Exploring the Mysteries of Time and Spacetime

[Azaravicius]

Hi.
Recently I was watching youtube about time and spacetime. And I have few questions about it.

If I well understood no one knows what is the time. Here are no formulas to describe time. If it is true how we know if other watches measure time and not our perception on time which can be subjective?

Official definition of a second is 9,192,631,770 cycles of the radiation that gets an atom of the element called cesium to vibrate between two energy states. But this is based on assumption that cycles are constant and do not change changing physics parameters like gravity or object speed in space.

From all videos I know that time slows down in more powerful gravity field and it slows down if objects speed in space increases. But how do we know if it's time slows down and not physics? It's like running in air and in water. Your running speed in water will be slower nor in air, because the water density greater than air. Spacetime in greater gravity and at greater speed should compress and may slow down all physics, but not time. As we don't know that time is.

Such thinking may solve problems and discussions about time travel. Because you will not travel back in time if your speed greater nor c and wormholes will not help time travelers. As only perception of time is affected by gravity and speed not the time.

 

[Ibix]

Time is one direction in four-dimensional spacetime. It is different from the other three directions which are space-like directions. You have a certain amount of flexibility in which direction you call time for you, which is where a lot of the "time flows at different rates" comes from.

I've no idea what compressed space would be. More space in the same space? It doesn't really make sense.

Honestly, if you want to do any real thinking about relativity you are going to have to pick up a textbook and study it properly. YouTube videos won't tell you anything except hand wavy pictures that are distantly related to the reality. It's like trying to learn geography from landscape paintings.

 

[Nugatory]

What @Ibix said.
A good start might be "Spacetime Physics" by Taylor and Wheeler.

 

[Ibix]

Or Ben Crowell's Relativity for Poets, for a maths-free introduction. Freely downloadable from www.lightandmatter.com

 

[Azaravicius]

YouTube videos won't tell you anything except hand wavy pictures that are distantly related to the reality. It's like trying to learn geography from landscape paintings.

Yes. From watch youtube documentary movies I realized that it is very simplified and many things are missing. The same go with articles on blogs and similar. It is made so simple that if you start to think about it will start to make less and less sense.

 

[Azaravicius]

I've no idea what compressed space would be. More space in the same space? It doesn't really make sense.

I mean this one

https://www.google.lt/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjdqZHDjpLQAhVJhSwKHQmADQAQjRwIBw&url=https%3A%2F%2Fwww.dreamstime.com%2Fstock-photo-space-time-gravity-image11748480&psig=AFQjCNFrtb59mb7LFZVpXPEzWzelsIbeDw&ust=1478452364975773

 

[Dale]

If I well understood no one knows what is the time. Here are no formulas to describe time.

Most formulas with a "t" in them describe time.

It sounds like you are being deceived by your pop-sci sources.

 

[Nugatory]

I mean this one

That would be an especially good picture to forget about - it is seriously misleading. You'll find many threads about why if you search here for "rubber sheet", as well some much better explanations.

 

[Ibix]

I mean this one

As @Nugatory says, that picture is very poor. For a start, it implies that the grid is a 2-dimensional model of spacetime, then draws a 3-dimensional Earth sitting outside spacetime.

The picture is very pretty, but it's not even internally consistent, let alone much like the reality.

 

[Azaravicius]

That would be an especially good picture to forget about - it is seriously misleading. You'll find many threads about why if you search here for "rubber sheet", as well some much better explanations.

Really? Rubber sheet is so popular I was thinking it is standard in thinking. Even this video uses it.


So majority of internet sources are misleading.

 

[PeroK]

So majority of internet sources are misleading.

Yes!

 

[phinds]

So majority of internet sources are misleading.

I think that is MUCH too weak a statement. Many of them are misleading and the rest are just flat wrong.

 

[PeroK]

I think that is MUCH too weak a statement. Many of them are misleading and the rest are just flat wrong.

In these days of political correctness, perhaps we should call them "geodesically challenged"?

