Time is weird

[halfeye]

Well it is. Then weirder still the more you think about it.

Gravity is due to curved space-time.

When I think of curved spacetime, I can see how something moving could find it's path changed by curved space. However, with something stationary it's not so clear that curving space would move it. So it seems to me that it's the curve in time that causes relatively stationary bodies to accelerate under gravity.

Have we yet worked out how much mass curves time? It doesn't seem to be very much, the change in the speed of time at the surface of the Earth and a few miles above it is apparently very little, but detectable.

[Yora]

Gravity bends the path of ight, even though light has no mass. Not because it pulls on things, but because it bends the space through which it passes.

But if you don't move through space relative to the mass that creates the gravity, you're still getting pulled in. Because even though you don't move through space, you're stil moving through space-time. You can not stop in space-time. You are always moving in space-time.

So it really is inevitable that gravity also warps time. There is no time. There is no space. Only space-time.

[halfeye]

You are always moving in space-time.

If there is no ether, then that shouldn't be true, you can eliminate the spacial aspect of your motion, then it is only time that is trying to make you move, and on Earth, that's at 9.8 ms^2 (with slight variations due to changes in the density of the Earth). We can determine the difference between time on Earth and interstellar time, and it's pretty small. The total dilation of time at a black hole implies that the warping of time there is 90 degrees, what is it at the surface of the Earth? one degree? much less than that? It might be useful to work in radians, in which case the 90 degrees I mentioned above is pi/2

So it really is inevitable that gravity also warps time. There is no time. There is no space. Only space-time.

That sounds mystical, I'm not generally inspired by mystical interpretations of things.

[Bavarian itP]

you can eliminate the spacial aspect of your motion

In relation to what?

[halfeye]

In relation to what?

Your frame of referance, e.g. the surface of the Earth.

[factotum]

Your frame of referance, e.g. the surface of the Earth.

And then as soon as you look at another frame of reference (say, the Earth's centre) you're moving again. Also, he said that you can never be stationary in *spacetime*, which is entirely true--even if you're supposedly stationary in space, you're still moving into the future one second at a time and can't stop doing that whatever you try.

[halfeye]

And then as soon as you look at another frame of reference (say, the Earth's centre) you're moving again.

Well yeah, except you are allowed to talk about your local frame of reference.

Also, he said that you can never be stationary in *spacetime*, which is entirely true--even if you're supposedly stationary in space, you're still moving into the future one second at a time and can't stop doing that whatever you try.

Right, so it's only time that is moving, and it's rate of movement is time plus 9.8 ms^2, where the acceleration is the curvature in time, aka the difference in the passage of time between Earth and time in free space (intergalactic space?).

[georgie_leech]

Well yeah, except you are allowed to talk about your local frame of reference.



Right, so it's only time that is moving, and it's rate of movement is time plus 9.8 ms^2, where the acceleration is the curvature in time, aka the difference in the passage of time between Earth and time in free space (intergalactic space?).

Your mistake is thinking that space, time, and spacetime are distinct ideas; the latter encompasses the former two, which are just different aspects of the latter. We are always moving in spacetime regardless of frame of reference, even if our motion through the spatial dimensions is 0.

[The Extinguisher]

Well it is. Then weirder still the more you think about it.

Gravity is due to curved space-time.

When I think of curved spacetime, I can see how something moving could find it's path changed by curved space. However, with something stationary it's not so clear that curving space would move it. So it seems to me that it's the curve in time that causes relatively stationary bodies to accelerate under gravity.

Have we yet worked out how much mass curves time? It doesn't seem to be very much, the change in the speed of time at the surface of the Earth and a few miles above it is apparently very little, but detectable.

If you have a trampoline with a bunch of stationary balls on it, pressing down on the surface (curving it) will cause those balls to move around (and in fact those balls also curve the trampoline)

Its just that instead of curving the plane of the trampoline, mass curves space-time

[LordEntrails]

It gets ever weirder when people start applying "daylight savings" in order to save time.

[halfeye]

If you have a trampoline with a bunch of stationary balls on it, pressing down on the surface (curving it) will cause those balls to move around (and in fact those balls also curve the trampoline)

Its just that instead of curving the plane of the trampoline, mass curves space-time

That is a bad analogy, because it involves using the thing you are trying to explain in the explanation.

[The Extinguisher]

That is a bad analogy, because it involves using the thing you are trying to explain in the explanation.

