Determining a stationary point of reference to base all absolute motion

[tsslieberman]

TL;DR Summary

Determining a stationary point of reference to base all absolute motion

We can determine the earth’s movement in relation to other astronomical objects, but we have no idea of our absolute movement, correct?

The earth is spinning and revolving around the sun which is revolving around a galaxy that is expanding from other galaxies in the universe.
The entire universe could be spinning or moving, correct?

But wouldn’t we need to know our velocity in absolute terms to know our relative time?
Wouldn’t our relative time be dependent on our ‘absolute’ velocity?

The constant of light speed measured as distance/time (m/s). What time is this based? Is this time unit based on the observer's absolute movement (which we don’t know) or our time as we perceive it at our current velocity?

 

[Ibix]

but we have no idea of our absolute movement, correct?

There is no such thing to have an idea of.

The entire universe could be spinning or moving, correct?

I'm not really sure this even makes sense. If you are referring to all the matter in the universe, yes you can at least conceive of it moving or rotating. Bulk motion of matter is unmeasurable, but rotation would be detectable.

But wouldn’t we need to know our velocity in absolute terms to know our relative time?

No, just look at clocks and compare them.

Wouldn’t our relative time be dependent on our ‘absolute’ velocity?

Nothing can be dependent on "absolute time" because there is no such thing.

The constant of light speed measured as distance/time (m/s). What time is this based?

Any time. The whole point about $c$ is that it's invariant - as long as you use clocks and rulers that are at rest with respect to each other you will always get the same value of the speed of light (even in non-SI unit systems where measuring the speed of light is non-tautological).

A question that does make sense and may be related to what you are trying to ask is what is our speed relative to the coordinates used to state that the universe is 14bn years old. That's measurable (about 600kps, from memory). Does it affect our estimate of the age of the universe? Technically it means we've experienced less time since the beginning of the universe than an observer who sees an isotropic CMB, but the effect is several orders of magnitude less than our experimental uncertainty in the age, so it's not relevant in practice.

 

[Nugatory]

TL;DR Summary: Determining a stationary point of reference to base all absolute motion

We can determine the earth’s movement in relation to other astronomical objects, but we have no idea of our absolute movement, correct?
The earth is spinning and revolving around the sun which is revolving around a galaxy that is expanding from other galaxies in the universe.
The entire universe could be spinning or moving, correct?
But wouldn’t we need to know our velocity in absolute terms to know our relative time?
Wouldn’t our relative time be dependent on our ‘absolute’ velocity?
The constant of light speed measured as distance/time (m/s). What time is this based? Is this time unit based on the observer's absolute movement (which we don’t know) or our time as we perceive it at our current velocity?

The only time that we can directly measure is that measured by a clock that is where we are and at rest relative to us. Everything else is inferred or calculated, although in every day life these calculations may be so obvious that we aren't aware that we're making them. Thus the notion of absolute velocity isn't needed - the clock is at rest relative to us no matter whether we have some hypothetical absolute velocity.

All speed of light measurements are two-way measurements in which we bounce the light off of a mirror, reflect it back to the source, so we use the same clock at relative to us to measure the travel time. (Anything that appears to be a one-way measurement will on closer examination turn out to be based not on a directly measured time but rather something that is inferred based on the assumption that we already know the speed of light).

 

[PeroK]

We can determine the earth’s movement in relation to other astronomical objects, but we have no idea of our absolute movement, correct?

There is no concept of absolute motion. All motion is relative. That's essentially the first postulate of SR.

The earth is spinning and revolving around the sun which is revolving around a galaxy that is expanding from other galaxies in the universe.

That's all relative. Although, the Earth's rotation involves non-inertial motion (i.e. proper acceleration) that can be detected (e.g. through Foucault's pendulum).

The entire universe could be spinning or moving, correct?

It doesn't make sense to talk about the entire universe moving. Motion is a measure of rate of change of spatial displacement with respect to time. Those concepts apply within the universe, but there is no concept of observing the universe from the outside.

But wouldn’t we need to know our velocity in absolute terms to know our relative time?

There is no such thing as relative time. The laws of physics apply equally in all inertial reference frames - that's another way of stating the first postulate of SR.

Wouldn’t our relative time be dependent on our ‘absolute’ velocity?

No. Neither concept exists in physics.

