# Speed of heat

Thicko2006 | 02:42 Sun 19th Nov 2006 | Science
At 186,000 miles per second it takes about 8 minutes for light to reach here from the sun. Heat, I believe, does not travel so fast, so 1. How does it seem to get here at the same time? and 2 .How does it not dissipate in the intervening 93 million miles of space?

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No best answer has yet been selected by Thicko2006. Once a best answer has been selected, it will be shown here.

Light and heat are both in the electromagnetic spectrum and all radiation in the em spectrum travels at the same speed in a vacuum. Hence heat will get to us at the same time as the light.
Heat is three things (let's keep it simple):

conduction
convection

conduction is when an atom at some point starts getting warmer, which means it starts jumping up and down on its spot more. This then convinces atoms near it to start doing the same, and on an on. If you have a metal bar (metal is a good heat conductor), and put one end in a fire, then slowly the entire bar will get hot, due in large part to conduction.

convection is where atoms get warmer but due to the substance they're in (a liquid or gas), they can move upwards, as they kick out more at the other atoms so become less dense (and thus rise). you can see this if you turn your radiator on and put your hand over the top of it -- you'll feel the warm air.

radiation is due to the excited particles shaking off photons of energy (photons are the particles of the electromagnetic spectrum, such as light, radiowaves, microwaves, x-rays and gamma rays). these are typically in the infra-red part of the spectrum for heat.

now, both conduction and convection require atoms or something in order for the phenomena to happen. space is (in a simplified sense) a vacuum, and so conduction and convection won't work. radiation, however (electromagnetic waves), doesn't require any medium, so can happily move across a vacuum, hence why we get light. we also get radiation from the sun too.

all electromagnetic waves travel at the same speed in vacuum, so radiation gets here just as fast as light.

finally, we only get a small amount of the radiation. it's given off all over the sun, like a concentric sphere moving outwards from the sun. we only make up a small part of that, so what part of the radiation passes through us is grabbed by us. the rest just travels on.
Question Author
Thicko2006
The heat we get from the sun is infra-red light, so it travels at the same spped as the visible light, hence we experience the two at the same time, sunshine is warm.

The conduction and convection are due to 'hot' particles, usually a gas like hot air moving around and colliding with other colder air particles, sharing their heat energy and so dissipating the heat. Conduction and convection cannot come from the sun as there are no particles in the vacuum of space.
well done fo3nix for your briliant in depth explanation - I'm just gonna try and break it down. What a team.
Something else for you all to ponder on.

We are about 93million miles from the sun giving our orbital diameter of 186 million miles, the velocity of light in a vacuum ( space ) is said to be 186000 mph. Hmmm. does the velocity of light vary on different planets? Or indeed if we were part of a larger ( or smaller ) system, would the velocity change?
TeeGee seems to have noticed something that is almost a coincidence. It would be a coincidence if the diameter of the earth's orbit were exactly 186 million miles and if the speed of light were exactly 186,000 miles per second (not per hour). Why would he think that the velocity of light would be different on different planets? It does slow down just a little bit in different media, but the speed of light in vacuo is a universal constant.

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