Black Holes and the Arrow of Time

Black Holes and the Arrow of Time

What does heat have to do with time? How did Einstein try to comfort the family of a deceased friend?

Transcript Black Holes and the Arrow of Time

KEVIN HARRIS: What does all of that have to do with today’s podcast? Keep listening and find out. Welcome to Reasonable Faith with Dr. William Lane Craig. I’m Kevin. By the way, before we get started, let me remind you to check out all the latest things on our website, If you haven’t been there in a while, stop by. Lots of people are talking about the animated videos that illustrate some of Dr. Craig’s major work. They are fantastic. Those are available right there on the site. In the Q&A section, I noticed Dr. Craig interacts with a person who is an agnostic yet has questions about the Trinity. It is mind-boggling to say the least. As always, you can give to Reasonable Faith to support the very important work Dr. Craig is doing all over the world. If you get a chance, donate right on the website. Thanks for not only keeping us in prayer but blessing us financially. Thanks for stopping by today.

“Hot black holes and the arrow of time” - Aeon Magazine did a little interview question with Carlo Rovelli.[1] You are familiar with Rovelli.

DR. CRAIG: Yes. Carlo Rovelli is an important contemporary physicist, and one who thinks about the deeper metaphysical and philosophical issues raised by contemporary science. In this interview we see exactly that sort of interplay of philosophy and science.

KEVIN HARRIS: The question asked was, “Does the scientific notion that time is an illusion change your perceptions, as it did for Einstein?”

DR. CRAIG: Notice the loaded question - “the scientific notion” that time is an illusion. That is not a scientific notion. That is a philosophical notion. That is a metaphysical view that time is illusory. It is simply false that science implies or requires that time is somehow illusory. That is a metaphysical interpretation.

KEVIN HARRIS: He answers in part,

One of the biggest mysteries in modern physics is something that we experience literally every moment of our lives. Physics describes the world by means of formulas that tell us how things vary as a function of time, but it does not explain what time is.

DR. CRAIG: Now that, I think, is perhaps the most insightful sentence of the entire interview, and one which unfortunately Rovelli himself does not really take seriously because he goes on to dismiss the reality of time on the basis of scientific reasoning. But if it is true that physics does not tell us what time is as, I think, Isaac Newton rightly said, then what physics gives us at best are measures of time. But time itself is a metaphysical reality, not a physical reality. At best what physics tells us is how clocks measure time – how our physical instruments register time. But it does not tell us what time itself is. I think a knockdown argument for this is we can imagine time existing wholly in the absence of any physical reality. If we think of God existing alone without the universe, we can imagine that God is counting; say, counting down to creation – 3, 2, 1, let there be light. Clearly there would be a sequence in the contents of consciousness of God that would be sufficient for time to exist even though there is no physical reality whatsoever. If you don’t like God in this example then just simply use a finite consciousness which is counting or has a succession in the contents of consciousness. I think that is a knockdown argument for the statement that Rovelli makes here – that physics doesn’t tell us what time is. Time is something that transcends physics. Physics at best tells us how things vary with time, as he says, or what our measures of time are, but it doesn’t tell us what time is.


Another way of posing the problem is to ask: what is the present? We say that only the things of the present exist; the past no longer exists and the future doesn’t exist yet.

DR. CRAIG: OK. Let’s pause there.[2] This is describing a philosophical view called presentism. Presentism is the view that the only temporal items that exist are those that are present. This is a view of time that is committed to the objectivity of temporal becoming – that things really do come into being and go out of existence. This would be in contrast to a view of time that has been variously called the tenseless view of time or the static view of time or, in a convenient nomenclature borrowed from the British idealist J. M. E. McTaggart, the B-theory of time. That is the view that temporal becoming is an illusion of human consciousness, all things in time are equally real and existent whether past, present, or future. That is in contrast to what everyone, including Rovelli, recognizes is the commonsense view that the past no longer exists, the future doesn’t yet exist, that the only temporal items that exist are those that exist presently.

KEVIN HARRIS: And that’s the A-theory?

DR. CRAIG: Right.

KEVIN HARRIS: Then he says,

But in physics there is nothing that corresponds to the notion of the ‘now’. Compare ‘now’ with ‘here’. ‘Here’ designates the place where a speaker is. For two different people, ‘here’ points to two different places. Consequently ‘here’ is a word whose meaning depends on where it is spoken. The technical term for this kind of utterance is ‘indexical’. ‘Now’ also points to the instant in which the word is uttered and is also classed as indexical.

DR. CRAIG: Here Rovelli gets into philosophy of language and begins to talk about indexical terms. As he notes, there are spatial indexical terms like “here.” We would all agree that there is no objective “here.” “Here” is where the speaker is. If I am in Atlanta and I would say, “Atlanta is here,” that would be true, but someone that is in La Mirada would say, “Biola University is here and Atlanta is there.” So the spatial indexical doesn’t describe an objective reality.

