# Stargazing Our absolute speed in the cosmos

#### DeusExMachina

Hey everyone,

I've watched some youtube videos on special and general relativity.

A sentence that I heard over and over again puzzles me. "If I were in a closed space, there would be no way of distinguishing between acceleration and gravity." And there is supposed to be no way of telling how fast we are moving in absolute space.

I believe that we should be able to tell at what speed the earth is moving (with respect to the center of the Milky Way or the Center of the Universe or the fabric of space... I have no idea what the reference system would be)

Here is my thought experiment:
We we are standing in one line between two observers, one in front of us, one behind us, each at a distance of one 'light minute'
If we are not moving, they should receive the light beam at the same time.
However, if we are moving forward (all three of us in the same direction at half of the speed of light) the one behind us should receive the light beam after 40 seconds because he is travelling at 0,5c and the light is moving towards him at c ==> 60 light(c) seconds / 1,5c ==> 50 seconds
The one in front should receive the light after 2 minutes because the observer is 60 light seconds away and the light is only catching up with 0,5c (He is moving at 0,5c and the light at c)

So, shouldn't you be able to tell in which direction we're moving or have I grossly misunderstood something about relativity? Mind you, I'm not a physicist, just an economist and linguist with access to youtube ;-)

Thank you guys for any clarification.

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#### Orodruin

Staff Emeritus
Homework Helper
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I believe that we should be able to tell at what speed the earth is moving (with respect to the center of the Milky Way or the Center of the Universe or the fabric of space... I have no idea what the reference system would be)
Then you believe wrong. At least unless you want to contradict general relativity. It is fine determining relative speed to the Milky Way galaxy center - that is a relative speed. There is no such thing as the "center of the Universe".

However, if we are moving forward (all three of us in the same direction at half of the speed of light) the one behind us should receive the light beam after 40 seconds because he is travelling at 0,5c and the light is moving towards him at c ==> 60 light(c) seconds / 1,5c ==> 50 seconds
No. This is a fundamental misunderstanding of relativity. You are trying to artificially impose your "classical" Galilean thinking on top of special relativity. The Universe simply does not work like that.

So, shouldn't you be able to tell in which direction we're moving or have I grossly misunderstood something about relativity?
The latter.
The entire point of the second postulate is that the speed of light is independent on the reference frame but your entire argument is based on that it is not and therefore in conflict with actual observations.

That is dangerous if you want to learn actual physics.

#### phinds

Gold Member
I've watched some youtube videos on special and general relativity.
As Orodruin stated, that is your fundamental mistake if you actually want to learn science. These shows are strictly entertainment, not science education. They ALWAYS have a lot of stuff wrong, sometimes in the extreme.

#### PeroK

Homework Helper
Gold Member
2018 Award
I believe that we should be able to tell at what speed the earth is moving (with respect to the center of the Milky Way or the Center of the Universe or the fabric of space... I have no idea what the reference system would be)
A number of experiments were carried out in the 19th century to try to determine precisely this. At that time it was assumed that there must be an "aether", which is what space would be "made of". Nothing could ever be found to identify an absolute speed through the Cosmos. Look for Michelson-Morley as the most famous of these experiments.

Eventually, Albert Einstein proposed that there was no aether and that there was nothing in physics that required the concept of absolute motion. This led to the theory of Special Relativity.

To be fair, it was shocking and revolutionary at the time. But, 114 years later it seems strange that anyone could ever have thought otherwise.

#### BvU

Homework Helper
Hello DeusExMachina $\qquad$ $\qquad$ !

(although you might feel you've taken a spanking in your first thread here).

To be fair, your access to Utube and thereby to the internet(*) also allows you to learn about the Lorentz transform, with which you might work out your thought experiment in a correct way (if necessary with some help from PF).

(*) it's not all crap -- just nearly

#### Janus

Staff Emeritus
Gold Member
Hey everyone,

I've watched some youtube videos on special and general relativity.

A sentence that I heard over and over again puzzles me. "If I were in a closed space, there would be no way of distinguishing between acceleration and gravity." And there is supposed to be no way of telling how fast we are moving in absolute space.

I believe that we should be able to tell at what speed the earth is moving (with respect to the center of the Milky Way or the Center of the Universe or the fabric of space... I have no idea what the reference system would be)

Here is my thought experiment:
We we are standing in one line between two observers, one in front of us, one behind us, each at a distance of one 'light minute'
If we are not moving, they should receive the light beam at the same time.
However, if we are moving forward (all three of us in the same direction at half of the speed of light) the one behind us should receive the light beam after 40 seconds because he is travelling at 0,5c and the light is moving towards him at c ==> 60 light(c) seconds / 1,5c ==> 50 seconds
The one in front should receive the light after 2 minutes because the observer is 60 light seconds away and the light is only catching up with 0,5c (He is moving at 0,5c and the light at c)

So, shouldn't you be able to tell in which direction we're moving or have I grossly misunderstood something about relativity? Mind you, I'm not a physicist, just an economist and linguist with access to youtube ;-)

Thank you guys for any clarification.
Let's say you perform this experiment and get an answer that the light reaches both at the same time. All three now accelerate to 0.5c relative to where they started. You perform the same experiment. You will measure the same result. You will measure that the light reaches both of the other two at the same time.
However, if you left an observer back where you first performed the experiment, this observer would record that the light beam you sent out reached one of the other two before it reached the other.
Both you and the observer you left behind will measure the light beam as moving at c relative to themselves. As counter-intuitive as this seems, it is the way our universe is put together. Taking this into account is what gives us Relativity and leads to those effects we associate with with it (time dilation, length contraction, and even E=mc^2).

#### DeusExMachina

Thank you guys... I'll do some more reading on what you gave me and get back to you with more questions once I've processed all of it...
Don't be so hard on the internet! After all, you're part of it ;-)

#### phinds

Gold Member
Don't be so hard on the internet! After all, you're part of it ;-)
Yeah, but we're the good part

"Our absolute speed in the cosmos"

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