B Time Dilation: Is This a Reasonable Explanation?

[PeroK]

Why do you argue always with the aether, which is abolished since 118 years?

The question is: if there is no aether, in which reference frame does light propagate at $c$? Before Einsten, no one had a satisfactory answer to this question. Because: a) everyone assumed that light needed a medium in which to propagate; b) the invariance of $c$ was impossible owing to the laws of classical physics, as outlined above.

Einstein's amazing answer to the question: "in which reference frame does light propagate at $c$?", was "in all of them!". That took incredible insight, and considerable courage to publish. Not least, because in avoiding the need for an aether, Einstein had to overturn the assumed classical nature of space and time.

 

[Vanadium 50]

OP's hypothetical five-year-

I don't think thats the OP. But my advice in #9 applies to everyone.

 

[vanhees71]

The problem is that this misses out the key property of light, which is that its speed is frame invariant. The source independence of speed, which is what you describe above, is also a property of waves in a medium - so this alone is enough to rule out a pre-relativistic ballistic model of light, but not enough to arrive at relativity.

Of course that's right only for waves with a (phase) velocity of the speed of light in a vacuum. Sound waves or em. waves in a medium also relativistically behave differently. For a treatment of the relativistic Doppler effect of sound waves and other waves with phase velocities less than the speed of light in a vacuum, see

https://itp.uni-frankfurt.de/~hees/pf-faq/rela-waves.pdf

 

[Sagittarius A-Star]

that one light pulse does not overtake the other. (In your words, the "speed of light" is not dependent on the reference frame in which it is measured.

From the first statement does not follow the second. See @Ibix's posting #28.

 

[mpresic3]

I see that Gatztopher has a degree in physics. I know from the posts that the OP has some misconceptions regarding time dilation, and although I have not read all the responses has not addressed simultaneity.

I want to say, Gatztopher, take heart, my first conceptions of Special Relativity were very different and wrong from what I know today, after a few months on study. I think I had less than a week on relativity is freshman physics, and perhaps a week more in junior level classical mechanics etc. To become better versed in SR, find a good book, and read and think about all the concepts very deliberately. This is not a field you can learn overnight.

As far as having friends and family eyes pop out, this is understandable. My coversations with friends go something like this:

Me: Suppose I am going 25 MPH and a car passes me going 30 MPH, the car passing me looks like it is going at 5 MPH. (Using your example)

Friend: What difference does it make how the car looks to you, The car is going 30 MPH.

Me: Don't you see, to me, the car is going 5 MPH.

Friend: But you are moving so you cannot be a good judge of the speed. The cop on the side of the road isn't moving. That is why he/she can come after you after he gets you on the radar. Now where should we go for pizza.
'Your friends/family most likely did not have the lectures on SR that you have had.

As far as explaining relativity to children, I do not think you should get to far in the weeds, although children will likely shut out information they are not ready for. When I was 12, I found out a mu-meson (it was called that, back then), lives longer than expected because time slows down when you are moving close to the speed of light. This is a statement of the phenomenology, without a detailed explanation. I was pretty advanced but I did not begin to understand it until I was 18 to 19.

Don't get me wrong, I think it is excellent to expose (older) children to somewhat sophisticated concepts, but consider whether they are receptive or can comprehend rather advanced explanation. For example, we may tell 12 year olds the straight line is the shortest distance between two points, or the pythagorean theorem, but we generally do not introduce
geometric proofs for those statements till much later

There is good reason, apart from teachers understanding or misunderstanding that relativity is not taught in high schools, and even some colleges and universities.

 

[phinds]

time slows down when you are moving close to the speed of light

No, it does not. You, right now as you read this, are going at about 99.99% of c relative to cosmic a radiation particle. Do you feel your time is slowed down?

Others may see you as moving close to the speed of light but you see yourself at rest. That is at the heart of Special Relativity.

 

[pervect]

TL;DR Summary: The speed of light is always the same no matter how fast you're going, so the closer to the speed of light you get, the slower time passes for you so that light you see continues to go the speed it's supposed to.

I thought of this description recently and I think it's pretty intuitive, but I've gotten some side eye telling it to friends and family (maybe because relativity is screwy, maybe because I'm confused, maybe both?) so I want to get some confirmation that it's reasonable. Here goes:

If you're in a car and there's a car going faster than you, its speed from your point of view is its speed minus your speed. So, say, if you're going 20mph and it's going 25mph, then it looks like it's just going 5mph. The speed of light, however, looks like it's going the same speed no matter how fast you're going. So what happens if you're going the speed of light minus 5mph? Instead of light looking like it's going 5mph, the passage of time slows down for you until you see light going the speed it's supposed to. That's time dilation.

What do you all think, does this explanation track?

