different "Slices of Now" Observable from Earth?

    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.)
  9. Nov 18, 2011 #8


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    Re: Different "Slices of Now" Observable from Earth?

    From another thread...
  10. Nov 18, 2011 #9
    Re: Different "Slices of Now" Observable from Earth?

    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.
  11. Nov 18, 2011 #10
    Re: Different "Slices of Now" Observable from Earth?

    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?
  12. Nov 19, 2011 #11
    Re: Different "Slices of Now" Observable from Earth?

    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.
  13. Nov 19, 2011 #12
    Re: Different "Slices of Now" Observable from Earth?

    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.
  14. Nov 19, 2011 #13
    Re: Different "Slices of Now" Observable from Earth?

    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?
  15. Nov 19, 2011 #14
    Re: Different "Slices of Now" Observable from Earth?

    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?
  16. Nov 20, 2011 #15
    Re: Different "Slices of Now" Observable from Earth?

    [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.
    Last edited: Nov 20, 2011
  17. Nov 20, 2011 #16
    Re: Different "Slices of Now" Observable from Earth?

    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.
  18. Nov 20, 2011 #17
    Re: Different "Slices of Now" Observable from Earth?

    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.]
    Last edited: Nov 20, 2011
  19. Nov 20, 2011 #18
    Re: Different "Slices of Now" Observable from Earth?

    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:

    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.


    When we try to talk about "now" we are also clearly talking about the nature of our own consciousness.
  20. Nov 20, 2011 #19
    Re: Different "Slices of Now" Observable from Earth?

    "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."
  21. Nov 20, 2011 #20
    Re: Different "Slices of Now" Observable from Earth?

    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.
    Last edited: Nov 20, 2011
  22. Nov 20, 2011 #21
    Re: Different "Slices of Now" Observable from Earth?

    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, lets 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.
  23. Nov 21, 2011 #22
    Re: Different "Slices of Now" Observable from Earth?

    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.
  24. Nov 21, 2011 #23
    Re: Different "Slices of Now" Observable from Earth?

    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.
  25. Nov 21, 2011 #24
    Re: Different "Slices of Now" Observable from Earth?

    A complementary perspective on a portion of post #22

  26. Nov 21, 2011 #25


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    Re: Different "Slices of Now" Observable from Earth?

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