The constant of light speed measured as distance/time (m/s). What time is this based? Is this time unit based on the observer's absolute movement (which we don’t know) or our time as we perceive it at our current velocity?

The invariance of the speed of light applies to measurements made using any inertial reference frame. That's the second postulate of SR.

 

[tsslieberman]

There is no concept of absolute motion. All motion is relative. That's essentially the first postulate of SR.

That's all relative. Although, the Earth's rotation involves non-inertial motion (i.e. proper acceleration) that can be detected (e.g. through Foucault's pendulum).

It doesn't make sense to talk about the entire universe moving. Motion is a measure of rate of change of spatial displacement with respect to time. Those concepts apply within the universe, but there is no concept of observing the universe from the outside.


There is no such thing as relative time. The laws of physics apply equally in all inertial reference frames - that's another way of stating the first postulate of SR.

No. Neither concept exists in physics.

The invariance of the speed of light applies to measurements made using any inertial reference frame. That's the second postulate of SR.

 

[tsslieberman]

Thank you for your thoughtful response.

Again, I’m a layman & I appreciate your patience with my questions.

When you say, “It doesn't make sense to talk about the entire universe moving. Motion is a measure of rate of change of spatial displacement with respect to time. Those concepts apply within the universe, but there is no concept of observing the universe from the outside.” Could we not use light to determine our absolute motion (not relative to other objects)?

The second postulate of SR, the speed of light in a vacuum is constant for all observers, regardless of the motion of the light source or the observer. I assume that means all observers are traveling at the same relative velocity. For arguments sake, let’s say that the earth is moving at a very high velocity.. the time light took to go between two objects on earth taken in different directions would be different, no? Light would travel at constant, but since the objects are moving, light would take longer to travel the same distance if the objects were traveling in the direction of the light, no?

 

[PeroK]

Thank you for your thoughtful response.

Again, I’m a layman & I appreciate your patience with my questions.

When you say, “It doesn't make sense to talk about the entire universe moving. Motion is a measure of rate of change of spatial displacement with respect to time. Those concepts apply within the universe, but there is no concept of observing the universe from the outside.” Could we not use light to determine our absolute motion (not relative to other objects)?

The second postulate of SR, the speed of light in a vacuum is constant for all observers, regardless of the motion of the light source or the observer. I assume that means all observers are traveling at the same relative velocity. For arguments sake, let’s say that the earth is moving at a very high velocity.. the time light took to go between two objects on earth taken in different directions would be different, no? Light would travel at constant, but since the objects are moving, light would take longer to travel the same distance if the objects were traveling in the direction of the light, no?

No, you can't use light to determine an absolute motion. That is what the Michelson Morley experiment showed. There is no special universal reference frame in which the speed of light is $c$. The speed of light is $c$ in all inertial reference frames.

Note that this contradicts classical notions of space and time, as used in Newtonian mechanics.

Relativity is a not a subset of Newtonian physics. It requires that you rethink your concepts of space and time.

 

[tsslieberman]

Well, considering I’ve never heard of the Michelson Morley experiment, I don’t feel so stupid for considering a similar idea.
Thank you

 

[Ibix]

The second postulate of SR, the speed of light in a vacuum is constant for all observers, regardless of the motion of the light source or the observer. I assume that means all observers are traveling at the same relative velocity.

No. It means exactly what it says: all observers measure light to be passing them at $c$ in vacuum, always. What we call "the theory of relativity" is essentially the implications of that assumption and the principle of relativity.

For arguments sake, let’s say that the earth is moving at a very high velocity.. the time light took to go between two objects on earth taken in different directions would be different, no?

No. The concept of "moving at high velocity" does not make sense even in a Galilean universe. You can only have a velocity relative to something. In everyday life we tend to treat our surroundings as fixed and assume we mean "high velocity relative to our surroundings" unless otherwise specified, and that's fine for driving your car but won't work in more general scenarios where there's no always applicable convention for what is "at rest".

The previously mentioned Michelson-Morley experiment assumed that light travelled through a medium (the "ether") and the Earth moved relative to it. Michelson and Morley looked for variation in the speed of light relative to Earth as a result of Earth's motion through the ether. Their null result is a key plank in the evidence base that led to Einstein proposing his two postulates.

 

[tsslieberman]

You can only have a velocity relative to something.

So, light has no velocity if not relative to something? Would that not contradict the concept of a constant?

