# Action and Reaction

BruceW
Homework Helper
Even if there is zero curvature, (i.e. no massive objects warping spacetime), then we can still have acceleration along geodesics. It just depends on what coordinate system you choose. For example, if you use yourself as the spatial origin of a coordinate system, drop a ball in free space, then get in a rocket and zoom off, then according to your coordinate system, the ball accelerates away from you. So the ball accelerates even though no forces are acting on it. This is another example of why it is not surprising that as soon as we allow non-inertial reference frames, it is kind of obvious that Newton's laws (as we once knew them) no longer apply.

edit: this post isn't really a reply to any other post, just I hope this might be useful to Naveen, to see the kind of crazy stuff that general relativity allows us to do.

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curves which locally can be made to look like the usual straight lines; these curves are called geodesics. The physics comes in when we state that freely falling particles travel on these geodesics.
I am getting something of this. it has been 2 months since I have been 'studying' relativity primarily from the Internet ( I was having holidays) and some 'awakening' has already dawned upon me. But clouds of doubt/curiosity keep on shadowing this awakening.

My next question. Can all curves be made to look like straight lines and then physics applied to them?

Consider a solid 'curved' particle of the size of an electron. can all this be done to this particle also. Why not?

You're right, if we simply curved 3D space, we wouldn't expect any acceleration along geodesics, but since we are curving 3 dimensions of space and one of time, we get time derivatives of spatial variables which give us velocity and acceleration. Not acceleration due to any force, but due to the fact that following a 4 dimensional geodesic demands it. This is in perfect keeping with the Newton's laws, because objects travel in straight lines, and a curved, accelerated path is what actually constitutes a straight line in curved 3+1 space.
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Does it mean there can be no motion without time?

Does it mean time has 'independent' existence and is just not the rate of 'change' of things..

I thought all the motion resulting from the big bang created our universe. It's just motion from moving sub-atomic particles to moving galaxies. 'Time' appears to be rate of change of all this.

I think now I have to spend many days studying about 'time' even.

By the way, who was he who said that ignorance is bliss. JUST JOKING:shy:

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mfb
Mentor
Does it mean there can be no motion without time?
I don't see how "motion" could be defined without time.

Does it mean time has 'independent' existence and is just not the rate of 'change' of things..
How do you mean that question?

I thought all the motion resulting from the big bang created our universe.
No.
'Time' appears to be rate of change of all this.
We define the length of our timescales (seconds, for examples) based on rates of change of something. I don't think you can say "time is the rate of change" of anything.

WannabeNewton
My next question. Can all curves be made to look like straight lines and then physics applied to them?
No, this will only work for geodesics; in particular, with regards to massive particles, it will only work for freely falling particles (and keep in mind they can be made to look like straight lines only locally, that is in a sufficiently small region of space-time). I'm not sure what you mean by "and then physics applied to them" however; the framework of GR works for any and all worldlines, not just geodesics (so for example the dust grains sitting on the surface of the Earth).

Does it mean there can be no motion without time?
If time doesn't pass, how could things change position? Even in Newtonian physics, acceleration is the second derivative of a spatial coordinate with respect to time...the difference only being that in Newton's world, the passing of time is absolute.

Does it mean time has 'independent' existence and is just not the rate of 'change' of things..
In the framework of general relativity, yes, that is essentially the case. Time is treated as a separate dimension from space, with some interesting mathematical treatment that differs from that of space because time is a negative term in our spacetime metrics.

I thought all the motion resulting from the big bang created our universe. It's just motion from moving sub-atomic particles to moving galaxies. 'Time' appears to be rate of change of all this.
In the Newtonian view of things, this is basically the case: time is just a parameter that we use to track the change of systems, there isn't NECESSARILY any physical realism added to time (though I think Newton believed that there should be). I'm not sure I understand what you mean about the big bang...

In relativity, we have time dilation, which demonstrates that time is not an absolute parameter, but rather a sort of dimension that an observer will travel through differently relative to other observers depending on the strength of the gravitational field they are in and the speed at which they are moving relative to that other observer.

