APOD: The Observable Universe (2018 May 08)

[Chris Peterson]

Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?

Space-time is 4D. 3D Space plus time.

No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.

Nope, that's definitely not what I'm talking about. The 4D universe is defined by three spatial dimensions and one time dimension. I don't even know what four spatial dimensions means, outside of pure mathematics.

The geometry of the Universe can be a hypersphere or some other 4D geometry without the need for a fourth "spatial" dimension.

[geckzilla]

Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?

Space-time is 4D. 3D Space plus time.

No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.

Space is time. Just stop trying to separate them, and it'll make more sense. Or less.

[Ann]

Space-time is 4D. 3D Space plus time.

No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.

Space is time. Just stop trying to separate them, and it'll make more sense. Or less.

This is how I visualize it. It may be right, it may be wrong.

Click to view full size image

Ötzi, the iceman.

This is Ötzi, the iceman. He was found in the Ötzal Alps on the border between Austria and Italy in 1991. He is Europe's oldest natural mummy, and he was alive some 5,000 years ago.

I think of Ötzi as a man who has been traveling on "spaceship Earth" for about 5,000 years. He was alive for only about 45 of those years, but his death didn't prevent his body from continuing his journey on spaceship Earth.

The Earth is "dragged along" in space by the Sun, which is moving at 72,000 kilometers per hour (45,000 miles per hour). Let's see. There are 24 hours in a day, which means that the Sun moves 1,728,000 kilometers in a day. Then there are 365 days in a year, which means that the Sun moves 630,720,000 kilometers per year, or almost 631 million kilometers per year, if I did the math right. Allow the Sun to travel at that speed for five thousand years, and it will have moved some three trillion, one hundred and fifty-three billion, six hundred million kilometers through space. And dragged the Earth along with it.

My figures may be correct, or not. But it is true without a shadow of a doubt that the Sun has dragged the Earth along a vast distance since Ötzi was alive on the Earth.

Here is my point. We can't travel back in time and meet Ötzi when he was alive, or visit the Earth at the time when Ötzi was alive. But crucially, we also can't travel back in spacetime and visit the 4D region in the Universe in which Ötzi was alive. Theoretically at least, we could indeed travel to the general region of space through which the Earth was moving when Ötzi was alive. But if we managed to locate that region in space and travel there, we would find that the Earth would not be there. It would have moved more than a trillion kilometers away from the 4D location in spacetime where Ötzi was alive.

Ötzi's mummified body exists in the same geographic location and time frame as we do, but he was alive in a 4D spacetime that is inaccessible to us. The spacetime coordinates of the Earth that he lived on are out of reach for us forever.

Ann

[Chris Peterson]

Theoretically at least, we could indeed travel to the general region of space through which the Earth was moving when Ötzi was alive. But if we managed to locate that region in space and travel there, we would find that the Earth would not be there. It would have moved more than a trillion kilometers away from the 4D location in spacetime where Ötzi was alive.

Another way to think of it, since spacetime dimensions have different units, is that we would have moved a trillion kilometers along the Universe's XYZ axes, and 5000 years along its time axis. We have freedom of movement on the former, but are basically just being carried along on the latter.

[neufer]

Theoretically at least, we could indeed travel to the general region of space through which the Earth was moving when Ötzi was alive. But if we managed to locate that region in space and travel there, we would find that the Earth would not be there. It would have moved more than a trillion kilometers away from the 4D location in spacetime where Ötzi was alive.

Another way to think of it, since spacetime dimensions have different units, is that we would have moved a trillion kilometers along the Universe's XYZ axes, and 5000 years along its time axis. We have freedom of movement on the former, but are basically just being carried along on the latter.

---

The Sun moves ≈ 370 km/s = 0.00123 c
(relative to the cosmic microwave background).

Hence, Ötzi would have moved about 6.2 (light) years along the Universe's space axes, and ~5000 years (+33 hours) along its (imaginary?) time axis.

Theoretically, we have freedom of movement to any spacetime point lying between our own future light cone and Minkowski's hyperbola (where the unit of time is defined by our own remaining life span).

[Guest]

All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.

Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in. Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist. The fact that we are here indicates that the probability of a 'big bang' is greater than one. So I suggest that you consider that a second other big bang had happened perhaps a hundred trillion years ago, in which case time and space would be in existence from that time forward. Or you could consider that time and space are infinite (in all directions) and rethink your perspective of time, the universe and everything... I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...

[Ann]

I need to rectify what I said about the Sun (and therefore the Earth) having moved more than a trillion kilometers in 5,000 years.

Clearly we have moved much farther than that from the 3D region of Ötzi's life span. Because it is not only the Sun and the Earth that have moved. The Milky Way has also moved! Our galaxy is experiencing a fairly strong pull from Andromeda, the other large galaxy of the Local Group.

But that's not all. The Virgo Cluster is also pulling on our galaxy and our Local Group. And there is the Great Attractor, which is pulling not only on the Local Group of galaxies, but also on the entire Virgo Cluster and a lot of other galactic groups as well.

And of course, as Art pointed out, there is the general expansion of the Universe pulling on us. So, can we really find the 3D region in space where the Earth was located 5,000 years ago?

Maybe we can. But with all these tidal pulls and flows in space, the 3D distance to the Earth's location in space some 5,000 years ago is getting larger and larger.

