Light and time

Main Question or Discussion Point

Hi, so I was wondering what specifically the link is between light and time, I know that the faster you get the slower time goes for you, and if you go light speed time does not progress for you, but what is it when light bends around something without interaction? Would it cause no time to pass within the space that light bends around?

Answers and Replies

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Drakkith
Staff Emeritus
Science Advisor
Light's path is bent by gravity, just like any other object. There is nothing special about this.

Ok

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Light's path is bent by gravity, just like any other object. There is nothing special about this.
So does light effect time at all?

Drakkith
Staff Emeritus
Science Advisor
So light dosent effect time at all?
Your question is not very clear. Light has energy, so it can warp the surrounding region of spacetime just like an object with mass can. This will increase the amount of time dilation in the region, but so will anything else you put there.

Nugatory
Mentor
I know that the faster you get the slower time goes for you
You'll hear non-scientists saying that, but it's not quite right. Consider that right now, even as you're reading this, you are moving at 99% of the speed of light relative to some observer in some far distant galaxy. Do you think that time is going slower for you? Or that if that distant observer were to change his speed so that you were moving at only 98% of the speed of light relative to him, time would speed up a little bit for you?
What relativity does say is that if you can arrange to measure the time between ticks of a clock that is moving relative to you (and this is harder than it sounds) you will find that that while your clock, at rest relative to you, ticks ten times the moving clock will tick somewhat less than ten times. However, this effect is completely symmetrical - someone at rest relative to the other clock will consider hiself to be at rest and find that your clock, moving relative to him, is the one that is running slow. Thus, time does not "get slower for you" as you go faster - instead, you find that clocks moving relative to you run slower than a clock at rest relative to you.

and if you go light speed time does not progress for you
Again, that's not quite right. As far as you are concerned, you are at rest and everyone else is moving relative to you... and it is not possible for some other clock to be moving at the speed of light relative to you, so the question never arises. It is true that a light signal is always moving at the speed of light relative to you (that's actually the basis of all of special relativity) but a light signal isn't a clock. There's an FAQ on this already: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

Your question is not very clear. Light has energy, so it can warp the surrounding region of spacetime just like an object with mass can. This will increase the amount of time dilation in the region, but so will anything else you put there.
Oh ok, thank you!

You'll hear non-scientists saying that, but it's not quite right. Consider that right now, even as you're reading this, you are moving at 99% of the speed of light relative to some observer in some far distant galaxy. Do you think that time is going slower for you? Or that if that distant observer were to change his speed so that you were moving at only 98% of the speed of light relative to him, time would speed up a little bit for you?
What relativity does say is that if you can arrange to measure the time between ticks of a clock that is moving relative to you (and this is harder than it sounds) you will find that that while your clock, at rest relative to you, ticks ten times the moving clock will tick somewhat less than ten times. However, this effect is completely symmetrical - someone at rest relative to the other clock will consider hiself to be at rest and find that your clock, moving relative to him, is the one that is running slow. Thus, time does not "get slower for you" as you go faster - instead, you find that clocks moving relative to you run slower than a clock at rest relative to you.

Again, that's not quite right. As far as you are concerned, you are at rest and everyone else is moving relative to you... and it is not possible for some other clock to be moving at the speed of light relative to you, so the question never arises. It is true that a light signal is always moving at the speed of light relative to you (that's actually the basis of all of special relativity) but a light signal isn't a clock. There's an FAQ on this already: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/
Thank you! I don't understand it all yet ( I'm not taking my high school physics class until next year ) but I will try to understand relativity better :)

Nugatory
Mentor
Thank you! I don't understand it all yet ( I'm not taking my high school physics class until next year ) but I will try to understand relativity better :)
When it comes to relativity, you will be way ahead of the game if you can just train yourself to always, always, always say what speeds are relative to.

You're at rest relative to the surface of the earth underneath you (unless you happen to be walking, running, or in some sort of vehicle right now), but that earth underneath you is spinning around around the planet's axis, moving around the sun at many kilometers/sec, the sun is wandering through the galaxy at even greater speeds, and the whole galaxy is racing through the universe at absurd speeds relative to other distant galaxies. So it makes no sense to talk about a speed without saying what it's relative to.

When it comes to relativity, you will be way ahead of the game if you can just train yourself to always, always, always say what speeds are relative to.

You're at rest relative to the surface of the earth underneath you (unless you happen to be walking, running, or in some sort of vehicle right now), but that earth underneath you is spinning around around the planet's axis, moving around the sun at many kilometers/sec, the sun is wandering through the galaxy at even greater speeds, and the whole galaxy is racing through the universe at absurd speeds relative to other distant galaxies. So it makes no sense to talk about a speed without saying what it's relative to.
Thanks ! Also, so in the way of light, so using light as a form of mass, or energy to distort the relativity in time ( and matey from this I mean in a place that has little gravity so that that isn't effecting anything ) if you where to bend the light around an excellerating object what effect would that have on the time relativity of the space being spiraled around?

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Nugatory
Mentor
if you where to bend the light around an accelerating object what effect would that have on the time relativity of the space being spiraled around?
As phrased, that question cannot be answered because the phrase "time relativity of space" makes no sense and I'm not sure what you do mean by it. I will suggest, though, that you'll find it easier to discuss general relativity is you first nail down your understanding of special relativity, which covers the behavior of time and light in flat space-times.

As phrased, that question cannot be answered because the phrase "time relativity of space" makes no sense and I'm not sure what you do mean by it. I will suggest, though, that you'll find it easier to discuss general relativity is you first nail down your understanding of special relativity, which covers the behavior of time and light in flat space-times.
Ok, also by time relativity of space, I meant the time for that specific part of space as compared to its surroundings, also sorry in the last thing I posted I didn't mean I'm I meant in.