This is about as abstract as I can go without saying things that are just meaningless. Questions like "what is time?" or "is time real?" are more or less outside of the scope of physics right now. Really, we just know how to treat time mathematically, and can compare our mathematical knowledge of it to other things we think we understand to try to draw some comparisons between time and other things that are a bit easier for us to grasp. Anything else is just semantics/speculation.

Regarding the Big Bang. No, it is not the source of motion in our universe, it is simply what expanded space at the beginning of the universe. When I push this pencil on my desk forward, it certainly does NOT move because of the big bang...

Does it mean there can be no motion without time?
Time keeps everything from happening at once.

Space keeps everything from happening to me.

Regarding the Big Bang. No, it is not the source of motion in our universe, it is simply what expanded space at the beginning of the universe. When I push this pencil on my desk forward, it certainly does NOT move because of the big bang...
In my opinion motion is related to big bang as follows:

1. Big bang happened, space expanded. This is motion.

2. First there was just energy. It was not static. This is motion

3. Next, sub atomic particles and light atoms formed. They were also moving (including electrons and other nuclear particles in a single atom). This is motion.

4. After that, particles started coming together and various structures formed. This is motion.

5. Then atoms on a particular planet called Earth, started coming together and formed unicellular organisms. This is motion.

6. Then complex beings like human beings formed when more cells gradually developed bigger structures. This is motion.

7. These cells in my brain indulge in synaptic firing and other 'processes' which lead to a decision being made by me. All these processes involve motion. This is motion.

So, IMHO, if you push a pencil on a desk, it seems to be due to big bang only.

A.T.
ISo, IMHO, if you push a pencil on a desk, it seems to be due to big bang only.
Everything is due to big bang.

In my opinion motion is related to big bang as follows:

1. Big bang happened, space expanded. This is motion.
But in the comoving sense, the Big Bang doesn't cause any motion. Motion is caused by the energy and the potential that existed in the universe at the time of the Big Bang expansion. Those distant galaxies aren't really moving away from us at a speed faster than light, it's just the space between us and them expanding.

As long as you understand that, that what you're saying here is basically right, but let's look more in depth at what happens...

Universe begins in a very hot, dense state. The Big Bang occurs at t = 0 and spreads out all this energy. Conditions in the early universe allow for creation of quarks, neutrons and protons, and electrons. These combine to create Hydrogen, Helium, Deuterium, etc. We now have a giant cloud of H-He gas somewhere in the universe separated from everything else in the universe. But, we look at the center of momentum of the cloud, there is NO total momentum or kinetic energy here to begin with, just a lot of potential due to gravity. This potential pulls the gas together into a star which ignites and fused H and He into heavier elements like Carbon, which later is ejected when the star supernovas and disperses throughout the galaxy to infuse other clouds with the elements required for life. The process repeats, with terrestrial planets and life now being formed. Life on Earth evolves, I am born, I push a pencil on my desk.

This all began from a cloud of gas that has no kinetic energy in our frame of reference. The energy for creating life came from gravity, and the potential it created inside the gas cloud, not from the big bang, though all the matter did come from the big bang, that is, if you consider the big bang to be the creation of energy in the universe, and not just the mechanism for expansion of the universe. We really DONT know where the energy came from in the first place...

I'm still kind of confused as to what this has to do with your original question :tongue:

PeterDonis
Mentor
2019 Award
Universe begins in a very hot, dense state. The Big Bang occurs at t = 0 and spreads out all this energy.
No, you have this backwards. The Big Bang occurs at t = 0, and *then* the Universe is in a hot, dense state, with the hot, dense matter and energy expanding. But "expanding" must be interpreted carefully; there is no pre-existing space that it is expanding into, just as there was no pre-existing matter and energy before t = 0 that the Big Bang started expanding.