 

[Mister T]

But this is based on assumption that cycles are constant and do not change

No, it is not based on that assumption. It's based on that conclusion. A conclusion reached from building machines that count these cycles, and observing that machines constructed using identical or totally different constructions, agree with each other. These machines are clocks, the thing they measure is called time, and the closer they agree with each other the better they are, by definition.

Metrologists don't assume clocks are good at measuring time, they demonstrate it.

 

[Azaravicius]

No, it is not based on that assumption. It's based on that conclusion. A conclusion reached from building machines that count these cycles, and observing that machines constructed using identical or totally different constructions, agree with each other. These machines are clocks, the thing they measure is called time, and the closer they agree with each other the better they are, by definition.

Metrologists don't assume clocks are good at measuring time, they demonstrate it.

This is exactly what I had in mind. If you make ruler in 20°C environment and measure size of object in 100 °C environment will it show size of object correctly? All materials have different expansion properties. So ruler size (length) will differ in 20°C environment and in 100 °C environment. Measured object size also varies in different temperatures and if it is made from different material it's coefficient of expansion differ from ruler. So will ruler show good measurements. If we make ruler in 20°C environment and always measure object in 20°C environment it will show correct measurements. Like clocks made in equal gravity or speed. But we put one clock on Earth and other in rocket and let it go to Mars and back. Will both clocks show the same time? And why?

 

[Ibix]

But we put one clock on Earth and other in rocket and let it go to Mars and back. Will both clocks show the same time? And why?

No. Google for "twin paradox".

 

[PeroK]

But we put one clock on Earth and other in rocket and let it go to Mars and back. Will both clocks show the same time? And why?

Try looking at the Hafele-Keating experiment:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html

 

[phinds]

But we put one clock on Earth and other in rocket and let it go to Mars and back. Will both clocks show the same time? And why?

If we put an odometer in a car that goes from Florida to Maine by way of New York it will show a milage. If we put one in a car that goes from Florida to Maine by way of California will it show the same number of miles? Why?

 

[A.T.]

I've no idea what compressed space would be. More space in the same space?

More volume within the same boundary.

 

[Grinkle]

Rubber sheet is so popular I was thinking it is standard in thinking.

Not all discussions that use this rubber sheet imagery as an aid are misleading or wrong.

You are being advised to look at a complete introductory discussion of the topic - you won't be able to infer the very much about relativity from one persons rubber sheet imagery discussion, even if everything they said was correct in the context within which they said it. Whether rubber sheet is right or wrong is a red herring for you. Forget about rubber sheet until you know enough to decide for yourself if it makes sense.

 

[Nugatory]

Will both clocks show the same time? And why?

No. Google for "twin paradox".

And when you google for "twin paradox", pay particular attention to this search result: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html

It is important to understand that the two clocks will show different times, but neither of them is wrong and both are accurately measuring the time that has passed. Suppose the twins are both growing beards at the rate of one centimeter per week, and suppose their hearts are both beating 75 times per minute. They're also both carrying samples of radioactive material that decays at a fixed rate, and they're both carrying potted plants that grow at some fixed rate. If when they compare clocks on their return the traveling twin's clock shows half as much time has elapsed... The traveling twin's beard will only be half as long as stay-at-home's, stay-at-home's heart will have beaten twice as often as traveller's, stay-at-home will have only half as much of his radioactive material left, traveller's plants will be only half as tall as stay-at-home's. By far the easiest interpretation of these facts is that both clocks have accurately measured the time that passed for each twin, and it's not the same.

It is also important to understand that this result is something completely different than time dilation, which you've also probably read about.

 

[Mister T]

This is exactly what I had in mind.

But it's the opposite of what you said!

So ruler size (length) will differ in 20°C environment and in 100 °C environment.

The only way to establish that is to have a length standard that doesn't change with temperature!

We have such length standards so that's why we know that thermal expansion is real.

Likewise, we have precise enough clocks to know that time dilation is real. Things live longer when they travel. That's a fact of everyday life for any engineer working with a particle beam.

