Why is the speed of light a constant?

last SG · 2009-12-31T13:57:13+00:00

Id like to make a correction to one of the ststements:"If we consider that electric field is an element(or whatever word you prefer), then it becomes clear s...

Mrruixiangz SG

Well, this actually has got nothing to do with my homework at all (in fact, I'm not even studying). But it's the closest forum I can find for my question.

deepak.c SG

Cause light cannot be accelerated or decelerated.

MaNyZeR SG

yes it is constant at 300million metres per second...

which means that light can travel around the earth diameter 8 times per second...

however, you must know that light travels straight..

deepak.c SG

yes it is constant at 300million metres per second...

which means that light can travel around the earth diameter 8 times per second...

however, you must know that light travels straight..

Light do bend.

dkcx SG

3X10^8m/s is the speed of light in vacuum, in air its slightly slower.

Light has no mass so gravity generally does not affect it. Only very strong gravity such as that from a black hole will really affect light and cause it to bend so for general cases, we will not consider gravity when talking about light.

Mrruixiangz SG

Cause light cannot be accelerated or decelerated.

Even so, it doesn't mean speed of light has to be a constant.

Okay, for example sake: let's say a person switch on a flashlight on a moving train.

If you're standing IN the train, the light moves at a certain speed. But if you're standing OUTSIDE the train, shouldn't the light appear to move faster relative to where you're standing?

Or let's say you're walking when someone next to you switch on his flashlight, compared to when you're travelling damn freaking fast in a near-speed-of-light vehicle (i.e. time slow down for you), how can the measured speed be the same?

deepak.c SG

Even so, it doesn't mean speed of light has to be a constant.

Okay, for example sake: let's say a person switch on a flashlight on a moving train.

If you're standing IN the train, the light moves at a certain speed. But if you're standing OUTSIDE the train, shouldn't the light appear to move faster relative to where you're standing?

Not under general relativity.

dkcx SG

Even so, it doesn't mean speed of light has to be a constant.

Okay, for example sake: let's say a person switch on a flashlight on a moving train.

If you're standing IN the train, the light moves at a certain speed. But if you're standing OUTSIDE the train, shouldn't the light appear to move faster relative to where you're standing?

If i'm not wrong, what you are trying to say is related to doppler effect http://en.wikipedia.org/wiki/Doppler_effect

When you are moving, the frequency and the wavelength will change but both will change in such a way that the speed is not changed.

Do note that light is both a particle as well as a wave so it does have and follow wave properties.

Mrruixiangz SG

3X10^8m/s is the speed of light in vacuum, in air its slightly slower.

From what I've read gravity does affect light, abeit in a very very small way. As light travel away from the Earth (i.e. gravitational field), it looses energy, hence, it's wavelength increase.

Thus, for a person living at a higher level, everything on the bottom will seem to move slower, therefore, time relative to this person is faster.

But then again, I could have interpreted the book wrongly.

deepak.c SG

Abstrack from a book:

What if the source of light is moving? One might, for example,

expect light to behave like a shell being fired from a passing

warship where an observer on the seashore would expect the

speed of the ship to be added to the shell’s muzzle speed if being

fired in the forward direction, and subtracted if being fired to the

rear. The behaviour of light in this regard was checked at the

CERN laboratory in Geneva in 1964, using tiny subatomic

particles called neutral pions. The pions, travelling at 0.99975c,

decayed with the emission of two light pulses. Both pulses were

found to have the usual speed of light, c, to within the

measurement accuracy of 0.1%. So, the speed of light does not

depend on the speed of the source.

x4dish SG

light is made up of photons and does have mass, just that its much too minute for anyone to measure it satisfactory.

Dejomel SG

I'm just content as a cucumber to wait for eagle's reply :)

deepak.c SG

Wave particle duality.

Shotgun SG

Speed of light is constant and is mathematically proven to be so. Despite whatever reference frame, you will always see and measure speed of light to be c.

