# Absolute Zero & Speed of Light

## Main Question or Discussion Point

Hi everybody, I would like to begin with apologizing for anything stupid I might say or ask because I am not a qualified physicist. I'd like some minutes of your time because i have a question that's been on my mind far too long and i would really like some help with this.

Am I correct when I think about Absolute zero and the speed of light along these lines;

the energy needed to slow atoms down increases towards infinity the closer you approach the lower cosmic speed limit, 0 km/s > no atomic movement is absolute zero

the energy needed to speed up atoms increases towards infinity the closer you approach the upper cosmic speed limit, 300 000 km/s > the speed of light

So is there a connection? If there is one I would think it is to be found in what sets those limits for speed? What controls the speed of light? Why is it 300 000 km/s? At the lower speed limit matter seems to get wonky (BECs) and at the upper limit space-time gets all wonky. Why? :p

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I can't shake the feeling that I am missing something here. Does the uncertainty principle play a role in all this? Do BECs exist in order to make sure that you can't violate the principle? As you cool an atom down you can determine its speed increasingly accurate so in order to balance speed vs location it just starts blurring out the location making it exist over a wider variety of possible locations at the same time? If this view is correct then why does it do that? What controls these limits, why can't we know speed and location at the same time?

I know many questions but i would really appreciate some answers. ^^

Thx for you time!

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K^2
the energy needed to slow atoms down increases towards infinity the closer you approach the lower cosmic speed limit, 0 km/s > no atomic movement is absolute zero
This is not true. The energy there is completely finite.

So what exactly are you saying? It's not possible to get to absolute zero right? Couldn't we invest ever more energy in building bigger and better machines to take us closer to it? If it's not an energy requirement than what is stopping us? Entropy?

This could turn out to be a really long thread because i have a ton of questions but i don't want to overwhelm you guys. Teach me! ^^

DaveC426913
Gold Member
if it's not an energy requirement than what is stopping us? Entropy?
http://en.wikipedia.org/wiki/Uncertainty_principle" [Broken].

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I am sorry if i continue to ask really weird questions but how exactly would this work? I know that heat is just molecules vibrating but how does heat move from one object to the next?

Situation 1) What happens to my hand on a molecular level when i put it on something hot (besides a burn of course :p). Do the molecules of the hot object bump into mine with such a force that they set the molecules in my hand in motion?

Situation 2) What happens in the shadow in a vacuum? To make it interesting let's say we build a machine in deep space that would try to cool matter to absolute zero blocked from all sunlight by mirrors or god knows what. How would heat then get transfered to the machine? If it's not a matter of energy then shouldn't we be able to just cool it all the way down without any heat leaking in there?

DaveC426913
Gold Member
I am sorry if i continue to ask really weird questions but how exactly would this work? I know that heat is just molecules vibrating but how does heat move from one object to the next?
There are 3 forms of heat transfer: radiation, conduction and convection.

Radiation: infrared light is emitted from an object and absorbed by another
Conduction: kinetic energy from vibrating and moving atoms is directly transferred to other atoms.
Convection: this is simply conduction with an intermediary fluid such as gas or liquid.

Situation 1) What happens to my hand on a molecular level when i put it on something hot (besides a burn of course :p). Do the molecules of the hot object bump into mine with such a force that they set the molecules in my hand in motion?
Pretty much, yes.

Situation 2) What happens in the shadow in a vacuum? To make it interesting let's say we build a machine in deep space that would try to cool matter to absolute zero blocked from all sunlight by mirrors or god knows what. How would heat then get transfered to the machine?
Precisely. How indeed.

If it's not a matter of energy then shouldn't we be able to just cool it all the way down without any heat leaking in there?
What is cooling? It is the transfer of heat. But heat flows from hot to cold. So you'd need something colder than the thing you're trying to cool.

(There are exceptions, such as adiabatic cooling, but basically, you've got to tackle them one by one)

Ultimately the problem is, you can cool a lot, but it becomes exceedingly difficult to remove all the heat.

Man I love science. I am trying to catch up on the basics but i am feeling very silly about not being interested in it when i was younger and still in school :(

You are saying the uncertainty principle is stopping us from reaching absolute zero? How exactly does that work? I thought we couldn't violate HUP by cooling because of Bose Einstein Condensation? My first thought was that the only reason a BEC exists is to enforce HUP, that a BEC is a manifestation of HUP in action obscuring location because we are gaining increasingly accurate values for its speed? If we would be able to measure an extremely accurate temperatue of a large amount of atoms at the same time... Would you see them smear out to hide their location? I don't get how reality can exist out of such a weird soup. :/

kinda silly topic title now that the connection i thought existed between lightspeed and absolute zero got debunked in the second post but I am still wondering as to what sets lightspeed. What sets the value at 300 000 km/s?

Thx a lot for your help btw.

To answer your simpler question, "What sets the value [of the speed of light] at 300 000 km/s?"; nothing does. It simply happens to be the upper limit based on the SI system's values of length and time. When the universe came into existence, the speed of light could have ended up being much slower, which would have lead to odd instances of time dilation all around us. It also could have been much faster, which would have diluted the effects of time dilation and allowed us to see much further into the outer reaches of the universe. What if it were only 500 km/s? 1 km/s? Even just 5 km/h? Although it is interesting to think about the implications of a different speed of light, there is nothing that sets it at 300 000 km/s besides the laws of physics that came with the formation of space-time.