 

[tsslieberman]

No. It means exactly what it says: all observers measure light to be passing them at $c$ in vacuum, always. What we call "the theory of relativity" is essentially the implications of that assumption and the principle of relativity.

No. The concept of "moving at high velocity" does not make sense even in a Galilean universe. You can only have a velocity relative to something. In everyday life we tend to treat our surroundings as fixed and assume we mean "high velocity relative to our surroundings" unless otherwise specified, and that's fine for driving your car but won't work in more general scenarios where there's no always applicable convention for what is "at rest".

The previously mentioned Michelson-Morley experiment assumed that light travelled through a medium (the "ether") and the Earth moved relative to it. Michelson and Morley looked for variation in the speed of light relative to Earth as a result of Earth's motion through the ether. Their null result is a key plank in the evidence base that led to Einstein proposing his two postulates.

I appreciate you taking the time to respond to an absolute layman.
If I were traveling at near light speed, would I not perceive the light traveling from the sun to the earth faster than someone on earth?

 

[phinds]

I appreciate you taking the time to respond to an absolute layman.
If I were traveling at near light speed, would I not perceive the light traveling from the sun to the earth faster than someone on earth?

NO. Again and again, the speed of light is c as detected by all observers regardless of their motion relative to the source of the light.

What DOES change is the frequency of light's wavelength. Google "red shifted" and "blue shifted".

You have GOT to let go of this erroneous concept of there being any special frame of reference or any way that light is not seen at traveling at c.

 

[PeroK]

If I were traveling at near light speed, would I not perceive the light traveling from the sun to the earth faster than someone on earth?

No. The speed of light in vacuum is $c$. That's on page 1 in every book on relativity.

 

[Ibix]

So, light has no velocity if not relative to something? Would that not contradict the concept of a constant?

Light's velocity is $c$ relative to anything. That is not a contradiction.

If I were traveling at near light speed

Relative to what? You are currently travelling at near light speed relative to a solar neutrino and virtually nothing with respect to the floor.

would I not perceive the light traveling from the sun to the earth faster than someone on earth?

No. The speed of light is always $c$ relative to you.

 

[phinds]

No. The speed of light in vacuum is $c$. That's on page 1 in every book on relativity.

and @tsslieberman what you have to understand is the light is not like anything else. The speed of sound, as perceived by someone moving relative to the source is different than that of someone not moving relative to the source. Light doesn't work that way. Light is seen as having speed c, REGARDLESS of the motion of the observer relative to the source of the light.

I suggest that before you ask any more questions, you reread this thread and make sure you understand what you are being told.

Asking the same question over and over is not going to change the answer.

 

[Nugatory]

If I were traveling at near light speed, would I not perceive the light traveling from the sun to the earth faster than someone on earth?

That's a very natural assumption based on our experience with relative velocities that are small compared with $c$: I can throw a rock at a speed of 100 km/hr relative to myself, I'm standing in an open truck driving down the road at 100 km/hr, I throw the rock straight ahead, we expect the rock to be moving at 100 km/hr relative to the truck and 200 km/hr relative to someone standing at the side of the road - the speeds just add, right? And then I do the same thing with a flashlight which is "throwing" light straight ahead at speed $c$ relative to me holding the flashlight, we expect the light is moving at speed $c$ relative to me and $c$+100 km/hr relative to the person standing on the side of the road, right?

No. It turns out that speeds don't add that way. If something is moving at speed $u$ relative to me, and I am moving at speed $v$ relative to you, its speed $w$ relative to you is not $w=u+v$ but instead $w=\frac{u+v}{1+uv/c^2}$. If you play with this formula a bit you will see two interesting things:

First, if either $u$ or $v$ is equal to $c$, then no matter what the value of the other one is, the value of $w$ comes out to be $c$ - the light is moving at speed $c$ relative to both of us even though we are moving relative to one another.

Second, for any of the speeds that we're used to experiencing the difference is too small to measure in a any normal way (it would be a good exercise to do the calculation for my example above with $u=v=100 km/hr$). That's why it took so long to realize that the $w=u+v$ is not exactly correct, just a really really good approximation for speeds that are small compared to $c$.
(As a historical note, the physicist Fizeau was able to measure this discrepancy in the mid-1800s, but the signifcance of this measurement was not recognized until Einstein developed special reativity a half-century later).

 

[Dale]

would I not perceive the light traveling from the sun to the earth faster than someone on earth?

