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# Expansion Faster Than The Speed of Light? T or F?

#### fmdog44

##### Active Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?

#### JoeMoma

##### Platinum Member
I am not going to pretend to have a deep understanding of the special theory of relativity. However, plugging into the formula for objects traveling in opposite directions (opposite Rays), the vector sum of their velocities would be 0.96C (Where C is the speed of light.)

Velocity-addition formula - Wikipedia the free encyclopedia

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#### SwimExpert

##### Gold Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?
I think you're confusing the expansion of the universe with the expansion of space. As the universe expands, so to does space. Thus, the one meter distance between two objects expands with the universe, remaining one meter, but being a "larger" one meter to an observer outside of the universe.

OP
F

#### fmdog44

##### Active Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?
I think you're confusing the expansion of the universe with the expansion of space. As the universe expands, so to does space. Thus, the one meter distance between two objects expands with the universe, remaining one meter, but being a "larger" one meter to an observer outside of the universe.
This is why I stated you are taking a position between two stars, the space between the two is greater but at what speed? Is this not valid because perhaps one cannot claim to be able to take a position between to stars?

#### SwimExpert

##### Gold Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?
I think you're confusing the expansion of the universe with the expansion of space. As the universe expands, so to does space. Thus, the one meter distance between two objects expands with the universe, remaining one meter, but being a "larger" one meter to an observer outside of the universe.
This is why I stated you are taking a position between two stars, the space between the two is greater but at what speed? Is this not valid because perhaps one cannot claim to be able to take a position between to stars?
What you are not understanding is that as the universe expands, the distance of the two stars will remain the same. So the light traveling between the two objects will continue to travel at light speed.

Think of it this way....imagine a large elastic sheet. This sheet represents the universe. Take two pins with large heads and stick them on the sheet somewhere. Those two pins are your two stars. Between those two pins, there are a set number of fibers. Those fibers are the distance.

Now, if you stretch the elastic sheet, the number of fibers between the two pins remains constant. Even though the sheet (i.e. the universe) has expanded, the distance (i.e. the number of fibers) between the two objects has remained the same. The only way that distance between the two objects can be changed is by one or both objects going into motion.

#### JoeMoma

##### Platinum Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?
I think you're confusing the expansion of the universe with the expansion of space. As the universe expands, so to does space. Thus, the one meter distance between two objects expands with the universe, remaining one meter, but being a "larger" one meter to an observer outside of the universe.
This is why I stated you are taking a position between two stars, the space between the two is greater but at what speed? Is this not valid because perhaps one cannot claim to be able to take a position between to stars?
What you are not understanding is that as the universe expands, the distance of the two stars will remain the same. So the light traveling between the two objects will continue to travel at light speed.

Think of it this way....imagine a large elastic sheet. This sheet represents the universe. Take two pins with large heads and stick them on the sheet somewhere. Those two pins are your two stars. Between those two pins, there are a set number of fibers. Those fibers are the distance.

Now, if you stretch the elastic sheet, the number of fibers between the two pins remains constant. Even though the sheet (i.e. the universe) has expanded, the distance (i.e. the number of fibers) between the two objects has remained the same. The only way that distance between the two objects can be changed is by one or both objects going into motion.
You might want to double check that. It's my understanding that stars and galaxies are not stationary. But I am not an astrophysics.

#### SwimExpert

##### Gold Member
You might want to double check that. It's my understanding that stars and galaxies are not stationary. But I am not an astrophysics.
I never said they were.

***Before I go on, I just looked back and realized that I said something horribly clumsy in my initial post. To correct myself, the expansion of the universe should not be confused with the movement of objects through space.***

My example demonstrates two objects occupying a given space at a given time. Any change in distance between the two object requires motion. Expanding the universe does not change the distance between the two objects because what constitutes a meter is expanding.

This is a crude explanation, and I realize that this is a confounding and counter-intuitive subject. But what you have to understand is that object moving outward from the center of the universe is not the same thing as the expansion of the universe itself.

#### JoeMoma

##### Platinum Member
You might want to double check that. It's my understanding that stars and galaxies are not stationary. But I am not an astrophysics.
I never said they were.

***Before I go on, I just looked back and realized that I said something horribly clumsy in my initial post. To correct myself, the expansion of the universe should not be confused with the movement of objects through space.***

My example demonstrates two objects occupying a given space at a given time. Any change in distance between the two object requires motion. Expanding the universe does not change the distance between the two objects because what constitutes a meter is expanding.

This is a crude explanation, and I realize that this is a confounding and counter-intuitive subject. But what you have to understand is that object moving outward from the center of the universe is not the same thing as the expansion of the universe itself.
I have a very limited understanding of the expanding universe theory. From what I understand it has to do with the distance between galaxies to be ever increasing since the Big Bang. In other words, the galaxies are moving outwardly from the Big Bang.

What does it mean when they say the universe is expanding Everyday Mysteries Fun Science Facts fromthe Library of Congress

OP
F

#### fmdog44

##### Active Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?
I think you're confusing the expansion of the universe with the expansion of space. As the universe expands, so to does space. Thus, the one meter distance between two objects expands with the universe, remaining one meter, but being a "larger" one meter to an observer outside of the universe.
This is why I stated you are taking a position between two stars, the space between the two is greater but at what speed? Is this not valid because perhaps one cannot claim to be able to take a position between to stars?
What you are not understanding is that as the universe expands, the distance of the two stars will remain the same. So the light traveling between the two objects will continue to travel at light speed.

