Help me understand why “Block Universe” is not scientifically proven

[Dale]

is it true that, according to relativity, all events in spacetime are “fixed”?

The concept of an event being "fixed" or not is not part of the math, nor is it part of any experiment. It is only part of the block universe interpretation. The math just tells you that if $(t,x,y,z)$ are the coordinates of the event in one frame then $(t',x',y',z')$ are the coordinates in another frame. More physically, it says that the quantity $ds^2=-c^2 dt^2+dx^2+dy^2+dz^2$ is invariant. There is no mathematical representation of the block universe concept of "fixed", nor is there an experimental meaning to it.

 

[ojitojuntos]

The concept of an event being "fixed" or not is not part of the math, nor is it part of any experiment. It is only part of the block universe interpretation. The math just tells you that if $(t,x,y,z)$ are the coordinates of the event in one frame then $(t',x',y',z')$ are the coordinates in another frame. More physically, it says that the quantity $ds^2=-c^2 dt^2+dx^2+dy^2+dz^2$ is invariant. There is no mathematical representation of the block universe concept of "fixed", nor is there an experimental meaning to it.

Thank you for the clarification. I think that part of my confusion comes due to the adamant defense of the block universe concept by some speakers, like Sean Carrol.
I’ve read about SR in the last months since I asked this question, and I don’t see why the block universe is treated as a “natural conclusion” of relativity.

 

[phinds]

I don’t see why the block universe is treated as a “natural conclusion” of relativity.

Any such statement is an OPINION (or possibly an interpretation --- same thing), not a scientific fact.

 

[Dale]

And regardless of the expertise or assertiveness of the opinion holder, it remains an opinion.

I don’t see why the block universe is treated as a “natural conclusion” of relativity.

It is a natural conclusion in the sense that given the math of relativity it is natural that many people would hold that opinion.

 

[Ibix]

’ve read about SR in the last months since I asked this question, and I don’t see why the block universe is treated as a “natural conclusion” of relativity.

I'd say that the block universe has the least philosophical baggage (not counting "shut up and calculate"). The observation that the maths of coordinates in relativity is the same as the maths of Minkowski space is unarguable, so the interpretation aspect is basically "let's take that literally". On the other hand, LET and hybrid ideas like a growing block universe also need a global boundary between past and future which is undetectable even in principle. So something that is absolutely critical to the model but of zero consequence in practice and hence completely arbitrary.

So I think one can argue for the block universe as being the simplest interpretation in an Occam's Razor sense. I don't know that I would call it the "natural" interpretation though - that would be a matter of opinion, in my opinion anyway.

 

[ojitojuntos]

Thank you for your clarification! I have the impression that physics is a field in which the math is as important as the experimental data. This being the case, if taking the math literally can lead to understanding the universe as a Block Universe, why is this not treated as a given in a similar way to how we treat probabilities in quantum mechanics?
Also, please correct me if I'm wrong, but if Block Universe doesn't necessarily entail determinism, could this suggest that the "structure" of space-time can be thought of as a block, while still events being ontologically statistical within this structure? I hope this makes sense. I'm not trying to propose a theory of everything, or to discuss well-stablished physics, I'm just asking because your answers help me wrap my head around these complex concepts.

 

[Ibix]

This being the case, if taking the math literally can lead to understanding the universe as a Block Universe, why is this not treated as a given

Most people more or less do take it as a given - you don't get huge interpretational debates in GR the way you do in quantum. But you have to keep in mind that other interpretations are possible.

To an extent, I think one can see debates over the utility of pseudotensor measures of gravitational energy (which privilege one definition of "now") as arguments over whether something like a Lorentz ether is legit.

Also, please correct me if I'm wrong, but if Block Universe doesn't necessarily entail determinism, could this suggest that the "structure" of space-time can be thought of as a block, while still events being ontologically statistical within this structure?

No, because the distribution of the stuff in the universe affects the structure of spacetime. If there's randomness in one it communicates itself to the other.

I would think one obvious approach to try to have a non-deterministic block universe would be something like Many Worlds, where there would be a different block universe in every world. I don't know if anyone seriously considers such things, though.

 

[PeroK]

Let's take non-determinism to mean that two identical experiments can have different outcomes.

The universe as a whole is essentially a single experiment. You'd have to consider multiverse theories for it to be otherwise. In that sense asking whether the universe itself is deterministic has no meaning. You can't rerun the experiment. If someone says no other evolution was possible, then you cannot disprove it. And if someone says it could have evolved differently from some point in its history, then you can't test that either.

You may, however, set up two local experiments that are identical in their critical criteria and find two different outcomes. That's non-determinism within the universe. The superdeterminist can, and would, still claim that the outcome of each local experiment was predestined.

That's one reason why the block universe has little or no bearing on QM, other than as a superdeterminism that cannot experimentally be ruled out.

 

[Dale]

why is this not treated as a given in a similar way to how we treat probabilities in quantum mechanics?

The debates about probabilities in QM are far stronger than the debates about the block universe.

However, I think that you miss the point of your own statement.

if taking the math literally can lead to understanding …

Both interpretations are completely compatible with the math. They are mathematically equivalent. So to me, taking the math seriously would be to say that since they are mathematically equivalent they are also physically equivalent. The distinction between the two is a purely mental human conceit.

