Sean Carroll

If you can at time t1 select an event, you can't at time t1 have a record of the event. So it seems that either lack of influence is a precondition or else a co-condition of records. In Janan Ismail's book, How Physics Makes Us Free, she offers a united explanation of both arrows of time, influence, and records.

Local macroscopic changes to the present state of the world propagate asymmetrically into the past and future. So who's right about the direction of explanations here? Or is this a case where, depending on what the audience already understands, the explanation can go either way?

Local macroscopic changes to the present state of the world propagate asymmetrically into the past and future. So who's right about the direction of explanations here? Or is this a case where, depending on what the audience already understands, the explanation can go either way?

I do think there's a unified explanation here, and I might have been, or the specific language that I used might have been chosen sloppily here. When I say because we have memories of records of the past, we can't change them, what I mean is because we have what we think of as reliable memories and records of the past, we know we can't change them.

I do think there's a unified explanation here, and I might have been, or the specific language that I used might have been chosen sloppily here. When I say because we have memories of records of the past, we can't change them, what I mean is because we have what we think of as reliable memories and records of the past, we know we can't change them.

But our knowledge of the fact that we can't change them is different than the fact we can't change them. I don't love the language of – macroscopic changes to the present state of the world propagating asymmetrically into the past and the future. I'm not quite sure what it means that they propagate. But of course, you can invent a meaning for it.

But our knowledge of the fact that we can't change them is different than the fact we can't change them. I don't love the language of – macroscopic changes to the present state of the world propagating asymmetrically into the past and the future. I'm not quite sure what it means that they propagate. But of course, you can invent a meaning for it.

I'm not saying that it's meaningless, but I'm saying that you have to be very clear. There's more words that would need to be stated to go into what exactly is being said. But anyway, I don't think there's any... weirdness or mysteriousness or true disagreement here. The overall unmistakable fact is you have some macroscopic incomplete information about the present.

I'm not saying that it's meaningless, but I'm saying that you have to be very clear. There's more words that would need to be stated to go into what exactly is being said. But anyway, I don't think there's any... weirdness or mysteriousness or true disagreement here. The overall unmistakable fact is you have some macroscopic incomplete information about the present.

You have some hypothetical information about the past in the form of a low entropy boundary condition. You have no information about the future. There's no boundary condition that you are imposing there. And given those three ingredients, you will get an asymmetry both of memory and causality.

You have some hypothetical information about the past in the form of a low entropy boundary condition. You have no information about the future. There's no boundary condition that you are imposing there. And given those three ingredients, you will get an asymmetry both of memory and causality.

Ryan Hibbs says, the light we see from stars represents the object as it was in the past due to the speed of light and the further away the object is, the faster it's moving away from us. How do we know from just this data that expansion is accelerating and not that expansion just used to be faster in the past than it is in the present? You know, I mean, cosmologists aren't dummies, okay?

Ryan Hibbs says, the light we see from stars represents the object as it was in the past due to the speed of light and the further away the object is, the faster it's moving away from us. How do we know from just this data that expansion is accelerating and not that expansion just used to be faster in the past than it is in the present? You know, I mean, cosmologists aren't dummies, okay?

They have ways of thinking about this. They know what the issues are. And the thing is, you don't simply—these words that we use when we talk about these in ordinary language don't map exactly onto what real cosmologists actually do. The point is that cosmologists have a model. They don't just use words like, ah, the universe is expanding, the universe is accelerating.

They have ways of thinking about this. They know what the issues are. And the thing is, you don't simply—these words that we use when we talk about these in ordinary language don't map exactly onto what real cosmologists actually do. The point is that cosmologists have a model. They don't just use words like, ah, the universe is expanding, the universe is accelerating.

You say, I have Einstein's equation in general relativity. Under assumptions of homogeneity and isotropy, this turns into a differential equation for the scale factor of the universe, the relative size of the universe at different times, called the Friedmann equation. And the Friedmann equation depends on the sources of matter and energy and curvature in the universe.

You say, I have Einstein's equation in general relativity. Under assumptions of homogeneity and isotropy, this turns into a differential equation for the scale factor of the universe, the relative size of the universe at different times, called the Friedmann equation. And the Friedmann equation depends on the sources of matter and energy and curvature in the universe.

the amount of ordinary matter, dark matter, radiation, vacuum energy, et cetera. And with all of these ingredients, the specification of what those ingredients are, how much matter, how much radiation, et cetera, at any one moment of time allows you to predict what the observed relationship should be between a redshift and a distance.

the amount of ordinary matter, dark matter, radiation, vacuum energy, et cetera. And with all of these ingredients, the specification of what those ingredients are, how much matter, how much radiation, et cetera, at any one moment of time allows you to predict what the observed relationship should be between a redshift and a distance.

And then you match the data to the observations and you figure out, aha, the data will match the observations if 70% of the universe is dark energy, vacuum energy, 30% is matter, and 10 to the minus 4 is radiation, something like that. Now, when we explain this to people, they don't want to hear that. They want to hear the universe is accelerating or the universe is not accelerating.