Theseus with Ariadne's thread in the labyrinth
of the Minotaur. Source:
https://www.icompositions.com/music/song.php?sid=223531

Click to view full size image

Hansel's pebbles. Source:
http://www.thejanusproject.ie/2014/08/2 ... s-pebbles/

Also bear in mind that there are no "roads" or "road signs" in space that we could use to "follow our own footsteps" back to where we were 5,000 years ago. We don't have Ariadne's thread to help us find our way back to where we started. We don't even have pebbles or bread crumbs like Hansel and Gretel.

Finding our 3D location in space 5,000 years ago would be very hard indeed, and traveling there would be harder.

Ann

[geckzilla]

All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.

Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in. Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist. The fact that we are here indicates that the probability of a 'big bang' is greater than one. So I suggest that you consider that a second other big bang had happened perhaps a hundred trillion years ago, in which case time and space would be in existence from that time forward. Or you could consider that time and space are infinite (in all directions) and rethink your perspective of time, the universe and everything... I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...

Hi Guest, most of the perceived strictness and rigidity of Chris's posts are due to the forum rules reinforcing mainstream concepts in science. We're really not about exploring alternative theories here in any capacity beyond idle speculation. So if someone asks a question, most responses will be a kind of by-the-book answer if not some kind of creative allusion from Art. It's not that any of us don't harbor feelings that well, maybe the Big Bang isn't all there is, or maybe we're wrong. It's just what we have to work with. Quite frankly, alternatives have been hashed and rehashed over, and it's boring at this point.

Kindly set aside your righteous indignation and take a gander at the rules.

[Ann]

All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.

Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in. Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist. The fact that we are here indicates that the probability of a 'big bang' is greater than one. So I suggest that you consider that a second other big bang had happened perhaps a hundred trillion years ago, in which case time and space would be in existence from that time forward. Or you could consider that time and space are infinite (in all directions) and rethink your perspective of time, the universe and everything... I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...

Could you spare a moment and consider how extremely tiny and short-lived we humans are compared with the size and scope of the Universe in space and time? And still, we have managed to find out as much about our Universe as we actually have. Personally, I think that humanity's feat of uncovering many of the aspects of the grand Universe that we are a tiny, tiny part of is absolutely amazing and truly awe-inspiring.

Our tiny size compared with the size of the Universe, our short lifespans, the incomprehensible value of the speed of light, yet at the same time the relative "snail's pace" of the speed of light in relation to the vastness of the Universe, plus the fact that the earliest Universe was wrapped in an impenetrable fog completely hiding it from our view - all these factors make it terribly difficult for us to ascertain the properties of the Universe, let alone the factors that made the Universe come into existence.

All we can do is observe the Universe with better and better instruments and make better and better models of how the Universe works, based on our ever-improving observations. With more and more powerful computers, we can test our theories by running them through our most powerful computers and see if our computers appear to come up with a Universe like ours, based on the models we fed into them based on our observations that we used to make our models.

That's what we can do, really.

We can make many observations that clearly demonstrate that the Universe was much smaller and more compact in its infancy than it is now, and it was also hotter. From these observations, we can extrapolate backwards in time and conclude that there must have been a time when the Universe was really, really tiny, and perhaps - but who knows? - infinitely tiny, as a mathematical point in a non-existing space in a non-existing time frame. From this mathematical point our Universe burst forth. This is what our models suggest.

But we can make no observations that suggest that there were other Big Bangs before the one that gave birth to our own Universe. Therefore, the question of previous Big Bangs and other universes than our own are not scientific questions, but philosophical ones.

Ann

[Chris Peterson]

All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.

Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in.

It's not a question of what I "believe". It's simply that we have very good theory which is extremely well supported by a large amount of independent evidence. In other words, it's the best explanation we have.

Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist.

As previously noted, the understanding that time and space came into existence together is unrelated to frames of reference. And in fact, the excellent theory that we have very much does argue for there not being any time or space until the formation of the Universe.

The fact that we are here indicates that the probability of a 'big bang' is greater than one.

How can the probability of anything be greater than one? What does that even mean?

I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...

I believe it is your inability to get past the biases imposed by your simple worldview that reflects a lack of imagination and a limitation.

[JohnD]

Which is why I suggested some reading and study, rather than relying on previously acquired ideas.

Chris, and others here including myself, follow Russell and his teapot. Bertrand Russell, philosopher and another Nobel prize winner, pointed out that it was up to someone who proposed that a teapot was in orbit around the Sun to provide the evidence, not for others to disprove it. We have evidence on the Big Bang that started this Universe. Speculation on prior BBs is just teapots.
John

[Ann]

Which is why I suggested some reading and study, rather than relying on previously acquired ideas.

Chris, and others here including myself, follow Russell and his teapot. Bertrand Russell, philosopher and another Nobel prize winner, pointed out that it was up to someone who proposed that a teapot was in orbit around the Sun to provide the evidence, not for others to disprove it. We have evidence on the Big Bang that started this Universe. Speculation on prior BBs is just teapots.
John

Click to view full size image

Teapot in orbit?

I had forgotten about the teapot! That's a really good one!

Ann

[JohnD]

Thank you, Anne, for the quote of Russell's words! He puts it so much better, but then he got the Nobel and I haven't!
John