Was reading the 'brief history of time'. It is a very difficult read considering my level of knowledge of physics (A'level knowledge), but it's interesting anyhow.
Anyway, the first few pages mentioned that it has been theorised that there is no absolute space and no absolute time. However, it also mentioned that the speed of light was found to be constant no matter which direction it's coming from. I'm lost: how can this be?
Speed is distance traveled within a certain time, i.e. change in distance divided by change in time. However, from the way I interprete the book, change in distance is relative to where you're standing in space, while change in time is relative to how fast your body is moving, and how far you are away from a gravitational field. So how can a variable divided by a variable give a constant (assuming the two variables are not proportional).
Again, my physics knowledge is only at pre-university stage so try to keep answers simple :p
Why is the speed of light a constant?
-
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.
-
Cause light cannot be accelerated or decelerated.
-
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..
-
Originally posted by MaNyZeR:
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.
-
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.
-
Originally posted by deepak.c:
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?
-
Originally posted by Mrruixiangz:
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?
Not under general relativity.
-
Originally posted by Mrruixiangz:
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?
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.
-
Originally posted by dkcx:
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.
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.
-
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.
-
light is made up of photons and does have mass, just that its much too minute for anyone to measure it satisfactory.
-
I'm just content as a cucumber to wait for eagle's reply :)
-
Wave particle duality.
-
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.
-
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."
-
Originally posted by Shotgun:
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."
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.
-
Originally posted by Dejomel:
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.
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
-
i love this kind of threads.so interesting and enlightening!
-
Ask the late Einstein....
-
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.
-
Originally posted by Mrruixiangz:
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.
Was reading the 'brief history of time'. It is a very difficult read considering my level of knowledge of physics (A'level knowledge), but it's interesting anyhow.
Anyway, the first few pages mentioned that it has been theorised that there is no absolute space and no absolute time. However, it also mentioned that the speed of light was found to be constant no matter which direction it's coming from. I'm lost: how can this be?
Speed is distance traveled within a certain time, i.e. change in distance divided by change in time. However, from the way I interprete the book, change in distance is relative to where you're standing in space, while change in time is relative to how fast your body is moving, and how far you are away from a gravitational field. So how can a variable divided by a variable give a constant (assuming the two variables are not proportional).
Again, my physics knowledge is only at pre-university stage so try to keep answers simple :pThe speed of Light is constant because time and distance (to put it simply) "changes" "to accommodate" (to put it simply) Light.
That is to say, if the distance is double (due to relativity, not static measurement) time would have slowed to half; the time it takes light to travel the doubled distance is half the "normal time it takes".
Therefore the speed of light remains constant.
You might think that this is ridiculous, that Time and distance can be relative because in your world (our world) Time and Distance is fixed.
Just as Newtonian gravity applies very well to earth (small objects) you need Einstein's gravity to apply to inter galactic objects.
-
If you think all that is mind boggling, ...
....there are small (earth size) objects, large (intergalactic) objects ...and then there are QUANTUM objects... behaving differently hahahaha....
-
Speed of light is constant is a theory. There are experiments and observations that validate this theory.
In simple terms, the experiment is something like this.
You travel in a space ship at 0.1 times the speed of light, for 10 months. When you come back to earth, you compare your time on the watch with your wife, you realize your watch registered 10 months has passed, but your wife one registered 1 year has passed.
And for both you and your wife, using your own equipment, you measured your speed to be 0.1 times the speed.
of course, the actual experiment was done at a more minor scale.
And they come up with the theory that time is not constant, but speed of light is.
-
You guys are mixing up general relativity and doppler effect and many other issues at the same time.
Speed of light is constant in a controlled environment. In this case, a vacum with zero or very low gravitational pulls.
Taking your train example, lets say there is a car travelling at 80kmph and the train is going at 120kmph. For a guy standing on ground both are 80 & 120 respectively, but for the guy on the train, the car is just going slower, perhaps 40kmph slower. But the fact is the car is still travelling 80kmph relative to the ground.
Speed of light is constant to a high extent in an environment with controlled variables.
You introduce Earth's atmosphere, gravitational pull e.t.c, then the speed is varied, but the variation is very negligible. Hence it is still approximated to be constant.
Only in presence of a black hole, the statement completely falls apart..
-
LOL. It's hard to understand indeed.
Again, for example sake:
Let's say a observer is standing by the road (Guy A), and another person is inside a moving car (Guy B)
Let's say Guy B, while inside the moving car, throws a ping-pong ball forward and switch on a flashlight at the same time.
To Guy A, the ping-pong ball will seem to travel faster than it is to Guy B. Yet, to both observer, the light will seem to travel at the same speed?
Or is this example wrong in the first place cuz a ping-pong ball travels a gazillion time slower than light?
Huat ah!
Anyway, I've read about the LHC also. Apparently, it was ready a long time ago but has been delayed time and again due to many accidents (conspiracy theory: ppl from the future trying to sabotoge the experiment) as well as lawsuits. Others are afraid how it may destroy the world or something. Well...
There are people who were once scared the computers might destroy the world someday too.