Questions about the Speed of Light

[Thunderfoot]

Galilean relativity was pretty well known by the mid-20th century.

True - but largely ignored (Why I have no clue...)

 

[Thunderfoot]

<SNIP>
Something flying at mach 4 is not lighter than something flying at mach 2... in fact, it has slightly (very, very slightly) more mass and more weight. Now, if said jet has used up a lot of fuel to get to mach 4 obviously it will have less mass, and if it is higher up gravity will have a smaller effect, though that will not affect the mass of the jet.
<SNIP>QUOTE]
Actually - there have been tests on the SR71 done with equivilant fuel loads at both speeds - other than that I can agree with most of your post. I'm just Trolling along to bring up conjecture - as any good scientist will tell you - if it weren't for the nay sayers - I wouldn't have cared.

 

[Staffan]

I've got nothing against science, at all. Its just when people start saying things that aren't possible when we really have such a limited scope on what IS and ISN'T possible that gets me. Again, the universe is a big place.

Clarke's first law: When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

 

[fuindordm]

If we're going to be picky - that's the popular hypothesis, but it isn't the only one. It isn't as if our universe isn't already full of constants for which we don't have an origin. Maybe the constant "just is". Einstein first proposed the constant as "it just is", without knowing where it might come from, like the charge on an electron.

Speaking as a researcher in cosmology... it's pretty clear that the 'dark energy' component has a negative energy density; the astronomical observations that support the acceleration of the cosmic expansion also place limits on the equation of state of this component.

You're right, however, that we don't know what it is. It could be a cosmological constant (a fundamental number written into the laws of nature), or it could be a form of zero-point quantum energy that we don't understand yet, or it could be an unknown field (that some are calling quintessence in a fit of poetic sensibility)... it will take large surveys of distant supernovae and the large-scale distribution of galaxy clusters at the very least to figure out which of the three is more likely.

But considering that 10 years ago, cosmologists didn't even know how much matter there was in the universe, I consider the progress pretty good and the future bright.

Ben

 

[Ankh-Morpork Guard]

But considering that 10 years ago, cosmologists didn't even know how much matter there was in the universe, I consider the progress pretty good and the future bright.

Just give it a few years and the stars will be right...then things will change!

 

[bolen]

Speaking as a researcher in cosmology... it's pretty clear that the 'dark energy' component has a negative energy density; the astronomical observations that support the acceleration of the cosmic expansion also place limits on the equation of state of this component.

That seems a bit early to say isn't it. I have seen the plot of the red shift of high Z supernova and it seems like it does certainly favor some kind of acceleration but I would like to see those error bars ccme down before I would make such a statement. (Although I am a relativist not a cosmologist so if I am missing something tell me)

 

[Umbran]

Speaking as a researcher in cosmology... it's pretty clear that the 'dark energy' component has a negative energy density; the astronomical observations that support the acceleration of the cosmic expansion also place limits on the equation of state of this component.

My work is more in mathematical and simulations, but I'm reasonably familiar with the cosmology.

I'd put it this way - the data are mostly consistent with it being a negative energy density, but it is also consistent with other explanations. I haven't myself seen any models use the negative energy density that also predict somethign else unrelated and get it right. A model that predicts only what it is specifically designed to predict isn't much of a model.

The other way I've come to think of it is rather like QM - we have math that predicts, but we also have interpretations of what the math actually means, like the Copenhagen Interpretation of QM. For a few of the cosmological models, you have math that gets the right results, and then comes the question of interpretation - and depending how you manipulate it, you can interpret it as negative energy density, or as a cosmological constant that "just is".

 

[fuindordm]

My work is more in mathematical and simulations, but I'm reasonably familiar with the cosmology.

For a few of the cosmological models, you have math that gets the right results, and then comes the question of interpretation - and depending how you manipulate it, you can interpret it as negative energy density, or as a cosmological constant that "just is".

I thnk that's what I said, in another way...

I'm more of a simulationist too, and unfortunately my QM isn't strong enough to get deep understanding of the possible nature of the dark energy. From the perspective of cosmologists, the evolution of the universe can be modeled as the adiabatic expansion of a perfect fluid with more than one component. The radiation component has one equation of state, the matter and dark matter components have another equation of state (zero pressure, since it's so sparse), and the dark energy has a third exerting negative pressure proportional to the energy density. The equations that model the distribution of supernovae redshift come from following the evolution of these three components--these are the curves drawn through the data.

The dark energy component has some wiggle room, since we don't know the constant of proportionality between energy density and pressure. But if the pressure weren't negative then the curves would swing the opposite way and lie well outside the data, large error bars or no. (And keep in mind, even if the error bars are large when there's a lot of data you can still get pretty good statistical constraints.)

As for whether the energy density of the dark energy is *really* negative, from my point of view the difference between saying that it creates negative pressure or that it has negative energy density is purely semantic. A substance with an equation of state like Pressure = -C*density will have the effect in general relativity of promoting an expansion of space-time, wheras normal matter will have the effect of contracting space-time. So what is needed for a warp bubble is something like the dark energy, but in a controlled form. The surveys going on right now are trying to constrain the value of C.

A cosmological constant would still fit the data. The difference between a cosmological constant and dark energy is that the cosmological constant behaves like a 100% uniform field of dark energy with equation of state P = -density (C=1) --for example, the quantum mechanical zero-point field energy. I do know enough about quantum field theory, however, to say that all calculations of this field energy so far have either given a value of zero or a number dozens of orders of magnitude higher than the observed universe allows. So if it is a cosmological constant, its physical origin begs explanation.

To sum up, I would give a cosmological constant even odds against a new, exotic field. Both explanations have problems that can't be resolved right now, but both have the correct equation of state and fit the data. Both also have "negative energy density" in the sense that they provoke an expansion of space rather than the usual contraction, but you're right that if it turns out to be simply a cosmological constant there's probably little we can do with that.

On a final note, however, the cosmological constant isn't really constant; its value evolves along with the universe. In the far distant future, when the universe has expanded so much as to be effectively empty, its value will approach 1.

Ben

 

[fuindordm]

Just give it a few years and the stars will be right...then things will change!

Ah... you've uncovered the cunning plan of the world's cosmologists. In another ten billion years, we'll usher in a new age!

Ben

 

[Umbran]

Both also have "negative energy density" in the sense that they provoke an expansion of space rather than the usual contraction, but you're right that if it turns out to be simply a cosmological constant there's probably little we can do with that.

*nod*. And that's the crux of it, really - the negative energy density interpretation implies that there is something out there that we could potentially grab and use, the cosmological constant does not. While at the moment, the difference seems academic, in the long term, the differences may be stunning.

On a final note, however, the cosmological constant isn't really constant; its value evolves along with the universe.

As I understand it, current thinking is that there may have been variation among a number of the things we currently think of as "constants", so this merely puts it on par with the rest of the models.