Sean Carroll

My father always wanted to understand the answer to the question, if mass, a large object, follows the curvature of the fabric of space, wouldn't then there be some kind of small measurable background heat due to friction of its motion through space? So yes and no. Strictly speaking, no.

My father always wanted to understand the answer to the question, if mass, a large object, follows the curvature of the fabric of space, wouldn't then there be some kind of small measurable background heat due to friction of its motion through space? So yes and no. Strictly speaking, no.

There's not friction due to the motion of objects through space because the idea that you have that there should be friction comes from a very higher level, non-fundamental description of reality, right?

There's not friction due to the motion of objects through space because the idea that you have that there should be friction comes from a very higher level, non-fundamental description of reality, right?

If I push a tire down the road, there is friction because the tire is made of atoms and the road is made of atoms and the air through which it moves is made of atoms and there are photons bouncing off the tire. And in all of these invisible ways, there's noise and friction and dissipation and energy gets lost from the tire.

If I push a tire down the road, there is friction because the tire is made of atoms and the road is made of atoms and the air through which it moves is made of atoms and there are photons bouncing off the tire. And in all of these invisible ways, there's noise and friction and dissipation and energy gets lost from the tire.

And so you perceive that as a kind of frictional force that eventually slows down the tire. If I have an elementary particle or a single object moving through the universe, space itself is not made of atoms in the same sense that the tire or the road is made of atoms. There isn't any way for that object to sort of give off energy to the medium around it.

And so you perceive that as a kind of frictional force that eventually slows down the tire. If I have an elementary particle or a single object moving through the universe, space itself is not made of atoms in the same sense that the tire or the road is made of atoms. There isn't any way for that object to sort of give off energy to the medium around it.

The medium around it is as empty as it's possible to be. So strictly speaking, no. There's no friction of that kind because the medium we're talking about is a much more basic element of reality, it's not this collective thing that you get by taking many, many atoms and jiggling them together.

The medium around it is as empty as it's possible to be. So strictly speaking, no. There's no friction of that kind because the medium we're talking about is a much more basic element of reality, it's not this collective thing that you get by taking many, many atoms and jiggling them together.

Having said all that, there's one kind of tiny caveat, which is that if you move an object back and forth, rather than just like letting it move through space, actually, sorry, let me back up because I realized I missed a chance to explain something. I can prove that an object moving through space does not slow down. The proof is the following.

Having said all that, there's one kind of tiny caveat, which is that if you move an object back and forth, rather than just like letting it move through space, actually, sorry, let me back up because I realized I missed a chance to explain something. I can prove that an object moving through space does not slow down. The proof is the following.

There's no such thing as a reference frame for velocity in relativity. There's no preferred velocity to the universe. So slow down compared to what? Said in other words, if there was only one object in the universe, I could always describe that object in its rest frame, in the frame in which it's not moving at all. And there it's just not moving, so there's no need for it to give off—

There's no such thing as a reference frame for velocity in relativity. There's no preferred velocity to the universe. So slow down compared to what? Said in other words, if there was only one object in the universe, I could always describe that object in its rest frame, in the frame in which it's not moving at all. And there it's just not moving, so there's no need for it to give off—

energy and slow down. And if it just stays there perfectly ordinarily in its rest frame, then in some frame that is moving with respect to it at constant velocity, you will always see it moving at constant velocity. Okay, that's one way of saying it.

energy and slow down. And if it just stays there perfectly ordinarily in its rest frame, then in some frame that is moving with respect to it at constant velocity, you will always see it moving at constant velocity. Okay, that's one way of saying it.

Which is why if instead I'm shaking it back and forth, rather than just having it move at a constant velocity, then it's a different story, because then it is coupled to gravity. Everything is coupled to gravity. There's a gravitational field for this massive object that you're shaking back and forth, and there it will emit gravitational waves.

Which is why if instead I'm shaking it back and forth, rather than just having it move at a constant velocity, then it's a different story, because then it is coupled to gravity. Everything is coupled to gravity. There's a gravitational field for this massive object that you're shaking back and forth, and there it will emit gravitational waves.

It might also emit photons or something like that, electromagnetic waves. Because every object creates gravity, if it's moving on a non-uniform trajectory, it can lose energy by emitting through gravitational waves. Indeed, when you get a detection of gravitational waves at a gravitational wave observatory like LIGO, Why do you do it?

It might also emit photons or something like that, electromagnetic waves. Because every object creates gravity, if it's moving on a non-uniform trajectory, it can lose energy by emitting through gravitational waves. Indeed, when you get a detection of gravitational waves at a gravitational wave observatory like LIGO, Why do you do it?