# Is the speed of light constant

1. Jul 29, 2011

### John15

A hypothetical senario. As velocity increases time slows, time dilation.
2 observers 1 in normal earth time 2nd traveling at velocity where time passes at 1/2 rate. Both observers maintain same relative observational point. For sake of senario Light speed = 100 units per second.
Measured course of 200 units. Beam of light shone down course, both observers start timers at start and stop timers as light passes the 200 unit marker.
Observer 1s timer shows 2 seconds passed, observer 2 shows 1 second passed (1/2 normal time rate)
Speed of light is constant for all observers so how can observer 2 time light at twice normal rate? Where is the above senario flawed?
Please remember this is a hypothetical senario so how observer 2 maintains velocity is not important.

2. Jul 29, 2011

I don't think your described scenario is clear. I especially wonder what is meant by the above. How do two observers "maintain the same relative observational point" when one of them is moving extremely fast relative to the other?

Another questionable part: "Measured course of 200 units."

Measured by whom? In what frame of reference?

I think that, somewhere in your formation of this problem, you may have made the error of considering one frame of reference as privileged.

3. Jul 29, 2011

### Ryan_m_b

Staff Emeritus
I agree with zadignose, your scenario is a bit confusing. Hopefully I can offer a scenario that will clear it up;

Let's say that light travels at 200 units per second.

1000 units away from Alice is a mirror that is at rest relative to her (i.e. they are a constant distance away from each other). Alice shines a torch at the mirror, taking the speed of light and the distance in ten seconds Alice will detect the light from her torch. If Alice measures the distance between her and the mirror she can calculate the speed of light using distance/time.

In another scenario Bob is in a spaceship. At rest relative to him is a mirror 1000 units away. Bob shines a torch at the mirror and at the same time immediately accelerates up to 0.5c (lets say the acceleration time was instantaneous and ignore the impossibilities for a second). By the time the light has hit the mirror Bob is only 500 units away. Eventually Bob detects the light from his torch. The time between Bob turning on his torch and the time between him detecting the reflected light is 6.66 seconds. But this does not mean that Bob measures the speed of light to be faster because he was moving relative to the mirror, if Bob was to take his and the mirror's closing speed into account he would measure the speed of light as the same as Alice!

4. Jul 29, 2011

### ghwellsjr

You are assuming that both observers can instantly know when the light passes the 200 unit maker but they cannot see that happening until the light reflects off the marker and comes back to them so that they can stop their timers.

So observer 1's timer will reach 4 seconds by the time the light travels to the marker an back to him, correct?

Now for observer 2, things are a little bit more complicated because we have to figure out where he will be when the light reflects off the marker and gets back to him and for that we will have to know how fast he is going. So why don't you figure that out instead of leaving all the work up to us. Then figure out where he will be when the reflected light reaches him.

After you do that, you will have a description of the scenario in terms of several events in the rest frame of observer 1. Then you can apply the Lorentz Transform to see what the coordinates of those same events are in the rest frame of observer 2 and you will see that he will also see that the speed of light is the same for him as it is for observer 1. Hint: in addition to time dilation there is also length contraction for observer 2.

5. Jul 29, 2011

### Staff: Mentor

The Lorentz transform involves more than just time dilation. You have forgotten to include length contraction and the relativity of simultaneity.

6. Jul 29, 2011

### John15

Thanks for the replies. To be honest I think in concepts so unless I can picture things in shall we say a real life situation then equations, like lorentz transformation, are difficult to understand unless they are explained and as I am on a teach yourself physics with no-one to help explain things it is not easy. For example why does moon orbit the earth. An answer of - the moons speed balances the gravitational force acting between them is fine I dont at this point need a lot of equations explaining exactly all the forces involved it just complicates things although I accept the maths is necessary at times.

Perhaps the senario should have been a hypothetical racecourse 200 units round with hypothetical horses capable of light speed and both observers timing at the start/finnish line.
I tried this on another forum and got absolutly nowhere apart from no-one can observe light going faster than c which as an explaination is worse than useless. After thinking about it myself I came up with the answer that in order for light to remain constant for both observers then the relative distance must 1/2 to counteract the reduction in time, which I was told was rubbish. Looking at the above replies I suspect that I may not be to far out.
Would it be possible to explain relativity of simultaneity in understandable language so I can fit it into the picture, hopefully.
Distance and time dilation as you approach c obviously opens some interesting approaches to how the universe works.

7. Jul 29, 2011

### Ryan_m_b

Staff Emeritus
The big problem is that hypothetical analogies with impossible objects can't help you up to a point but can also prevent you from getting a proper understanding. It's important to remember that analogies are simply that.

8. Jul 29, 2011

### Staff: Mentor

Do you know how to plot a straight line on a piece of graph paper given an equation of the form y=mx+b

9. Jul 29, 2011

### rede96

Hi John, I'm with you mate!

I've only been learning this stuff for a short time, but decided to do it with as little math as possible for very similar reasons. I know to really understand it I'll have to learn the math at some point, but I have to be able to visualise it first as well I guess.

For me relativity of simultaneity is simply saying that if two events are separated by any distance, and as there is speed limit to how fast we can observe those events, then it makes sense that if I am nearer to one of them, I will see it before you and vise-versa.

So if I see two events happen simultaneously, and you are moving with respect to me, then you can't see those two events happen simultaneously.

So you would need to re-think your initial thought experiment, as both observers won't agree on when the light beam was at the start and finish points.

