B Is time out there?

Hey!

I wanted to ask and learn more about what time is, and whether it even exists. Can relativity be understood solely in terms of rates of motion and subjective experiences, or do we really need to say this 4th dimension is a real thing. And if it is real, is it material?
Thank you
 

Ibix

Science Advisor
Insights Author
5,125
3,422
whether it even exists
If it doesn't, when do you expect a reply?
Can relativity be understood solely in terms of rates of motion and subjective experiences, or do we really need to say this 4th dimension is a real thing.
Not sure what you mean. You can certainly do relativity's maths without interpreting it as geometry, but it'll be really hard to learn because more or less everybody uses that interpretation and you'd have to translate everything everyone says into different terms.
And if it is real, is it material?
No.
 
And if it is real, is it material?
Would you even consider any of the spacial dimensions to be “material”? Material generally means something you can touch.
 
Well you can't touch a wave, only particles. But can't they change into particles and thus be considered material in principle? This discussion can get really philosophical, but all I am looking for is the basic science facts that we know are true. Various interpretations are interesting but unessential. Relativity can be described by a situation where two people see each other. To one of them the other is going strangely fast, while the other sees the first as going strangely slow. But each experiences themselves as moving in a normal way. Some people define time as just the way we count things, not as real. If we are to avoid Newtonian time, I wanted to know more about what would replace. Am I right in anything I've said in this post?
 

pervect

Staff Emeritus
Science Advisor
Insights Author
9,488
795
"What time is" is a philosophical question.

And science, in general, can't truly answer philosophical questions.

For instance, science can't really answer the question of whether you exist, whether I exist, whether this post I just wrote exists, or whether bricks exist. Those aren't scientific questions, they are philosphical ones. So, taking your question literally, science can't answer the question of whether time exists or not.

However, some philosophical assumptions make things much easier to teach and understand. If you're trying to understand rigid body mechanics, for instance, it's helpful to believe that bricks exist, even if the philosphers can't quite settle the point.

It's very common to philosophically assume that the space-time continuum exists, and that it is four dimensional. And it makes special relativity much easier to understand. This is rather similar to the way that it's convenient to assume that bricks exist when trying to learn the Newtonian physics of rigid body mechanics.

There are theories, such as some versions of string theory, that suggest that there are more than just four dimensions. There are also theories that suggest that space-time might not be a continuum, such as the "quantum foam" idea. But in the context of special relativity it's convenient to assume space-time exists, and that it's a four dimensional continuum.

If you are actually wondering about why special relativity talks about space-time as a four dimensional continuum, rather than a separate three dimensional continuum plus time, there are some reasons for this point of view. The "parable of the surveyour" gives some insight. <<link>> has some information about this, (though it's unclear how long the link will be accessible). The textbook reference, which being in print will be around longer, is Taylor & Wheeler's "Spacetime physics", in chapter 1. You can find an older edition of that text <<here>>.

To give a quick summary of the parable of the surveyor, I would say that it explores the question of why we consider the surface of the Earth to be two dimensional, rather than two one dimensional quantities. I.e., why do we think of north- south distances and east-west distances as being unified into a single conceptual entity, rather than two different things.

Then it draws an analogy between this case and the space-time case, and suggests that the argument is similar. A key point in this argument / analogy is something that is notoriously to get laymen to understand, though - the idea of "the relativity of simultaneity".

Note that much of this is in the context of special realtivity. If you move onto something else, at some later date, like string theory, you might have to revisit these assumptions - in the string theory case, you might have to add more dimensions. Some people have difficulty with keeping all the contexts of the assumptions straight - I can have some sympathy for that, but I don't have a solution.
 

Ibix

Science Advisor
Insights Author
5,125
3,422
Well you can't touch a wave, only particles.
It's perfectly possible to derive radiation pressure in a classical picture where light is a wave. So I'd have to disagree with this.
To one of them the other is going strangely fast, while the other sees the first as going strangely slow.
No. In general, both will see (if you mean "see" literally) the other either apparently fast or slow, depending on whether they are approaching or moving away from one another, due to the Doppler effect. If they correct for changing distance then they will both determine that the other is moving slowly.
But each experiences themselves as moving in a normal way.
Yes.
Some people define time as just the way we count things, not as real.
I don't recognise this statement - who says that?
If we are to avoid Newtonian time, I wanted to know more about what would replace.
The easiest way to interpret relativity's description of time is as a dimension in Minkowski space. I gather you can interpret Newtonian time as a dimension also, if you want, but the maths is considerably nastier.
 
The parable of the surveyor is very enlightening! I always get confused by what exactly Leibniz thought time was, but Aristotelian writers (Edward Fesser is one) almost always say time is something human's make up in their minds. But they say this for philosophical reasons. Philosophers are often able to say to physicists "maybe that interpretation is not necessary" and such, but maybe its not always helpful.

An old encyclopedia from the 60's says "According to the theory of relativity, it is possible for different observers to see the same chain of events in different order. Such a possibility does not, however, mean that there is no actual order of cause and effect in the physical world. That is to say, the relationship existing between an effect and its cause is not a conventional way of thinking. The theory of relativity merely asserts that events casually UNRELATED and occurring in separate frames of reference may appear in different sequences to different observers. Causally related events, on the other hand, ALWAYS follow a definite sequence. The time interval between the events may appear longer or shorter, depending on the reference systems used by different observers."

I read that entry to understand relativity better. Is this paragraph still accurate (setting aside quantum paradoxes like the quantum eraser problem)?

