Exploring the Connection Between Light & Time

In summary, the speed of light is the fastest that information can travel, making it the most reliable way to measure time. However, due to the relativity of simultaneity, observers in relative motion will not agree on when an event occurred, even when taking into account the difference in time it takes for light to reach them. This is why we treat things differently when talking about events happening on a train, as the perceived time of the event may be different for observers in different frames of reference. Light is also easier to work with compared to sound, as its speed is constant in all reference frames.
  • #1
31moppie
2
0
I have read several books on relativity and find the subject fascinating. But there are a couple of things that I hope someone can help me understand. The big question on my mind is "why is light always associated with time"? It seems to my simple mind that light just portrays a visual representation of events and that the peculiarities of light just alter our perception of the timing of events but not of the actual time that the event occurred at d=0 Why pick light over sound for instance? I know that we can view a cosmic event and say that it occurred 93 million years ago and we acknowledge that the event didn't just occur now but much earlier in time. So why do we treat things differently when talking about events happening on a train for instance? I am not disputing the theories and experimental results discovered about light to date although I sometimes wonder if using an atomic clock, which works on electromechanical principles, can be misleading us.
 
Physics news on Phys.org
  • #2
31moppie said:
Why pick light over sound for instance?
It's a lot easier to work with, since the speed of light is the same in all reference frames. The speed of sound is not.
I know that we can view a cosmic event and say that it occurred 93 million years ago and we acknowledge that the event didn't just occur now but much earlier in time. So why do we treat things differently when talking about events happening on a train for instance?
Why do you think things are different for events on a train? You always take into account the light travel time (as needed) when interpreting observations. Do you have a specific example that you find puzzling?
 
  • #3
I think this can be a common misunderstanding, an idea that relativity is an optical illusion or that it describes how things "look". Relativistic effects are what happens after accounting for the travel time of light, whether we are talking about interstellar distances or "train car" distances. I think the usual "thought experiments" help promote this misunderstanding.
 
  • #4
Hi moppie...thats the name of a famous make of boat by Bertram, the first "deep vee" design .

The big question on my mind is "why is light always associated with time"?

The basic reason is that light (electromagnetic radiation) is the fastest that information can travel...nothing can travel from the cosmological horizon, for example, to us faster than electromagnetic waves. So the cosmic background radiation gets here as fast as anything can, yet it is a fixed finite speed, not instantaneous which is assumed in Newtonian physics.
But the actual time of travel is NOT obvious because while the light is traveling toward us space it traverses is expanding and so the light takes longer to arrive than one might otherwise think. See the Cosmology forum here for more discussion.

So why do we treat things differently when talking about events happening on a train for instance?

When two observers in relative motion "see" an event, even accounting for the difference in the time it might take for light to reach them over different distances, they still will not in general agree on when the event occurred. this is called the relativity of simultaneity. Two people in relative motion measure the same light speed, but see the other person via length contraction and time dilation (shorter distance, slower time)...time and distance vary in such a way, via lorentz transformations, that they compensate for each other reflecting a fixed speed of light (in the absence of gravity). Try reading about the pole and barn paradox as an example...
 
Last edited:
  • #5
Doc Al said:
It's a lot easier to work with, since the speed of light is the same in all reference frames. The speed of sound is not.

I think you missed my point here. It is absurd to use sound to determine how we should set our watches for obvious reasons. It "seems" to me that light is equally as absurd for the same reasons.

Doc Al said:
Why do you think things are different for events on a train? You always take into account the light travel time (as needed) when interpreting observations. Do you have a specific example that you find puzzling?

My thoughts are that although the events are perceived to happen at different times, why does anyone have to change their watch? Can't they just acknowledge that when they meet again that they saw events occur at different times and that their watches still have the same time?

Naty1 said:
Hi moppie...thats the name of a famous make of boat by Bertram, the first "deep vee" design .

You are correct and I am the proud owner of one made in 1963.

Naty1 said:
The basic reason is that light (electromagnetic radiation) is the fastest that information can travel...

It is the fastest that we know of. I still don't see how it relates to time.

Naty1 said:
When two observers in relative motion "see" an event, even accounting for the difference in the time it might take for light to reach them over different distances, they still will not in general agree on when the event occurred. this is called the relativity of simultaneity. Two people in relative motion measure the same light speed, but see the other person via length contraction and time dilation (shorter distance, slower time)...

Understood (I think) but consider this thought experiment.

