Is the speed of light the same for all observers?

In summary: Note that this doesn't mean that we can answer the question "why do the conservation laws hold?" - it is assumed that they do hold (and that's an observation not a theory). There are also some other theories that try to explain it with more fundamental reasons (e.g. causal sets, loop quantum gravity, string theory, ...). But as far as I know there is no generally accepted explanation yet. P.S.: Of course, this is only a short summary and I am not sure how accurate it is. If you want more information then I suggest you ask questions about the conservation laws in the theoretical physics forum.In summary, the statement that the speed of light is the same for all observers, regardless
  • #1
Dewbo
6
0
Is this statement true? The speed of light (about 300,000,000 meters per second) is the same for all observers, whether or not they're moving.

If so could someone please explain it to me, because it was my understanding that if an object was moving at a constant velocity it would appear to be traveling slower to an observer which is stationary than an observer traveling at any constant velocity.

Found on http://science.howstuffworks.com/warp-speed2.htm

Thanks
 
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  • #2
Dewbo said:
Is this statement true? The speed of light (about 300,000,000 meters per second) is the same for all observers, whether or not they're moving.

If so could someone please explain it to me, because it was my understanding that if an object was moving at a constant velocity it would appear to be traveling slower to an observer which is stationary than an observer traveling at any constant velocity.

Found on http://science.howstuffworks.com/warp-speed2.htm

Thanks
I didn't see anything in that link that says that light or anything else "would appear to be traveling slower to an observer which is stationary than an observer traveling at any constant velocity". Can you please copy and post the text in that link that says anything like that?
 
  • #3
The statement is false. The speed of light is constant (299,792,458 m/s) regardless of reference frame and velocity of an observer.
 
  • #4
ghwellsjr said:
I didn't see anything in that link that says that light or anything else "would appear to be traveling slower to an observer which is stationary than an observer traveling at any constant velocity". Can you please copy and post the text in that link that says anything like that?

That wasn't a quote, that was what I thought, as i said "because it was my understanding that...".
read the question more thoroughly please.
 
  • #5
Dewbo said:
That wasn't a quote, that was what I thought, as i said "because it was my understanding that...".
read the question more thoroughly please.

But then you put a link right after that statement. It is naturally implied that you learned that from that link!

So you need to thoroughly read your post and make sure you convey your message clearly.

IF that link wasn't your reference, what is the purpose for it being presented there?

Zz.
 
  • #6
AmanDhaliwal said:
The statement is false. The speed of light is constant (299,792,458 m/s) regardless of reference frame and velocity of an observer.

This doesn't answer my question you just re quoted the statement with a more accurate speed of light and different context.
 
  • #7
Dewbo said:
Is this statement true? The speed of light (about 300,000,000 meters per second) is the same for all observers, whether or not they're moving.
That statement is true (aside from AmanDhaliwal's point about it being 299,792,458 m/s instead of "about 300,000,000" m/s).

If so could someone please explain it to me, because it was my understanding that if an object was moving at a constant velocity it would appear to be traveling slower to an observer which is stationary than an observer traveling at any constant velocity.

Your understanding is wrong - speeds do not add and subtract that way. Let's say that you're at rest, somebody ("Slowguy") passes by you at speed u; and then a moment later "Fastguy" moving at some faster speed v passes by you in the same direction. You're expecting that Fastguy's speed relative to Slowguy will be (v-u). It's not; it's [tex]\frac{v-u}{1-\frac{uv}{c^2}}[/tex]
Two interesting things about this formula:
1) For speeds that are small compared with the speed of light, it comes out REALLY close to the v-u that you expected; that's why you've never noticed.
2) If v=c (that is, fastguy is actually a pulse of light) the formula comes out to give fastguy moving at speed c relative to both you and slowguy, which makes the statement that you found on the web consistent.
 
  • #8
Dewbo said:
This doesn't answer my question you just re quoted the statement with a more accurate speed of light and different context.

In a way, it did answer your question. It is one of the fundamental postulate of SR that the speed of light is a constant in ALL reference frame.

