Have a question about speed of light

[whyonlyme]

Everyone know speed of light is nearly 1079252848.8 km/hr .

Assume that I am driving car at 40 km/hr. then I turn on the light. so why speed of light doesn't goes +40 km/hr. Why it will still nearly 1079252848.8 km/hr .

I think you will understand what I am trying to say. Help soon...

 

[Curl]

It is the speed limit of the universe. If it worked like you think it should, Maxwell's equations wouldn't make sense and the whole universe would be screwed.

 

[Redbelly98]

whyonlyme, welcome to Physics Forums.

A "simple" answer is that time runs differently for observers who are in motion relative to each other. Because of this, everybody observes a beam of light to travel at the same speed.

 

[whyonlyme]

I am driving a car. I can observe that lights speed is same. But the person who is stand on the road, why don't it observe light speed + 40...?

 

[jtbell]

Velocities don't "add" the same way in relativity as in classical physics.

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

 

[Rap]

I am driving a car. I can observe that lights speed is same. But the person who is stand on the road, why don't it observe light speed + 40...?

Because clocks and rulers don't behave the way you would expect for the person on the road. To you in your car, their rulers are shortened (Lorentz contraction) and their clocks run slower (time dilation). To you, the reason they say the speed of light is the same is because they are using shorter rulers and slower running clocks. But they will say the same about you. To them, YOU are using shorter rulers and slower running clocks, and to them, that's why you measure the same speed of light as they do.

 

[Drakkith]

Here's an interesting way of looking at it. It's probably not 100% perfect, but I think it gets the gist of it.

The speed that light travels is sort of like a universal speed limit. Nothing can travel faster than this. Light MUST travel at this speed and nothing else, no faster or slower. Now, when you are heading towards something, the light you emitted travels at the speed of light to an object ahead of you, let's say Lisa, the observer. Now Lisa measures the speed of the light you emitted as it passes her. She ALSO measures it at exactly the speed of light. How you ask? Well, since the speed of light is the speed limit, that extra "Kick" of energy that the light received from your velocity is added into the energy of the light. Not in speed though, but in frequency. Lisa will see the light coming from you as being BLUESHIFTED.

When you pass Lisa and you are headed away from her, the light from you still travels at light speed for both you and Lisa. However, this time when Lisa observes the light she finds that it is REDSHIFTED. Since you were going away from her, the light had to "borrow" energy from itself to travel at light speed and thus had its energy reduced by reducing the frequency.

Does that make sense?

 

[Nabeshin]

Here's an interesting way of looking at it. It's probably not 100% perfect, but I think it gets the gist of it.

The speed that light travels is sort of like a universal speed limit. Nothing can travel faster than this. Light MUST travel at this speed and nothing else, no faster or slower. Now, when you are heading towards something, the light you emitted travels at the speed of light to an object ahead of you, let's say Lisa, the observer. Now Lisa measures the speed of the light you emitted as it passes her. She ALSO measures it at exactly the speed of light. How you ask? Well, since the speed of light is the speed limit, that extra "Kick" of energy that the light received from your velocity is added into the energy of the light. Not in speed though, but in frequency. Lisa will see the light coming from you as being BLUESHIFTED.

When you pass Lisa and you are headed away from her, the light from you still travels at light speed for both you and Lisa. However, this time when Lisa observes the light she finds that it is REDSHIFTED. Since you were going away from her, the light had to "borrow" energy from itself to travel at light speed and thus had its energy reduced by reducing the frequency.

Does that make sense?

A cute explanation, but fails to account for things like the transverse doppler shift. Is sort of a neat semi-classical way of thinking of it though.

 

[Drakkith]

A cute explanation, but fails to account for things like the transverse doppler shift. Is sort of a neat semi-classical way of thinking of it though.

Yep. I find that it seems to make a lot of sense to people when I explain it to them like this. At least compared to most of the other ways I've seen it described.

 

[GrayGhost]

I am driving a car. I can observe that lights speed is same. But the person who is stand on the road, why don't it observe light speed + 40...?

Velocity is distance/time. Let's consider light's velocity ... c. The poor fellow on the road hitch-hiking measures the speed of light at x/t = c. You in your car at 40km/hr measure light's speed at X/T = c. You both get the same measurment of c. Therefore ...

x/t = c = X/T​

The reason you both obtain the same value of c, is because they each measure distance and time differently. That is x<>X and t<>T. So you will always measure space and time differently to "just the right tune" such that light's speed will always be measured at c. Einstein began with the assumption that light's speed was invariant c (ie. it was raised to a postulate), and then derived how each of 2 inertial observers of relative motion must measure space and time such that an invariant c was maintained. This derivation led to his spacetime transformations known as the Lorentz transformations, which map every point in space and time of the one POV to a corresponding unique point of the other's POV.

GrayGhost

 

[harrylin]

Everyone know speed of light is nearly 1079252848.8 km/hr .

