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#### DrJoePesce

Verified Expert
Dr Joe, I believe that many of us might like some direction as to when to use "space" and when to use "spacetime". Also, in string theory, is there a similar unity/difference between stringspace (11 dimensions) and "string time"?

In a similar vein, starting from matter in spacetime, due to expansion, (inflation) there comes a 'point' when ST expansion exceeds c, and presumably leaves the matter behind. Have I got this right? If ST later slows down again below c (after inflation?) how (if) does it reconnect with matter?

Cat

Hi Cat, good to hear from you! The main difference between space and spacetime is….time! “Space” is where everything is located. You can define positions with coordinates. There are three dimensions, x, y, and z (or left/right, forward/back, up/down). “Spacetime” consists of the three spatial dimensions, plus time, so four dimensions in total.

I am not a string-theory expert, so go easy on me! String theory describes space/spacetime and how the universe works. I think it is not analogous, then, to space and spacetime. I would say that one does not have string space and string time.

Our intuition, experience, and language all make analogizing and understanding universal expansion extremely difficult. (I’ve been teaching this for almost 30 years in general education/introductory astronomy courses and it is one of the most difficult concepts for the instructor to describe and for students to understand!) Part of the problem is that universal expansion was discovered by noting that galaxies appeared to be all moving away from us, with those at larger distances moving faster than those more nearby galaxies. We want to think It’s the galaxies themselves rushing into the distances. Instead, and it’s a subtle distinction, it’s not the galaxies moving away from us, it's the space in between expanding. Moreover, the expansion velocity is not the same everywhere: Nearby space expands slowly, and more distant space expands more rapidly.

There can indeed be a situation where two very distant galaxies are “moving apart” at velocities greater than the speed of light. This is not a violation of the cosmic speed limit because the galaxies themselves are not moving (which is what Einstein is concerned with: Individual objects moving through space from one point to another).

Dr. Joe

#### DrJoePesce

Verified Expert

Welcome billiejean!

#### Catastrophe

##### "There never was a good war, or a bad peace."
Dr Joe, thank you very much for your kind, and lengthy, reply. I do apologise for the multipart question, but they seem to have some elements in common.

With regard to spacetime, I appreciate what you say, but my difficulty is in understanding why (indeed how) they can be combined. Space and time are very different. I mean (as you obviously know ) space has (at least) three dimensions that can be travelled in any direction. We can say that the Battle of Hastings took place near the present town of Hastings, and we can make a good stab at providing a 'time' - this being quite different in nature. We do not define an event by Lat Long Date and Time (of day) together. Sounds like mixing three apples and one pear, and trying to describe THE taste of fruit. Perhaps there is some scientific reason which demands such a terminological inexactitude? (Note for US viewers. Terminological inexactitude is a polite term in English English for a lie. I don't know whether you have the same? I do not intend to imply lie here, but simply inexact terminology). I find this particularly interesting in relation to the above.

Is this whole area, perhaps, verging on the metaphysical? In relation to the third part, is not matter 'tied to' the space it occupies? Can they be congruent below c, separate at c, and then rejoin on slowing below c again? Does matter exist in space below c, holiday above c, but join space again on decreasing velocity? Sounds like my old mentor, Korzybski, could have a field day here? "The map is not the territory".

Please do not think I am trying to be difficult. These are honestly matters of interest to me, as, rightly or wrongly, I am looking at these questions as perhaps being borderline science/metaphysics, and can they be settled (separately, if necessary) as one or the other?

Cat

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#### DrJoePesce

Verified Expert
Dear Cat, I love the questions (even if sometimes I'm unable to fully answer!).

And I think you might not be satisfied with this: Our universe is just that way - space and time are combined. We have Einstein to thank for this. I can't provide a response to WHY, but that is just my ignorance. Others may be able to do so.

In our everyday, human, existence, we don't tend to give all four dimensional coordinates to an event, BUT we could: Everything in the universe is happening at a location and at a specific time. This is very real; I don't think we need to get in the metaphysical realm for this particular discussion.

And before we get into the metaphysical, we need to get into quantum mechanics, which I'm avoiding 🙂, but which is where the true descriptors/explanations lie.

As for stuff in the universe: We must remember that in a expanding universe, the universe is doing the expanding, not the stuff (matter) within it. The universe is not stuff, so can expand at the speed of light (or above). (As someone else noted, Einstein (and the universal speed limit) only cares above moving matter and in universal expansion no matter is moving.)