10. Jul 29, 2011

### ghwellsjr

If you want some pictures to help you get going on what Special Relativity is all about, I made a series of animations to help someone else try to understand how the Michelson-Morley Experiment lead to SR. You can see them at post #78 and #79 on page 5 of this thread:

11. Jul 29, 2011

### John15

Following on. Time effectively stops at c, if distance also shrinks to 0 then what are the implications, I understand that at c mass becomes infinite and needs an infinite amount of energy to push it but if distance reduces to 0 then it must be density that becomes infinite and energy is trying to push it into 0 space or effectively a singularity, which obviously cannot happen, if that makes any sense.
Understand about the differences in observation, obviously not what it was called.

12. Aug 1, 2011

### rationalist76

It would not be the same observational point as one is moving. And either way, it is relative to the second observer that it is taking 1 second as he is already moving .5c

13. Aug 5, 2011

### John15

Quick question, following on.
Do length and time reduce to 0 at the speed of light (a singularity)?
If so is this related in any way to a black hole singularity.
Personally I dont see any way realistically for an infinite singularity to exist.

14. Aug 5, 2011

### ghwellsjr

No matter how much you accelerate, you can never achieve the speed of light. As soon as you stop accelerating and make a measurement of the speed of light, you will get the same answer as you got before you started.

This has nothing to do with black holes.

You are right, in special relativity, you cannot reach the speed of light.

15. Aug 5, 2011

### John15

Light obviously reaches the speed of light. Question is then what happens to it at this speed that stops it going any faster?

16. Aug 5, 2011

### Staff: Mentor

It never goes any slower either. That is just its speed, it does not suddenly stop at c because it never goes at anything other than c.

17. Aug 5, 2011

### John15

I understand that that is just its speed and that its constant whatever the speed of its source but that does not explain why. Whereas if time = 0 at that speed then we would have a time barrier much the same as the sound barrier, you cannot go backwards in time, also if length = 0 then that would also help explain why mass cannot travel at c, hence why photons have no mass, mass must have a volume.

18. Aug 5, 2011

### John15

Another thought, if t = 0 at c then after the BB when all was energy traveling at c then inflation could have taken as long as it liked as time would only have come into existance with the formation of mass which travels at less than c.

19. Aug 5, 2011

### ghwellsjr

Did you read any of the FAQs, especially this one?

That FAQ points out that the idea of time and length for a photon has no meaning. It's not that they are zero, it's pointless to discuss the concept.

As I said in an earlier response, no matter how much you accelerate, when you stop and measure the speed of light, you get the same answer as before you started. That is the reason you can never reach the speed of light, not because something wierd happens when you get there. You're always just as far away from going the speed of light as you were before you started. There is no barrier ahead that prevents you from achieving the speed of light.

I'm not sure what kind of a response to your question of why would satisfy you. Are you asking why we believe these things about light or are you asking why nature behaves this way?

20. Aug 5, 2011

### ghwellsjr

Since the idea of "t = 0 at c" is a meaningless idea, then the "if" in your thought cannot lead to a meaningful conclusion after the "then" in your thought.

Why don't you focus your thoughts on trying to understand the Theory of Special Relativity which can be understood instead of on meaningless ideas? Until you do that, I doubt that you will be able to understand why we say that "t = 0 at c" is a meaningless idea and you'll be forever confused and wondering if scientists really know what they are talking about.

21. Aug 5, 2011

### John15

Have read the FAQ. To me it says that a photon moves at the speed of light because relativity says so and time and length cease to have any meaning. How can time and length cease to have any meaning to me that means the universe ceases to exist at c.
Relativity says that a photon would have to be moving at c in its own rest frame, as everything is relative I actually see no problem with this statement even though the FAQ says it does not make sense. From what I have read nothing can exceed c so if you are traveling at c then relatively speaking light would be travelling at 0 exactly the same way as if 2 cars are both travelling at 60mph in the same direction their relative speed is 0 and the relative distance travelled would also be 0 unless they are moving in different directions. The speed and distance travelled by the cars are obviously relative to all observers in all directions moving at any speed. At c we have a different situation as observers can only observe from the side or behind and at the same or lesser speed, so to all observers you can only appear to be standing still or moving away from, you cannot be observed by anything you are moving towards so to them you would cease to exist untill you stopped in front of them, and you cannot observe anything you are moving away from so observation is limited. I assume this is where relative time and length come from.
While most people seem to accept the physical constants for what they are I cannot do so, to me there must be a reason why c is c , G is G an electron is the mass it is etc.
Other things like density going to infinity in a black hole do not make sense but seem to be accepted, nor does the BB singularity for that matter.

22. Aug 5, 2011

### ghwellsjr

More meaningless thoughts and ideas. That's what happens when you refuse to believe the answers in the FAQs.

This forum is to help people learn and understand relativity. If you don't want do that, you should quit posting or you'll likely get yourself banned as you're violating the rules.

23. Aug 5, 2011

### rede96

Hi John,

This video might help understand the reason.

It is in 3 parts and is about 30 mins in length.

Last edited by a moderator: Apr 26, 2017
24. Aug 5, 2011

### Staff: Mentor

In classical mechanics the "why" is explained by Maxwell's equations, and in quantum mechanics the "why" is explained by the fact that photons are massless.

25. Aug 5, 2011

### Staff: Mentor

Really? You see no problem with the statement that 299792458=0