[eli5] Why temperature is not relative if velocity is relative


So, we all know whether a train is moving depends on where the observer is. For passengers riding on it, the train is not moving. And high temperature are just particle moving with high speed. Is it possible that some observers will find a hot object cold? Just like the passengers on the train Thanks in advance!

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No, because it doesn’t work like that.

Relativistic effects are seen between objects moving at a high relative velocity, such as a distant galaxy retreating from Earth.

The molecules of a substance move in all different directions, and the temperature of that substance is the *average* kinetic energy of the molecules. (They also aren’t moving anywhere near light speed. Air molecules in a room at room temperature move at about 300 to 400 metres per second.) So the temperature of the object is, to all intents and purposes, identical to any observer.

How it *feels* to different observers depends not only on temperature but various physiological factors, so what is warm to one person may be cold to another. But this isn’t because of the relative motions of the molecules of the substance.

Temperature is a scalar value measuring the average thermal energy of a substance. Velocity is a vector value measuring the change in displacement. To measure the change in something you need a starting point.

> Is it possible that some observers will find a hot object cold?

No, because the hot object has particles moving randomly in many different directions at any one time. You could match the speed of some particles but that would mean all the others were moving faster and the average speed, which is what gives temperature, wouldn’t change, if anything increasing slightly.

By moving very fast relative to gases it can seem as if their temperature increases. Given that air molecules are moving roughly 1000 mph, you have to be going very fast to feel this effect; we’re talking about jet aircraft and rockets.

your perception is relative.

Cold and Hot to you IS relative, if you chill your hand then dip in lukewarm water, that water will feel scalding hot.

Temperature (how we measure things) DOES NOT rely on your senses, it relies on mercury, which is why it is not relative.

How fast something is going is relative to you the observer. But if someone aims a speed gun at the object while not moving, that velocity is not relative.

One answer is that temperature is always relative, it’s just to a well defined zero.

A more interesting way of seeing it is that it is actually kind relative luke you describe too. Temperature is basically how “riled up” a set of molecules are. The hotter they are, the more they’re vibrating and crashing into each other. When colder, they’re more stable and still.

Imagine what happens for a meteor entering our atmosphere. It blazes up in a ball of fire that’s many thousand degrees hot. But that sort of temperature wasn’t actually anywhere to be found. The meteor is cold, and the atmosphere is cold. So where does the sudden heat come from? Well, it comes from the meteor crashing into air molecules at very high speed.

What did we say temperature was again? Molecules at high speed crashing into each other…

So we *could* argue that the meteor was in fact hot all along, except all the molecules were moving in the same direction, so it’s only hot relative to our atmosphere and not hot by itself. Or we could argue that it was in fact our atmosphere that was very hot if we use the meteor’s perspective. Or we could argue that as long as it doesn’t hit our atmosphere, it’s cold. That would make it relative.