Assume that I am driving car at 40 km/hr. then I turn on the light. so why speed of light doesn't goes +40 km/hr. Why it will still nearly 1079252848.8 km/hr .
I think you will understand what I am trying to say. Help soon...

I'm not sure that the others explained it in a for you easy to understand way (I'm afraid they skipped the introduction), so here's my attempt.

You think perhaps that light is made up of particles that are emitted like bullets, with a fixed speed relative to it, perhaps also depending on the kind of source that emits them.

Instead, the speed of light is independent of the source and the speed of the source. That is a hypothesis from Maxwell's electromagnetic wave theory, and it has never been disproved. It was even raised to a postulate. So, the simple model of light as a wave in a medium works rather well for that riddle.

However, no model is perfect. For example, it turned out to be impossible to detect any speed of a light medium, and according to most(?) quantum physicists, light excitations do not spread out like a sound wave.
Still, concerning the aspect of light speed, the concept of light as a vibration provides a better intuition than the concept of light as made up of bullets.

Regards,
Harald

 

[HallsofIvy]

By the way, the fact that the speed of light is the same for any observer influences how we see (and "add") all speeds. If you are moving (relative to me) at speed u and you throw something in the direction you are moving at speed v, then its speed relative to me is NOT 'u+ v'. It is, rather,
$$\frac{u+ v}{1+ \frac{uv}{c^2}}$$
where "c" is the speed of light. In particular, if v= c (you shine a light in front of you) I will see its speed at
$$\frac{c+ v}{1+ \frac{cv}{c^2}}= \frac{c+ v}{1+ \frac{v}{c}}$$
$$= \frac{c+ v}{\frac{c+ v}{c}}= c$$
going back to the fact that the speed of light is c relative to any observer.

 

[icester]

Everyone know speed of light is nearly 1079252848.8 km/hr .

Assume that I am driving car at 40 km/hr. then I turn on the light. so why speed of light doesn't goes +40 km/hr. Why it will still nearly 1079252848.8 km/hr .

I think you will understand what I am trying to say. Help soon...

Because it is not possible to measure the speed of light in your example with sufficient accuracy. So, it is assumed based on established science that it is constant.
You can claim that it is c+40km/h and no one can disprove you statement.

Mathew Orman

 

[Naty1]

No one knows why light speed is fixed...seen as the same by all observers...while both space and time are variables depending one reference (speed). Before Einstein, everybody thought the reverse was true.

Even at low speeds, say 40km/hr...things don't REALLY add as you imply...but it is such a close approximation, that we use the "approximate" addition of velocitites all the time...In fact as someone noted above, time is passing differently for one observer who is "stationary" relative to another moving at 40km/hr,,,,each sees the other's time as passing more slowly! It's kind of CRAZY! But there are MANY experimental confirmations.

All we know is what we observe: everybody sees light at a fixed, constant, universal velocity. Einstein developed the math to explain that...but he had the intuitive insight first, then developed the math to explain what HE thought...

It does NOT necessarily make "common sense"...that's why everybody had it wrong for thousands of years until Einstein.

 

[Dmitry67]

Because it is not possible to measure the speed of light in your example with sufficient accuracy. So, it is assumed based on established science that it is constant.
You can claim that it is c+40km/h and no one can disprove you statement.

Mathew Orman

Ligth speed is not 'nearly', but exactly 299'792'458m/s
299'792'458.0000000000000000000000000000000000 m/s
Exactly.

 

[harrylin]

[..] Even at low speeds, say 40km/hr...things don't REALLY add as you imply...but it is such a close approximation, that we use the "approximate" addition of velocitites all the time...In fact as someone noted above, time is passing differently for one observer who is "stationary" relative to another moving at 40km/hr,,,,each sees the other's time as passing more slowly! It's kind of CRAZY! But there are MANY experimental confirmations.
All we know is what we observe: everybody sees light at a fixed, constant, universal velocity.

Right. But note:

Einstein developed the math to explain that...but he had the intuitive insight first, then developed the math to explain what HE thought...
It does NOT necessarily make "common sense"...that's why everybody had it wrong for thousands of years until Einstein.

Actually it can make common sense if you want, as it was understood already before Einstein gave his presentation of it.

 

[Rap]

time is passing differently for one observer who is "stationary" relative to another moving at 40km/hr,,,,each sees the other's time as passing more slowly! It's kind of CRAZY! But there are MANY experimental confirmations.

All we know is what we observe: everybody sees light at a fixed, constant, universal velocity. Einstein developed the math to explain that...but he had the intuitive insight first, then developed the math to explain what HE thought...

It does NOT necessarily make "common sense"...that's why everybody had it wrong for thousands of years until Einstein.

It makes more common sense knowing that relativity sees time as part of a larger spacetime, not something isolated from space. If you hold a yardstick in front of you going left to right, and somebody does the same with their yardstick standing next to you, and you take a picture of your yardstick and their yardstick, your yardstick will be longer in your picture than theirs. If they take the pictures, their yardsticks will be longer in their picture than yours. Space and time are like snapshots of spacetime. People traveling at different velocities take their snapshots at different angles.