The question is: what about the temporal indexical “now” or “today” or “presently?” When we say, “It is presently hot” or “The universe is presently of such-and-such a density or temperature” is that an objective fact about reality or does that simply express the standpoint of the speaker? You can’t just write off the objectivity of the now or the present because spatial indexicals don’t designate objective reality. A good many philosophers would say that temporal indexicals do designate an objectively real present.

The analogy here would be with personal indexical terms. I say something like, “I am making this podcast with Kevin Harris.” When I say that, I designate a first-person perspective that I call “I” but you wouldn’t say that. You would say, “I am making a podcast with William Lane Craig” or “I am making a podcast with you.” You would use the personal indexical term “you” when talking to me whereas I would use the personal indexical “I” when speaking of myself. This first-person perspective is privileged. No one else has access to my first-person perspective. At best you can say, “You” or “he.” But you can’t say “I” referring to William Lane Craig. You will say “I” and it will be Kevin Harris – your first-person perspective.

Now, are these personal indexical terms non-objective like “here?” Do you not really exist? Is that what you are prepared to say? That I don’t really exist? It seems to me that that is patently absurd. Of course I exist. So I would say that you can give a reductive analysis of spatial indexical terms like “here” and “there” . . .

KEVIN HARRIS: Indexical – index.

DR. CRAIG: Yeah, they are indexed to a certain person, time, or place.

KEVIN HARRIS: Indexical cards. Index cards. Category? Index.

DR. CRAIG: “Index” means they are connected to a certain reference point.[3] Like a personal indexical is a term that has reference to or is connected with a personal perspective. A spatial indexical to a place. Temporal indexical to a time. What I want to say – and I’ve argued in my published work – is that we can give a reductive analysis of spatial indexicals in terms of the objective reality of personal and temporal indexicals. In particular, “here” is where “I now am.” Given the objectivity of the “I” and the “now” you can explain the subjectivity of the “here.” So I think that “now” designates an objective reality just like “I” designates an objective reality.

KEVIN HARRIS: And it is not an illusion.

DR. CRAIG: Not at all illusory – no more illusory than my own existence.


No one would dream of saying that things ‘here’ exist, whereas things that are not ‘here’ do not exist. So then why do we say that things that are ‘now’ exist and that everything else doesn’t? Is the present something that is objective in the world, that flows, and that makes things exist one after the other, or is it only subjective, like ‘here’?

DR. CRAIG: That is the question between the A- and the B- theory of time. Though I would say that the flow of time is a metaphor. Time doesn’t literally flow. The flow of time is a metaphor for temporal becoming. The question is: is temporal becoming an objective feature of reality? Do things really come into being and go out of being or not?

In my published work I give a number of reasons why I think that we should affirm the A-theory of time and the objectivity of temporal becoming as opposed to the B-theory of time and the tenseless view of reality. There are both good arguments for the A-theory and good objections against the B-theory. Moreover, there are no good objections against the A-theory, and there are no good arguments for the B-theory.

KEVIN HARRIS: He says that this is more than just a little obscure mental exercise. There is more to it than just this fun thought process and thinking about it. Physics makes this “a burning issue.” How?

DR. CRAIG: He says here,

Einstein’s theory of special relativity has shown that the notion of the present is also subjective.

I think this is undoubtedly the principal motivating reason for people to adopt the tenseless or B-theory of time. But here I have to say shame on Professor Rovelli for saying that Einstein’s theory of relativity has shown that the notion of the present is subjective. That is false! What it shows, if anything, is that the present is relative; that is to say, what is present is relative to a reference frame or an inertial frame. But it doesn’t show that it is subjective. I am not talking here from the standpoint from an A-theorist. I’m just talking here from the standpoint of a physicist – what does the theory say? What it says is that what is present is relative to a reference frame. But that reference frame is a hypothetical inertial frame. When relativity talks about hypothetical observers, it doesn’t mean people or consciousnesses. It could be a machine. It could even just be, as I say, a system of coordinates – an inertial frame. So in no way is it true that relativity shows that the present is something subjective like “the here.” Rovelli knows that. I’m sure he would retract that statement if he could.

But I would argue further that neither does the special relativity theory show that the notion of the present is relative. That is false. There are various physical interpretations of the mathematical equations of special relativity.[4] In particular, the interpretation of H. A. Lorentz, who was Einstein’s contemporary and collaborator, holds that there is an absolute reference frame with respect to which temporal becoming occurs, and that the effects described by the special theory of relativity are the result of the deformations that are caused in our measuring instruments by uniform motion relative to this fundamental frame. Moving clocks run slow. Moving measuring rods shrink up in the direction of motion. This is right in line with what Rovelli says earlier when he says that physics doesn’t explain what time is, it just tells us how things vary as a function of time. Clocks run slow when they are in uniform motion; measuring rods shrink up. What physics describes is the physical behavior of clocks and rods. It doesn’t tell us about time itself, which is a metaphysical quantity independent of these physical measures of time.

