Light speed barrier / closing speed

In summary, the conversation discusses the concept of relativistic velocities and the invariance of the speed of light. The participants question whether two spaceships can approach each other at speeds faster than the speed of light and whether there is an absolute state of rest in the universe. They also talk about how the speed of light would appear to an observer on a stationary space station as two spaceships approach each other at .75c and turn on their headlights. The conversation ends with a recommendation to research the topic further and provides links for further exploration.
  • #1
Mojo-electro
2
0
This is something that has always bugged me about relativity... I must be missing something...

Lets say you have a space station sitting stationary out in space... a spaceship approaches it from one side at .75c...another ship approaches the station from the other side, also at .75c, still no laws of physics violated yet that I can think of... The closing speed of the 2 ships relative to each other is now 1.5c (unless I am missing something)... the 2nd ship turns on it's headlights, what would the the first ship see?

Given the invariance of the speed of light, it seems that the light from the other ship would approach at c (which, as is my understanding, it would in any reference frame) but the ship itself is approaching at 1.5c...

This leads me to something I have never understood fully... according to relativity, all descriptions of motion only make sense in terms of motion relative to some other frame of reference... yet there is an absolute speed limit, the speed of light... "nothing can move faster than the speed of light"... nothing can movine faster than light relative to what exactly? Can 2 spaceships not approach each other at faster than c? If so, than can c be exceded in some reference frames?

Is there really a state of absolute rest relative the the background universe.. is this the reference frame I am missing?
 
Physics news on Phys.org
  • #2
Mojo-electro said:
Lets say you have a space station sitting stationary out in space... a spaceship approaches it from one side at .75c...another ship approaches the station from the other side, also at .75c, still no laws of physics violated yet that I can think of... The closing speed of the 2 ships relative to each other is now 1.5c (unless I am missing something)...
Right. But that closing speed is according to the space station. It is not the relative speed of the spaceships.
the 2nd ship turns on it's headlights, what would the the first ship see?
Light coming at it at its usual speed.

Given the invariance of the speed of light, it seems that the light from the other ship would approach at c (which, as is my understanding, it would in any reference frame) but the ship itself is approaching at 1.5c...
From the frame of spaceship A, spaceship B is not approaching at 1.5c. The relative speed--given by the relativistic addition of velocities formula--is only 0.96c.
 
  • #3
Doc Al said:
Right. But that closing speed is according to the space station. It is not the relative speed of the spaceships.

Light coming at it at its usual speed.


From the frame of spaceship A, spaceship B is not approaching at 1.5c. The relative speed--given by the relativistic addition of velocities formula--is only 0.96c.

Interesting.. thanks!.. so would it be correct to say that an observer at the station would see ship B beat it's own light to ship A, but the reverse is true as seen by the pilot of ship A? I'll have to research the addition of velocities a bit more...
 
  • #4
Mojo-electro said:
so would it be correct to say that an observer at the station would see ship B beat it's own light to ship A,
No, why would you think that? An observer at the station sees ship B traveling at 0.75c. The 'closing speed' is not really the speed of anything, it's just the calculated rate at which the two ships close the distance between themselves according to the station frame. The 'closing speed' of the ships is 1.5c, but the 'closing speed' of ship A and the light from ship B is 0.75c + c = 1.75c. The light wins the race no matter how you look at it.

I'll have to research the addition of velocities a bit more...
You'll find many threads on that topic.
 
  • #5
A few links to explore:

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

http://www.edu-observatory.org/physics-faq/Relativity/SR/velocity.html"
 
Last edited by a moderator:

1. What is the light speed barrier?

The light speed barrier, also known as the speed of light, is the maximum speed at which any object with mass can travel in the universe. It is approximately 299,792,458 meters per second in a vacuum and is considered to be the fastest speed possible.

2. What happens when an object reaches the light speed barrier?

According to Einstein's theory of relativity, an object with mass cannot reach the speed of light. As an object approaches the speed of light, its mass increases and its length contracts, making it more and more difficult to accelerate. At the speed of light, an object would have infinite mass and zero length, which is physically impossible.

3. How does the light speed barrier affect time?

The closer an object travels to the speed of light, the slower time passes for that object relative to an outside observer. This phenomenon is known as time dilation and is a fundamental aspect of Einstein's theory of relativity. At the speed of light, time would essentially stop for the object, making it impossible to travel through time.

4. Is it possible to break the light speed barrier?

According to our current understanding of physics, it is not possible for an object with mass to exceed the speed of light. However, scientists are constantly exploring new theories and technologies that could potentially allow for faster-than-light travel.

5. How does the light speed barrier affect the universe?

The light speed barrier is a fundamental limit that affects the behavior of objects and the passage of time in the universe. Without this limit, the laws of physics as we know them would not hold true. The speed of light also plays a crucial role in the study of space and the observation of distant objects in the universe.

Similar threads

I Is the concept of a one-way speed of light meaningful and measurable?
    • Special and General Relativity
Replies
25
Views
2K
I Battery life on VERY fast moving object
    • Special and General Relativity
4
Replies
128
Views
2K
B Speed of Light in Free Space - Einstein's Second Principle
    • Special and General Relativity
Replies
8
Views
1K
B Questions about the Speed of Light, Time, etc.
    • Special and General Relativity
3
Replies
74
Views
3K
I One-Way Speed of Light: Is it Possible?
    • Special and General Relativity
Replies
12
Views
2K
B Why does it require an infinite amount of energy to reach the speed of light?
    • Special and General Relativity
4
Replies
130
Views
8K
B New Paradox Discovered, I Think
    • Special and General Relativity
3
Replies
98
Views
2K
I Relative Speed of Light Between Objects
    • Special and General Relativity
Replies
3
Views
946
B Speed of light during acceleration
    • Special and General Relativity
Replies
30
Views
3K
B Energy, Mass, Speed of Light: Can We Reach It?
    • Special and General Relativity
Replies
9
Views
1K

Hot Threads

Recent Insights

Back
Top