We have our train rolling through the station, flashlight on the floor, target on the ceiling. We also have a sensor built into the target on the ceiling and a connection down to the rails of the track which our observer on the platform can connect to to monitor the event on the train. The person on the train will "see" the light leave the flashlight at t1 and hit the target/sensor at t2. The person on the platform will "see" the event start at the same time(he is right beside the train at the instant the flashlight is turned on) but sees the light hit the target at t3. He looks down at his monitoring equipment, however, and sees that the events actually occurred from t1 to t2. Why does the person on the train need to adjust his watch?
 
  • #6
31moppie said:
I think you missed my point here. It is absurd to use sound to determine how we should set our watches for obvious reasons. It "seems" to me that light is equally as absurd for the same reasons.
There's nothing "absurd" about using sound to set your watch. (Of course, it would be quite impractical.) Say you are across a field and we want to synchronize watches. You shout out at the exact moment that your watch reads 1:00 pm. When I hear the shout I set my watch to read 1:00 pm plus the time it took for the sound to travel from you to me.

My thoughts are that although the events are perceived to happen at different times, why does anyone have to change their watch? Can't they just acknowledge that when they meet again that they saw events occur at different times and that their watches still have the same time?
No one "changes" their watch. But if you want to correctly predict what happens, then you'd better know how to translate measurements made on a moving clock to measurements made on your clocks.

Understood (I think) but consider this thought experiment.

We have our train rolling through the station, flashlight on the floor, target on the ceiling. We also have a sensor built into the target on the ceiling and a connection down to the rails of the track which our observer on the platform can connect to to monitor the event on the train. The person on the train will "see" the light leave the flashlight at t1 and hit the target/sensor at t2. The person on the platform will "see" the event start at the same time(he is right beside the train at the instant the flashlight is turned on) but sees the light hit the target at t3. He looks down at his monitoring equipment, however, and sees that the events actually occurred from t1 to t2. Why does the person on the train need to adjust his watch?
Why do you think that the platform observer's equipment would measure the same time interval as the train observers? Why do you think train observers have to "adjust" their watches?
 
  • #7
31moppie said:
I think you missed my point here. It is absurd to use sound to determine how we should set our watches for obvious reasons. It "seems" to me that light is equally as absurd for the same reasons.
Time passes differently for different observers in relative motion, and the difference is related to c. It's not related to the speed of sound.

One thing that should be mentioned is that the invariant speed c isn't special because light travels at that speed, it's the other way around.
 
  • #8
Can't they just acknowledge that when they meet again that they saw events occur at different times and that their watches still have the same time?

Correct, they saw events occur at different times; BUT if one observer is stationary in a train station and the other observer leaves, takes the train for experiments, and returns, their respective watches will NOT read the same time when they again compare elapsed time!...The traveler's watch will reflect less elapsed time...
 

What is the concept of light and time?

The concept of light and time is the relationship between the speed of light and the passage of time. According to Einstein's theory of relativity, the speed of light is constant and time is relative, meaning that the passage of time is affected by the speed of an object. As an object approaches the speed of light, time slows down.

How does light affect time?

Light affects time by causing time dilation, which is a difference in the passage of time between two observers due to their relative speeds. The faster an object travels, the slower time passes for that object compared to a stationary observer. This is known as the time dilation effect.

How does time affect light?

Time affects light by causing light to undergo redshift or blueshift. Redshift occurs when light is emitted from an object that is moving away from an observer, causing the wavelength of the light to lengthen and appear more red. Blueshift occurs when light is emitted from an object that is moving towards an observer, causing the wavelength of the light to shorten and appear more blue.

What is the relationship between light and the speed of time?

The relationship between light and the speed of time is that the speed of light is constant, and time is relative depending on the speed of an object. As an object approaches the speed of light, time slows down for that object. This is known as the time dilation effect.

How does the study of light and time impact our understanding of the universe?

The study of light and time has greatly impacted our understanding of the universe. It has led to the development of theories like relativity, which have revolutionized our understanding of space, time, and gravity. It has also allowed us to make accurate measurements and observations of distant objects in the universe, helping us to unravel the mysteries of the cosmos.

Similar threads

  • Special and General Relativity
2
Replies
58
Views
3K
  • Special and General Relativity
Replies
20
Views
747
  • Special and General Relativity
Replies
17
Views
440
  • Special and General Relativity
2
Replies
35
Views
3K
  • Special and General Relativity
Replies
16
Views
615
  • Special and General Relativity
Replies
9
Views
1K
  • Special and General Relativity
Replies
27
Views
2K
  • Special and General Relativity
Replies
6
Views
1K
  • Special and General Relativity
Replies
4
Views
1K
  • Special and General Relativity
5
Replies
144
Views
6K
Back
Top