 

[Orodruin]

Just going to nitpick a few things.

have beaten twice as often as traveller's,

Technically, it will have beaten twice as many times. Whether this means twice as often depends on how you define "often".

stay-at-home will have only half as much of his radioactive material left,

Are you familiar with the exponential function?
Also consider the case when the mean life of the material is much longer than the trip takes.

 

[Azaravicius]

Try looking at the Hafele-Keating experiment:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html

Hi,
I have read Hafele-Keating experiment. Don't know if I understood it right. So I will try to explain how I understood it.

Clock on Earth surface have speed v in space sum of Earth rotation about it's axis, rotation about sun, sun rotation and so on.
Clock on flight going eastward will have greater speed in space compared to clock on Earth surface and so it will have slower time.
Clock on flight going westward will have slower speed in space compared to clock on Earth surface and so it will have faster time.

Is it right?

 

[PeroK]

Hi,
I have read Hafele-Keating experiment. Don't know if I understood it right. So I will try to explain how I understood it.

Clock on Earth surface have speed v in space sum of Earth rotation about it's axis, rotation about sun, sun rotation and so on.
Clock on flight going eastward will have greater speed in space compared to clock on Earth surface and so it will have slower time.
Clock on flight going westward will have slower speed in space compared to clock on Earth surface and so it will have faster time.

Is it right?

Those are the velocity-based variations in what the clocks read; there were also the effects of the difference in height, hence gravitational potential.

 

[Nugatory]

Are you familiar with the exponential function?
Also consider the case when the mean life of the material is much longer than the trip takes.

Sigh... Yes, you're right. Thanks.
(first draft of that post had specific numbers, chosen so that the shorter trip was one half-life).

 

[Ibix]

Further to @PeroK's comment, note that the experiment treats the center of the Earth as stationary. All velocities are being measured with respect to that. And the point is more about different routes through spacetime having different durations than about "time runs slowly", which is a heuristic that leads to the "paradox" part of the twin paradox.

 

[nitsuj]

Really? Rubber sheet is so popular I was thinking it is standard in thinking. Even this video uses it.


So majority of internet sources are misleading.

It's a good "first step" in a journey of thousands of miles. This is why it is used so often.

Though it's to often presented as the start and end as a description of gravity...by the time a decent understanding of gravity is accomplished that rubber sheet thing looks silly.

 

[nitsuj]

This is exactly what I had in mind. If you make ruler in 20°C environment and measure size of object in 100 °C environment will it show size of object correctly? All materials have different expansion properties. So ruler size (length) will differ in 20°C environment and in 100 °C environment. Measured object size also varies in different temperatures and if it is made from different material it's coefficient of expansion differ from ruler. So will ruler show good measurements. If we make ruler in 20°C environment and always measure object in 20°C environment it will show correct measurements. Like clocks made in equal gravity or speed. But we put one clock on Earth and other in rocket and let it go to Mars and back. Will both clocks show the same time? And why?

First post you seemed to be asking about the specific rate of time, now it seems to be about comparatives. There is something called proper time.

A second is merely a unit. Nothing inherently fundamental, unlike the basis from which it is defined.

 

[A.T.]

...by the time a decent understanding of gravity is accomplished that rubber sheet thing looks silly.

Unfortunately many never arrive at this stage, and some conclude GR itself is nonsense because of the obvious flaws of that analogy.

 

[PeroK]

It's a good "first step" in a journey of thousands of miles. This is why it is used so often.

Though it's to often presented as the start and end as a description of gravity...by the time a decent understanding of gravity is accomplished that rubber sheet thing looks silly.

The journey may be 1000 miles, but the rubber sheet is a step in the wrong direction. The obvious flaw is that it relies on the preconceived notion that things move "down". The only reason things move down into the sheet is gravity: take that sheet to the space station and the ball would refuse to follow the sheet. The other flaw is there is no time dimension on the sheet. It's the geometry of spacetime (not space) that defines gravity, so at least one of the dimensions of the sheet would have to be time. The rubber sheet reinforces the classical concept that space and time can be separated.

The first step in learning GR is to learn SR. Part of that journey will be to redefine your understanding of space and time. The rubber sheet analogy is definitely something you have to unlearn.

