# Why doesn't light go through walls then?

1. May 15, 2006

### disregardthat

If x-ray beams, which has less wavesize than light, go through walls. And radiowaves, which has higher wavesize than light, go through walls. Why don't light go through walls then?

2. May 15, 2006

### Danger

It will, if the wall is made of glass. Almost everything is opaque to some EM frequencies. Most of our building materials just happen to be opaque to the visible spectrum. It depends upon how the atomic structure within the material interacts with particular wavelengths.

3. May 17, 2006

### disregardthat

ah, ok, thought it the answer would be something like that. I just have another question about light:

I read that lights speed in vacuum is a little under 300 000 km\h.

If you throw a rock forward in 20km\h from a car that is driving in 20km\h the rock will go in 40 km\h if you observe it from the ground. But if light is sent from the earth forward in the direction the earth goes. (lets say that the earth are moving in 10 000 km\h (not that i know) ). will the light speed up 10 000 km\h then? 300 000 + 10 000 = 310 000 km\h. will the light be going in 310 000 km\h then?

4. May 17, 2006

### Neohaven

The basis for relativity was the fact that light goes at a constant speed, regardless of the frame of reference.

We have object A going at 0.5c and it sends out a beam of light. the beam of light goes at c, from the point of view of any observer, on the ship, or "stationary".

(c is the named constant for the speed of light in a vacuum.)

5. May 17, 2006

### rbj

$$v = v_1 + v_2$$

it's more like

$$v = \frac{v_1 + v_2}{1 + \frac{v_1 v_2}{c^2}}$$

if you're in a car going 100 km/hr and you throw ahead of you a ball at 30 km/hr, the person on the ground will measure it to be flying by him at 130 km/hr only because of the imperfect precision of measurement. it is actually moving slightly slower than 130 km/hr. the closer either your car or your ball gets to lightspeed, the more pronounced the effect.

6. May 17, 2006

### disregardthat

you said that the light would go at "c" speed in the observation of every point? so you mean that a man standing still, will see it in 300 000 km\h and the man i movement of 200 000 km\h would ALSO see it in 300 000km\h?? wouldnt the moving man see the light in a speed of 100 000 km\h?

7. May 17, 2006

### clj4

No, this incorrect, "rbj" has already given you the correct formula of speed addition:

$$v = \frac{v_1 + v_2}{1 + \frac{v_1 v_2}{c^2}}$$

So, for $$v_1=v_2=20$$ it is:

(20+20)/(1+20*20/300,000^2), a little less than 40km/s

No, this is incorrect as well, light speed has always the same speed regardless of the relative motion of the source and of the observer. So , it is 300,000km/s. If you don't believe it, make $$v_1=c$$ in the above formula, see what you get (this is not a proof, it is a form of verifying that what I told you is true).

Last edited: May 17, 2006
8. May 17, 2006

### Neohaven

A man staying still in regard to the earth, for example, would see the light beam travelling at 300000km/h. Another man, following that same beam of light at 200000km/h would also see it at 300000km/h in front of him. Time extends and length contracts when you go faster and faster, keeping the speed of light constant.

9. May 18, 2006

### disregardthat

:S that is un-understandable. The light is constant i have understood that, but a man travelling at a speed of 200 km\h in the same direction of the light, should see the light 100 000 km\h faster than him.

10. May 18, 2006

### disregardthat

What is the photons that is light made of?

11. May 18, 2006

### Ich

I don´t think you understood. It means that everyone will measure the speed of light to be the same value c, no matter where the light came from or whether he moves relative to the source. That´s a fact.
Given that, one has (of course) to conclude that v = v1+v2 is wrong. That is understandable. There are other consequences, all of them are called SR.

12. May 18, 2006

### GOD__AM

The speed of light in a vacuum is a little less than 300,000 km/sec, not km/h, as it has been improperly stated a few times in this thread.

13. May 18, 2006

### rbj

i changed "km\h" to "km/s".

no. that is precisely what one of the main postulates of special relativitly speaks to.

the postulates say that no inertial frame is qualitatively different (or "better") than any other inertial frame of reference and that we can't tell the difference between a "stationary" vacuum and a vacuum "moving" past our faces at a high velocity, that there is no difference and that Maxwell's Equations should work the same for any and all inertial frames so then the speed of E&M must be measured to be the same in all inertial frames, even if it is the same beam of light viewed by two observers moving relative to each other.

from these postulates, we get time dilation, then length contraction, then Lorentz transformation, then mass dilation (or relativistic momentum), then relativistic kinetic energy, and from that E=mc2.