The reason why you think it might move slower is because you are using what we call "Galilean transformation", i.e. the way we add velocities. It has already been shown that this is only valid for velocity that is very much smaller than c, i.e. an approximation.

If you start with the postulate of SR, then there is a more generalized way to add velocities. If you are asking WHY c is a constant in all reference frames, we have no answer to that (yet), because that is how Nature behaves as we know it now.

Zz.
 
  • #9
Dewbo said:
Is this statement true? The speed of light (about 300,000,000 meters per second) is the same for all observers, whether or not they're moving.

If so could someone please explain it to me, because it was my understanding that if an object was moving at a constant velocity it would appear to be traveling slower to an observer which is stationary than an observer traveling at any constant velocity.

Found on http://science.howstuffworks.com/warp-speed2.htm

Thanks
That site is inaccurate, but it does explain the basics. Not clear enough? Regretfully I don't understand your (mis)understanding about observing a moving object, or what that has to do with the speed of light. No object can reach the speed of light.
 
  • #10
ZapperZ said:
If you are asking WHY c is a constant in all reference frames, we have no answer to that (yet), because that is how Nature behaves as we know it now.

Ahh I see, so no one knows why it happens but we just have to accept that it's true.

Thank you for you help.
 
  • #11
Dewbo said:
Ahh I see, so no one knows why it happens but we just have to accept that it's true. [..]
Different people give different answers as to the "why". For example, assuming that Maxwell's electrodynamics is correct then it has been shown that special relativity (incl. the aspect that you mentioned) follows from the conservation laws (conservation of momentum and energy). Perhaps it can also be shown to follow from the wave nature of matter (I'm not sure about that one).
 
  • #12
Nugatory said:
it's [tex]\frac{v-u}{1-\frac{uv}{c^2}}[/tex]

Could you please expand on the equation, I'm not familiar with the terms "tex" and "frac".
 
  • #13
Dewbo said:
Could you please expand on the equation, I'm not familiar with the terms "tex" and "frac".
What you should see is something like:

v - u
---------
1 - uv/c2

If you see terms like "tex" and "frac", then there is a problem with your browser or internet reception.
 
  • #14
Dewbo said:
Could you please expand on the equation, I'm not familiar with the terms "tex" and "frac".

See here:

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

There's also Wikipedia, but it probably has more "extra" stuff than you really want:

http://en.wikipedia.org/wiki/Velocity-addition_formula

As Harrylin noted, your browser isn't decoding LaTeX code for you for some reason. If you're going to hang around here a lot, you really should get that fixed, because we use LaTeX a lot for equations. Try asking about it in "Forum Feedback and Announcements", down at the bottom of the list of forums on our home page.
 
  • #15
I believe MathJax doesn't work in tapatalk.
 

1. What is Einstein's theory of relativity?

Einstein's theory of relativity is a scientific theory that describes the relationship between space and time. It states that the laws of physics are the same for all observers in uniform motion, and the speed of light is constant regardless of the observer's frame of reference.

2. What are the two main components of Einstein's theory of relativity?

The two main components of Einstein's theory of relativity are the special theory of relativity and the general theory of relativity. The special theory of relativity deals with the laws of physics in non-accelerating frames of reference, while the general theory of relativity includes the effects of gravity and acceleration.

3. How did Einstein come up with the theory of relativity?

Einstein developed the theory of relativity by combining his own insights with the work of other scientists, including Max Planck and Hendrik Lorentz. He also drew inspiration from thought experiments and his famous equation, E=mc².

4. What are some real-world applications of Einstein's theory of relativity?

Einstein's theory of relativity has had many practical applications, including the development of nuclear energy, GPS technology, and space travel. It has also played a crucial role in our understanding of the universe and the behavior of objects at high speeds.

5. Is Einstein's theory of relativity still relevant today?

Yes, Einstein's theory of relativity is still considered one of the most important and influential theories in modern physics. It has been extensively tested and confirmed through experiments and observations, and it continues to be used in various fields of science and technology.

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