To me, that is the main purpose of math. Math shows us the equivalence of 2+2 and 4, we don’t have to prefer 2+2 or 4 because they are equivalent.

Similarly, taking the math seriously would mean that you don’t have to prefer LET or Block Universe because they are mathematically equivalent. Not coincidentally, they are also experimentally equivalent, which is another reason to not prefer either.

 

[Herman Trivilino]

I have the impression that physics is a field in which the math is as important as the experimental data.

Theory and experiment are both essential to physics. In physics, math is a tool.

The block universe is an interpretation that's consistent with both theory and experiment. But that is true of other interpretations.

 

[ojitojuntos]

Thank you all for the thorough answers. I know that I'll have to study a lot more math and physics if I want to look further into these issues, but it is very refreshing to get a guiding discussion from experts, especially because a lot of "pop-sci" media likes to overstate the implications and reach of interpretations of scientific theories.

Basically, and please continue correcting me when I'm wrong, the Block Universe is an interpretation of SR, one of many, that is useful to visualize spacetime. Experimentally, it doesn't differentiate from other interpretations of time in SR, so choosing between the BU, and others is mostly due to personal preference on how we want to think about time, more like a philosophical stance, or a useful tool.
My initial mistake came from thinking that, if this interpretation is the "simplest" one of SR, then we should assume that it is an accurate description of the universe, which I thought conflicted with the observed probabilistic behavior of QM.

Part of my confusion on how accepted the interpretation was as the description of the universe was due to how some articles framed that the block universe was the accepted de-facto stance, something that has been supported by science communicators like Sean Carroll and Sabine H. (although I'm not a fan of their approaches, but that is a different discussion).

 

[PeroK]

Part of my confusion on how accepted the interpretation was as the description of the universe was due to how some articles framed that the block universe was the accepted de-facto stance, something that has been supported by science communicators like Sean Carroll and Sabine H. (although I'm not a fan of their approaches, but that is a different discussion).

For me, these questions are more philosophical than physical. And there are, effectively, no answers in philosophy - only opinions and more questions. Perhaps that's a strength of philosophy - that nothing is ever decided and everything is open to debate. In a way, you are supposed to be confused by philosophy.

Whereas, despite the different possible foundations, you are not supposed to be confused by mathematics. Mathematicians aren't confused by the Axiom of Choice (AC). They don't argue in terms like "the AC is absurd" or "only an illusion" or "the only valid approach to mathematics".

Physics is somewhere between the two. We should keep an open mind, but equally there are practical consequences of going in one direction or another. For example, Fred Hoyle insisted that the universe must be steady state and defied the Big Bang Theory as far as he was able. That was a mistake, although perhaps his graduate students who worked for years to make the data fit his model, got some benefit out of it. But, eventually, they gave it up as a lost cause.

The equivalent philosophical questions will never be resolved. I honestly think you need to speak to philosophers to establish what the endgame is here. I don't think we humble physicists (whether amateur or professional) can help you. If you did a PhD in GR or Cosmology, the block universe question would most likely be an irrelevance, You might be doing something like modelling neutron star mergers. Or, studying galaxy formation. It's only a PhD in the philosophy of phyiscs that would agonize over the block universe.

 

[ojitojuntos]

The equivalent philosophical questions will never be resolved. I honestly think you need to speak to philosophers to establish what the endgame is here. I don't think we humble physicists (whether amateur or professional) can help you. If you did a PhD in GR or Cosmology, the block universe question would most likely be an irrelevance, You might be doing something like modelling neutron star mergers. Or, studying galaxy formation. It's only a PhD in the philosophy of phyiscs that would agonize over the block universe.

You are correct. While my questions were mostly philosophical, it was very enriching for me to read you and others who engaged with me from a scientific perspective. I was having trouble differentiating the difference between the scientific approach and the proposed implications shed by the interpretations, but I think I'm clear now on how to approach these types of inquiries.

Thanks a lot!

 

[ojitojuntos]

Me again. Just felt like writing again to appreciate your patience, and to kind of explain where I was coming from after you've helped me get a better understanding of the differences between theories and the interpretations' arguments.

I understand my mistake in thinking that the BU interpretation was necessarily the most accurate representation of our universe. I think my confusion came from videos such as this one, and my lack of understanding of the matter made it that, when watching this interview with Jacob Barandes, I kind of got the impression that the relativity of simultaneity and SR precluded the possibility of an "open future" (as in, the whole history of the universe being out there).

I think I understand now that this is a wrong assumption, but I wanted to provide more context on why I was also asking questions about determinism and how/if the probabilistic aspect of quantum mechanics challenged this. PeterDonis' Insight was super helpful too! I'm not a native English speaker, so I appreciate even more that you're so patient with your explanations and corrections.

 

[ojitojuntos]

Finally, I wanted to ask, is anyone here familiar with Emily Adlam's All-At-Once interpretation? How is it significantly different from a Block Universe view?

 

[PeterDonis]

Emily Adlam's All-At-Once interpretation

If you can find a published reference (textbook or peer-reviewed paper), we can discuss this in a new thread. I'm closing this thread since it looks like your original question has been answered.