Is speed of light relative to eather flow?

[schiz0ai]

Is speed of light relative to "eather" flow?

If 2 rockets fly from Earth in opposite directions.
Both end up flying at 60% the speed of light.
Does that mean they flay faster then the speed of light, compared to each other?
Or is it relative to spacetime or eather or whatever?

 

[jtbell]

If 2 rockets fly from Earth in opposite directions.
Both end up flying at 60% the speed of light.
Does that mean they flay faster then the speed of light, compared to each other?

No. In relativity, velocities don't "add" the same way as in classical physics.

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

The result is always less than the speed of light.

 

[schiz0ai]

so would an observer standing on 1 rocket be able to see light coming from the other rocket?

 

[Doc Al]

so would an observer standing on 1 rocket be able to see light coming from the other rocket?

Why not?

 

[schiz0ai]

how fast would light travel from 1 rocket to the other? and compared to what? the earth? the rocket emitting the light? or the one receiving it?

 

[HallsofIvy]

The basic concept of relativity is that the speed of light is the same in any coordinate system. So the answer to all of your questions is "c".

 

[schiz0ai]

so does that mean that when we think we shot something at 90% speed of light.
That seen from the center of the universe, the Earth was moving so fast to begin with, that the thing we shot went only little bit faster?

 

[Doc Al]

Something like that. Say we shoot a spaceship at 90% of the speed of light with respect to earth. Then those on that spaceship shoot a rocket in the forward direction at 90% of the speed of light with respect to the spaceship. According to us on earth, that rocket is moving at about 99% of the speed of light (not 180%).

 

[phinds]

so does that mean that when we think we shot something at 90% speed of light.
That seen from the center of the universe, the Earth was moving so fast to begin with, that the thing we shot went only little bit faster?

There is no "center of the universe".

 

[schiz0ai]

i thought the radius of the universe was the speed of light times the time since the big bang, and asumed herefor it had a calculatable center, but something tells me i really need to do some serious years of school to understand that kinda stuff. And hopefully some day il get the time to dive into it.

 

[phinds]

i thought the radius of the universe was the speed of light times the time since the big bang, and asumed herefor it had a calculatable center, but something tells me i really need to do some serious years of school to understand that kinda stuff. And hopefully some day il get the time to dive into it.

Try this for a start:

www.phinds.com/balloonanalogy

 

[tensor33]

i thought the radius of the universe was the speed of light times the time since the big bang.

That is correct. However, it is true for any point in space. There is no special point that is the center of the universe, any point in space can be considered the center. This is called the cosmological principle.

 

[phinds]

That is correct. However, it is true for any point in space. There is no special point that is the center of the universe, any point in space can be considered the center. This is called the cosmological principle.

No, it is NOT true. You are using sloppy terminology. What you have described is NOT "the universe", which is what the statement was about, but the "OBSERVABLE universe". There is a big difference.

 

[tensor33]

What you have described is NOT "the universe", which is what the statement was about, but the "OBSERVABLE universe". There is a big difference.

You're right. That was a sloppy choice of words. Let me rephrase that. Any point in space can be cosidered the center of the universe it can observe (The observable universe)

 

[phinds]

You're right. That was a sloppy choice of words. Let me rephrase that. Any point in space can be cosidered the center of the universe it can observe (The observable universe)

Excellent recovery

 

[Austin0]

No, it is NOT true. You are using sloppy terminology. What you have described is NOT "the universe", which is what the statement was about, but the "OBSERVABLE universe". There is a big difference.

I am not sure I understand the distinction you are making here.
It would seem that unless there is an assumption that our observable sector of a total universe is somehow privileged that it would follow that we (or any other point) are effectively at the center of the whole shebang whatever it's extent.
What am I missing?

 

[phinds]

I am not sure I understand the distinction you are making here.
It would seem that unless there is an assumption that our observable sector of a total universe is somehow privileged that it would follow that we (or any other point) are effectively at the center of the whole shebang whatever it's extent.
What am I missing?

Yeah, I think I got carried away by the use of the terms "radius" and "universe" together and immediate said to my self, NO ... that's the OBSERVABLE universe (which HAS a radius), not "the universe" which does not have a radium, but you are of course correct. Any point anywhere is the center of its own observable universe

 

[Austin0]

That is correct. However, it is true for any point in space. There is no special point that is the center of the universe, any point in space can be considered the center. This is called the cosmological principle.

Yeah, I think I got carried away by the use of the terms "radius" and "universe" together and immediate said to my self, NO ... that's the OBSERVABLE universe (which HAS a radius), not "the universe" which does not have a radium, but you are of course correct. Any point anywhere is the center of its own observable universe

Just a point of clarification. There is an obvious distinction between observable universe and total universe. But if we assume global (total universe) isotropy and homogeneity it seems to me that tensor33's unqualified statement above, is not only valid as it stands but is a logical necessity. IMHO
I.e. Any point must be considered the center of the total universe.
What do you think?

 

[Dickfore]

eather

What's this?

 

[phinds]

Just a point of clarification. There is an obvious distinction between observable universe and total universe. But if we assume global (total universe) isotropy and homogeneity it seems to me that tensor33's unqualified statement above, is not only valid as it stands but is a logical necessity. IMHO
I.e. Any point must be considered the center of the total universe.
What do you think?

Yes, if the universe is infinite in extent. My own belief is that it likely is, but I figure my personal belief with no facts to back it up is worthless, and I don't have any. Since the topology of the universe is unknown, I'm hesitant to say that EVERYWHERE is the center although it may well be.

If the universe is finite but unbounded, I don't know enough about possible topologies to understand whether the statement would always hold or not.

 

[Austin0]

Yes, if the universe is infinite in extent. My own belief is that it likely is, but I figure my personal belief with no facts to back it up is worthless, and I don't have any. Since the topology of the universe is unknown, I'm hesitant to say that EVERYWHERE is the center although it may well be.

If the universe is finite but unbounded, I don't know enough about possible topologies to understand whether the statement would always hold or not.

Yeah me too as far as lacking in facts or any knowledge of what's really out there.
I apologize if I projected any unwarranted certainty of the reality in this regard.

I did not mean to imply that the statement was necessarily true in reality. Only that based on our current models and theories it was a logical consequence without any theoretical reason for denying it.
Not even that I particularly believe it. I think that it is not only you and I lacking in factual basis for prediction but it is the case for all theoretical speculation at this point regarding the topology and nature of the universe as a whole or even within our observable slice.
We seem to be stuck with using creative imagination and logic in seeking answers to many questions IMO

 

[phinds]

Yeah me too as far as lacking in facts or any knowledge of what's really out there.
I apologize if I projected any unwarranted certainty of the reality in this regard.

I did not mean to imply that the statement was necessarily true in reality. Only that based on our current models and theories it was a logical consequence without any theoretical reason for denying it.
Not even that I particularly believe it. I think that it is not only you and I lacking in factual basis for prediction but it is the case for all theoretical speculation at this point regarding the topology and nature of the universe as a whole or even within our observable slice.
We seem to be stuck with using creative imagination and logic in seeking answers to many questions IMO

I agree w/ you in general, with the strong exception that I completely DISagree regarding the observable universe, which is well known to be flat to within our ability to measure it.

Personally I find it REALLY unlikely that out of all the possible values of curvature that the observable universe could take, it is flat to within our ability to measure it but NOT actually totally flat. That is, I'll be surprized should it turn out that the observable universe is not flat, and of course that has a huge ramification because if the OU is exactly flat, then so is the total universe (well ... there IS some argument about that conclusion but I'm dubious about the arguments).