 

[nitsuj]

The journey may be 1000 miles, but the rubber sheet is a step in the wrong direction. The obvious flaw is that it relies on the preconceived notion that things move "down". The only reason things move down into the sheet is gravity: take that sheet to the space station and the ball would refuse to follow the sheet. The other flaw is there is no time dimension on the sheet. It's the geometry of spacetime (not space) that defines gravity, so at least one of the dimensions of the sheet would have to be time. The rubber sheet reinforces the classical concept that space and time can be separated.

The first step in learning GR is to learn SR. Part of that journey will be to redefine your understanding of space and time. The rubber sheet analogy is definitely something you have to unlearn.

It's arbitrary if one takes a single step in the wrong direction when the whole journey is 1,000 miles...

Perhaps the flaw is bringing in some presumption that things are moving down; to that point the representation is that the sheet itself is the gravity. If that's an issue, what of the friction between the ball and rubber. There is a million issues with it, however I'd say the least of which is that it doesn't work on the space station. :/

With that said, it teaches orbits quite well; of course forgiving many many comparatives to the reality...as is typically true with analogies. Which is all it is, not a 1,000 mile journey. Not a method to explain spacetime, but it's effect under the influance of a mass.

k, can we agree enough with analogies in a science forum ;) The real issue with this damned rubber sheet being referenced here.

SR a single step in the 1,000 mile journey towards understanding GR! I guess GR is far far more complicated than I thought! :D

I watched that vid and I agree completely with you about the presenter focusing on the bending of space. If I understand right the vast majority of what is experienced (the acceleration) is due to time...and that it takes vast amounts of mass to bend space...such as with light bending. However I also noticed this was high school

 

[Max364]

Time may be an emerging quantity?, ie, arises from mental construct? also from another discussion elsewhere i commented:
Taking the speed of light as a unity value of 1

then v + t = C = 1
ie the sum of total speed and time must always be 1.

so for a photon (speed of light) v is at maximum so v=1 must mean that t=0 for photon, it experiences no passage of time.
But for everything else our speed on Earth is very small compared to light so our v is tiny (even at 1000s mph)
so that means we are almost at the maximum rate of passage for time (as v is almost zero) - so for us on Earth v= almost 0 and t= almost 1, we are at maximum rate of passage for time (opposite to that of a photon)!

Also there is no simultaneity of events - (eg, Andromeda Paradox).
There is no universal special moment "Now" (Einsteins famous Train thought experiment with light)

some quotes:
Stannard 1987: In four-dimensional space-time nothing changes, there is no flow of time, everything simply is . . . It is only in consciousness that we come across the particular time known as ‘now’ . . . It is only in the context of mental time that it makes sense to say that all of physical space-time is. One might even go so far as to say that it is unfortunate that such dissimilar entities as physical time and mental time should carry the same name!

 

[Ibix]

Time may be an emerging quantity?, ie, arises from mental construct?

This is incorrect, at least in the framework of relativity. If you are considering another theory, please cite your source.

then v + t = C = 1
ie the sum of total speed and time must always be 1.

This is clearly incorrect. $U_\mu U^\mu=1$ is true for objects on timelike worldlines (the modulus of the four-velocity is one), which may be what you are referring to, but...

so for a photon (speed of light) v is at maximum so v=1 must mean that t=0 for photon, it experiences no passage of time.

...photons do not follow timelike worldlines, so that would seem to be irrelevant.

Also there is no simultaneity of events - (eg, Andromeda Paradox).

There is no global definition of simultaneity, I presume you mean.

 

[Max364]

By emerging , i mean each person/object experiences its own rate of time passage (hence, can not agree on simultaneity of events etc), my time is unique to me even though it is almost exactly the same as yours, as we occupy almost the same space (relatively)
v+t=1, yes for wordlines or even worldtubes (solid 4d object)
Two people passing each other can not agree on what is defined by exact moment "now" - extended to Andromeda galaxy this gives difference of days not just nanoseconds like on Earth etc.