Regarding whether these questions are settled, it allows me to delve a bit into how science is done. Nothing is ever settled in science. It can get darn close to being settled, but there's always a possibility of a new observation that changes things (minutely or fundamentally).

So, are the topics we've discussed here, settled? Absolutely not. But what underpins them helps us explain what we observe in the universe (on the micro and macro scales, from fusion reactions in the centers of stars to the universal expansion) REALLY well. If they are wrong, my expectation is that they only need to be tweaked (rather than a wholesale rejection and the need to start from scratch). These hypotheses, laws of nature, theories, are subject to constant testing through experimentation on Earth (in labs, in particle accelerators, etc.) and in space (studies of strong gravity regions, all other astronomical observations, etc.).

Dr. Joe

#### Catastrophe

##### "There never was a good war, or a bad peace."
Dear Dr Joe,
Many thanks for your long reply. Just a little about Minkowski please. Did he not arrive at spacetime (no pun intended), this then being found useful to relativity? That is, rather than being derived from relativity?
Vide
Our universe is just that way - space and time are combined. We have Einstein to thank for this.
In 1907 the mathematician Hermann Minkowski explored a way of visualizing these processes that proved to be especially well suited to disentangling relativistic effects.

Items 1 and 7.
These figures have a venerable history in relativity theory. Here is how they appeared in Minkowski's original publications.
. . . . . . . . .
Hypebolas are the analogs in Minkowski geometry of circles in Euclidean geometry.
Spelling sic.

What does concern me (Please pardon my ignorance), is where two totally different dimensions are treated as equivalent and fused viz. 3 space dimensions and one time dimension.

Our universe is just that way - space and time are combined. We have Einstein to thank for this.
You state that space and time are combined. On what authority? We can combine apples and pears, designating them fruit. And then say they must be fruit-shaped. Which does not follow. All fruit does not have the same shape. What is the difference here. Oh how I wish that Korzybski were still around.

With best wishes and my grateful thanks for your time in responding.

Cat

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#### Catastrophe

##### "There never was a good war, or a bad peace."
This might help some non-scientific people:

Cat

#### Jzz

Many thanks for your long reply. Just a little about Minkowski please. Did he not arrive at spacetime (no pun intended), this then being found useful to relativity? That is, rather than being derived from relativity?
Vide
I think it was Poincare, who far before Minkowski even thought about it, who introduced the idea that different geometries might exist.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Jzz,
Poincare was certainly a mathematician (inter alia), and may be better known for his interactions with the 3-body problem. [Pun maybe intended].

Different geometries, including 4 (space) dimensions, maybe, but spacetime? I need convincing on that one!

Were you, perhaps, thinking of this?
The Poincaré group is the group of Minkowski spacetime isometries. It is a ten-dimensional noncompact Lie group. The abelian group of translations is a normal subgroup, while the Lorentz group is also a subgroup, the stabilizer of the origin.

Cat

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#### Catastrophe

##### "There never was a good war, or a bad peace."
On further reflection, my Korzybski background [the map is not the territory = the words are not the reality] has led me to ask

Dimension Definition & Meaning | Britannica Dictionary

Is time actually a dimension anyway? Are we verbally deluding ourselves?
Are we arguing with non-words?

Cat

#### Marcd2k

Happy Monday dear astronomy enthusiasts!

Given the great turnout on my previous AMA appearances, it looks like now you will have me here on a more regular basis!

I look forward to your questions and our continued discussions. And, as always, I will try to answer as many as I can (though there might be some I can't answer!).

In the meantime, keep looking up!

Dr Joe
Hello, Dr. Joe,
I just read today's Space Dot Com article about the planet Smertrios and it's suprisingly heavy elements in its atmosphere, according to the JWST.

I am wondering why there are two descriptors for this planet:
1. It is called a "hot Jupiter" in more than one point of the article.
2. It is also said to be the "mass of Saturn" in another point in the article.

If the planet (Smertrios) is the mass of Saturn, then why is it being called a "hot Jupiter"?

Thank you,
Marc

#### Catastrophe

##### "There never was a good war, or a bad peace."
Please excuse me if I see this before Dr. Joe, but it may be helpful to you meanwhile:
You might also Google "hot jupiters" for more similar explanation.

Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (P < 10 days). The close proximity to their stars and high surface-atmosphere temperatures resulted in their informal name "hot Jupiters".
Hot Jupiter - Wikipedia

Cat

#### Think twice

Hi Joe. Maybe not exactly on topic but related. I’m trying to find out what nm difference there is between the zeeman effect lines on a hydrogen spectral line, sometimes seen in astronomical observations of stellar spectra. . There is a zeeman page on wiki obviously aimed only at top genius quantum mathematicians working at the max planck institute and no one else. Which has tons of basically unfathomable maths, formulas and Byzantine algorithms describing how to calculate what nm the zeeman split lines are...and yet no actual simple answer as to a specific sample line (for instance h-alpha Balmer 656.3 ) and what nm the 3 split lines would be.

(Why does wiki physics pages only address its data to a select few top doctorate holders in maths? Wiki is for the masses interested in *physics* not maths. And those amateur researchers of us looking for basic empirically observed data. Not maths. After all the original data comes as spectral lines that are measured for nm values. Not as complex Formulas echoing across the cosmos. )

My best try from trying to understand the nonsense maths gobbledygook on various wiki pages about line splitting etc is that the zeeman split lines are very close to the original line on the order of only 1/1000 th of a nm or so.
So H-alpha 656.3 might be split into maybe 2 lines 656.3 +-.0005 nm on either side of an original central line?
Unfortunately for my home research I need to be sure exactly what the nm values are. But can’t find any.
Do you know any specific nm values for any of the split lines from a specific H Balmer spectral line split by zeeman effect ? Let’s say H -alpha 656.3 for example.

#### StarmanG10

Hello! What is more likely: Heat death of the universe or a vast, cold, infiite expansion of the universe?

#### Helio

Hello! What is more likely: Heat death of the universe or a vast, cold, infiite expansion of the universe?
Perhaps Dr. Joe will bounce in here, but he hasn't been here in a while. I do expect he will visit and give you a great answer.

Your question is a good one because there has been a lot of claims of a "heat death", as well as, a cold one. The "heat death" view, these days, notes how sytems (e.g. engines) produce heat. Heat at low temperatures cannot be used to produce useful energy, but all systems, due to entropy (think friction), will produce useless heat. This waste heat gets dumped outside, including into space.

But the universe, as was determined in the late 90s, was found to be expanding, and that it has an accelerating expansion rate. So for the universe to not die a cold death, due to further expansion, then it would have to stop accelerating, then slow down, then contract. Contraction is what is required to die of a heat death. Of course, this is the unlikely scenario.

Dark Energy is the term invented to put a label on whatever it is that is causing this accelerated expansion. There are dozens of theories of what is behind it, but for now, all we can argue is that the universe is being pushed outward in all directions, and at a slightly accelerated pace. This strongly suggests a cold demise in a few trillion years, perhaps.

#### joshuawestman

Many thanks, Dr. Joe. May I repost my question from the previous week?

>DrJoePesce said:
>Thanks for the follow-up XinhangShen. My understanding (maybe incorrect) is that the
>GPS satellites' clocks are synchronized. I don't think they are synchronized with the
>ground clocks. I'll see if I can get someone better versed in this subject to weigh in.

I think that all the clocks including the ground clocks should be synchronized so that they all have the same time that can be used to determine the position. Some people argue that the clock on a satellite and the clock on the ground are synchronized only relative to the ground reference frame and not synchronized relative to the satellite reference frame. If it was true, then the difference between the clocks observed from the satellite would monotonically grow but can't be corrected because the clocks are still synchronized observed from the ground frame, which is obviously not the case on the clocks of the GPS.

I think, Lorentz Transformation is a redefinition of space and time (called relativistic time in the following) which is no longer the clock time, but a fake time without physical meaning.

In fact, Lorentz Transformation is mathematically equivalent to the following definitions:

t' = (1/γ)T' - (γv/c^2)X'
x' = γX'
y' = Y'
z' = Z'

where γ = 1/sqrt(1 - v^2/c^2), (X', Y', Z', T') is the Galilean spacetime of the inertial reference frame moving at speed v in the X-direction relative to aether, (x', y', z', t') is the relativistic spacetime of the same inertial reference frame. When v = 0, relativistic spacetime (x, y, z, t) becomes the same as Galilean spacetime (X, Y, Z, T).

