Another physicist here. I see that the issue of traveling at the speed of light has already been addressed. So I’ll ignore that bit. Otherwise, yes, the time dilation would make it appear to an observer that the traveler is speaking slowly. It would also make it appear to the traveler that the observer is speaking slowly.

The actually true explanation is that special relativity only applies to reference frames without acceleration or nearby gravity.
So the fact that your clock runs slow as seen from earth and earth's clocks run slow from your POV doesn't cause a paradox.
There's no way to compare clocks at this time, since information can't travel faster than light.
When you decelerate/accelerate back towards earth, you leave the realm of symmetric time dilation and earth's clocks will appear to jump ahead as you switch reference frames, so it'll show more elapsed time when you're back. But I've yet to find an intuitive illustration for that.

I'd probably just assume the traveler and receiver aren't experiencing any maleffects in this situation. Else the traveler would be cooked by all of the radiation that they would be receiving all at once.

So both sides of the conversation would view the other participant in the conversation to be speaking slowly?

The exact opposite…

I see

Yep.

That's interesting. How would this work with length contraction? Would both sides view the other participant in the conversation to be experiencing length contraction?

Yep. Relativistic effects are generally not what we would intuitively expect.

Okay, so I guess a takeaway here is that each person only observes relativistic effects in the other conversation participant, but not in themselves.

I'm still about confused though with how this would work with time dilation. Like, imagine a scenario where I go in a spaceship and approach lightspeed for a while and then come back (for me, subjectively) a short while later, only to find that I had grandkids that were all senior citizens. It makes sense in that scenario that, if I were to view Earth, time would seem to be moving slowly over there. But I don't understand why, if people on Earth were observing me, they would also observe my time to be going very slowly. Intuitively it would seem that they should observe my time as moving very fast, since relative to them it is

Correct. Everyone thinks their second and meter are unchanged. Everyone else’s second is slower and their rulers are compressed.

Hard to explain the details without using math. Relativity is not intuitive as we don’t encounter relativistic effects in everyday human life.

Relativity build upon the fact that there are no absolute reference frames. If time was absolute then sure, one person would appear slow while the other appears fast. But it isn’t absolute, it is relative. This means outcomes need to be symmetric. So a stationary observer checking out a spacecraft going fast is the same as going fast while observing a stationary spacecraft.

That's interesting, thanks for explaining.

This means outcomes need to be symmetric.

I guess I'm just confused as to how you can have situations where someone ages faster than another due to time dilation. That doesn't seem to be a symmetrical outcome. Like this scene in Interstellar (which, as I understand it, is scientifically accurate). In this scene Matthew McConaughey goes away for 3 hours and finds that, due to time dilation, his kids have aged 23 years. There doesn't seem to be a symmetry here. Because Matthew McConaughey aged slower than his children, it would be weird if, from his point of view, his kid's clocks were ticking slower than his (his kids can't also think that he has aged 23 years in 3 hours). So how do you resolve this lack of symmetry with the requirement that outcomes need to be symmetrical from both these reference frames?

Granted, in this scene Matthew McConaughey ages due to gravitational time dilation. Is that somehow make things different? Would a similar scenario not be possible with time dilation solely caused by travelling at very fast speeds?

(Please let me know if I'm not making sense with these questions, and I'll try to reword them)

Well, parts of interstellar are accurate. :) That being said, time dilation due to gravity is real. Go someplace heavy for awhile and then leave and you will travel far into the future. The spaceship-observer example is special relativity. The gravity thing is general relativity. I’m not sure I have a non math explanation here so, simply put, time dilation due to gravity is different.

You can get a similar outcome by going somewhere real fast, then turning around, and going real fast again back towards the start. In the rocket frame that may take, say 10 years, but more years will have passed by on Earth.

You may think this breaks the symmetry I brought up earlier, and it does, but that symmetry breaking occurs when the rocket accelerates a whole bunch turning around and heading back home. On the outbound journey though the rocket will think the earth clock is slow, and vice versa. Similarly, on the return journey the same thing occurs. During the acceleration phase though things gets real weird. Or weirder I should say.