Thank you
 

jbriggs444

Science Advisor
Homework Helper
7,493
2,556
Old Encyclopedia said:
An old encyclopedia from the 60's says "According to the theory of relativity, it is possible for different observers to see the same chain of events in different order. Such a possibility does not, however, mean that there is no actual order of cause and effect in the physical world. That is to say, the relationship existing between an effect and its cause is not a conventional way of thinking. The theory of relativity merely asserts that events casually UNRELATED and occurring in separate frames of reference may appear in different sequences to different observers. Causally related events, on the other hand, ALWAYS follow a definite sequence. The time interval between the events may appear longer or shorter, depending on the reference systems used by different observers."
I read that entry to understand relativity better. Is this paragraph still accurate (setting aside quantum paradoxes like the quantum eraser problem)?
The wording could use a little tweaking. Rather than "chain of events", I would have used "set of events" so as to avoid implying a causal sequencing.

One should never refer to events as occurring in a frame of reference. For most purposes, a frame of reference amounts to a coordinate system. An event is not tied to a particular coordinate system. It exists in all of them. It has different coordinates and, in particular, a different time coordinate in most of them. You as the analyst get to pick which frame of reference you would like to use. The event does not get to pick for you.
 

Martin Scholtz

Gold Member
14
9
If it doesn't, when do you expect a reply?

Not sure what you mean. You can certainly do relativity's maths without interpreting it as geometry, but it'll be really hard to learn because more or less everybody uses that interpretation and you'd have to translate everything everyone says into different terms.

No.
Ok, I think that geometrical interpretation is the most beautiful, natural and fruitful one. But the question itself isn't that meaningless.

Except for mathematics, Carlo Rovelli in his lectures on Loop Quantum Gravity spends"a lot of time" in order to clarify what we mean by time. Is it purely relational quantity which parametrizes relative changes of configuration? As I understand, Rovelli advocates this interpretation. Mathematically, general relativity can be formulated as evolution of 3D geometries in time (Hamilton formalism in ADM variables) but here the time is just auxiliary gauge quantity. This leads to very non-trivial constraints making theory difficult to quantize.

But if you do it covariantly, you formally get rid of equations of motion and the Hamiltonian (normally generator of time translations) vanishes, thus we are left with the constraint of Wheeler-de Witt type
## \mathcal{H} = 0 ##
Only choosing a specific gauge, the constraint becomes the equation of motion, but that breaks covariance again.

Similarly, in models of emergent gravity, you can e.g. construct a 3D geometry from the underlying Hilbert space, but how does the time emerge?

I have almost none knowledge on models based on dualities (AdS/CFT) so I have no idea how they deal with the problem of time.
 
25,779
6,625
I read that entry to understand relativity better.
Unfortunately, it doesn't look like a good source from which to do that. In addition to the issues @jbriggs444 raised, there is this problem:

According to the theory of relativity, it is possible for different observers to see the same chain of events in different order.
The article left out a crucial qualifier: this can only happen with events that are spacelike separated. And such events cannot be causally connected, so you wouldn't normally use the term "chain of events" to refer to them. The article does later on say this:

Causally related events, on the other hand, ALWAYS follow a definite sequence.
Which is correct, but contradicts the earlier statement I quoted. So I think this is a poorly written and poorly checked article (unfortunate in an encyclopedia; even in the 1960s SR was well enough understood that these errors should have been caught).
 

pervect

Staff Emeritus
Science Advisor
Insights Author
9,488
795
In relativity, it's useful to distinguish between "proper time" and "coordinate time".

Proper time is what a clock measures. The SI definition of the second as a unit of time is a measure of proper time. Proper time is generally regarded as having physical significance because it can be measured by a clock. It's often informally called "wristwatch time", as it's the sort of time a small portable clock measures.

Coordinate time is part of a system of labelling events. Time and space are combined into a 4 dimensional continuum in relativity. In either special or general relativity, the position and location of an "event" is specified by 4 numbers. Typically, 3 of the numbers specifiy the location of the event (where it is), and one of the numbers specifies when the event occurs.

A practical example of coordinate time would be "Universal coordinate time", which is based on International Atomic Time (TAI time). TAI time takes place on a rotating, gravitating Earth, it's an important practical example of coordinate time, but it's not the simplest example to study as an introduction to the theory of special relativity. In particular, the effect of gravity on the time coordinate can't be dealt with properly without General relativity.

Because coordinate time in the end revolves around a set of labels to describe events, it's regarded as a convention, rather than something physical. Many familiar physical formulas are valid only if the proper conventions are followed, however. It's possible to use different conventions, but then the formulae have to be modified.

At important feature of special relativity is the relativity of simultaneity. There is a lot written about the topic. Einstein's introduction to the idea was famoulsy "Einstein's train", as espoused in chapter 9 of his book, "Relativity, the special and general theory", <<link>>.

Einstien's exposition may not be the easiest to follow, but typically there is a fair amount of initial confusion regardless of exactly what exposition one first studies.
https://www.bartleby.com/173/9.html
 
Last question: accepting Newtonian space and time as real implies a change in understanding Einstein's space-time continuum? Would the latter be simply contained in the former? (I don't see how it would work because of relativity's doing away with objective location and size)
 
I didn't know that Newtonian space and time have been disproven as not overarching (time) and containing the universe (space). I am not espousing any position. Just looking for more information
 
25,779
6,625
I didn't know that Newtonian space and time have been disproven as not overarching (time) and containing the universe (space).
You are aware that relativity exists, but you didn't know that Newtonian mechanics has been falsified? I find that very difficult to believe.
 
25,779
6,625
I am not espousing any position. Just looking for more information
Asking vague questions and giving highly dubious responses to the answers is not a good way to look for more information. At this point I think the posts already made in this thread are as good as you're going to get.

Thread closed.
 

Related Threads for: Is time out there?

  • Posted
2
Replies
34
Views
22K
Replies
4
Views
2K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top