KEVIN HARRIS: He seems to be asking, why do we have this sense of a flow of time?

DR. CRAIG: I want to say one other thing. This is so great. I was amazed when Rovelli quotes Einstein’s letter to Michele Besso’s widow when Michele Besso died. Einstein’s lifelong friend – the only person he cites in the original special relativity paper of 1905. When Besso died Einstein tried to comfort his family by saying that,

Michele has left this strange world a little before me. This means nothing. People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.

If this was meant to bring comfort . . .

KEVIN HARRIS: That’s a Hallmark card if I’ve ever heard one! [laughter]

DR. CRAIG: Yeah! If this was meant to bring comfort to his grieving family, this is the most lame and obtuse thing you could possibly write. I’m sure that Besso’s family were not very comforted by knowing that his worldline in spacetime had ended only somewhat earlier than their worldline will, but that it still exists. This was a lame attempt by Einstein to bring comfort. And yet Rovelli refers to this as a moving letter to Besso’s family, which I think most people that I’ve read would say this was anything but a moving letter!

KEVIN HARRIS: Maybe his heart was in the right place, but not very tactful.

DR. CRAIG: And not very comforting. There’s no comfort in knowing that a person’s tenseless worldline ends earlier than somebody else’s worldline.

KEVIN HARRIS: Fascinating.

DR. CRAIG: Yeah, it is. It really is.

KEVIN HARRIS: He says, “Illusion or not, what explains the fact that for us time flows?”

DR. CRAIG: Right. The presentist, or A-theorist, has an obvious explanation of that – the objectivity of temporal becoming. It is because things really come into being and go out of being that we experience the so-called passage of time. It is a metaphor for the reality of temporal becoming. The A-theorist has a very ready and obvious explanation for our experience of time’s passage, namely, the objectivity of temporal becoming.

KEVIN HARRIS: He ends the article here on the second page – Bill, you’ll have to explain it. He thinks, “At one level, I think that the answer lies in an intimate connection between time and heat.” Is this where black holes come in?

DR. CRAIG: Not really. He tries to draw it in, but this is just the old hackneyed attempt to explain the arrow of time by thermodynamics. According to the second law of thermodynamics entropy increases over time. It increases in the later-than direction. So many, many scientists think that the direction of time is determined by the thermodynamic era. But as philosophers of time like Lawrence Sklar of the University of Michigan have pointed out, this is a viciously circular explanation because the very claim that entropy increases presupposes a direction to time rather than saying that entropy decreases.[5] You are postulating or presupposing that there is one direction of time in which entropy grows, and another direction of time in which it declines. So this attempt to explain away time’s arrow thermodynamically is really quite hopeless. It is viciously circular.

KEVIN HARRIS: Is he hoping that . . . because the last sentence here says,

In the end, it seems that the heat of black holes is like the Rosetta stone of physics, written in a combination of three languages – quantum, gravitational, and thermodynamic – just awaiting decipherment in order to reveal the true nature of time.

Is he holding out hope?

DR. CRAIG: Well, this is just mumbo-jumbo – it explains nothing nor does it explain how physics could explain time when he says time is not something that physics can explain. What is important here in this sentence, however, is that what it draws our attention to is that special relativity is only one aspect of contemporary physics. Even if in special relativity what is present is relative to reference frames, that isn’t true in other fields of physics like quantum mechanics – that other pillar of contemporary physics. In general relativity when you apply general relativity to the cosmos as a whole you get an expanding universe (beginning in the Big Bang and then expanding) which has a cosmic time parameter associated with it. When scientists say that the universe is around 14 billion years old, they are not talking just in Earth-time. They don’t mean relative to our reference frame. Rather, this cosmic time parameter is independent of space. It is independent of spatial coordinates. Therefore, cosmic time is the same for every hypothetical observer in the universe regardless of his state of motion. In other words, cosmic time is a kind of reinstatement of Newton’s absolute time. It measures the duration of the universe in a frame-independent way. So special relativity is simply not the final theory on the nature of time even in physics. Elsewhere in physics, you do have the notion of an absolute present and a time parameter that is independent of your frame of reference.

KEVIN HARRIS: I want to encourage our listeners to go to and read some of the articles, and get your books that you’ve devoted to this topic.

DR. CRAIG: Time and Eternity would be the book that I would recommend to laypersons who want to get into this very question that we’ve been talking about today, and then, of course, the theological implications then – what does all this mean for God’s relationship to time?[6]

[1] See (accessed January 22, 2016).

[2] 5:02

[3] 10:11

[4] 15:04

[5] 20:09

[6] Total Running Time: 23:46 (Copyright © 2017 William Lane Craig)