The speed of light, etc

[Sephiroth]

On the balloon analogy:

That just doesn't wash. In that analogy, the matter in the universe is on the outer perimeter with nothing inside. The universe we are observing is full of stuff. What have I misunderstood?

[downquark1]

The balloon analogy does have problems but there isn't really a better one. You need to learn the math for a better understanding - which I never have done btw.

[daveeb]

Originally Posted by Sephiroth

On the balloon analogy:

That just doesn't wash. In that analogy, the matter in the universe is on the outer perimeter with nothing inside. The universe we are observing is full of stuff. What have I misunderstood?

You're probably right in that it should have matter represented inside it also, a bit difficult with a balloon. It just shows rather crudely that everything is moving away from everything else. That said it's a 4D (apparently) representation of space-time not just space so perhaps the inside is the time vector and the surface represents 3D space. Difficult to represent 4 dimensions using 3.

[Sephiroth]

But what the heck is "space-time"? 3D is quite simple, with a force causing or having caused expansion. Time in this context is the number of time unit (seconds, minutes, years, etc) for a galaxy to move from 3D location "A" to 3D location "B".

In my assessment, for what it is worth, "space-time" is a confection of convenience for scientists who know quite a lot about the square root of all this, but not the bit underneath it all.

"Time vector" - don't we all understand this as time moving in one direction only? As in forwards? It has to be measured by some entity that can do such measurements or imagine them with a sound basis in fact. "Space-Time" - includes a "time vector"? Might as well be a triaxellated amplification of a sub-space message from Starfleet.

Imo!

[idi banashapan]

Originally Posted by daveeb

You're probably right in that it should have matter represented inside it also, a bit difficult with a balloon. It just shows rather crudely that everything is moving away from everything else. That said it's a 4D (apparently) representation of space-time not just space so perhaps the inside is the time vector and the surface represents 3D space. Difficult to represent 4 dimensions using 3.

I think someone mentioned the raisin bread analogy as an alternative, which I actually think is a better one as that does describe for matter on the inside.

[Sephiroth]

Originally Posted by idi banashapan

I think someone mentioned the raisin bread analogy as an alternative, which I actually think is a better one as that does describe for matter on the inside.

Do the raisins expand in space-time? When an analogy is provided, it's usually intended to simplify a more complex situation to help understand what's going on. But really (and absolutely), no such analogy has any bearing on the universe's forces.

That's why I say we should apply logic to the question of the speed of light and what we can see in the universe.

The frequency shift of light emanating from a moving object proves (to me at least) that the speed of light is fixed.

In the theory of accelerating universe expansion, the logical situation is that somewhere, everything is moving away from us (ignoring local gravity effects) and some galaxies are approaching recession speeds that cannot be measured with existing detection systems. If the universe's expansion is truly accelerating, then the corollary is, that, relative to us as an observer, some galaxies are receditn at a speed faster than light. A lot of bottoming out still to be done!

[idi banashapan]

Well, that’s a fascinating point, and it touches on some of the fundamental aspects of relativity and cosmology. But let’s break it down carefully.

You're absolutely right that analogies, while useful for simplifying concepts, don't alter the fundamental laws of physics. They help us build intuition, but ultimately, the universe follows mathematical principles that don;t depend on our human need for simplification.

Now, regarding the speed of light, yes, the Doppler effect, or more specifically the redshift of light, is strong evidence that the speed of light is constant. This is central to Einstein’s special relativity, which tells us that the speed of light in a vacuum is the same for all observers, regardless of their motion or the motion of the source of the light.

As for the accelerating expansion of the universe, this is where general relativity comes into play. Space itself is expanding, and that’s not the same as objects moving 'through' space. When we talk about galaxies receding from us faster than the speed of light, we’re not violating relativity because it's not the galaxies themselves moving at superluminal speeds through space; rather, the space between us and them is expanding. This means that light from these galaxies will nevre reach us, because the fabric of space is stretching too quickly for the photons to make headway.

So, in a way, we have a sort of observational horizon, the so-called 'cosmic event horizon'. Beyond this, light from galaxies can’t reach us because space is expanding too fast. This is a direct consequence of the universe's expansion being governed by what's called 'dark energy', which is driving this acceleration.

