# The Illusion of Time and distant observers

• mjames

#### mjames

The "Illusion of Time" and distant observers

In Brian Greene's "Illusion of Time" on NOVA, he cited a case where a distant alien rides a bicycle away from us at a constant speed and then looks back at the earth. She will see a slice of spacetime in our past. Similarly, if she rides towards us at a constant speed, she will see a slice of spacetime in our future.

What happens if she accelerates instead of rides at a constant speed? I know this is the realm of GR so weird things might happen. But as she looks back at us, won't she see events on Earth going backwards as her instantaneous speed increases? In other words, won't she see entropy decreasing, not only on the Earth but also in our general area of the universe? And if she looks in her forward direction, won't she see events speeding up? But in both directions, if she looks out far enough will she still see the Big Bang?

I'm not sure time is any more "illusory" than mass or energy or forces...although at "times" it appears that way. Who would think "mass" is 99.9% empty space? or that most energy and mass is "dark"...unobservable? Or that there may be an infinite number of universes?

Similarly, if she rides towards us at a constant speed, she will see a slice of spacetime in our future.

I have a strong suspicion that's not what Brian Greene said...
I'll have to watch the program.

Because constant motion is relative, what would you suppose the observer on Earth sees of the distant alien?? Can the Earth observer see the alien's future before it occurs?

Whether the observer is stationary, moving towards or away from earth, that observer will not see into the future of the earth...nor their own future. All the light (information) that reaches the observer from Earth will have originated from the past relative to the observer. What the observer sees is red shift or blue shift due to the relative motion with the earth.

Also, recall that accelerated motion and gravity are indistinguishable ...so the Earth in your example can also be thought of as accelerating already. So gravity affects the passage of time

But in both directions, if she looks out far enough will she still see the Big Bang?

No one will be able to ever observe the big bang because for the first few hundred thousand years following the bang, the universe was opaque...photons could not reach an observer.

Here is a closely discussion:

Last edited:

Did he say that an alien 10 billion light years away can look at the earth? The Earth didn't exist 10 billion years ago so when she looks over here, she won't have any idea that our solar system, our earth, or humans would eventually come into existence.

When we look at something from this far away, we see it as it was a long time ago. If we travel toward or away from it, we will see it aging faster or slower as it was 10 billion years ago, not as it is now.

Same thing for the distant alien. If she wants to see our future, she will have to travel towards us at very close to the speed of light, at which point she will be able to watch the past 10 billion years of our history plus the next 10 billion years of our future unfold before her eyes in very rapid succession by the time she gets here. So for the first half her trip, she sees our history and for the second half she will see our future.

This is all assuming, like Brian is assuming, that we are analyzing this using Special Relativity from a Frame of Reference in which the Earth and the alien share a common rest frame. What Brian is really talking about is coordinate times in different Frames of Reference which don't help anyone instantly see distant things at earlier or later times.

I have thought in depth about things I've read in B. Greenes books.

The things he says are abstracts from the theory applied to imaginary scenarios. Often thinking in depth about those scenarios becomes even further removed from the actual physics.

Because of this site being (and thankfuly) such a stickler for accuracy, pretty much all of what Brian Greene says I take as physics poetry. And it's quite good, at producing a sense of wonderment and awe, but in NO sense is it a subsitute for study.

I feel that is all I'm left with after reading / watching something like his books / shows. A sense of wonderment, not understanding.

I'm glad to see this thread because I came here just now to open one just like it. That alien on a bicycle illustration has been haunting me ever since I saw it. Firstly, though a lot of the talk here has been about one actor "observing" the other, the program didn't say anything about observation. The situation was purely about simultaneity, about whether and how different events in different parts of space are simultaneous in time or not. I don't believe observing was spoken about. Also I don't believe anything was spoken about a lag due to the time it takes light to get from one place to the other. To check me, and to give those who didn't see the program an idea of what was said, here is a transcript of that section of the program ([http://www.pbs.org/wgbh/nova/physics/fabric-of-cosmos.html#fabric-time).(Look for 'transcript'). I will make another comment or two after it.

Begin transcript selection:

... BRIAN GREENE: To see what I mean, think of spacetime as a loaf of bread. Einstein realized that, just as there are different ways to cut a loaf of bread into individual slices, there are different ways to cut spacetime into individual "now" slices. That is, because motion affects the passage of time, someone who is moving will have a different conception of what's happening right now, and so they'll cut the loaf into different now slices. Their slices will be at a different angle.

