# If time slows at speed of light

## Main Question or Discussion Point

It is said that the light we see from a star a million light years away is actually what happened a million years ago, that we are not seeing it at it's present state. For example, we see an explosion from a star that is a million light years away, then that means that star in real time had not really existed for the past million years, we are only now seeing its destruction because the light took a million years to reach us.

Now it is also said that as I reach the speed of light, that time slows down for me. It effects every molecule of my body somehow. I approach the speed of light, people on earth will still continue to grow old faster, or there cells will continue to decay at the normal rate, where mine somehow will be affected and not decay as fast.

Now with all this taking place as we approach the speed of light, wouldn't the same effect be the same with what we where seeing from the light of the planet that is 1 million light years away? Wouldn't time slow down around the light that we see coming from that star making it possible that what we are seeing really didn't take place million years ago, but maybe billions of years ago? Or maybe the image we see is only a few thousand years old because things slow down as we approach the speed of light.

My question is, how do we know for sure that the light from a star a million light years away really took a million years to reach us?

## Answers and Replies

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Nugatory
Mentor
It is said that the light we see from a star a million light years away is actually what happened a million years ago, that we are not seeing it at it's present state.
True. We don't see the explosion until the light from it hits our eyes.

As I reach the speed of light, that time slows down for me.
Not true. Time does not slow down for you. Right now you are moving at many miles a second, and you're changing your speed all the time as the earth rotates and orbits the sun, but nothing funny is happening with time for you.

(Also you can't reach the speed of light - the laws of physics that prohibit faster-than-light travel also prohibit travel at the speed of light).

My question is, how do we know for sure that the light from a star a million light years away really took a million years to reach us?
Well, seeing how "a million light years" is defined to be the distance that light travels in one million years, it kind of has to turn out that way.... But of course that's not really what you're asking

We don't know "for sure" because there could be some unknown physics out there that's messing up our observations. However, there is zero evidence for that possibility, and lots of evidence against it. For example, the speed of light follows from the laws of electricity and magnetism; the only way the speed of light won't be the same everywhere would be if the laws of electricity and magnetism could vary from place to place, and we have centuries of evidence that they don't.

True. We don't see the explosion until the light from it hits our eyes.

Not true. Time does not slow down for you. Right now you are moving at many miles a second, and you're changing your speed all the time as the earth rotates and orbits the sun, but nothing funny is happening with time for you.
Thank you.

(Also you can't reach the speed of light - the laws of physics that prohibit faster-than-light travel also prohibit travel at the speed of light).
Thank you for pointing out that mistake by me. I would should have said as I near the speed of light.

Well, seeing how "a million light years" is defined to be the distance that light travels in one million years, it kind of has to turn out that way.... But of course that's not really what you're asking
Thank you again. So the theory that if I where on a spaceship traveling .99999999999.5 of c and I travel that speed for two years out then two years back, I would have aged normal rate for four years. Nothing changes for me, but 20 years would have passed on earth. (I know the math is incorrect just stating the principle). So with that principle is not applied to the light that traveled, at c and would not be affected correct? Is that because the photons in the light are not affected by time as we would be?

We don't know "for sure" because there could be some unknown physics out there that's messing up our observations. However, there is zero evidence for that possibility, and lots of evidence against it. For example, the speed of light follows from the laws of electricity and magnetism; the only way the speed of light won't be the same everywhere would be if the laws of electricity and magnetism could vary from place to place, and we have centuries of evidence that they don't.
Thank you for answering my questions. I am just trying to get someone far smarter than I to shore up the fact that if light traveled six millions years, then six million years it is, period.

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Dale
Mentor
I am just trying to get someone far smarter than I to shore up the fact that if light traveled six millions years, then six million years it is, period.
The problem is that the statement "X took six million years" is an incomplete statement. It is like saying "Bob is taller than", you need something else to complete the statement.

A complete statement would be "X took six million years in Earth's reference frame". If it took six million years in Earth's frame then six million years in Earth's frame it is, period. But if it took six million years in Earth's frame then in another frame it took a different amount of time.

ghwellsjr
Science Advisor
Gold Member
I am just trying to get someone far smarter than I to shore up the fact that if light traveled six millions years, then six million years it is, period.
The problem is that the statement "X took six million years" is an incomplete statement. It is like saying "Bob is taller than", you need something else to complete the statement.

A complete statement would be "X took six million years in Earth's reference frame". If it took six million years in Earth's frame then six million years in Earth's frame it is, period. But if it took six million years in Earth's frame then in another frame it took a different amount of time.
Here is a spacetime diagram in Earth's reference frame depicting light from two exploding stars, each six million light-years away from the blue Earth on opposite sides. The dots mark off a million years of time for each body.

Note that both stars exploded six million years prior to time zero and that it took six million years for the images of the explosions to reach Earth.

Now here is a spacetime diagram for a reference frame moving to the left of the first one at 0.6c:

Note that the two stars are closer to the Earth by a factor of one over gamma. Since gamma is 1.25 at 0.6c, this puts the stars at 6/1.25 = 4.8 light-years. (You can measure this at the Coordinate Time of 0.) But that doesn't mean that it takes 4.8 years for the images of the explosions to reach Earth. In this frame the explosions don't occur at the same time and as a result, the image of the red star takes only three years while the image of the green star takes twelve years.

Now if we use a frame that is traveling at 0.999998c, we would find that the distance to the two stars would be 12,000 light-years and the time for the light to arrive from the red start would be 6000 years while the time for the light to arrive from the green star would be 6 billion years.

So you can have your thousand to one ratio and your one to a thousand ratio both in the same frame. In fact you can't have only one of the ratios for all the stars in the universe.

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ghwellsjr
Science Advisor
Gold Member
I made an obvious mistake in the previous post. I forgot to add on the "million" prefix when talking about the coordinates in the paragraph right after the last image. I should have said:

Note that the two stars are closer to the Earth by a factor of one over gamma. Since gamma is 1.25 at 0.6c, this puts the stars at 6000000/1.25 = 4.8 million light-years. (You can measure this at the Coordinate Time of 0.) But that doesn't mean that it takes 4.8 million years for the images of the explosions to reach Earth. In this frame the explosions don't occur at the same time and as a result, the image of the red star takes only three million years while the image of the green star takes twelve million years.

phinds
Science Advisor
Gold Member
2019 Award
Now it is also said that as I reach the speed of light, that time slows down for me.
As has already been pointed out, and I know you understand, you can't actually reach the speed of light.

OK, suppose you could reach .99999999999% of the speed of light? What would happen?

Well, guess what? You ARE, right now as you read this, moving at .99999999999% of the speed of light* Do you feel any different?

* relative to an accelerated particle in the LHC.