Now, your intuition that we still have a lot to work out is absolutely correct! We don’t yet fully understand what dark energy is, or why the universe’s expansion rate changed over time. But the mathematical framework of relativity does an incredibly good job of describing what we 'can' observe. But there's nothing to say that what we can see is everything that is there.

So yes, there’s definitely a lot of bottoming out still to be done, but people far more clever than myself making good progress, I should think.

[Sephiroth]

Originally Posted by idi banashapan

Well, that’s a fascinating point, and it touches on some of the fundamental aspects of relativity and cosmology. But let’s break it down carefully.

You're absolutely right that analogies, while useful for simplifying concepts, don't alter the fundamental laws of physics. They help us build intuition, but ultimately, the universe follows mathematical principles that don;t depend on our human need for simplification.

Now, regarding the speed of light, yes, the Doppler effect, or more specifically the redshift of light, is strong evidence that the speed of light is constant. This is central to Einstein’s special relativity, which tells us that the speed of light in a vacuum is the same for all observers, regardless of their motion or the motion of the source of the light.

As for the accelerating expansion of the universe, this is where general relativity comes into play. Space itself is expanding, and that’s not the same as objects moving 'through' space. When we talk about galaxies receding from us faster than the speed of light, we’re not violating relativity because it's not the galaxies themselves moving at superluminal speeds through space; rather, the space between us and them is expanding. This means that light from these galaxies will nevre reach us, because the fabric of space is stretching too quickly for the photons to make headway.

So, in a way, we have a sort of observational horizon, the so-called 'cosmic event horizon'. Beyond this, light from galaxies can’t reach us because space is expanding too fast. This is a direct consequence of the universe's expansion being governed by what's called 'dark energy', which is driving this acceleration.

Now, your intuition that we still have a lot to work out is absolutely correct! We don’t yet fully understand what dark energy is, or why the universe’s expansion rate changed over time. But the mathematical framework of relativity does an incredibly good job of describing what we 'can' observe. But there's nothing to say that what we can see is everything that is there.

So yes, there’s definitely a lot of bottoming out still to be done, but people far more clever than myself making good progress, I should think.

This is a direct consequence of the universe's expansion being governed by what's called 'dark energy', which is driving this acceleration.

We don’t yet fully understand what dark energy is, or why the universe’s expansion rate changed over time

If nobody understands what "dark energy" is, there is no possibility of certainty that this is driving the universe's acceleration. To my mind, it's the unknown coefficient "K" in the formula that tries to explain it all.

But the mathematical framework of relativity does an incredibly good job of describing what we 'can' observe.

That is no surprise. What we can observe can fit a mathematical bmodel of behaviour.

[idi banashapan]

Correct, there is no certainty it exists. It is just the best explanation (that dark matter does not interact with light, but does have mass, as something is holding things together) that fits the driven theories used in physics (Newton’s laws of motion and Einstein’s theory of relativity). Keep in mind dark matter has not been show horned into the theories, more the observations in relation to the theories show that something is missing. It’s the most logical answer to what is being seen.

The other option is that physics needs a new theory, such as Modified Newtonian Dynamics.

[joglynne]

Although reading this thread is still giving me a headache I find it fascinating, and I am never surprised, by our member's depth of knowledge when they get to discuss/debate subjects outside of the normal scope of the forum. Respect to you all.

[Sephiroth]

Originally Posted by idi banashapan

Correct, there is no certainty it exists. It is just the best explanation (that dark matter does not interact with light, but does have mass, as something is holding things together) that fits the driven theories used in physics (Newton’s laws of motion and Einstein’s theory of relativity). Keep in mind dark matter has not been show horned into the theories, more the observations in relation to the theories show that something is missing. It’s the most logical answer to what is being seen.

The other option is that physics needs a new theory, such as Modified Newtonian Dynamics.

First, I thank Joglynne for the kind words! The "depth of knowledge" does, however, stretch into sheer speculation.

@Idi suggests that the non-shoe-horning of dark matter into theories is the "most logical answer".

Obviously, this is where I contend otherwise. Logic says that you can't delve back to the beginning of the formation of matter because that's happened and we (our galaxy) must have happened much later. Or did it? Maybe it was always there in which case all sorts of light from everywhere that is not shifted outside our detection ranges can be observed.

The bit that's teasing the people who appear in money earners such as How the Universe Works, is why is the universe, according to their measurements, expanding at an increasing rate (acceleration)? So, "dark matter" and "dark energy" are invented (coefficient K) that balances the equations.