Well, parts of interstellar are accurate. :)

Wait, you're telling me it's not scientifically accurate to say that love is the one thing we're capable of perceiving that transcends the dimensions of time and space? /s

You may think this breaks the symmetry I brought up earlier, and it does, but that symmetry breaking occurs when the rocket accelerates a whole bunch turning around and heading back home.

Ah, okay that answers my question then. It's the acceleration and deceleration that changes things. Thanks for bearing with me so far and answering all my questions!

You are welcome. Thanks for the interest in physics.

Though I know in real life that wouldn’t be possible because entanglement still can’t be used to transmit information faster than light.

The entire point of it, is that it can...

I want to push back on this - quantum entanglement cannot be used to transmit information faster than the speed of light. The entanglement effect does allow for instantaneous correlations, but you can’t use those correlations to transmit information on their own.

For example, imagine Alice and Bob have a pair of entangled particles. The spins are anti-correlated, so if one is spin up the other is spin down. Alice measures the spin of her particle and sees its spin up. She now knows that Bob’s is spin down. She can learn this even if he is many light years away, without needing to communicate with him.

But what information was sent? If Alice or Bob wanted to communicate a simple “yes or no” to each other, how does knowing the spin of each others particles help?

What could happen is Alice and Bob could agree that spin up is yes, down is no. And Alice could measure her particles spin, then call Bob and say “my answer is the spin of your particle” to communicate “no”. This would be physically secure encryption of her answer which is a big advantage, but it is still communicated at classical speeds.

Actually methods of quantum cryptography are more complicated and involve measuring many entangled particles to prove that no one is intercepting the information.

Oh boy, I went down a rabbit hole. This has been bothering me to the point where I called in some favors and found some public info for what the current hypothesis on "near" light speed communications is. Because light speed is impossible...for now haha. But let's use some assumptions.

First, light speed travel is possible. Second, that we have superior tracking technology. To the point where we can find the exact location of a traveling at light speed object in space.

The best option we have now is using photonics for communications. This bypasses using radio waves by using an array of lasers that are pointed toward a receiver, which is in turn pointed to the transmitter. We can see why space and light speed travel makes this difficult. It's hard to track objects in space at very high speeds with the precision needed for this level of communication.

The reason photonics is the best option is, it opens up significantly more frequencies to work with. And some frequencies travel farther and faster than others. If we ever master light speed and light speed communications, I would bet money now it's photonics. Because what's faster than light?

Let me know if you have any questions, I'd be happy to answer. Please do not ask about FTL, crazy things start happening then. <- in jest, if you wanna ask you can. Engineers I know argue about it to the point of almost fighting. But that's engineers for you.

Oh boy, I went down a rabbit hole. This has been bothering me to the point where I called in some favors and found some public info

I appreciate the dedication!

The lasers idea is interesting.

And some frequencies travel farther and faster than others.

Don't all frequencies of light travel at the same speed?

You are correct, my sincerest apologies. Light at different frequencies can carry more/less data. I will leave my mistake, as failures are the price of progress. I will not make that mistake again and be more careful. Thank you for your kindness.

Edit: Clarification; different frequencies affect data transfer rate

different frequencies affect data transfer rate

Ah that makes sense

You are correct, my sincerest apologies.

No problem mate. Thanks for all your info and the detailed responses

The last thing I'll say is, you still need to over come the Doppler effect. But if you can send large enough packets quick enough, you can mitigate some of the issues. The biggest thing is sending something across long distances quickly. That's the biggest issue. For that, unfortunately, you need faster than light. I hope the information provides a more exciting journey through your story.

In my opinion, the only way to have faster than light communications is a massive breakthrough on the quantum scale. Entanglement is so new and unknown, that I have no idea if that'll be the answer. It doesn't seem so to me, though. But quantum mechanics breaks so many laws of classical mechanics, it's crazy. You'd almost need some wormhole to communicate using classical techniques. I.e radio, photonics, etc. Which I think is where I rest. Photonics for high data transfer pointed at a wormhole. Then someone receives it in the other side. You still have Doppler effect, but much more mitigated. Hope you enjoyed my diatribe. Much love and peace to you, friend.

A wormhole is a cool idea.

Much love and peace to you, friend.

Much love and peace to you as well