Galilean spacetime follows Galilean Transformation:

T' = T
X' = X - vT
Y' = Y
Z' = Z

We can verify that the speed of light defined by Galilean spacetime follows Newton's velocity addition law, while the speed of light defined by relativistic spacetime is constant relative to all inertial reference frames:

C = X/T

C' = X'/T' = (X - vT)/T = X/T - v = C - v

c = x/t = X/T = C

c' = x'/t'
= (γX')/[(1/γ)T' - (γv/c^2)X']
= (X'/T')/[(1/γ^2) - (v/c^2)(X'/T')]
= C'/[(1 - v^2/c^2) - (v/c^2)C']
= (C - v)/[1 - v^2/c^2 - (v/c^2)C + v^2/c^2]
= (c - v)/(1 - v/c)
= c

That means, all what Lorentz Transformation does is to redefine spacetime.

Since the clock time Tc is defined by the number N of cycles of a physical periodical process:

Tc = N/k

where k is a calibration constant and equals 9,192,631,770 for a cesium atomic clock.

The clock times Tc and Tc' of two clocks attached to the inertial reference frames defined by Galilean spacetime (X, Y, Z, T) and (X', Y', Z', T') are

Tc' = N'/k = (T'/Τ' )/k = (T/Τ)/k = N/k = Tc

where the Galilean periods Τ and Τ' of the two clocks are the same because Galilean time is absolute.

This equation tells us that clock time is also absolute same as Galilean time.

In special relativity, the clock times Tc and Tc' of two clocks attached to the inertial reference frames defined by relativistic spacetime (x, y, z, t) and (x', y', z', t') can be calculated by:

N' = t'/𝜏'
N = t/𝜏
t' = t/γ
𝜏' = 𝜏/γ

Tc' = N'/k = (t'/𝜏')/k = (t/𝜏)/k = N/k = Tc

where N', t' and 𝜏' are the number of cycles, elapsed relativistic time and the relativistic period of the moving clock, respectively, N, t and 𝜏 are the number of cycles, elapsed relativistic time and the relativistic period of the stationary clock respectively.

This equation tells us that clock time is Lorentz invariant and thus is still absolute and independent of the reference frame, which confirms that the property of the clock time (i.e. the absoluteness of the clock time) won't change with the change of the definition of time. Thus, the relativistic time is no longer the clock time but a meaningless mathematical variable.

Therefore, based on such a fake time, special relativity is wrong and so are all relativistic spacetime based theories including the Big Bang theory.
This reply is not directly correlated but has the possibility to have some ties. Is it possible that gravity "knows" so much about quantum states because gravity, space time, and size are intertwined? Could time not only be affected by speed and gravity but size as well? Studies show that smaller organisms with higher metabolic rates experience time more slowly, however I see no notes of time dilation or relativity anywhere in these studies. Is it possible that another cause of this is a relationship between ambient mass vs. body mass? Could an object experience Earth’s (or any other celestial body’s) gravity well differently based on the size of the object and the micro-gravity well that it creates? If this could be indeed an explanation could a formula be dirived and extrapolated down to the quantum level explananing the stragness observed in the quantum realm and possibly be a bridge from relativity to quantum mechanics?

#### StarmanG10

Perhaps Dr. Joe will bounce in here, but he hasn't been here in a while. I do expect he will visit and give you a great answer.

Your question is a good one because there has been a lot of claims of a "heat death", as well as, a cold one. The "heat death" view, these days, notes how sytems (e.g. engines) produce heat. Heat at low temperatures cannot be used to produce useful energy, but all systems, due to entropy (think friction), will produce useless heat. This waste heat gets dumped outside, including into space.

But the universe, as was determined in the late 90s, was found to be expanding, and that it has an accelerating expansion rate. So for the universe to not die a cold death, due to further expansion, then it would have to stop accelerating, then slow down, then contract. Contraction is what is required to die of a heat death. Of course, this is the unlikely scenario.

Dark Energy is the term invented to put a label on whatever it is that is causing this accelerated expansion. There are dozens of theories of what is behind it, but for now, all we can argue is that the universe is being pushed outward in all directions, and at a slightly accelerated pace. This strongly suggests a cold demise in a few trillion years, perhaps.
Thank you!

Helio

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