DaveC426913
Gold Member
...there is nothing that sets it at 300 000 km/s besides the laws of physics that came with the formation of space-time.
I'm fairly confident that the question he is looking for an answer to is indeed 'what are the laws of physics that came with the formation of space-time?', not 'why is 300,000 a nice round number?'

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You are saying the uncertainty principle is stopping us from reaching absolute zero? How exactly does that work? I thought we couldn't violate HUP by cooling because of Bose Einstein Condensation? My first thought was that the only reason a BEC exists is to enforce HUP, that a BEC is a manifestation of HUP in action obscuring location because we are gaining increasingly accurate values for its speed? If we would be able to measure an extremely accurate temperatue of a large amount of atoms at the same time... Would you see them smear out to hide their location? I don't get how reality can exist out of such a weird soup. :/
I wouldn't take the connection between BEC:s and the uncertainty principle too far. First of all, remember that BEC:s only exist for bosons and not for fermions so it is not a general phenomenon. My understanding is that BEC happens because bosonic particles start to fall down into the lowest energy state at a very small, so called critical temperature. This results in weird quantum mechanical behavior. And sure, it is quantum mechanics - the physics of microscopics - appearing as a macroscopic phenomenon.

But I would say that the reason it is happening is not the uncertainty principle, but the property of bosons to be able to occupy the same state.

I'm fairly confident that the question he is looking for an answer to is indeed 'what are the laws of physics that came with the formation of space-time?', not 'why is 300,000 a nice round number?'
Indeed it is. It might be a bit too complex for my level but i get shivers just trying to understand all this crazy stuff. ^^

I don't get why they don't teach all this stuff in school. :(
It shouldn't go too deep, just a rough explenation, skim the surface of what is out there. It's so easy to become interested in science but I had never even heard of quantum mechanics, relativity or astronomy (as in neurtron stars, pulsars, stellar nurseries, ...). I had biology but they never even discussed evolution. We could be making a generation of scientists if we just updated our schools and stopped teaching them insanely outdated stuff such as there are only 3 states of matter. :/

I think I am overlooking something big because as i understand it all normal matter is made up out of fermions. Quarks are fermions and bosons are the so called force carriers like photons right? Don't they make these BECs out of normal fermionic matter? I know I am wrong because wiki too says that a BEC is made out of bosons but I also found something about a fermionic condensate which seems to be closely related. http://en.wikipedia.org/wiki/Fermionic_condensate

Another silly questions but could you guys draw out a basic path of understanding? Right now I am getting lost and it's really hard to see the trees through the forest. I keep digging down in specific topics but it's hard to see how it all fits together. So what I am saying is basically where should i start? I started out with basics like atomic structure and the electro magnetic spectrum. I really want to learn more about quantum realm but I feel I need to learn more about the basic structure of everything before i focus on "details".

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If absolute zero where ever achieved, would light travel at all? My feeling is that at absolute zero atoms/electrons would be 'frozen' by which I mean movement and thus transfer of energy would be impossible.

K^2
A finite pulse of light will always have finite temperature, so in a sense, you can't have absolute zero and light. It doesn't mean that light won't propagate at absolute zero, though. It means that if you have light propagation, you don't have absolute zero.
http://en.wikipedia.org/wiki/Uncertainty_principle" [Broken].
Uncertainty principle doesn't prevent absolute zero, just wanted to clarify that.

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There are some good thoughts here from the OP, but unfortunately the questions are uneducated. I'd recommend a book on statistical mechanics so that you understand what temperature means, what entropy is, what heat is, etc.

Basically, the temperature is related to how energy changes with respect to entropy, all other variables fixed. The whole point of statistical mechanics is that you are looking at a large number of particles, you can't single one out and analyze it that way.

K^2 - Thank you for your response and being patience with my stupid and uninformed question. I understand what you say (just!) but I'll like to take this further, but I'm going to need a little more time than I have available at the moment to express my views in a form that willmake any sense.

Rap
The answer to "why does light have the speed that it does" is "because the dimensionless constants of the universe (fine structure constant, gravitational coupling constant, elementary particle mass ratios, etc) have the values that they have".

In other words, as long as the dimensionless constants have the value that they have, the speed of light will always seem the same to us. There are a number of fundamental constants (speed of light, Planck's constant, charge of electron, etc). These can be combined to form the dimensionless constants. You could say that the speed of light could be varying all over the place, but if the other "fundamental constants" were also varying in such a way that the dimensionless constants remained the same, then we would never notice anything different. There would be no way to measure it, because our meter sticks would be changing, our clocks would be changing, etc. in such a way that it always came out the same. A scientist would say if you cannot measure it, it isnt worth thinking about.

The bottom line is, if you want to know why light has the speed that it does, then you have to ask why do the dimensionless constants have the values that they have? And that is a question that physicists cannot answer (yet).