# The velocity of light is constant?

1. May 13, 2004

### Amphi

Hey,
I feel like pulling the hair off of my head right about now.
As I understand, and as it is posted in SR, the speed of light stays the same regardless of which frame of reference one is...well, okay. So, if I were to be in motion towards a light source or away from a light source. The light wave will always be travelling at its constant of roughly 300,000km/s in reference to me? But then again if I am between two light sources (A and B), and decide to travel towards light source "B" at a decent rate of speed, I will see the event of "B" before "A", right. So, according to my frame isn't light source "B" covering the distance much faster than "A". I know about time dilation, but it just isn't sinking in. hmmm, I can't even say what I don't understand, okay, Looking at light source "B" from my reference, time is progressing slower for "B", right? Still though, it covered the distance to reach me before "A" did,..gee,...help!!!!

2. May 13, 2004

### billy_boy_999

Amphi, you could look at it that way but is it really the same distance? If you're moving toward source "B" (and you started in the middle of "A" and "B") the distance the light from "B" has to cover is much less, is it not? in fact this non-simultaneity is a good illustration that light travels at a constant speed...

3. May 13, 2004

### Amphi

hmm, maybe I, no, let's say I'm travelling 150,000km/s towards "B". Then being in my frame of reference it would seem like "B" is coming towards me at 450,000km/s. That isn't what SR states about light's constant. I just can't get the classical physics out of the back of my head. It doesn't click.

4. May 13, 2004

### arildno

You seem to be adding velocities in the Galilean manner; this is incorrect!

5. May 13, 2004

### billy_boy_999

(don't listen to me, but this is how i comprehend relativity)

you are in a spaceship travelling away from earth at 155,000km/s. your friend flashes a ray of light at you. your friend sees the light leaving at the speed of light (300,000km/s). you see the light going past you at the speed of light. where did the 155,000km/s go? something's gotta give?

the thing that gives is time, and space...for the spaceship, space shrinks and time goes more slowly...this is how you are able to measure the same speed - speed is just a measure of distance/time...

6. May 13, 2004

### Amphi

Man,...I don't know, hmm, so the light ray will pass me, because the distance between me and earth shrinks and it has more time to cover the distance?
Am I starting to get it? But what about me? doesnt' my space-time change too? What if I reach the speed of light?

7. May 13, 2004

### Staff: Mentor

Right. The constant speed of light can be thought of as a postulate of SR.
Right.
I'm not sure what you mean by "seeing the event of B before A". I presume you mean that the moving observer "sees" the light from B before the light from A. But that depends on: When the light pulses were emitted and where the moving observer is, as well as the speed of the moving observer.

I presume you are imagining that A and B emit pulses of light "at the same time"? According to SR, observers in relative motion will observe those "simultaneous" emissions as occurring at different times. So if we assume that A & B emit their pulses at the same time with respect to a frame in which A & B are at rest, then it is true that the moving observer (moving towards B) will calculate that B emits his pulse before A does.
What distance? Look at things from the moving observer's frame. At some moment (according to the moving observer) B emits his pulse. At that moment B is a certain distance away. If you assume that the moving observer happens to be right in the middle between A & B at the moment that he calculates that B has emitted his pulse, then the distance is just the distance between the midpoint and B as measured by the moving observer.
In all seriousness, the only way to understand this stuff (and not just run in circles) is to be very precise about how you define things. There are three relativistic "effects" that work together: time dilation, length contraction, and the relativity of simultaneity. You must consider all of these effects to make sense of things according to SR.

This topic comes up just about every week in this forum. If you've got the strength, you may find parts of the following thread useful: https://www.physicsforums.com/showthread.php?t=13889
But beware, cranks abound!

8. May 14, 2004

### Amphi

Okay, one more situation to hopefully finally grab the concept.
If I am in a spaceship leaving earth at the speed-of-light, 300,000km/s, right? And sometime which would be somewhere ( because it takes time to cover distance,right?) A beam of light is sent towards me from earth.
Now, that beam of light wouldn't be able to catch up to me because I am in the same time frame of reference? So it would appear to me that it is travelling 0km/s?
Now if I turn around and fly back towards earth, this light beam would now be travelling 300,000km/s (c) because I am in a different frame of reference, right?
Tell, am I starting to understand it? And also it would still be perceived as c because of time dilation, regardless of my velocity? Even if I am going the speed-of-light myself?