Think of it this way....imagine a large elastic sheet. This sheet represents the universe. Take two pins with large heads and stick them on the sheet somewhere. Those two pins are your two stars. Between those two pins, there are a set number of fibers. Those fibers are the distance.

Now, if you stretch the elastic sheet, the number of fibers between the two pins remains constant. Even though the sheet (i.e. the universe) has expanded, the distance (i.e. the number of fibers) between the two objects has remained the same. The only way that distance between the two objects can be changed is by one or both objects going into motion.
I understand that example and agree with it but I thought this is also true: the further away objects are from "something" the faster they are accelerating. That would mean objects in one location are moving at a greater rate of speed than other objects. Correct?

#### Delta4Embassy

##### Gold Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?

To better understand what's happening, you have to appreciate that it's not the objects in the universe moving faster than light, but rather the universe's "fabric" or space-time itself expanding in excess of C. Planets and stars can't exceed C, but the universe itself can. It's like a naturally-occuring warp drive.

Using the balloon analogy with inky dots on the surface then, if you inflate the balloon it gets bigger and the dots move further apart. But the dots aren't moving independently, rather it's the balloon that's moving making it appear they are.

#### SwimExpert

##### Gold Member
Can it be said in a blanket statement the universe is expanding faster than the speed of light if you state the following: Take a position between two stars each moving away from each other at a rate of 75% of the speed of light. Isn't the distance measured during a amount of time then not greater than the speed of light, like 1.5 the speed of light?
I think you're confusing the expansion of the universe with the expansion of space. As the universe expands, so to does space. Thus, the one meter distance between two objects expands with the universe, remaining one meter, but being a "larger" one meter to an observer outside of the universe.
This is why I stated you are taking a position between two stars, the space between the two is greater but at what speed? Is this not valid because perhaps one cannot claim to be able to take a position between to stars?
What you are not understanding is that as the universe expands, the distance of the two stars will remain the same. So the light traveling between the two objects will continue to travel at light speed.

Think of it this way....imagine a large elastic sheet. This sheet represents the universe. Take two pins with large heads and stick them on the sheet somewhere. Those two pins are your two stars. Between those two pins, there are a set number of fibers. Those fibers are the distance.

Now, if you stretch the elastic sheet, the number of fibers between the two pins remains constant. Even though the sheet (i.e. the universe) has expanded, the distance (i.e. the number of fibers) between the two objects has remained the same. The only way that distance between the two objects can be changed is by one or both objects going into motion.
I understand that example and agree with it but I thought this is also true: the further away objects are from "something" the faster they are accelerating. That would mean objects in one location are moving at a greater rate of speed than other objects. Correct?
You are halfway correct. Expansion itself is accelerating, and that is an result that was quite confounding for scientists to find. The prevailing wisdom was that expansion should decelerate went on. Current research indicates that after the big bang expansion began to slow down for a long time. But after about 5 billion years, IIRC, expansion began to speed up again. Explaining this has been difficult because such acceleration needs to be justified by some kind of energy. This has led to the theory of dark energy and matter; energy and matter that is unseen and otherwise undetectable, which is said to be the otherwise unidentifiable thrust contributing to the accelerated expansion of the universe. IMO, this is the scientific equivalent of attributing it to God. But whatever.

What we know is that the acceleration cannot be identified on small scales. We can only observe the phenomenon in reference to objects whose distance from each other is quite large on a cosmological scale. What it all leads to is that the further from the center of the universe we go, the greater the acceleration of expansion.

#### SwimExpert

##### Gold Member
I have a very limited understanding of the expanding universe theory. From what I understand it has to do with the distance between galaxies to be ever increasing since the Big Bang. In other words, the galaxies are moving outwardly from the Big Bang.

What does it mean when they say the universe is expanding Everyday Mysteries Fun Science Facts fromthe Library of Congress
Don't worry, IMO the best scientists in the world have a limited understanding of it, partially due to the fact that many advanced concepts tend to rely on highly imaginative explanations, and unfortunately the trend lately has been to attach realism to models that ultimately entail logical contradiction.

But what you describe, from the link you've posted, is unfortunately one of the common misunderstood ways of articulating the idea of universal expansion. The expanding dough into a loaf of bread explanation is very crude. It does offer a slight metaphor for the expansion of the universe, but it has many problems.

All objects in the universe are in motion. The link suggests that objects do not move through space, they merely move along with the moving space that they occupy. But this is not true. The link would give the impression that all objects are moving on a direct trajectory outward from the center of the universe, when the reality is that movement occurs in along all three axes. Based on the link's explanation movement should only be possible on a single outward trajectory and could never move left or right, up or down, relative to their "expansion" trajectory. But we know this is not the case. Objects collide with each other. Stars collide with each other. Entire galaxies collide with each other. In about 4 billion years our own galaxy will collide with the Andromeda galaxy.

Movement through space is a separate thing from movement of space. Both occur independently of each other. Movement through space results in the distance between to objects changing. The movement of space results in the distance between two objects remaining the same even though intuitively we feel like the de facto distance would have to be greater.

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