DAVID KAISER: That person who's moving will, will tilt the knife, will be carving out these slices at a different angle. They won't be parallel to my slices 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.

Once we know that your now can be what I consider the past, or your now can be what I consider the future, and your now is every bit as valid as my now, then we learn that the past must be real, the future must be real. They could be your now. That means past, present, future…all equally real; they all exist.

SEAN CARROLL: If you believe the laws of physics, there's just as much reality to the future and the past as there is to the present moment...

End transcript selection

Now if this is valid and accurate, and not just relativity poetics, I would like to know a little more about how that tiny angle is magnified into 200 years. I mean, I get the general idea of spacetime as a melded whole where a line at an angle across space also cuts across time, sort of like an analytic geometry diagram with x as space and y as time...but it's not something I can quite wrap my brain around, the alien "now" in "my" far future, although I've been trying to for days on end. Can it possibly be true that if the alien bikes a few meters towards me that she hurtles so far into "my" future time just because of that tiny original angle on the diagram? And she has moved only a few seconds in her time but is now simultaneous with people on Earth not born yet? If I walk a few meters I am now in the far future of someone in another star system, who was alive a few of my seconds ago but is now long dead and buried? And if I switch direction and/or the alien switches direction, we are going back and forth in each others' pasts and futures like tennis balls?

Last edited:

It's nothing more than plugging numbers into the Lorentz Transform. Look at this statement:

Once we know that your now can be what I consider the past, or your now can be what I consider the future, and your now is every bit as valid as my now, then we learn that the past must be real, the future must be real. They could be your now. That means past, present, future…all equally real; they all exist.​
He's claiming that because my past is real and from far away, he can plug in numbers that cause events in my real past to pop out that means his process is real. Well, he can also go fast enough away from me and make events from 20 billion years ago pop out, so that means the universe is really older than scientists claim it is, correct?

Now if this is valid and accurate, and not just relativity poetics, I would like to know a little more about how that tiny angle is magnified into 200 years.

As others have remarked, just look at the Lorentz transforms. Specifically

$t' = \gamma \left(t - v\,x/c^2\right)$

where gamma = 1/sqrt(1-v^2/c^2)

see for instance http://en.wikipedia.org/w/index.php?title=Lorentz_transformation&oldid=459780578, or any SR textbook.

Convert all the units in the problem to meters, (use google to find that 1 light year = 9.4605284*10^15 meters) and then compute t', in the moving frame. We know that t=0, and that x= 10 billion light years = 9.4605284*10^25 meters. Now all we need is v, the velocity of the bicycle, which I didn't see in the transcript.

To get t' = 200 years you'd need v = 6 meters/second which is a reasonable number for a bicycle, (it's about 13 miles/hour if you like that system of units.)

You might also want to look at Einstein's "lightning and the train" thought experiment, see http://www.bartleby.com/173/9.html, or try any of the arious and sundry discussion threads about the topic.

Last edited:

In Brian Greene's "Illusion of Time" on NOVA, he cited a case where a distant alien rides a bicycle away from us at a constant speed and then looks back at the earth. She will see a slice of spacetime in our past. Similarly, if she rides towards us at a constant speed, she will see a slice of spacetime in our future.
If you check the quotation in post #5, 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.

This is where the light-cone comes in right, since the distance is so great and the light takes so long to get there, there is no, "now, over there", if now is an instant, dt = 0 then the only size of the universe that we could say surely exists to us is 0c = 0m big, it would just be sure that a now exists at every point, not what is happening or what could happen

time can never be future. It appears to be past due to distance and because light takes time. something may appear to be in the future if the distance is reduced quickly, but time can be no sooner than present if there is no distance from the reference point. All things happen now. Future is a fun word, but it doesn't exist; it's a concept. why do i bother...

Greene using an example with people and terms like right now, future and past is misleading.

And I'm pretty sure Greene typically wraps up that example with a comment how all this is non consequential to the observers. Such as Dr. Greg says it takes billions of years for the light to arrive to the observers location.

A light cone diagram shows that the space (or slice) of the "right now moment" is least significant to an observers perspective. Sometimes its even labeled "elsewhere" lol

I just find it a terrible way to explain special relativity, and shoves the person into the sometimes difficult conundrum of conscious perspective (light cone) of "now".

With billions of ly of distance between observers, what does a purely space like event at location 1 have to do with past or future of location 2?

I like what Wiki says
"When a space-like interval separates two events, not enough time passes between their occurrences for there to exist a causal relationship crossing the spatial distance between the two events at the speed of light or slower. Generally, the events are considered not to occur in each other's future or past."

