# Dimensions and dark matter

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## Main Question or Discussion Point

Is it correct that dimensions more than the three (4th being time) that we perceive can be mathematically modeled but as yet remain unobserved?

If so, is it possible that dark matter/energy are "elements" which exist in those higher dimensions, and as such remain invisible to those constrained to four?

PeterDonis
Mentor
2019 Award
Is it correct that dimensions more than the three (4th being time) that we perceive can be mathematically modeled but as yet remain unobserved?
Yes; string theory does this. But we have no experimental evidence for any such model.

is it possible that dark matter/energy are "elements" which exist in those higher dimensions, and as such remain invisible to those constrained to four?
They can't remain "invisible" in our four dimensions, since we observe their effects.

WWGD
Gold Member
2019 Award
It may be the case, speculating, that we may just perceive 'shadows' or projections into lower dimensions, as in the book 'Flatland'. But this is just an educated guess.

It may be the case, speculating, that we may just perceive 'shadows' or projections into lower dimensions, as in the book 'Flatland'. But this is just an educated guess.

It has nagged at me that there is a known constant being the speed of light, and feel free to correct me that this is the point at which matter can change into energy and vise-versa. Dark energy/matter may be outside the bounds of that constant.

The speed of light being expressed as C in E=M(C x C), it puzzles me that the constant in this case can be squared as that is an impossible number?

Ibix
It has nagged at me that there is a known constant being the speed of light, and feel free to correct me that this is the point at which matter can change into energy and vise-versa.
Absolutely not. Things with mass cannot reach the speed of light, for a start. The $c^2$ in $E=mc^2$ is a conversion factor between energy units and mass units - that's all. In a less artificial unit system than SI, the conversion factor is 1, and the equation is just $E=m$.

It's best to think of $c$ primarily as a constant of nature, and the fact that light travels at that speed as a consequence of its masslessness. If you think of $c$ as the speed of light primarily, you'll always be puzzled about the places it turns up.

Incidentally, it's possible to describe a universe where light does not travel at $c$. $E=mc^2$ still holds.

Absolutely not. Things with mass cannot reach the speed of light, for a start. The $c^2$ in $E=mc^2$ is a conversion factor between energy units and mass units - that's all. In a less artificial unit system than SI, the conversion factor is 1, and the equation is just $E=m$.

It's best to think of $c$ primarily as a constant of nature, and the fact that light travels at that speed as a consequence of its masslessness. If you think of $c$ as the speed of light primarily, you'll always be puzzled about the places it turns up.

Incidentally, it's possible to describe a universe where light does not travel at $c$. $E=mc^2$ still holds.
Thanks, got it regarding being used as a conversion unit and that c is a natural constant.
Why is c expressed as the speed of light if it's not true?

phinds
Gold Member
2019 Award
Thanks, got it regarding being used as a conversion unit and that c is a natural constant.
Why is c expressed as the speed of light if it's not true?
Well, first off, c IS the speed of light, as far as we know. More importantly, there is a "universal maximum speed" and THAT is the constant "c". Experimentally, and theoretically, there is no reason to believe that light travels slower than c, but it is not impossible. If light were found to travel slower than c, that would have no effect on c or equations using it, it would just mean we'd have to come up w/ a new symbol for the speed of light.

Ibix
Thanks, got it regarding being used as a conversion unit and that c is a natural constant.
Why is c expressed as the speed of light if it's not true?
It is true. But the constant $c$ appears all over the place as a conversion factor, and would continue to appear in the maths even if we were to discover that photons had a (tiny) mass and hence that light did not travel at $c$.

So $c$ is the speed of light but it isn't only the speed of light, and its other jobs are probably more important.

Thanks, that's really cleared up some definitions for me.

Question regarding the Darks...
Is it plausible that DM/DE were not part of the BB, rather pre-existing?
Are conditions leading up to the BB being a mystery still? Or is the term "leading up to" not correct?

phinds
Gold Member
2019 Award
Thanks, that's really cleared up some definitions for me.

Question regarding the Darks...
Is it plausible that DM/DE were not part of the BB, rather pre-existing?
Are conditions leading up to the BB being a mystery still? Or is the term "leading up to" not correct?
Actually, the "Big Bang Theory" starts at the end of the (presumed but not 100% confirmed) inflation in the first tiny portion of a second after the creation event (whatever THAT was) and moves forward from there. What went before inflation is unknown. Some models say that spacetime was created as part of the creation event but that's a model. Nature doesn't care about our models and we don't know what was really going on.

PeroK
Homework Helper
Gold Member
Thanks, that's really cleared up some definitions for me.

Question regarding the Darks...
Is it plausible that DM/DE were not part of the BB, rather pre-existing?
Dark energy is the energy of the vacuum. That is part of the GR/BB model. It's why dark energy has the value it does that is the unsolved puzzle. QM predicts a non-zero vacuum energy, but it doesn't agree with the observed cosmological data. QM generally predicts far more vacuum energy than we appear to measure. See

https://en.wikipedia.org/wiki/Vacuum_energy

There's no relationship between dark energy and dark matter, other than they have "dark" in their name. You could include dark chocolate if you wanted.