This question could be understood a few different ways. The light will be moving at c in your frame. But the time that it takes to go from the earth to the sun will be longer or shorter or the same depending on which direction you are going.

 

[PeterDonis]

So, light has no velocity if not relative to something?

All velocity is relative in relativity. It's just that, for timelike objects (objects like us and rocket ships), their velocity relative to different things can be different. For lightlike objects (like light), their velocity relative to everything is the same--$c$.

The more basic way of looking at this is that thinking of $c$ as "the speed of light" is really a misnomer. It's really a conversion factor between different systems of units for measuring the geometry of spacetime. In "natural" units, which you will find used a lot in relativity, $c = 1$ by definition: time and space are measured in the same units. To put it another way, the real meaning of "the speed of light is always $c$" is that the geometry of spacetime contains structures called light cones, which mark the boundaries of causal propagation. And those structures are invariants; they're the same for all observers and for all frames of reference.

 

[PeroK]

This question could be understood a few different ways. The light will be moving at c in your frame. But the time that it takes to go from the earth to the sun will be longer or shorter or the same depending on which direction you are going.

This seems to unnecessarily complicate a simple matter. The speed of light is $c$ independent of the state of motion relative to the source.

 

[tsslieberman]

NO. Again and again, the speed of light is c as detected by all observers regardless of their motion relative to the source of the light.

What DOES change is the frequency of light's wavelength. Google "red shifted" and "blue shifted".

You have GOT to let go of this erroneous concept of there being any special frame of reference or any way that light is not seen at traveling at c.

Again, thank you for taking the time to respond.
I certainly don’t want to irritate you any further so if you don’t want to respond I understand.
So, if I’m traveling at near light speed and time the travel of light from the sun to the earth it would be the same as if I was on earth?
The time of that event, or any event involving light would not change dependent on my velocity, but all other events would seem to progress faster from my perspective?

 

[PeterDonis]

if I’m traveling at near light speed

Relative to what? Such a statement is meaningless unless you specify what the speed is relative to.

I think what you mean is, relative to the sun and earth (assuming both to be at rest relative to each other, which is not actually true, but is a reasonable approximation for a simple idealized scenario). But you should not leave us to guess. You should explicitly tell us.

and time the travel of light from the sun to the earth\

How? You're not on the sun or the earth. You can't just magically know times of things that happen at places where you aren't.

One of the key implications of the fact that light travels at a finite speed (or, if we use the more basic terminology of my earlier post, of the light cone structure of the geometry of spacetime that constrains causal propagation) is that you have to explicitly think through how you actually get information about events that happen at other locations. You can't just assume that it somehow happens and ignore the details, as you can in non-relativistic physics.

 

[PeterDonis]

The time of that event

Relative to what? Times are relative just like velocities are. There is no such thing as "the" time of any event. You have to specify how the time is measured and what reference frame is used to measure it.

 

[PeroK]

Again, thank you for taking the time to respond.
I certainly don’t want to irritate you any further so if you don’t want to respond I understand.
So, if I’m traveling at near light speed and time the travel of light from the sun to the earth it would be the same as if I was on earth?
The time of that event, or any event involving light would not change dependent on my velocity, but all other events would seem to progress faster from my perspective?

Among other things, the distance from the Sun to the Earth is not invariant. Again, this is something you are assuming from classical concepts of space and distance. In relativity, the distance from the Sun to the Earth depends on whether you are at rest relative to those objects or moving relative to those objects.

It's quite hopeless to talk about relativity, while still hanging on to classical physics concepts.

 

[Ibix]

Again, thank you for taking the time to respond.
I certainly don’t want to irritate you any further so if you don’t want to respond I understand.
So, if I’m traveling at near light speed and time the travel of light from the sun to the earth it would be the same as if I was on earth?
The time of that event, or any event involving light would not change dependent on my velocity, but all other events would seem to progress faster from my perspective?

I'm afraid you're flailing here, and us simply batting back your mistakes is a really slow way for you to learn. I think you really need a textbook. Morin's Relativity for the Enthusiastic Beginner is well liked and the first chapter is free online. Taylor and Wheeler's Spacetime Physics is my preference, and the whole thing is free for download from Taylor's website if you are happy with an electronic copy.

 

[Nugatory]

So, if I’m traveling at near light speed and time the travel of light from the sun to the earth it would be the same as if I was on earth?
The time of that event, or any event involving light would not change dependent on my velocity, but all other events would seem to progress faster from my perspective?