Actually, strictly speaking, saying that the Big Bang occurs at t = 0 isn't quite right either, because t = 0 is a singularity and isn't actually part of spacetime. The strictly correct statement is that the closer we get to t = 0, the smaller, hotter, and denser the Universe is. But that's often glossed over since it's a technical point that doesn't affect a lot of discussions.

This doesn't really affect the rest of what you said, but I think it's important to understand what the Big Bang model actually says.

No, you have this backwards. The Big Bang occurs at t = 0, and *then* the Universe is in a hot, dense state, with the hot, dense matter and energy expanding.
Ok, thanks for the clarification. Kinda feels like a chicken and the egg scenario to me.

But "expanding" must be interpreted carefully; there is no pre-existing space that it is expanding into, just as there was no pre-existing matter and energy before t = 0 that the Big Bang started expanding.
This I was well aware of. I hope I wasn't suggesting otherwise.

Dale
Mentor
In my opinion motion is related to big bang as follows
If you accept the big bang model then the statement "X is related to the big bang" is a tautology, regardless of X. It is true, but not very informative.

Actually, strictly speaking, saying that the Big Bang occurs at t = 0 isn't quite right either, because t = 0 is a singularity and isn't actually part of spacetime. The strictly correct statement is that the closer we get to t = 0, the smaller, hotter, and denser the Universe is. But that's often glossed over since it's a technical point that doesn't affect a lot of discussions.
lol I cracked up reading that. Perhaps it doesn't effect allot of discussions because they are made in the context of the theory's limits. That said without the physical universe, what's there to discus :rofl:

. We now have a giant cloud of H-He gas somewhere in the universe separated from everything else in the universe. But, we look at the center of momentum of the cloud, there is NO total momentum or kinetic energy here to begin with, just a lot of potential due to gravity. This potential pulls the gas together into a star which ignites and fused H and He into heavier elements like Carbon, which later is ejected when the star supernovas and disperses throughout the galaxy to infuse other clouds with the elements required for life. The process repeats, with terrestrial planets and life now being formed. Life on Earth evolves, I am born, I push a pencil on my desk.

This all began from a cloud of gas that has no kinetic energy in our frame of reference. The energy for creating life came from gravity, and the potential it created inside the gas cloud, not from the big bang,
Why don't you think is like this:

Gravity is the result of big bang. Gravity causes potential which pulls the gas together (causes it to move, and the story continues).

So, what's the problem, if I say big bang caused the gas to come together?

As regards the question of 'action and reaction' between matter and space, I think that 'space' can also in some vague way be considered to have the properties of matter.

I say this because scientists say that fluctuations of space lead to the production of 'material particles'.

I would think there is discontinuity between SR/GR and the statement "fluctuations of space lead to the production of 'material particles'." SR/GR is kinematics/geometry. comparatively, "material particles" from nothing is "magic". :rofl:

Nugatory
Mentor
I say this because scientists say that fluctuations of space lead to the production of 'material particles'.
Not really. Some pop-sci sources say things that could make you think that, but they're over-simplifying to such an extent that they're misleading if not outright wrong.

BruceW
Homework Helper
I'm guessing that 'fluctuations of space lead to production of material particles' is meant to be talking about fluctuations of a quantum field. This is not related to the curvature of space, or anything related to general relativity (as far as I'm aware). The reason they say 'space' is because they are talking about the vacuum fluctuation.

Why don't you think is like this:

Gravity is the result of big bang. Gravity causes potential which pulls the gas together (causes it to move, and the story continues).

So, what's the problem, if I say big bang caused the gas to come together?
Well, you could, but as DaleSpam already said, it's a tautology. It's a redundant statement that doesn't really tell you anything. For example, at the beginning of my life, I was brought into this world by my mother, and now I am sitting here pushing buttons on a computer keyboard. You COULD say that it is because of my mother that these buttons on the keyboard are being pressed, but that tells you nothing about the process by which these buttons are actually being pressed.

If it were enough to say "this is caused by the Big Bang", then consider physics solved!

In other words, what point are you trying to make by invoking the Big Bang?