That's why I think Greene's example is misleading. A good point could be that the right now slice is relative as a consequence of how slow your clock ticks due to relative motion. Instead Greenes example sometimes leaves one curious of the past / future comparison of right now slices (which are purely space like).

Last edited:

I think it's rather important to understand that the concept of "now" depends on the observer, and I hope Greene's example helps get this important point through to a wide audience, because it will help clear up significant points of confusion I see here on PF all the time. Confusion that seems to be difficult to dispel, I might add.

Its also useful to know that light cone diagrams, rather than the concept of "now", most directly relate to cause and effect. But regardless of whether or not causality and light cones are discussed, it's useful in and of itself to know that the concept of "now" is not universal.

Some of the emphasis I might not be wild about, (it's hard to tell secondhand, even from a section of transcript), but if it gets the main point home, it's a job well done.

I think it's rather important to understand that the concept of "now" depends on the observer, and I hope Greene's example helps get this important point through to a wide audience, because it will help clear up significant points of confusion I see here on PF all the time. Confusion that seems to be difficult to dispel, I might add.

I started off reading about physics with elegant universe. And by far he does more good in explaining these SR concepts than harm. But with Greene's fabric of the cosmos example...

As you mention some words are very important to understand in this context, such as "now". That still confuses me sometimes. It's difficult to absorb the constancy of c and the concept of now. Throw 10 billion light years in between events and discuss right now past and future and I think that's crossing the line towards misleading.

I found reading a book with less fanciful examples of SR while still being understandable for a layman was far more informative.

Now I can picture the diagram and see how x1 points higher up the ct axis of the other FoR with a velocity towards. I think that's physics, and the very creative 10 billion light year example is physics poetry. Taking the simple and very accurate graphical representation and adding all this "color" to the concept is "poetic" physics.

Last edited:

It's nothing more than plugging numbers into the Lorentz Transform. Look at this statement:

Once we know that your now can be what I consider the past, or your now can be what I consider the future, and your now is every bit as valid as my now, then we learn that the past must be real, the future must be real. They could be your now. That means past, present, future…all equally real; they all exist.​
He's claiming that because my past is real and from far away, he can plug in numbers that cause events in my real past to pop out that means his process is real. Well, he can also go fast enough away from me and make events from 20 billion years ago pop out, so that means the universe is really older than scientists claim it is, correct?

Graphically you can. As you joke, who reads his book imagining graphs instead of what he describes. That's the problem with it... being in the physics section at chapters. "Physics for Dummies" was far better

Last edited:

While there definitely is a special relativity issue at a given constant velocity, my question was more about what the alien would "see" if she was accelerating on the bicycle. It's more of a GR question I think.

I know using the word "see" in my original question is imprecise, and I should have been more specific. My understanding of what Dr. Greene said was that the alien's view of Earth would be some point in what we perceive to be our past that depended on her velocity away from us and her distance from us. At a greater constant velocity or at a greater distance from earth, she would she observe a more distant point in what we perceive to be our past. I think I understand that her view of our world and our view of our world don't have to match up (she would see clocks here on Earth going slower, etc.)

But if she were to accelerate on her bicycle (taking her into GR territory), I would naively assume that as her velocity increases, she would see increasingly distant points of our past, or at least see time slowing down here on Earth (beyond the Lorentz contraction of SR) if the effect isn't great enough. But if she was far enough from the Earth and accelerating fast enough, wouldn't there be a point at which she sees events going backward here on Earth (or whatever distant point she's looking at)? Or does it turn out somehow that no acceleration/distance combination will ever allow that to happen, no matter how close to c she gets, how fast she accelerates, or how far away she is?

There is no combination that gives that result, in order for it to look like time was going backwards on Earth the photons would have to get here in reverse order than they left. give a constant c, that would be impossible. There would be a point that time appears to almost stop, at almost c(correct?)

Because of this site being (and thankfuly) such a stickler for accuracy, pretty much all of what Brian Greene says I take as physics poetry. And it's quite good, at producing a sense of wonderment and awe, but in NO sense is it a subsitute for study.

I don't really agree. "pretty much all of what Greene says" IS genuine physics. Brian Greene is an authentic physicst and what he and others, like Michio Kaku, try to explain to a general audience IS subject to misinterpretation, but is not "poetry" unless that's how your describe reral physics. In addition what he describes can sometimes be carried to unintended consequences.