Take the distance between the sun and the earth, divide it by the time it takes the flash of light to get from the sun to the earth, you will get $c$. Everyone will, regardless of their speed relative to the sun and the earth. They will disagree about both the distance and the time, but not about the ratio between them.

 

[tsslieberman]

idea

There is no such thing to have an idea of.

Let’s change that

 

[PeterDonis]

Let’s change that

I'm not sure what you mean. @Ibix was making a simple factual statement. There is no way to "change" it.

 

[tsslieberman]

Take the distance between the sun and the earth, divide it by the time it takes the flash of light to get from the sun to the earth, you will get $c$. Everyone will, regardless of their speed relative to the sun and the earth. They will disagree about both the distance and the time, but not about the ratio between them.

Oh, I thought that if I traveled at near light speed, time would progress faster.
You’re saying that it wouldn’t. That the time time light traveled between objects would be irrelevant to my velocity?

 

[tsslieberman]

Take the distance between the sun and the earth, divide it by the time it takes the flash of light to get from the sun to the earth, you will get $c$. Everyone will, regardless of their speed relative to the sun and the earth. They will disagree about both the distance and the time, but not about the ratio between them.

I see. So time does not progress slower with increasing velocity near C?

 

[tsslieberman]

I'm afraid you're flailing here, and us simply batting back your mistakes is a really slow way for you to learn. I think you really need a textbook. Morin's Relativity for the Enthusiastic Beginner is well liked and the first chapter is free online. Taylor and Wheeler's Spacetime Physics is my preference, and the whole thing is free for download from Taylor's website if you are happy with an electronic copy.

Maybe, instead of sending me references, you could guide me? Let’s not regurgitate information we already have and instead, maybe come up with new ideas

 

[tsslieberman]

There is no such thing to have an idea of.

I'm not really sure this even makes sense. If you are referring to all the matter in the universe, yes you can at least conceive of it moving or rotating. Bulk motion of matter is unmeasurable, but rotation would be detectable.

No, just look at clocks and compare them.

Nothing can be dependent on "absolute time" because there is no such thing.

Any time. The whole point about $c$ is that it's invariant - as long as you use clocks and rulers that are at rest with respect to each other you will always get the same value of the speed of light (even in non-SI unit systems where measuring the speed of light is non-tautological).

A question that does make sense and may be related to what you are trying to ask is what is our speed relative to the coordinates used to state that the universe is 14bn years old. That's measurable (about 600kps, from memory). Does it affect our estimate of the age of the universe? Technically it means we've experienced less time since the beginning of the universe than an observer who sees an isotropic CMB, but the effect is several orders of magnitude less than our experimental uncertainty in the age, so it's not relevant in practice.

And the fact that we have no idea of our absolute movement doesn’t bother you?

 

[tsslieberman]

Relative to what? Such a statement is meaningless unless you specify what the speed is relative to.

I think what you mean is, relative to the sun and earth (assuming both to be at rest relative to each other, which is not actually true, but is a reasonable approximation for a simple idealized scenario). But you should not leave us to guess. You should explicitly tell us.


How? You're not on the sun or the earth. You can't just magically know times of things that happen at places where you aren't.

One of the key implications of the fact that light travels at a finite speed (or, if we use the more basic terminology of my earlier post, of the light cone structure of the geometry of spacetime that constrains causal propagation) is that you have to explicitly think through how you actually get information about events that happen at other locations. You can't just assume that it somehow happens and ignore the details, as you can in non-relativistic physics.

Oh, my bad. I thought we could think of “thought experiments” in this forum. I didn’t realize we had to have experimental evidence.

 

[tsslieberman]

Among other things, the distance from the Sun to the Earth is not invariant. Again, this is something you are assuming from classical concepts of space and distance. In relativity, the distance from the Sun to the Earth depends on whether you are at rest relative to those objects or moving relative to those objects.

It's quite hopeless to talk about relativity, while still hanging on to classical physics concepts.

At rest? That’s exactly what I’m proposing

 

[tsslieberman]

I'm not sure what you mean. @Ibix was making a simple factual statement. There is no way to "change" it.

Uh, I disagree. There is a way to change one’s perception of “reality”

 

[renormalize]

Uh, I disagree. There is a way to change one’s perception of “reality”

But changing one's perception affects only the perceiver, not the external reality.