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I was hoping that some genius could explain what we would observe in the universe around us in the following hypothetical scenario in which the earth itself were traveling at or near the speed of light. I am confident that somebody can tell me that my hypothetical situation cannot exist, but they are not really the point here, though I don't mind being proven dumb.

Scenario: Say that our solar system, jointly with the Orion Spur in which it resides, is traveling at near the speed of light as it prepares to merge with the adjacent Perseus Arm of our galaxy (some 5,000-10,000 ly away). How does that change how we see the Perseus Arm? What do we see as far as the rest of the Milky Way galaxy? How does our speed change what we see in distant galaxies? Or in other words, does the Milky Way galaxy as a whole seem to move faster or slower due to our speed? Do other galaxies seem to be moving faster or slower than their actual motion?

Hope that makes sense and hope somebody besides me finds it fascinating to consider light speed's impact on our perception.

EDIT1: I guess a component of this question is regarding red shifts and blue shifts, as well as how our observation of star evolution and galactic evolution would be impacted. Such as, would distant galaxies appear to be evolving faster or slower than actual and whether we would think they were moving toward or away from us, just due to our own near c motion?

EDIT2: I have reduced my scenarios to just one, as all responders are consistently telling me that such is better. Hope that the change aids the dialogue. My apology as a newby to this forum.

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    $\begingroup$ We are traveling arbitrarily close to the speed of light in some inertial frames. If you want to ask about scenarios in which we are traveling near the speed of light, you need to specify relative to what? $\endgroup$
    – G. Smith
    Commented Jun 14, 2019 at 0:06
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    $\begingroup$ @G.Smith : Of course you're right, but I think it's a fair bet that the OP effectively means "relative to the CMB", even if he doesn't know that he means that (and even if he doesn't know what it means). $\endgroup$
    – WillO
    Commented Jun 14, 2019 at 4:37
  • $\begingroup$ @WillO Ah, yes. That would be a good assumption for anyone trying to answer. $\endgroup$
    – G. Smith
    Commented Jun 14, 2019 at 4:46
  • $\begingroup$ Please avoid asking multiple distinct questions at once, otherwise your question may be closed for being too broad. $\endgroup$
    – PM 2Ring
    Commented Jun 14, 2019 at 8:01
  • $\begingroup$ I know there are a lot of questions, but really it is a picture which I hope to paint with my question. I am perhaps too ignorant to frame a proper "concise" question. I have read the spaceship examples that many offer, but I am wanting to understand this relative to stars and the galaxy, where large groups of objects move together relative to others. I am hoping that answers will likewise paint pictures, and not necessarily hit every unique question mark, though such is also welcome. $\endgroup$
    – Thomas
    Commented Jun 14, 2019 at 14:56

2 Answers 2

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In very very short: if the rest of the Universe travels at a velocity close to the speed of light relatively to us, everything will happen slower in the Universe, every length (or distance) in the direction of movement will be contracted (but not perpendicularly to it, hence a deformation), and electromagnetic waves will be heavily doppler shifted. Everything will be totally normal in the solar system, of course, as speed is not a thing in itself but has only a relative meaning.

This only addresses scenario 1. If our galaxy was spinning such as the outer limit travel at a speed close to c in an frame having its origin at the galactic centre and axes not spinning relatively to the average distribution of matter in the Universe, first you need quite some forces to prevent it to dislocate, and then some effects could be detected in our frame if those forces were not gravitational interaction.

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  • $\begingroup$ So, are you saying that the faster we travel, or the nearer we approach the speed of light, the slower everything around us appears to be traveling? As you mention electromagnetic waves being dopler shifted, is this a reference to certain objects looking red shifted or blue shifted? $\endgroup$
    – Thomas
    Commented Jun 14, 2019 at 14:50
  • $\begingroup$ "the nearer we approach the speed of light, the slower everything around us appears to be traveling?* - No. For example, if we move at $0.9c$ relative to the universe (e.g. relative to CMB or to distant stars), then we would see everything around us traveling at $0.9c$. However, all other movements out there would appear to us slower. For example, we would see the Earth rotating slower around the Sun. "As you mention electromagnetic waves being dopler shifted, is this a reference to certain objects looking red shifted or blue shifted?" - Correct. Blueshifted head on and redshifted behind. $\endgroup$
    – safesphere
    Commented Jun 14, 2019 at 19:23
  • $\begingroup$ @safesphere No. If the entire solar system travelled at a relativistic speed relatively to the CMB, we would definitely not see anything changed in the solar system. This is the very principle of relativity. $\endgroup$
    – Matt
    Commented Jun 16, 2019 at 9:18
  • $\begingroup$ @Thomas No, objects would have their speed, you can't say that the faster they go, the slower they go, it wouldn't work. The faster they go, the slower every process occurring in them slows down. $\endgroup$
    – Matt
    Commented Jun 16, 2019 at 9:18
  • $\begingroup$ @Thomas another thing: to better understand relativity, I suggest you should stop using the verb "to appear" and start using "to be". Things doesn't appear to be slower in CMB from our perspective and slower for us in CMB's perspective. They are, as far as "reality" can be adressed by a physicist. For the metaphysics part of this, I'm afraid you have to ask it on another board. :) :) :) $\endgroup$
    – Matt
    Commented Jun 16, 2019 at 9:29
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Massive objects cannot travel at the speed of light, so I will assume you are asking about speeds close to c. And I will assume this all is relative to the CMB.

I will address scenario 1, because there are too many questions in one.

As our solar system travels near speed c relative to the CMB, time in the rest of the universe will seem to speed up (relative to us). Now if you have a clock here on earth, it will still seem to tick normally.

It is when you try to compare this clock to another clock somewhere else (outside the Solar system) in the universe, that you will see our clock tick way slower. In fact, as you go arbitrarily close to the speed of light, our clock might never tick during the whole 13.8 billion years since our universe existed (assuming the clock always existed).

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    $\begingroup$ This is unfortunately wrong. The rest of the Universe will go at a speed close to c relatively to us, hence all clocks in the Universe will tick slower than ours. $\endgroup$
    – Matt
    Commented Jun 14, 2019 at 11:03
  • $\begingroup$ @ Matt, my questions are regarding our near c speed, but your comments are regarding the near c speed of other frames. Are you saying that it does not matter who is moving at these near c speeds, only their relative motion to one another? How does that impact a body whose motion is radial, such as the possible rotation of the galaxy? $\endgroup$
    – Thomas
    Commented Jun 14, 2019 at 15:00
  • $\begingroup$ @Thomas Matt's perspective creates what is called the SR twin paradox. Speed is symmetrically relative. Though, in your case, our Solar system is moving near speed c relative to the CMB. Thus, I was trying to create a frame where we could measure your clocks here in the solar system relative to the CMB (as the universal reference frame). I was just saying that your clocks here in the solar system will tick slower compared to clocks co-moving with the CMB, and I was suggesting that this should be treated as the rest of the universe (co-moving with the CMB in this case). $\endgroup$
    – Árpád Szendrei
    Commented Jun 14, 2019 at 15:35
  • $\begingroup$ @Thomas Matt has not replied to your comment likely because he did not receive a notification. Since you have put a space between @ and his user name, the system assumes that you are addressing to no one while "Matt" is a part of your message, but not the address. There should be no spaces in the @ address, even if the user name contains spaces. $\endgroup$
    – safesphere
    Commented Jun 14, 2019 at 19:11
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    $\begingroup$ @Thomas I am not saying that it doesn't matter who is really moving, I claim that there is no such thing as really moving. :) $\endgroup$
    – Matt
    Commented Jun 16, 2019 at 9:20

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