If you are near something hot, say an open flame, you can feel the heat radiating from the fire. I have a juvenile understanding of thermodynamics, but I understand that there is energy radiating from the heat source which is what I’m feeling.
By contrast, if you hover your hand near an ice cold drink, you can FEEL the cold before even touching the glass.
How is this possible since my hand is seemingly radiating more energy than the drink is? Is what I’m feeling just the energy leaving my hand and entering the drink?
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You’ve predictably gotten a few answers claiming that we can’t sense temperature – only the flow of heat. This isn’t true. It’s become one of those science memes that at this point serves more to hinder understanding than to facilitate it.
Homeostasis requires absolute sensing of body temperature, which IIRC is done by the hypothalamus. If you sit in a 100F hot tub for long enough, your body will stabilize at that temperature. There’s no more heat flow between you and the water, but you’ll still feel uncomfortable and hot as hell.
A cursory reading of how [thermoreceptors](https://en.wikipedia.org/wiki/Thermoreceptor#Mechanism_of_transduction) are thought to work indicates that they do sense absolute temperature, as well as relative temperature. [Another paper](https://www.sciencedirect.com/science/article/pii/S096098220600354X) on the topic. Your skin senses “heat flow” insofar as the magnitude of the heat flow is what determines the temperature of your skin cells (as well as the thermoreceptors). But it’s not correct to say that you can only sense heat flow.
Heat flux certainly explains why some surfaces feel colder or warmer than others despite being at the same temperature, but that’s not literally what your body is sensing. It can detect both absolute and relative changes in temperature.
As for the glass – a cold glass doesn’t just exist in a vacuum – although it can, and in that case it wouldn’t feel cold when you brought your hand near it. The glass is cooling the air around it, which increases its density, which causes it to flow down along the sides of the glass and warm air to take its place. This is convection and is happening constantly until the temperature stabilizes. The cold air is what you’re feeling.
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