Im explaining curving spacetime by comparing it to pressing down on a trampoline? I know its the internet, but not all analogies need to be convoluted and byzantine

[halfeye]

Im explaining curving spacetime by comparing it to pressing down on a trampoline? I know its the internet, but not all analogies need to be convoluted and byzantine

You are trying to explain gravity by gravity pulling down some balls on a trampoline. If something is an analogy, it doesn't have the thing it's trying to explain in it.

There is a water pipe analogy of electricity, it wouldn't work if you tried to use electricity instead of water.

This is a known problem with this analogy, and the few physicists who still use it get very apologetic about it nowadays, as they should.

[The Extinguisher]

You are trying to explain gravity by gravity pulling down some balls on a trampoline. If something is an analogy, it doesn't have the thing it's trying to explain in it.

There is a water pipe analogy of electricity, it wouldn't work if you tried to use electricity instead of water.

This is a known problem with this analogy, and the few physicists who still use it get very apologetic about it nowadays, as they should.

Sure, but im not trying to explain gravity to you. Im trying to explain the curvature of spacetime. Like if you were asking how water flowed through a particularly tricky pipe layout or something and i explained part of it by telling you about something similar but simpler.

you know, like an analogy

[halfeye]

Sure, but im not trying to explain gravity to you. Im trying to explain the curvature of spacetime.

You think gravity isn't curved spacetime?

[Anymage]

Most of what I've learned about physics, both relativistic and quantum, I learned from watching youtube channels from people who really knew their stuff. Spacetime, fermilab, and looking glass universe were especially handy.

From there, Great Courses Plus and brilliant.org both looked interesting when channels had their "sponsored by" ads.

I guess the main point I'm making is, if you want to develop stronger intuitions for these fields, either take a class or sign up for a service that's effectively a class on the subject. There's a limit to how much can be effectively explained over the internet, especially when the people trying to explain it have no idea your skill level. And given the nature of public message boards, are also wise to be mindful of the skill level of anyone who might stumble across the topic.

[Grey_Wolf_c]

Your mistake is thinking that space, time, and spacetime are distinct ideas; the latter encompasses the former two, which are just different aspects of the latter. We are always moving in spacetime regardless of frame of reference, even if our motion through the spatial dimensions is 0.

One of the most illuminating understandings I have acquired on this topic in the last few years is that not only are we moving through spacetime all the time, we are also always moving through it at the same speed: the speed of causality (i.e. the speed of light). That is why when we move through space, time gets slower: we are borrowing some of the speed used to advance through time to instead advance through space, but the length of the vector in spacetime remains constant.

As an analogy, imagine a car on cruise control on a freeway. You can change lanes and thus take longer to reach your destination, but your absolute speed remains unchanged.

I suspect it is a bit more complex than that, but at least this simplification gets me closer to reality than my previous "sometimes we are standing still" conception.

Grey Wolf

[shawnhcorey]

To clear up some misconceptions:

1. Space is flat. Space-time is curved. Big difference.

2. There is no such thing as an inertial frame in our universe. All frames of references are being accelerated by gravity and EMF.

3. Gravity is a force. Mass is effected by both gravity and General Relativity (GR). Light is effected by only GR.

4. Both space and time are emergent properties of our universe, not fundamental ones. It is the speed of light that is a fundamental property.

5. Time is not a real dimension. It is only a mathematical illusion that makes calculations easier. Time is the distance travelled by light in a perfect vacuum.

[halfeye]

3. Gravity is a force.

Are you sure about that?

Curved spacetime is not a force.

[shawnhcorey]

Are you sure about that?

Curved spacetime is not a force.

Yes, I'm sure. What causes space-time to curve?

[halfeye]

Yes, I'm sure. What causes space-time to curve?

The presence of mass.

[shawnhcorey]

The presence of mass.

And what gets the effect of mass out there to so far away? Gravity is an infinite field. Something must transport the presence of mass away from the mass for gravity to have an effect so far away.

[halfeye]

And what gets the effect of mass out there to so far away? Gravity is an infinite field. Something must transport the presence of mass away from the mass for gravity to have an effect so far away.

Unknown. Gravitons? Tachyons? it might be a field without a particle.

Whatever, curved space-time, a.k.a. gravity is the result.

[Fyraltari]

Unknown. Gravitons? Tachyons? it might be a field without a particle.

Whatever, curved space-time, a.k.a. gravity is the result.

ISn't that Higg's boson's job?

[halfeye]

ISn't that Higg's boson's job?

I don't understand it as being so, but even if it was, that'd still leave the result being curved spacetime.

[anhuangdi]

CHina has the best policy. No daylight savings. The Entire country is the same timezone.

I call you in Beijing when I'm anywhere, and we are working with the same time.