I would say no, because this explanation does not take into account the relativity of simultaneity. So, at best , it's incomplete. Missing this piece of the explanation is about like asking if the gizmo you took a part and reassembled was reaassembled correctly when there's still an extra part lying on the table...

If the term "relativity of simultaneity" isn't familiar, I'd suggest searching other PF threads, or google, for more information. See also "Einstein's Train".

 

[Peter Strohmayer]

Sagittarius A-Star #34 said:From the first statement ("that one light pulse does not overtake the other") does not follow the second ("the "speed of light" is not dependent on the reference frame in which it is measured"). See @Ibix's posting #28.

If one light pulse cannot overtake the other, and each of the light pulses was emitted by one of the two observers moving relative to the other, then the lengths of the propagation of each light pulse from the point of view of the respective observer can only be in the ratio of the Lorentz transformation to each other. There is no other solution, because also the light pulses in the opposite direction cannot overtake each other.

To tell the truth: this follows already from the first sentence I told my granddaughter in #15, the axium of the SRT: Time is the length of the propagation of a light pulse. Space is the length of the propagation of a light pulse.

It follows that both the observer S and the observer S' measure the light pulse emitted by them at the "speed of light".

It follows that one observer also measures the light pulse of the other observer at the speed of light, because the photon at the tip of his light beam has propagated together with the photon at the tip of the other light beam from the respective observer to the school gate.

So I suspect that proposition #34 does not apply. Or have I missed something?

 

[Sagittarius A-Star]

Or have I missed something?

Yes. As other mentioned, you missed the following:

Your first statement ("that one light pulse does not overtake the other") does not exclude i.e. the possibility of light propagation according to the pre-relativistic ether theory, while the second statement ("the "speed of light" is not dependent on the reference frame in which it is measured") does exclude this.

 

[Peter Strohmayer]

If the time is the length of the propagation of a light pulse and the space is the length of the propagation of a light pulse, then the "speed of light" in every inertial system is "1" by definition. (In other words, if a distance between two events in one system is light-like, then their distance in the other system must also be light-like).

If the light pulses cannot overtake each other, then the lengths of their propagation must be in the ratio of the Lorentz transformation. Therefore, from the point of view of observers moving relative to each other, there are different time spans between two events.

That is the whole SRT.

Why would I now also have to consider that in the aether, if it existed, the light pulses could not overtake each other either? What can we learn from a thought that has long been disproved?What is so remarkable and important about it, that the Lorentz-transfomation cannot be derived from the classical ether view, except that one becomes aware how important the first sentence above is? And of course the first sentence excludes the ether. The aether is old thinking, it is boring, annoying and disturbing.

 

[jbriggs444]

If the time is the length of the propagation of a light pulse and the space is the length of the propagation of a light pulse, hen the "speed of light" in every inertial system is "1" by definition.

By this reasoning, the speed of everything would be one by definition. Experience shows otherwise.

 

[Sagittarius A-Star]

Why would I now also have to consider that in the aether, if it existed, the light pulses could not overtake each other either?

To disprove the following statement:

And of course the first sentence excludes the ether.

 

[Peter Strohmayer]

By this reasoning, the speed of everything would be one by definition. Experience shows otherwise.

That is correct. But if I refer this definition to the propagation of the light, I cannot refer it to the movement of a certain ball, and it follows from it the nice definition that the velocity of the ball is a percentage of "1".

 

[Peter Strohmayer]

To disprove the following statement: ...

Only if the theorem „The time is the length of the propagation of a light pulse and the space is the length of the propagation of a light pulse.“ is correct, then also theorem „If the light pulses cannot overtake each other, then the lengths of their propagation must be in the ratio of the Lorentz transformation.“ is correct.
With an classic aether, the theorem "If light pulses cannot overtake each other, then the lengths of their propagation must be in the ratio of the Lorentz transformation" would not be correct, unless perhaps one makes contrived assumptions about the contraction and dilation of lengths and times that amount to the validity of the first theorem.
Since the validity of the Lorentz transformation is an proven fact, it results from the theorem „The time is the length of the propagation of a light pulse and the space is the length of the propagation of a light pulse.“ that there is no classic ether.

 

[Ibix]

If the light pulses cannot overtake each other, then the lengths of their propagation must be in the ratio of the Lorentz transformation.

I'm impressed your five year old understands ratios and square roots.

 

[Peter Strohmayer]

Bernhard Russel: „The same sort of change was demanded by Copernicus, when he taught that the earth is not stationary and the heavens do not revolve about it once a day. To us now there is no difficulty in this idea, because we learned it before our mental habits had become fixed. Einstein’s ideas, similarly, will seem easy to a generation which has grown up with them; but for our generation a certain effort of imaginative reconstruction is unavoidable.“ (The ABC of Relativity, S 2).