# B Professional debates about Spacetime

#### lucas_

Is it not Spacetime is akin to the Wave function in Quantum Mechanics where it is just a mathematical tool and no way to distinguish between different interpretations?

Why is that there are countless professional debates about interpretations of quantum mechanics while there is very few or almost non-existing debates about interpretations of spacetime?

I always tried to wrap my head around spacetime where time is a coordinate and daily visualizing how my passage of time in daily basis is depicted in terms of coordinates and time caused gravity.

Anyone care to give their two cents?

Related Special and General Relativity News on Phys.org

#### PeterDonis

Mentor
Is it not Spacetime is akin to the Wave function in Quantum Mechanics where it is just a mathematical tool and no way to distinguish between different interpretations?
What different interpretations are there of spacetime, corresponding to the different interpretations of QM?

#### lucas_

What different interpretations are there of spacetime, corresponding to the different interpretations of QM?
Copenhagen = Block Universe
Many Worlds = LET

Since spacetime interpretation choices are only few.. only two. There are more members of either and so debates are so hot that fighting occurs and I understand why spacetime interpretation is banned here.

In quantum mechanics. Interpretations are many and members are equally divided so less fighting hence not banned.

I'm not discussing about the specific interpretations of spacetime. Just inquiring why are there less professional debates regarding spacetime versus the quantum?

#### PeterDonis

Mentor
Copenhagen = Block Universe
Many Worlds = LET
If this is meant as an analogy, I don't think it's valid.

spacetime interpretation choices are only few.. only two
Those aren't the only two. At the very least, there is a "minimal" interpretation of spacetime that only claims "reality" for what has actually been observed or could have been observed (in other words, what's in the observer's past light cone). That is basically the interpretation I refer to in this Insights article refuting a common argument for the block universe interpretation:

why are there less professional debates regarding spacetime versus the quantum?
You would have to ask all the professionals. My own personal take on it is that all of the interpretations of spacetime still tell a reasonably acceptable story, whereas no interpretation of QM does; every QM interpretation requires you to accept something that is very hard to accept. I expand on that some in this Insights article:

#### lucas_

If this is meant as an analogy, I don't think it's valid.
Copenhagen - Hilbert space is just a tool to get observable
Block Universe (Minkowski) - Geometry is just a tool to get observation

Many worlds - wave function is real
LET - time dilation or length contraction is physical process

Don't ya agree with the analogy. What portion do you think it's not valid?

Those aren't the only two. At the very least, there is a "minimal" interpretation of spacetime that only claims "reality" for what has actually been observed or could have been observed (in other words, what's in the observer's past light cone). That is basically the interpretation I refer to in this Insights article refuting a common argument for the block universe interpretation:

You would have to ask all the professionals. My own personal take on it is that all of the interpretations of spacetime still tell a reasonably acceptable story, whereas no interpretation of QM does; every QM interpretation requires you to accept something that is very hard to accept. I expand on that some in this Insights article:

#### PeterDonis

Mentor
Block Universe (Minkowski) - Geometry is just a tool to get observation
This is not at all what the block universe interpretation says. It says that the entire 4-dimensional spacetime is real, not just the part in our past light cone.

LET - time dilation or length contraction is physical process
This is not what distinguishes LET from other interpretations. What distinguishes LET is its claim that one particular inertial frame is the "absolute rest" frame, even though there is no way even in principle to find out which one it is.

Btw, you don't need to quote portions of someone else's post that you're not responding to.

#### lucas_

This is not at all what the block universe interpretation says. It says that the entire 4-dimensional spacetime is real, not just the part in our past light cone.
I thought block universe was same as Minkowski original view. So let me correct it.

Different interpretations of QM corresponds to different interpretations in Relativity in the sense of the following:

Copenhagen = Minkowski
interpretation (where Hilbert space and spacetime are just mathematical tool and subjective).

Many Worlds = LET (both objective)

more accurate now?

#### PeterDonis

Mentor
Minkowski original view.
Copenhagen = Minkowski
interpretation (where Hilbert space and spacetime are just mathematical tool and subjective).
Do you have a reference for this "Minkowski original view" you are describing? Minkowski himself said spacetime was real as soon as he came up with the concept. As far as I know nobody has ever proposed an interpretation of relativity where "spacetime" was just a calculational tool, much less where it was subjective.

more accurate now?
I don't think so. I think you are simply mistaken about what interpretations of relativity actually say.

