Dark Matter and The Eather: What's the Difference?

[ComfortNumb]

Hi All,

First question so please go easy on me.

I'm currently reading 'Why Does E=mc2? at the moment and in an early chapter, Brian Cox referers to The Eather and how daft the concept was because it would cause drag in the universe and planets would lose orbital momentum etc, etc.

Now Eather was an unknown to explain a gap in a theory (as I understand it). So, what's the difference between the 19th century Eather concept and Dark Matter?

Will physicists in a 100 years time be looking back and thinking how quaint our 'belief' in dark matter was?

Happy Eather to you all!

ComfortablyNumb

 

[hkyriazi]

I would say that the luminiferous (light-carrying) aether of Kelvin and Maxwell was much less of an ad hoc creation than dark matter. Light clearly expresses wave properties, so it was natural to assume it was a wave in some material substance. No one was able to work out any consistent properties for it, but they were quite certain of its reality.

Dark matter, on the other hand, is proposed to exist in order to explain how galaxies and galaxy clusters stay together despite not having enough matter for the usual formulations of gravity to hold together. That's a purely ad hoc postulation.

 

[ComfortNumb]

Thanks for that. Made things a bit clearer in the murky waters of Dark Matter!

 

[Chalnoth]

I would say that the luminiferous (light-carrying) aether of Kelvin and Maxwell was much less of an ad hoc creation than dark matter. Light clearly expresses wave properties, so it was natural to assume it was a wave in some material substance. No one was able to work out any consistent properties for it, but they were quite certain of its reality.

Dark matter, on the other hand, is proposed to exist in order to explain how galaxies and galaxy clusters stay together despite not having enough matter for the usual formulations of gravity to hold together. That's a purely ad hoc postulation.

No, I'm sorry, but I can't understate how just wrong and misleading this post is (and, by the way, I am tempted to use much stronger language than this). We know dark matter exists, because dark matter is the simplest, most reasonable explanation for a wide body of observational phenomena. Perhaps the most stunning example is the Bullet Cluster, one explanation of what this means for dark matter can be found here:
http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/

Dark matter, in short, is evidenced by a wide variety of experimental evidence, and is thus in an entirely different class than the luminiferous aether.

 

[orange31]

I have a question related to this post: How exactly do cosmologists quantify the amount of kinetic energy in the universe? Or potential energy? Surely the equation E=mc2 implies that energy and mass are correlated, and that energy itself has a mass (and therefore a gravitational effect). So how do you measure the total potential energy or kinetic energy in the universe?

I suppose a talented mathematician could estimate the total thermal energy in the universe, but none of these energies can be visually observed and so is it possible that these energies are being overseen?? Could these be dark energy??

 

[Chalnoth]

I have a question related to this post: How exactly do cosmologists quantify the amount of kinetic energy in the universe? Or potential energy? Surely the equation E=mc2 implies that energy and mass are correlated, and that energy itself has a mass (and therefore a gravitational effect). So how do you measure the total potential energy or kinetic energy in the universe?

Well, kinetic energy is largely a function of temperature. With the current average temperature of the universe at 2.725K, the average kinetic energy of particles is effectively zero in a cosmological sense. Now, in the very early universe, when temperatures were higher than the rest masses of many particles, the precise opposite was the case, and it was the kinetic energy that was the most significant energy density in the universe.

As far as potential energy is concerned, the potential energy of an object in a gravitational potential well is negative, such that the negative gravitational potential energy largely offsets any gain in kinetic energy it might obtain from falling into said potential well.

I suppose a talented mathematician could estimate the total thermal energy in the universe, but none of these energies can be visually observed and so is it possible that these energies are being overseen?? Could these be dark energy??

It's actually quite easy to observe: it's the energy of cosmic microwave background. And no, kinetic energy acts extremely differently from dark energy.

 

[orange31]

Thank you for your explanation. I was trying to get my head around the scenario that if you had 2 planets and placed them side by side, they would perhaps crumble and merge together, but if you had the same 2 planets and put them 1,000,000 km apart, they would fall together and crash together with huge force. The same amount of matter, but more potential energy in the system, and as E=mc2, wouldn't that suggest more mass?

So from this I concluded that if 2 objects move away from each other, the mass of the collective objects would increase. If the universe is expanding, then isn't the mass of the universe also increasing?

 

[Chalnoth]

Thank you for your explanation. I was trying to get my head around the scenario that if you had 2 planets and placed them side by side, they would perhaps crumble and merge together, but if you had the same 2 planets and put them 1,000,000 km apart, they would fall together and crash together with huge force. The same amount of matter, but more potential energy in the system, and as E=mc2, wouldn't that suggest more mass?

So from this I concluded that if 2 objects move away from each other, the mass of the collective objects would increase. If the universe is expanding, then isn't the mass of the universe also increasing?

Well, if you want to take the situation where the only energy in question is rest mass energy, then the situation you're describing is two planets very far away at rest with respect to one another. As the two objects get closer to one another, they pick up kinetic energy as the gravitational potential energy gets more and more negative.

For the two planets to start out at rest closer to one another, they would actually have to have less total energy than their respective rest mass energy.