Eg, if you are travelling at the speed of light c, you will still see light to be travelling at c. Its a mind-fark yes. So whether you are on a train or on a speed of light vehicle, whether you measure inside or outside the vehicle, the speed of light you measure will still be 'c'. A constant speed that doesn't change. Amazing right?

In any case, the speed of light remains the same at all reference frames. So if light is constant, Einstein theorizes that it is time that dilates. That is the faster you move, time slows down. The theory is called "time dilation." Go check that out. =D

Also, gravity does affect light. The easiest example would be black holes where the gravity is so intense that light does not escape. The "event horizon" of a black hole is an imaginary "no-escape" boundary where the velocity of the gravitation is equal or higher than the speed of light. Hence, "no escape."

The other thing that was mentioned in the posts above is the doppler effect. Light traveling away from a intense gravity suffers "red-shift" if i remember my lessons correctly. http://en.wikipedia.org/wiki/Gravitational_redshift

Finally for the biggest mind-fark of all, as deepak mentioned is wave particle duality. There really is no explanation. Light or electrons are both waves and particles. And they behave differently when they are observed. Yes, observation alters the the behavior! http://www.youtube.com/watch?v=1iJG6qkeMxA

Unfortunately, there is no "real" explanation for why the speed of light is constant or any explanation to wave particle duality. This is why the more you study physics (quantum n particle), the less things make sense and add up. So even at university level, there is no real concrete explanation to the questions you asked. Questions lead to more questions but still no real answer.

Hopefully, this will change as CERN fires up its LHC... hopefully.

Shotgun SG

Just to add on and make things really even more bizarre... There is a theory that there is only ONE electron in the whole universe. It gives the illusion that there is many, but only one. A single electron "simply" travels back and forth in time.

Am I joking? No... Like I said, the more we dig into physics, the more bizarre it gets. I think there was this famous saying that goes... "science is either physics or stamp collecting."

Dejomel SG

Am I joking? No... Like I said, the more we dig into physics, the more bizarre it gets. I think there was this famous saying that goes... "science is either physics or stamp collecting."

I recall another theory, not exactly physics related.

Every second that passes, as we move foward in time, we are moving into another dimension, and there are greater powers that move us through into these dimensions.

Whenever we notice something missing or misplaced only to suddenly find it seconds later, it's because these "greater powers" had forgotten to move that particular object foward into the dimension we were in.

I lol-ed when I tried to piece that theory together with the theory of relativity.

I'm just an O' Level student dammit.

Shotgun SG

I recall another theory, not exactly physics related.

Every second that passes, as we move foward in time, we are moving into another dimension, and there are greater powers that move us through into these dimensions.

I lol-ed when I tried to piece that theory together with the theory of relativity.

I'm just an O' Level student dammit.

Haha, so much for "greater powers" when they suffer from the same "forgetfulness" as human beings.

The theory you mentioned doesn't sound like physics or science at all since those are not based on any observations. However, other dimensions are postulated to exist. We supposedly live in a 3 spatial dimensional world, and time as another dimension. Hence, we exist in a (x,y,z) coordinate in space, and (t) in time. Essentially, even if we sit still at coordinate x1, y1, and z1, our coordinate in time, will still move forward from t1 to t2.

Scientist today are still exploring to find the additional spatial dimension. Now they are looking the particle level. They think that perhaps the additional dimensions are so small that you can only "observe" or get a clue about them when you smash some particles such a photons together at the speed of light. Again, something that will have to wait til the LHC comes online.

We live in exciting times. =D

xingg SG

i love this kind of threads.so interesting and enlightening!

maxsee SG

Ask the late Einstein....

SBS2601D SG

Speed of light is not dependant on speed of source.

The implication of this is that nobody, nothing....can exceed the speed of light.

Another implication is that time therefore is relative. How fast one experiences time is therefore different from from what the other person experiences if both are moving at different velocities.

Light has a mass, and can therefore be captured by gravity.