If they can see events from 13.5 billion years ago with the latest space telescopes, why isn't the so-called "dark matter" visible? Presumably because it's been conveniently named "dark" and thus we cannot see it. So, might it not be there? But it has to be there because something must account for the expanding universe. And so we go round in circles looking for the coefficient K - which cannot be found.

But some on this thread and various scientists say that galaxies that are receding from us at greater than the speed of light will always be invisible to us. So why do all the scientists say that nothing can travel faster than light because of Einstein's E=MC^2? Does the receding galaxy "know" it's travelling faster than the speed of light relative to something that cannot see it?

[daveeb]

Originally Posted by Sephiroth

First, I thank Joglynne for the kind words! The "depth of knowledge" does, however, stretch into sheer speculation.

@Idi suggests that the non-shoe-horning of dark matter into theories is the "most logical answer".

Obviously, this is where I contend otherwise. Logic says that you can't delve back to the beginning of the formation of matter because that's happened and we (our galaxy) must have happened much later. Or did it? Maybe it was always there in which case all sorts of light from everywhere that is not shifted outside our detection ranges can be observed.

The bit that's teasing the people who appear in money earners such as How the Universe Works, is why is the universe, according to their measurements, expanding at an increasing rate (acceleration)? So, "dark matter" and "dark energy" are invented (coefficient K) that balances the equations.

If they can see events from 13.5 billion years ago with the latest space telescopes, why isn't the so-called "dark matter" visible? Presumably because it's been conveniently named "dark" and thus we cannot see it. So, might it not be there? But it has to be there because something must account for the expanding universe. And so we go round in circles looking for the coefficient K - which cannot be found.

But some on this thread and various scientists say that galaxies that are receding from us at greater than the speed of light will always be invisible to us. So why do all the scientists say that nothing can travel faster than light because of Einstein's E=MC^2? Does the receding galaxy "know" it's travelling faster than the speed of light relative to something that cannot see it?

Nothing can travel faster than light in space/matter. Space creation isn't subject to this limitation, I'm not pretending to understand it, the maths presumably allows it to be true.

[Sephiroth]

Originally Posted by daveeb

Nothing can travel faster than light in space/matter. Space creation isn't subject to this limitation, I'm not pretending to understand it, the maths presumably allows it to be true.

Then how do you explain galaxies that we cannot see because they are receding from us at faster than the speed of light? Yet galaxies closer to the one in question can see the receding galaxy?

The "maths presumably" suddenly becomes "truth", but always without anyone knowing what the coefficient K is.

Originally Posted by Sephiroth

Then how do you explain galaxies that we cannot see because they are receding from us at faster than the speed of light? Yet galaxies closer to the one in question can see the receding galaxy?

The "maths presumably" suddenly becomes "truth", but always without anyone knowing what the coefficient K is.

Because the galaxy’s absolute velocity is not greater than the speed of light. The distance between that galaxy and us may be increasing at greater than the speed of light because we are moving away from the galaxy at speed, just as it it’s moving away from us. But neither object is moving at greater than light-speed if observed from a fixed point.

There is a concept called the ‘future light cone’ - the region of spacetime within which nothing is moving at greater than light-speed relative to us. Anything light-emitting within this region can be observed by us. But anything whose speed relative to us exceeeds light-speed, was never within our future light cone, and never will be.

[joglynne]

OK. <, sigh>> Skip to my final comment and don't feel you have to read/reply to this as it's probably a silly question and, as before, being totally irrelevant.

If there was a straight line from A through B & C to
point D. (Have I lost you yet)

1) I am on point A with a speedometer oh and a mahoosive telescope.

2) If I were able to see points B & D, point C being invisible (big bang or whatever) and .......

..... I can see B & D are moving apart from each other at the speed of light what would my speedometer register as the relative speed that they were travelling apart...... I am assuming super dooper planets could move that fast

Give up reading now unless you want the same headache as I have.

3) My head is seeing 2 cars, driving in opposite directions, passing at 30 mph surely they would be moving apart at 60 mph

Off to go and lie down in a dark room.

Add// Just realised I'm probably confusing the speed of light with the speed of sound but I have spent so much time pondering about all this that I blowed if I'm going to delete the sodding post!!!!!!!!!!