9. May 14, 2004

### Staff: Mentor

Note that Einstein is describing things from the point of view of someone on the embankment!
:rofl: You don't get it at all, do you? I've looked through your previous posts--this has been explained to you over and over. Of course a person on the embankment will see an apparent "relative" velocity greater than c between the person and the light beam. (They travel towards each other.) But all observers see the light as moving at speed c with respect to their own frame. That's what "constant speed of light" means.

10. May 14, 2004

### Staff: Mentor

Material objects (like your spaceship) cannot travel at the speed of light. That's an accepted consequence of relativity.

But the point can still be made by having your ship move at 0.9c. Due to relativistic effects you will see that beam of light moving towards you at the usual speed of light, whether you are coming or going.

11. May 14, 2004

### billy_boy_999

why can't he travel at the speed of light? it just depends how you measure speed...(distance measured by stationary reference frame/time measured by inertial reference frame)...

anyways, Amphi, yes this is difficult to swallow but if you are travelling away from the earth at 300,000km/s and someone on earth flashes a beam of light that is travelling away at 300,000km/s, you will still measure that beam of light to be travelling past you at 300,000km/s!!!

this is undeniably weird!!! the fact that you're having trouble swallowing the constant speed of light means you are actually thinking about it, which is a good thing...

again, the reason you measure the same speed of light is because you are taking longer to measure it and you are measuring it with a shorter ruler...space shrinks and time goes more slowly...

Last edited: May 14, 2004
12. May 14, 2004

### Staff: Mentor

No, you can't just mix and match measurements made in different frames. (See my post in your "Relativity and Space Travel" thread.)

SR prohibits faster than light travel since it would require infinite energy.
Forget about the ship traveling at the speed of light--not going to happen. But the same point can be made with a physically realizable speed, say 0.9 c. And, yes, the speed of light will be observed by all inertial frames to be c.
I agree wholeheartedly. If you don't think this weird, you've probably missed the point. It takes a long time to develop a relativistic intuition, since things work so differently at high speeds.

13. May 14, 2004

### Staff: Mentor

Yep. You're asking the right questions, Amphi, which means you understand the concept. Accepting it is more difficult and anyone who hasn't struggled with it wasn't paying enough attetion when they first read it. Keep on it, you'll get it (you're most of the way there).

14. May 15, 2004

### Amphi

Okay, I can understand travelling towards light and observing "c". Because of time dilation caused by me being in a very different frame than the light.
Now though, again, very sorry here, you guys must be like "ohhhh not again!"
Let's say a light beam is sent chasing after another light beam ( I won't use the spaceship anymore because matter can't travel as fast as light). Aren't these two light beams in the same frame of reference concerning time, but not the same location. And wouldn't that then make the pursuing light beam appear stationary to the light beam being pursued? It would never catch up or pass during its chase, right.
Now, if the leading light beam would for some reason slow down by just a very minimul amount, the chasing light beam would catch up and pass it going 300,000km/s?
Am I still a hopeless case?

15. May 16, 2004

### Staff: Mentor

All good questions, Amphi! (Sorry for not responding earlier---it was your thread, after all!)

But the premise of your thought experiment is flawed. As soon as you say "I am in a spaceship leaving earth at the speed-of-light" you have violated SR, since no inertial frame (material body) can move at that speed with respect to anything! So for any realistic thought experiment, you must assume some less-than-light speed for your reference frame. And, yes, from any real inertial frame, the speed of light is c. It's a relativistic invariant: it's the same in every reference frame.

Also realize that this means that there is no inertial reference frame in which light is at rest! Imagine a photon's viewpoint (just for fun): there is no distance and time stands still.

But keep on thinking!

16. May 16, 2004

### Staff: Mentor

Once again, realize that there is no inertial frame in which light is at rest. From the light's point of view--there is no distance! Everything shrinks to zero.

From any normal inertial frame, those two light beams each just travel at speed c, as usual. They don't overtake each other.

As far as one beam slowing down---now you are making an assumption that is not physically realistic. So I don't know how to answer that one!

17. May 17, 2004

### Amphi

hmmmm, so, if one believes relativity (David, Doc Al, man you guys!!!!)
Okay, so if one believes in relativity, then being a light beam the whole universe shrinks down to one single little point? Being a light beam, you are everywhere at the same time. Ofcourse not for an outside observer (a true inertial frame)?

18. May 17, 2004

### Janus

Staff Emeritus