I will, howver readily concede that the loaf of bread discussion in his book is one I never did quite understand...it was so long, I figured it was probbably important, but it was the only part of his books that seemed uninteresting...perhaps because it rasied more questions for me than it answered. I would have preferred a lorentz transform discussion..

I found Brian Greene's FABRIC OF THE COSMOS and THE ELEGRANT UNIVERSE wonderful introductory descriptions of Cosmology. In fact those were among the first books I read which reintroduced me to physics and led me to explore further in these forums so I am grateful for them and other introductory texts.

If you think you can handle rather strict mathematical physics, try Roger Penrose's THE ROAD TO REALITY, but you had better have either advanced mathematical training or be prepared for years of self study to get much past the first 100 pages of this 1100 page masterpiece.
And you'll have to also be prepared for even more 'imprecision' because you'll find he discusses many disagreements amonf theoretical physicsts about the mathematical implictions of various theories.

I've read a number of sections later in Penrose's book for further understanding to discover that different theories give different insights...have different implications. For an introductory view on some of these, THREE ROADS TO QUANTUM GRAVITY by Lee Smolin is very good.

And for a fascinating perspective from the 1920's, UNCERTAINTY by David Lindley, captures the historical struggles to understand quantum theory among such luminaries as Einstein, Bohr, Heisenberg, Bohr and others...struggles and interpretational difficulties that continue to this day.

I don't really agree. "pretty much all of what Greene says" IS genuine physics. Brian Greene is an authentic physicst and what he and others, like Michio Kaku, try to explain to a general audience IS subject to misinterpretation, but is not "poetry" unless that's how your describe reral physics. In addition what he describes can sometimes be carried to unintended consequences.

Well stated, Naty1. Thanks for bringing a needed perspective to the discussion. Greene is certainly not just bringing us poetry. He has done a pretty good job of giving us a picture of an external objective reality--a picture of the universe that exists independently of us observers. Those who will not accept any reality that they cannot directly observe inhibit the fundamental pursuit of theoretical physics. Mach would not accept the concept of molecules until Einstein described to him the analysis of Brownian motion.

Special relativity certainly describes a universe as a 4-dimensional continuum. Einstein has described this explicitly. I don't begrudge anyone not embracing a 4-dimensional universe, and I really cringe at some of the implications you will see if you really think it through. But, objectively I have not found a way to dismiss what is so directly described by the mathematics of special relativity.

As Einstein commented, "...we physicists make no distinction between past, present and future."

In Brian Greene's "Illusion of Time" on NOVA, he cited a case where a distant alien rides a bicycle away from us at a constant speed and then looks back at the earth. She will see a slice of spacetime in our past. Similarly, if she rides towards us at a constant speed, she will see a slice of spacetime in our future.

Roger Penrose posed this exact situation in reverse as shown in the sketch below. Bill and Ruth pass each other on a street somewhere on earth. In Bill's simultaneous space a meeting is being held somewhere in the Andromeda Galaxy to decide whether to attack earth. But, in Ruth's simultaneous space, the Andromeda space fleet has already launched the attack. This is not poetry. This is a direct application of special relativity mathematics. That's the way it works in a 4-dimensional space-time continuum. Some folks like it and some folks don't. That's just the way it is here on the physicsforums.

Last edited:

I don't really agree. "pretty much all of what Greene says" IS genuine physics. Brian Greene is an authentic physicst and what he and others, like Michio Kaku, try to explain to a general audience IS subject to misinterpretation, but is not "poetry" unless that's how your describe reral physics. In addition what he describes can sometimes be carried to unintended consequences.

No need to put words in my mouth, I already defined "poetic" in a previous post;

"Taking the simple and very accurate graphical representation and adding all this "color" to the concept is "poetic" physics"

And the difference does not have to be elegant universe compared to an 1,100 page book by Penrose.

The difference between a "physics for dummies" type book and elegant universe illustrates the extra fluff added to SR concepts for the sole purpose of popularizing physics concepts (and/or a concepts of a particular theory).

In either case, this is purely opinion on how to best learn SR. And my opinion is Brian Greene's books are low on that list. I've read Brian Greenes books and read some that described SR graphically. I found the latter far more efficient and effective.

I don't at all mean to imply that Brian Greene's example of SR is wrong when I call it physics "poetry". Just a round-about-way of describing SR.

You're entitled to believe Brian Greene's books are a good place to learn SR.

Last edited:

This is not poetry. This is a direct application of special relativity mathematics. That's the way it works in a 4-dimensional space-time continuum.

I see that's the way it works in a 2D space time continuum graph, but how does that translate to 4D?