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Example: If a football game between the Pittsburgh Steelers @ Tennessee Titans will happen in 3 days Sunday 10/25/2020 at 1pm, why can't light leave today 10/22/2020 and go to where the Earth will be on Sunday and transmit that information back to an observer prior to them arriving at the location on Sunday?

This is an example of my question. I realize there is more to information transmitting. But, why can't light go to the future and transmit back in the simplest of senses prior to us arriving to that location? In my estimation light can travel 3 days into the future and back rather quickly. We know where the earth will be in exactly 72 hours. The math works out for light to be able to do this. What am i missing? Thank you.

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    $\begingroup$ Why do you assume light could do anything like that? In every physical frame of reference light travels only into the future (well, apart from strange constructs in general relativity, which as of now seem to have little to do with our physical reality). $\endgroup$
    – drfk
    Oct 22, 2020 at 16:19
  • $\begingroup$ Can you expand on what you mean by, "in every physical frame of reference light travels only into the future." is it impossible for us to send a light signal into a future location of the earth. I mean don't we know the exact location the earth will be at in 3 days? Does light not have this capability? Thanks for responding. $\endgroup$
    – Non Random
    Oct 22, 2020 at 16:40
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    $\begingroup$ I think it is possible (in principle) to send light to the future location of earth. However, it would be hard, since light travels in 8min to the sun and earth doesn't travel that distance even in three days, so you would have to build a huge contraption of mirrors or something alike to get the light to that position in three days... However my point was simply, you seem to assume light can travel backwards in time, but it cannot. $\endgroup$
    – drfk
    Oct 22, 2020 at 16:54
  • $\begingroup$ In my thought experiment i would want the light to get to the position as fast as it can, but i guess i don't understand why it can't reflect back from that position? Is that considered backwards in time? Because isn't it really reflecting back to our future state? Im not asking it to go to yesterday or 2 weeks ago. Im asking it to go to our future. Thank you, im obviously confused with how this works. These conversations help. $\endgroup$
    – Non Random
    Oct 22, 2020 at 17:07
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    $\begingroup$ How are you sending light backwards in time? $\endgroup$
    – BioPhysicist
    Oct 22, 2020 at 17:26

2 Answers 2

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Weird thing about light; it doesn't really follow Newtonian mechanics. So we can fire something in a specific position to where we will be such as some planes being able to shoot themselves down. Light doesn't work that way. You see, position is relative. Light moves relative to us. Hence, if the light is a distance away from us today, it's still a distance away from us tomorrow (Even more weirdly; it will move away from us at constant rate, despite our speed). If you're completely unfamiliar with this topic, it's very unlikely that a written answer will be as clear as something like this (highly recommended for better understanding)

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  • $\begingroup$ Thank you guys for the answers. $\endgroup$
    – Non Random
    Oct 27, 2020 at 1:31
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You seem to be confusing the idea of getting to the same position as earth in three days and getting to the same time as earth in three days. Light can go almost anywhere position-wise, but time-wise it only goes forward.

The consequence of this for your thought experiment is that because light moves far, far faster than the earth, if you aim it at where the earth will be in 3 days, it will arrive well before the earth does, and then it will depart just as quickly. By the time the earth arrives, the light will be long gone, as the light is the easy winner in this race, and the light will never reach the earth in the future. You could potentially bounce the light off of something before it reaches the future location of earth to add extra time to its journey, but then the light, now with information of earth’s future, will never be able to return to the past. Again I’ll mention that light can go just about any which way in space, but it only goes forward in time.

Hope this answers your question, and let me know if you’d like me to clarify on any of the above points.

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