As almost nothing is known about dark matter it's difficult to speculate. I imagine you are aluding to this:

https://www.sciencedaily.com/releases/2019/08/190807190816.htm

Ok, those references provide some interesting reading, thank you.

The wrong assumption on my part that there is correlation between the darks exists basically because of sloppy naming convention?

So, does the dark mean obscured from view in this context, awaiting revelation?

PeroK
Homework Helper
Gold Member
Ok, those references provide some interesting reading, thank you.

The wrong assumption on my part that there is correlation between the darks exists basically because of sloppy naming convention?

So, does the dark mean obscured from view in this context, awaiting revelation?
Dark matter makes sense because it doesn't interact with light. I don't know how dark energy came to be called that. I'm sure you could find out online!

Dark matter makes sense because it doesn't interact with light. I don't know how dark energy came to be called that. I'm sure you could find out online!
Will do, here seemed like the right place to ask tho.

Ibix
Dark matter was so called because all the matter we can see is the glowing stuff (stars, nebulas etc), but there wasn't enough. Since it doesn't glow... Transparent matter would be a more precise name, but we are where we are.

As far as I'm aware, dark energy acquired its name because it is another case of "all the stuff we see doesn't move quite the way it should if that's all there is". We see the glowing stuff, so...

It's unfortunate that it sounds like there's a link between the phenomena, when the link is primarily in how they were first detected.

timmdeeg
Gold Member
Transparent matter would be a more precise name
Whereby till now we couldn't confirm that what seems to act like matter is really matter.

Janus
Staff Emeritus
Gold Member
Ok, those references provide some interesting reading, thank you.

The wrong assumption on my part that there is correlation between the darks exists basically because of sloppy naming convention?

So, does the dark mean obscured from view in this context, awaiting revelation?
In the terms of dark matter, it basically means that it isn't detectable by light (or any other frequency of the electromagnetic spectrum). The common possible candidates are MACHOs (MAssive Compact Halo Objects), which could be objects like Black holes, etc, or WIMPs (Weakly Interacting Massive Particles) , These would be particles that have mass, but just don't interact electromagnetically, so they do not emit, absorb or scatter light(other than by gravitational lensing) The neutrino is such a particle, though there are reasons why the neutrino's we are familiar with don't make up dark matter.( though a hypothetical type of neutrino, the "sterile" neutrino has been proposed as a candidate).
DM could even be a mix of the two (Though there appears to be limit on just how much can be attributed to MACHOs)

"Dark" energy, likely just got its name from the fact that we already had "dark" matter coined as a term. Naming conventions in physics aren't always based on logic. The six types of Quarks* are Up, Down, Strange, Charm, Top and Bottom. They were named in order as needed to explain the model. Up and Down just used to distinguish between two types, Strange, because "Normal" matter such as Neutrons and Protons that make up atoms doesn't contain them. The Charm quark was needed to explain particles that couldn't be modeled by the existing 3.
The same for Top and Bottom quarks, they were added to the model to fit new observations. Now, since they already had Charm, there was a push to name the new quarks Truth and Beauty, but saner heads prevailed.

*Quarks themselves got their name from a line in "Finnegan's Wake"

PeterDonis
Mentor
2019 Award
It has nagged at me that there is a known constant being the speed of light
The actual physical constant isn't the speed of light; its value depends on your choice of units. The actual physical constant related to the behavior of light is the fine structure constant, which is dimensionless, so its value is independent of any choice of units and represents an actual physical property of the electromagnetic field.

feel free to correct me that this is the point at which matter can change into energy and vise-versa
This doesn't even make sense.

Dark energy/matter may be outside the bounds of that constant.
This doesn't make sense either.

PeterDonis
Mentor
2019 Award
it's possible to describe a universe where light does not travel at $c$. $E=mc^2$ still holds.
Can you be more specific about what you're thinking of here?

Ibix
Can you be more specific about what you're thinking of here?
I was thinking of the methodology we use to place experimental restrictions on the photon mass. As far as I understand, you use Proca's equation to derive some effect (e.g. a non-zero E-field inside a charged sphere) the strength of which depends on photon mass, and then go and measure that effect. So far the results are consistent with zero mass to ever tighter limits, but if it's ever found to be non-zero then light doesn't propagate at the invariant speed, $c$. But that wouldn't change the role or value of $c$ in relativity.

Experimentally, and theoretically, there is no reason to believe that light travels slower than c, but it is not impossible.
Light travel slower than c in a medium with an index of refraction that's less than 1

Ibix
Light travel slower than c in a medium with an index of refraction that's less than 1
@phinds is referring to light travelling in a vacuum.

phinds
Gold Member
2019 Award
Light travel slower than c in a medium with an index of refraction that's less than 1
As Ibix pointed out, I'm using standard of light traveling in a vacuum. Light traveling in a medium is irrelevant to this discussion.

As Ibix pointed out, I'm using standard of light traveling in a vacuum. Light traveling in a medium is irrelevant to this discussion.
Why did you say that it is not impossible for light to travel slower than c?

Ibix
See post #20. If photons turn out to have mass then they do not move at $c$ even in vacuum. We are currently unable to detect any mass, and therefore we model light as travelling at $c$.