# Can we seeing the past?

#### terencesiu

I am watching a movie about a person who can see the future. I have thought a lot of time and I think it is impossible, but how if we see the past? In fact, we can see the relative past of a planet which is far far away, since time is needed for light to pass to our eyes. But can we do that on a very short distance, by decreasing the speed of light/sound/etc? Does it possible? And are there any research about it? thank you.

My English is poor, hope you understand it.

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#### HallsofIvy

Homework Helper
There is no way to decrease the speed of light.

#### terencesiu

then, how about decrease the speed of sound, by making the medium tends to be vacuum? Can we hear the voice from the past?

#### DaveC426913

Gold Member
then, how about decrease the speed of sound, by making the medium tends to be vacuum? Can we hear the voice from the past?
Rarefying the medium slows the sound down but not appreciably, not practically.

Note that, even if you could find a way of making a light or sound signal travel slower, it would not allow you to see or listen to the past. It would simply mean any signal you sent after the device was in place would travel slower - which is pretty useless.

For example: email takes a few seconds to get from point to point around the Earth. If you slowed this transit time down to a year, you would not start getting emails from last year. No, it would simply mean that, when people started sending emails today, they would now take a month to arrive.

#### terencesiu

Rarefying the medium slows the sound down but not appreciably, not practically.

Note that, even if you could find a way of making a light or sound signal travel slower, it would not allow you to see or listen to the past. It would simply mean any signal you sent after the device was in place would travel slower - which is pretty useless.

For example: email takes a few seconds to get from point to point around the Earth. If you slowed this transit time down to a year, you would not start getting emails from last year. No, it would simply mean that, when people started sending emails today, they would now take a month to arrive.
I can mostly understand,
does it means that we can see the past of planets is because the medium "space" is already exists?
but, really thank you!

#### DaveC426913

Gold Member
I can mostly understand,
does it means that we can see the past of planets is because the medium "space" is already exists?
but, really thank you!
No. Light does not need a medium to travel through.

It is kind of academic to talk about seeing the past of a distant planet. Since c is the speed limit on any connection between distant points, it is kind of meaningless to compare a point at our "present" with some point at a distant planet's "present".

#### terencesiu

No. Light does not need a medium to travel through.

It is kind of academic to talk about seeing the past of a distant planet. Since c is the speed limit on any connection between distant points, it is kind of meaningless to compare a point at our "present" with some point at a distant planet's "present".
I think I have more and more questions. How does the people can observe the planet? Do they get the picture of the planet instantly, or just get the picture after the device have been set up for a long period? If they can get the message instantly, does it means that we can see the past after the speed slowing device have been operated for a long period? I think it will be funny if it can be made.

#### DaveC426913

Gold Member
I think I have more and more questions. How does the people can observe the planet? Do they get the picture of the planet instantly, or just get the picture after the device have been set up for a long period? If they can get the message instantly, does it means that we can see the past after the speed slowing device have been operated for a long period? I think it will be funny if it can be made.
What device? We don't have a device.

Our devices for observing planets are telescopes, which of course are limited to the speed of light.

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