the man traveling at a speed of 200000 km/s relative to what or whom? maybe he thinks he is stationary and not traveling at any speed at all! and there is nothing to contradict that. say you and he are both in space with no other objects around. if you think he is whizzing past you at 200000 km/s in some direction, he can just as legitimately say that it is you that is whizzing past him (but in the opposite direction) and that he is not moving at all. both perspectives are equally valid.

now say that you are both observing the very same beam of light. doesn't matter whether he is holding the flashlight or you are, once the E&M wave leaves the flashlight, it has no "knowledge" of the "boost" you think it might have gotten from a possibly moving source (relative to some observer). it's just a changing E field causing a changing B field which is causing a changing E field which is causing a changing B field, etc. that's it! that's what Maxwell's Equations say will happen. and the solution to Maxwell's Equations say that this propagation of the E and B field is at $c = 1/ \sqrt{\epsilon_0 \mu_0}$. doesn't matter if the E&M wave got a boost from a "moving" source or not. once it is in free space, that "boost" is not part of the mechanism for it's propagation. the only mechanism of propagation is Maxwell's Equations.

unlike with sound waves where the air (which is matter) is the medium of travel, if the wind is blowing by your face, from left to right, at a speed of 30 m/s, you will measure the speed of sound to be 60 m/s faster going from left to right than you will measure the speed of sound going from right to left. but there is no air or medium (called "aether") other than the vacuum that E&M waves travel in. that is what Maxwell's Equations say and that is what is confirmed experimentally with the Michaelson-Morley experiment. so if there is no medium that conducts the propagation of E&M waves and if you can't tell the difference between a "stationary" vacuum from a "moving" vacuum whizzing past your face at 200000 km/s, perhaps that concept of a "moving" vacuum is meaningless and just not there at all. if that is the case, then how do you construct a physical mechanism for why anyone observing the propagation of an E&M wave from different inertial frames of reference would see the propagation of that wave to be different in any manner?

Last edited: May 18, 2006
14. May 18, 2006

### finchie_88

Just to make sure, if there is an observer moving towards a "stationary" light source, then will there be blue shift? And likewise, if the observer is moving away from the source, would there be red shift? if so, how do you calculate it?

P.S. Is there something like a background medium that EM waves travel through, yet we can only see the 3D effects caused? Can this medium (if it exists) be detected?

Last edited by a moderator: May 18, 2006
15. May 18, 2006

### clj4

Yes, you are correct.
here is a very good treatment:

http://www.mathpages.com/rr/s2-04/2-04.htm

The answer is "no" to the first question , resulting in a "no" to the second .Lots of people spent a lot of time and a lot of experiments on this issue.

16. May 19, 2006

### finchie_88

I'm assuming that no one is still researching this area any more then. Also, one final question about relativity: Is space-time itself a physical thing (i.e. is it really there) or is it simply a convenient way of describing what is seen? As in, was it "invented" to help explain things like the orbit of Mecury and gravitational lensing of light around the sun?

17. May 19, 2006

Staff Emeritus
It wasn't invented as an epicycle, to account for some phenomenon, as you conjecture. Minkowski devised it basically as a natural expression of the symmetries of special relativity. Einstein originally resisted it, but then saw that by making it dynamic and allowing dynamics to curve it, he could explain gravity.

As to its reality, scientists differ. See the new thread on the "hole argument". Einstein came up with this and it caused him to doubt his theory for two years. He finally concluded that the only "reality" to it was in the equivalence classes of diffeomerphisms.

18. May 19, 2006

### pervect

Staff Emeritus
Once a result has been found and replicated a few times, there is not much interest in paying to keep replicating it endlessly.

Anyone who is paying attention and is capable of being convinced by experimental results is already convinced after a few replications.

However, this is not to say that funding is totally dead. Proposals to test the predictions of relativitiy more precisely and accurately than previous experiments are still done. To date, relativity has passed all the tests put to it.

This sounds like a good candidate for the philosophy forum to me. Philosophical questions can be distinguished from scientific ones by the fact that they can't be decided by experiment. As a result, they usually can be argued endlessly.

19. May 19, 2006

### masudr

I don't want to annoy pervect by answering this (and in fact I won't) but spacetime is definitely a dynamic thing; in fact as dynamic as everything else that we consider to be physical. I don't know if you knew that already (and hope not to patronise if you did) but whether or not that adds some element of physical existence is up to you.

20. May 20, 2006

### pervect

Staff Emeritus
Heh - well, sometimes I get a little testy with philosophy, but I suppose it has its place (it is a recognized human endeavor).

I just think that the best place for philosophy is the philosophy forums as a general rule:-)

If you think you can "answer" a philosophical question, don't let me stop you. But please don't blame me if you find you get sucked into a very long argument by trying to do so....

Last edited: May 20, 2006