 

[Naty1]

as it was understood already before Einstein gave his presentation of it.

I don't think so...never have read that. Any source for that view?

Lorentz tried to explain what he thought via length contraction alone...I think it's more accurate to say that Einstein adopted Lorentz contraction mathematics within special relativity, then went on to develop GR using other people's mathematics...but Einstein is given credit for the physical insight and interpretation by his peers of the era as far as I have read...

As I understood the development of things, the guy who coined "spacetime" after he found out about Einstein's new theory of relativity was ahead of Einstein in THAT respect...was it Minkowski??

 

[Rap]

I don't think so...never have read that. Any source for that view?

Lorentz tried to explain what he thought via length contraction alone...I think it's more accurate to say that Einstein adopted Lorentz contraction mathematics within special relativity, then went on to develop GR using other people's mathematics...but Einstein is given credit for the physical insight and interpretation by his peers of the era as far as I have read...

As I understood the development of things, the guy who coined "spacetime" after he found out about Einstein's new theory of relativity was ahead of Einstein in THAT respect...was it Minkowski??

I think Lorentz came up with the idea of the contraction, but others added to it the time dilation idea. You need both to keep the speed of light the same. Those others were all trying to force it into an idea of a motionless ether, where the ether was forcing the contraction. I don't know how they rationalized the time dilation. Einstein was the first to say there is no ether, these effect are way more fundamental, and that all physics was the same for inertial observers. He was the one who saw the true meaning of these effects. Minkowski was a mathematician who realized that Einstein was talking about a mathematical space, and cast Einstein's equations in geometrical terms. Einstein was motivated to learn the mathematics from Minkowski, and then said yes, absolutely. Same thing happened with general relativity. Einstein had this immense ability to understand intuitively what was happening, but I think it was Minkowski again who taught him tensor calculus so it could be very elegantly expressed.

 

[harrylin]

I don't think so...never have read that. Any source for that view?

Lorentz tried to explain what he thought via length contraction alone...I think it's more accurate to say that Einstein adopted Lorentz contraction mathematics within special relativity, then went on to develop GR using other people's mathematics...but Einstein is given credit for the physical insight and interpretation by his peers of the era as far as I have read... [..]

A source for the view that "it can make common sense if you want, as it was understood already before Einstein"?

Of course, the source for that are all the original papers of that time on that topic, in particular the last papers by Lorentz and Poincare just before that of Einstein. Many of those are increasingly easy to access nowadays:
- http://en.wikisource.org/wiki/Portal:Relativity

You can also find a rather imperfect overview here:
- http://en.wikipedia.org/wiki/Relativity_priority_dispute

And your claim of "length contraction alone" is definitely wrong, see for example Lorentz-1999:

"Michelson's experiment should always give a negative result, whatever transparent media wore placed on the path of the rays of light, and even if one of these went through air, and the other, say through glass. This is seen by remarking that the correspondence between the two motions we have examined is such that, if in S0 we had a certain distribution of light and dark (interference-bands) we should have in S a similar distribution, which might be got from that in S0 by the dilatations (6), provided however that in S the time of vibration be kε times as great as in S0."

Indeed, it is simply impossible to explain the relativistic effects with length contraction alone. What was your (wrong) source?

If you would like to discuss that topic more, please start a thread on it; let's not hijack this thread on the speed of light.

 

[PhilDSP]

Einstein was the first to say there is no ether

but I think it was Minkowski again who taught him tensor calculus so it could be very elegantly expressed.

Actually not. The general trend of the times just before the 1905 paper, at least outside of Britain, was to recognize that optical phenomena could not be directly correlated with a motion of an ether which then became superfluous whether it existed or not. Cohn and Bucherer for example, published models with that idea years earlier.

Wasn't it Hilbert who worked with Einstein with Tensors while developing GR?

 

[Rap]

Actually not. The general trend of the times just before the 1905 paper, at least outside of Britain, was to recognize that optical phenomena could not be directly correlated with a motion of an ether which then became superfluous whether it existed or not. Cohn and Bucherer for example, published models with that idea years earlier.

Wasn't it Hilbert who worked with Einstein with Tensors while developing GR?

Well, I'm not good with the history, just repeating the standard story line.

As for teaching Einstein tensor calculus, I remember that, just like with special relativity, Einstein had the physical insight, but then had to learn to express it mathematically. Hilbert makes sense.

 

[atomthick]

Imagine this as an analogy:

You are driving in your car having 40 km/h and there is a string of infinite length that passes through your car and you ride along the string line. At a moment you pich the string and the string vibrates the vibration is sent along the string with the same speed not depending on your velocity.

 

[bcrowell]

Einstein was the first to say there is no ether

but I think it was Minkowski again who taught him tensor calculus so it could be very elegantly expressed

[...]

Wasn't it Hilbert who worked with Einstein with Tensors while developing GR?

Einstein learned tensors from Levi-Civita's book, and I think that was long after SR was already published. He needed tensors for GR, not SR.