#### lucas_

Do you have a reference for this "Minkowski original view" you are describing? Minkowski himself said spacetime was real as soon as he came up with the concept. As far as I know nobody has ever proposed an interpretation of relativity where "spacetime" was just a calculational tool, much less where it was subjective.
I read it somewhere. I'll dig it up. Before Einstein. Time was a parameter. Separate from space. Now Minkowski made time a coordinate connected to space. This is not natural. Hence is it not spacetime (where space and time are both coordinate) is just calculational tool?
Or should you treat time as coordinate literally? But then is it not like the wave function as tool to get observable only. In relativity. Spacetime is a geometry to get observations only. So only the observable in QM and observations of the geometry in relativity is objective (a relativist in sci.physics onced emphasized relativity was simply geometry).

I don't think so. I think you are simply mistaken about what interpretations of relativity actually say.

#### PeterDonis

Mentor
Minkowski made time a coordinate connected to space.
No, he made both "space" and "time" coordinates on spacetime.

This is not natural.
Why not?

is it not spacetime (where space and time are both coordinate) is just calculational tool?
No. Spacetime is a real geometric object. "Space" and "time" are coordinates on it. Just as, for example, a flat sheet of paper is a real geometric object, but you can set up x and y coordinates on it. The coordinates are human calculational tools, but the geometric object itself is not.

#### lucas_

No, he made both "space" and "time" coordinates on spacetime.

Why not?

No. Spacetime is a real geometric object. "Space" and "time" are coordinates on it. Just as, for example, a flat sheet of paper is a real geometric object, but you can set up x and y coordinates on it. The coordinates are human calculational tools, but the geometric object itself is not.
So how does time dilation work? According to relativist Tom Roberts of sci.physics. Spacetime is just geometry. And observations produce reality. So time dilation works because of the geometry.

In your case. Spacetime is a real geometric object and not just geometry? But in our everyday life. Time is time. Space is space. Why should you join them together as coordinates. It's like trying to join cat and dog as coordinates. They don't jibe. If space and time are really coordinates. Why didn't anyone besides Einstein or Minkowski think of it?

#### PeterDonis

Mentor
time dilation works because of the geometry.
Yes. Time dilation just means the lengths of different paths through spacetime can be different. Just as the lengths of different curves on a piece of paper can be different.

in our everyday life. Time is time. Space is space
That's because everyday life does not involve objects with relative velocities close to the speed of light. So we don't observe relativistic effects in everyday life, and it appears to us that time and space can be separated. But as soon as we started doing experiments with objects moving at speeds close to the speed of light, we realized that what appears to us to be true in everyday life is only an approximation.

It's like trying to join cat and dog as coordinates. They don't jibe.
Sure they do; just use the speed of light to convert between them. In fact, in relativity, that's what the speed of light is: a conversion factor between space units and time units. And with appropriate choices of those units, the speed of light is 1 and the symmetry between space and time is obvious.

#### lucas_

Yes. Time dilation just means the lengths of different paths through spacetime can be different. Just as the lengths of different curves on a piece of paper can be different.

That's because everyday life does not involve objects with relative velocities close to the speed of light. So we don't observe relativistic effects in everyday life, and it appears to us that time and space can be separated. But as soon as we started doing experiments with objects moving at speeds close to the speed of light, we realized that what appears to us to be true in everyday life is only an approximation.

Sure they do; just use the speed of light to convert between them. In fact, in relativity, that's what the speed of light is: a conversion factor between space units and time units. And with appropriate choices of those units, the speed of light is 1 and the symmetry between space and time is obvious.
The speed of light is
299,792,458 meters/second.
Space is in meter.
Time is in seconds.

In what sense is the speed of light the conversion factor? Care to show some simple illustration or derivations.

And how could you convert between cat and dog using the speed of light?

thanks to you!

#### Dale

Mentor
Just inquiring why are there less professional debates regarding spacetime versus the quantum?
Because of the two major interpretations one is simpler, more useful, and more easily generalizable to GR.

Basically, in QM all of the interpretations involve some sort of trade-off. An interpretation that is beneficial for one thing is problematic for another. In contrast, with LET the scientific community sees no clear benefit whereas the block universe has several. In particular, the generalization to GR is an especially strong point.

And how could you convert between cat and dog using the speed of light?
This is exceptionally silly. He never claimed it was cats and dogs, that was you. It is not up to him to justify your absurd metaphor.

In what sense is the speed of light the conversion factor? Care to show some simple illustration or derivations.
Because that is how it shows up in the math. The spacetime interval is $ds^=-c^2 dt^2 + dx^2 + dy^2 + dz^2$. This is exactly how conversion factors show up. If you measured x distances in yards and y and z distances in feet then the Euclidean interval would be $ds^2=3^2dx^2+dy^2+dz^2$

As far as a derivation goes it is a simple exercise to calculate the Lorentz transform of the spacetime interval and show that it is an invariant.

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#### PeterDonis

Mentor
The speed of light is
299,792,458 meters/second.
In SI units, yes. But those are not the only possible units. Since the speed of light is a universal constant, we can pick units to make its value whatever we want.

#### Heikki Tuuri

Albert Einstein was not happy with quantum mechanics. Quantum mechanics is much more complex than general relativity. There are lots of ad hoc methods in quantum mechanics, like renormalization. That is the reason why the debate about the philosophy of quantum mechanics will never end.

General relativity, on the other hand, is a classical deterministic theory. It is a lot easier to grasp, in principle. An ugly thing in general relativity is singularities. The theory does not describe how they should behave. Einstein tried to prove that black holes cannot form. Now we know that black holes exist.

The philosophical debate in general relativity, combined with some rudimentary quantum mechanics, concentrates on the inner structure and destiny of black holes and singularities. Since we have no empirical data, the debate will continue indefinitely.

Staff Emeritus
Don't ya agree with the analogy. What portion do you think it's not valid?
That's not how it works. It's your idea. The burden is on you to convince us its right, not on everyone else to show that it's wrong.

I think "Colorless green ideas sleep furiously". Prove me wrong!

#### PeroK

Homework Helper
Gold Member
2018 Award
The speed of light is
299,792,458 meters/second.
Space is in meter.
Time is in seconds.

In what sense is the speed of light the conversion factor? Care to show some simple illustration or derivations.
We quite often talk about journeys in terms of time or distance. For example, London to Edinburgh is four hours by train. In that case, the speed of the train is the (approximate) conversion factor.

With light, in vacuum, it is exact: a journey of $299,792,458m$ is exactly $1s$, with the speed of light as the conversion factor.

#### PAllen

I should note that there are at least 4 spacetime interpretations that I know of, used by at least some reputable physicists:

Minimalist, as decribed by @PeterDonis
Block universe
LET
EBU (evolving block universe; there is a ‘real’ , possibly fuzzy, boundary between past and future, but it is not conceived in terms of any preferred reference frame. ‘Many fingered time’ is a variant of it. A major proponent of it is George Ellis, coauthor with Steven Hawking of “Large Scale Structure of Spacetime”. One may say that at the end of time it coincides with block universe.)

#### lucas_

If this is meant as an analogy, I don't think it's valid.

Those aren't the only two. At the very least, there is a "minimal" interpretation of spacetime that only claims "reality" for what has actually been observed or could have been observed (in other words, what's in the observer's past light cone). That is basically the interpretation I refer to in this Insights article refuting a common argument for the block universe interpretation:

The "minimal" interpretation of spacetime clams "reality"
for what has actually been observed or could have been observed, as you described. So it didn't give physical mechanism how exactly space and time produce
time dilation except via the geometry? Just like the "minimal" interpretation of quantum mechanics didn't give mechanism how the wave function was physical? Just trying to see both from a common perspective.

I was looking for an old Discover Einstein anniversary edition (Sept. 2004) magazine which showed how a spinning disc produced curve spacetime. I forgot the details. Do you know of the concept how spinning disc can produce curved spacetime?

#### PeterDonis

Mentor
it didn't give physical mechanism how exactly space and time produce
time dilation except via the geometry?
Geometry is how spacetime produces time dilation. There is no other physical mechanism. That's true regardless of which interpretation of relativity you adopt; interpretations don't differ at all on this point.

"minimal" interpretation of quantum mechanics didn't give mechanism how the wave function was physical
Huh? If the wave function is physical, that's just a basic fact; there is no mechanism that makes it so.

#### PeterDonis

Mentor
Do you know of the concept how spinning disc can produce curved spacetime?

#### PeterDonis

Mentor
Just trying to see both from a common perspective.
And as I've already said, I don't see any such common perspective. The very fact you point out in the OP, that there are endless debates about QM interpretation but not about relativity interpretation, argues against there being such a common perspective.

#### lucas_

Geometry is how spacetime produces time dilation. There is no other physical mechanism. That's true regardless of which interpretation of relativity you adopt; interpretations don't differ at all on this point.
But in LET, the physical mechanism of time dilation is due to the ether.

Huh? If the wave function is physical, that's just a basic fact; there is no mechanism that makes it so.
Oh. Let's not discuss LET as it is a hated word.
If your reasoning that no other mechanism produces time dilation in other relativity interpretations was true. So.

Geometry is how spacetime produces time dilation. There is no other physical mechanism.

Wave function is how matter produces observable. There is no other physical mechanism.

so Geometry and wave function is space, time, and matter.

#### Dale

Mentor
Oh. Let's not discuss LET as it is a hated word.
This is fairly disingenuous given the substantive reply I gave you above which you have ignored. It isn’t a hated word, it is simply not a useful concept.

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